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MANUAL HEAT TRANSFER LAB - Department of ME - …mech.gecgudlavalleru.ac.in/pdf/manuals/HEAT...

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MANUAL HEAT TRANSFER LAB (For III B.Tech.-II Sem. Mechanical Engineering) Department of Mechanical Engineering, Gudlavalleru Engineering College, Gudlavalleru 521356.
Transcript

MANUAL HEAT TRANSFER LAB

(For III B.Tech.-II Sem. Mechanical Engineering)

Department of Mechanical Engineering, Gudlavalleru Engineering College,

Gudlavalleru – 521356.

MANUAL

HEAT TRANSFER LAB

Department of Mechanical Engineering, Gudlavalleru Engineering College,

Gudlavalleru – 521356.

The role of experimentation in the teaching of subjects such as Heat

Transfer is well established. Properly designed experiments reinforce

the teaching of basic principles taught in lecture classes and help

students to understand these principles more thoroughly. There are

other advantages too: students learn to use basic instruments and get

acquainted with the method used to measure various physical

quantities; they also study techniques of analyzing experimental data

and presenting them in a proper form.

CONTENTS

Page

List of Experiments ...................................................................................................... i

Acceceries of Experiments .......................................................................................... ii

General Instructions to Students ................................................................................. iii

Experiments on Conduction

1. Composite Wall 05

2. Lagged Pipe 09

3. Thermal conductivity of insulating powder 12

4. Thermal Conductivity of Metal Rod 13

Experiments on Convection

5. Pin-Fin (Natural and Forced Convection) 18

6. Natural Convection 25

7. Forced Convection 31

Experiments on Radiation

8. Stefan Boltzman Constant 34

9. Emissivity of test plate 39

Experiments on two-phase flow

10. Drop and Film wise condensation 49

11. Critical Heat flux

Experiments on applications of heat transfer

12. Concentric tube Heat Exchanger 43

13. Heat Pipe (Unsteady state heat transfer) 60

i

LIST OF EXPERIMENTS

Experiments on Conduction

1. Determination of Thermal conductivity of insulation powder

2. Determination of over all heat transfer coefficient of Composite Wall

3. Determination of over all heat transfer coefficient of Lagged Pipe

4. Determination of Thermal Conductivity of given Metal Rod

Experiments on Convection

5. Determination of heat transfer coefficient of Pin-Fin (Natural and Forced

Convection)

6. Determination of heat transfer coefficient of Natural Convection

7. Determination of heat transfer coefficient of Forced Convection.

Experiments on Radiation

8. Determination of Stefan Boltzman Constant

9. Determination of Emissivity of test plate

Experiments on Applications of heat transfer and heat transfer with

phase change

10. Determination of effectiveness and overall heat transfer coefficient using

Parallel and Counter flow Heat Exchanger

11. Determination of heat transfer coefficient in drop and film wise

condensation

12. Determination of Critical Heat flux

13. Study of heat pipe and its demonstration

ii

ACCECERIES OF EQUIPMENTS

1. CRITICAL HEAT FLUX APPARATUS

i) Thermometer (0-1100C) - 1

ii) Bulb (100W) – 1

iii) Beaker (Glass) -1

2. CONCENTRIC TUBE HEAT EXCHANGER (PLAIN TUBE TYPE)

i) Thermometer (0-1100C) - 4

ii) Measuring Jar – 1

iii) Stop Watch (Glass) -1

3. HEAT PIPE APPARATUS

Thermometer (0-1100C) - 3

4. STEFAN BOLTZMANN APPARATUS

Thermometer (0-1100C) - 1

5. THERMAL CONDUCTIVITY OF METAL ROD

i) Measuring Jar (1000ml)- 1

ii) Stop Watch - 1

iii

GENERAL INSTRUCTIONS TO STUDENTS

1. Keep the variac to zero voltage position before starting the

experiments.

2. Increase voltage slowly.

3. Do not increase voltage above 150 V.

4. Keep all the assembly undisturbed.

5. Operate selector switch of temperature indicator slowly.

6. Operate all the switches and controls gently.

7. Always ensure that the equipment is earthed properly before

switching on the supply.

8. Ensure steady state heat transfer before noting down the

readings.

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Thermal conductivity of insulating powder

1

TTHHEERRMMAALL CCOONNDDUUCCTTIIVVIITTYY OOFF IINNSSUULLAATTIINNGG PPOOWWDDEERR

AIM

To determine the thermal conductivity of insulating powder at average

temperature.

INTRODUCTION

Conduction of heat is flow of heat which occurs due to exchange of

energy from one molecule to another without appreciable motion of

molecules. In any heating process, heat is flowing outwards from heat

generation point. In order to reduce losses of heat, various types of

insulations are used in practice. Various powders e.g. asbestos powder,

plaster of paris etc. are also used for heat insulation. In order to

determine the appropriate thickness of insulation, knowledge of thermal

conductivity of insulating material is essential.

APPARATUS

The apparatus consists of a smaller (inner) sphere, inside, which is fitted

a mica electric heater. Smaller sphere is fitted at the center of outer

sphere. The insulating powder, whose thermal conductivity is to be

determined is filled in the gap between the two spheres. The heat

generated by heater flows through the powder to the outer sphere. The

outer sphere loses heat to atmosphere. The input to the heater is

controlled by a dimmerstat and is measured on voltmeter and ammeter.

Four thermocouples are provided on the outer surface of inner sphere

and six thermocouples are on the inner surface of outer sphere, which

are connected to a multi channel digital temperature indicator.

SPECIFICATIONS

1. Inner sphere- 100mm O.D., halved construction

2. Outer sphere- 200mm I.D., halved construction

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Thermal conductivity of insulating powder

2

EXPERIMENTAL PROCEDURE

1. Keep dimmerstat knob at ZERO position and switch ON the

equipment.

2. Slowly rotate the dimmerstat knob, so that voltage is applied

across the heater. Let the temperatures rise.

3. Wait until steady state is reached.

4. Note down all the temperatures and input of heater in terms of

volts and current.

OBSERVAATIONS

Sl.

No.

Temperatures 0 C

Heat input

T1 T2 T3 T4 T5 T6 T7 T8 T9 T10 Volts Amps

THEORY

Consider the transfer of heat by heat conduction through the wall of a

hallow sphere formed of insulating powder (Ref.fig.)

Let, ri = raidus of inner sphere, m

r0 = radius of outer sphere, m

Ti = average inner sphere surface temp. OC

T0= average outer sphere surface temp. OC

Consider a thin spherical layer of thickness dr at radius r & temperature

difference of dT across the layer. Applying Fourier law of heat

conduction, heat transfer rate,

Q = -k. 4π . r2.[dT/dr]

Where k = thermal conductivity of insulating powder.

Therefore dTr

drx

k4

Q2

Integrating between ri to ro and Ti to To, we get

0

i

0

i

T

T

r

r

2dT

r

dr

k4

Q

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Thermal conductivity of insulating powder

3

)TT(r

1

r

1x

k4

Q0i

0i

or )rr(

)TT(rkr4Q

i0

oioi

From the measured values of Q, Ti and T0 thermal conductivity of

insulating powder can be determined as

)TT.(rr.4

)rr(Qk

oio.i

i0

CALCULATIONS

1. Heater input = Q = V x I Watts

2. Average inner sphere surface temperature, 4

4321 TTTTTi

0C

6

.3

10765

0

TTTTT

etemperatursurfacesphereouterAverage

4. Inner sphere radius = 50 mm = 0.05 m

5. Outer sphere radius = 100 mm = 0.1 m.

Now )TT.(rr.4

)rr(Qk

oio.i

i0

W / m K at C

TT ooi

2

PRECAUTIONS

1. Operate all the switches and controls gently.

2. If thermal conductivity of the powder other than supplied is to be

determined then gently dismantle the outer sphere and remove the

powder, taking care that heater connections and thermocouples

are not disturbed.

3. Earthling is essential for the unit.

RESULT

Thermal conductivity of insulating powder is ___________at temperature of

________

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Thermal conductivity of insulating powder

4

Fig. 1 Apparatus of thermal conductivity of insulating powder

1. Shell 2. Voltmeter

3. Ammeter 4. Temperature indicator

5. Selector switch 6. Main switch 7. Heater control

Fig. 2 Location of thermocouples in spherical shell

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Composite wall apparatus

5

CCOOMMPPOOSSIITTEE WWAALLLL AAPPPPAARRAATTUUSS

AIM

To determine the thermal resistance, thermal conductivity of composite

wall material and plot temperature gradient along composite wall

structure.

APPARATUS

The apparatus consists of a plates of different materials sandwiched

between two aluminum plates. Three types of slabs are provided on both

sides of heater, which forms a composite structure. A small hand press

frame is provided to ensure the perfect contact between the slabs. A

dimmerstat is provided for varying the input to the heater and

measurement of input is carried out by a Voltmeter and Ammeter.

Thermocouples are embedded between interfaces of input slabs, to read

the temperatures at the surface.

SPECIFICATIONS

Slab size:

a. M.S. - 25 cm x 25 mm thick

b. Bakelite - 25 cm x 10 mm thick

c. Brass - 25 cm x 10 mm thick

EXPERIMENTAL PROCEDURE

1. Start the supply of heater. By varying the dimmerstat adjust the

input (range 30- 70 Watts) and start water supply.

2. Take readings of all the thermocouples at an interval of 10 minutes

until steady state is reached.

WALL THICKNESS CONDUCTIVITY

a. M.S 2.5 cm 46 W / m K

b. Bakelite 1.0 cm 0.223 W / m K

c. Brass 1.0 cm 110 W / m K

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Composite wall apparatus

6

OBSERVATIONS

Sl.No. Heat Supplied

(Watts)

Temperatures 0C

Voltmeter Ammeter T1 T2 T3 T4 T5 T6 T7 T8

CALCULATIONS

1) Mean readings,

a) 2

)( 21 TTTA

0C

b) 2

)( 43 TTTB

0C

c) 2

)( 65 TTTC

0C

d) 2

)( 87 TTTD

0C

2) Rate of heat supplied, Q = V x I Watts

For calculating the thermal conductivity of composite walls, it is assumed

that due to large diameter of the plates, heat flowing through central

portion is unidirectional i.e. axial flow. Thus for calculations central half

diameter area where unidirectional flow is assumed is considered.

Accordingly thermocouples are fixed at close to center of the plates.

Now, Heat flux, 2/ mWattA

Qq

).(4

2 platesofdiahalfddxA

Total thermal resistance of composite slab:

WKmq

TTR oDA

total /2

i) Thermal conductivity of composite slab, kmWTT

bqK

DA

composite //.

b = Total thickness of composite slab = 0.045 m

ii) Plot thickness of slab material against temperature gradient.

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Composite wall apparatus

7

PRECAUTIONS

1. Keep the dimmerstat zero before start

2. Increase voltage slowly

3. Keep all the assembly undisturbed

4. Do not increase voltage above 200 V

5. Operate selector switch of temperature indicator slowly.

GRAPH

RESULT

Thermal resistance of composite wall =

Over all thermal conductivity of composite wall =

45 35 25

TD

TC

TB

TA

Bakelite

Brass

Mild Steel

Tem

pe

ratu

re,

0C

Thickness of slab

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Composite wall apparatus

8

Fig. 1 Composite wall apparatus

1. Voltmeter 2. Ammeter

3. Temperature indicator 4 Main switch

5. Heater Control 6. Water connection

Fig. 2 Thermocouple setting in composite wall apparatus

A. Heater B & B1 M.S. Plate C & C1 Bakelite D & D1 Brass plate E & E1

Cooling plate

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Lagged pipe

9

LLAAGGGGEEDD PPIIPPEE

AIM

1. To determine heat flow rate through the lagged pipe and compare it

with the heater input for known value of thermal conductivity of

lagging material.

2. To determine the approximate thermal conductivity of lagging

material by assuming the heater input to be the heat flow rate

through lagged pipe.

3. To plot the temperature distribution across the lagging material.

APPARATUS

The apparatus consists of three concentric pipes mounted on suitable

stand. The hollow space of the innermost pipe consists of the heater.

Between first two cylinders the insulating material with which lagging is

to be done is filled compactly. Between second and third cylinders,

another material used for lagging is filled. The third cylinder is

concentric to other outer cylinder. Water flows between these two

cylinders. The thermocouples are attached to the surface of cylinders

appropriately to measure the temperatures. The input to the heater is

varied through a dimmerstat and measured on voltmeter and ammeter.

SPECIFICATIONS

Pipes –i) GI pipe inside Φ6 cm. (O.D)

ii) GI pipe middle Φ8.5 cm. (Mean dia.)

iii) GI pipe outer Φ10.7 cm. (I.D)

iv) Length of pipes 1 meter.

PROCEDURE

1) Start the supplies of heater and by varying dimmerstat adjust the

input for desired value (range 60 to 120 Watts) by using voltmeter and

ammeter, also start water supply.

2) Take readings of all the 6 thermocouples at an interval of 5 min until

the steady state is reached.

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Lagged pipe

10

3) Note down steady readings in observation table.

OBSERVATIONS

1. Inner pipe O.D., D1 = 0.06 m

2. Middle pipe mean dia., D2 = 0.085 m

3. Outer pipe I.D., D3 = 0.107 m

Sl.No. Volt

meter Ammeter Thermocouple Readings

V I T1 T2 T3 T4 T5 T6

CALCULATIONS

Mean readings

C2

TTT

C2

TTT

C2

TTT

o65Outer

o43

middle

o21inside

ri = Inner pipe radius = 0.03 m

ro = outer pipe radius = 0.0535 m

rm = mean radius of middle pipe = 0.0425m

L = Length of pipe = 1m

k = thermal conductivity W / m K

Q= actual heat input = V x I Watts

Assumption: The pipe is so long as compared with diameter that heat

flows in radial direction only middle half length.

i) Now, first we find out the theoretical heat flow rate through the

composite cylinder

m

0e

2i

me

1

outsideinside

r

rLog

k

1

r

rLog

k

1

L2

1

TTQ

k1 = 0.22 W/moK & k2 = 0.13 W/m0K, where Actual heat input, Qact = V.I.

ii) Now from known value of heat flow rate, value of combined thermal

conductivity of lagging material can be calculated.

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Lagged pipe

11

)r/r(log

)ToTi(Lk2Q

i0e

act

mK/W)ToTi(L2

)r/r(logQk i0eact

The space between the pipes of φ6 cm and φ 8.5 cm contains commercial

asbestos powder and the space between pipes of φ8.5 cm and φ10.5 cm

contains saw dust.

.Km/W)TT(L2

)r/r(logQk

Km/W)TT(L2

)r/r(logQk

o

om

moeact2

o

mi

imeact1

iii) To plot the temperature distribution use formula

)r/r(Log

)r/r(LogS/V

TT

TT

ioe

ie

io

i

Where r is the selected radius for corresponding to temperature T,

between the two pipes of the same lagging material. Thus plot is made

for different values of ‘r’.

PRECAUTIONS

1) Keep dimmerstat to ZERO position before start.

2) Increase voltage gradually.

3) Keep the assembly undisturbed while testing.

4) While removing or changing the lagging materials do not disturb the

thermocouples.

5) Do not increase voltage above 150V

6) Operate selector switch of temperate indicator gently.

RESULTS

Theoretical Heat flow rate =

Thermal conductivity of lagged material =

Thermal conductivity of asbestos powder =

Thermal conductivity of saw dust =

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Lagged pipe

12

Fig 1 Lagged pipe apparatus

1. Voltmenter 2. Ammeter 3. Temperature indicator 4. Selector switch 5. Main switch 6. Heater control 7. Assembly

Fig. 2 Thermocouple settings in lagged pipe

Fig.3 Graph of temperature gradient

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Thermal conductivity of metal rod 13

TTHHEERRMMAALL CCOONNDDUUCCTTIIVVIITTYY OOFF MMEETTAALL RROODD

AIM

To determine the thermal conductivity of copper bar at various sections

to study the variation of thermal conductivity with temperature.

INTRODUCTION

Thermal conductivity is the physical property of the material denoting the

ease with which a particular substance can accomplish the transmission

of thermal energy by molecular motion. Thermal conductivity of a

material is found to depend on the chemical composition of the

substance or substances of which it is a composed, the phase (i.e. gas,

liquid or solid) in which it exists, its crystalline structure if a solid, the

temperature and pressure to which it is subjected, and whether or not it

is a homogeneous material. For pure copper thermal conductivity is 380

W/ m. K at 200C.

Thermal energy can be conducted in solids by free electrons and by

lattice vibrations. Large number of free electrons moves about in the

lattice structure of the material, in good conductors. These electrons

carry thermal energy from higher temperature region to lower

temperature region, in a similar way they transport electric charge. In

fact, these electrons are frequently referred as electron gas. Energy may

also be transferred as vibrational energy in the lattice structure of the

material. In general, however, this mode of energy transfer is not as large

as electron transport and hence, good electrical conductors are always

good heat conductors, e.g. copper, silver etc. However, with increase in

temperature, lattice vibrations come in the way of transport by free

electrons and for most the metals thermal conductivity decreases with

increase in temperature.

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Thermal conductivity of metal rod 14

APPARATUS

The apparatus consists of a copper bar, one end of which is heated by an

electric heater and the other end is cooled by a water-circulated heat

sink. The middle portion, i.e. Test section of the bar is covered by a shell

containing insulation. The bar temperature is measured at 8 different

section, while 2 thermocouples measure the temperature at the shell.

Two thermometers are provided to measure water inlet and outlet

temperatures. A dimmer is provided for the heater to control its input.

Constant water flow is circulated through the heat sink. A gate valve

provided controls the water flow.

SPECIFICATIONS

1. Metal bar – copper, 25mm O.D, approx. 430 mm long with

insulation shell along the test length and water cooled heat sink at

the outer end.

2. Test length of the bar – 240 mm

3. Measuring flask to measure water flow.

EXPERIMENTAL PROCEDURE

1. Start the electric supply.

2. Start heating the bar by adjusting the heater input to say 80 V or

100 V

3. Start cooling water supply through the heat sink and adjust it to

around 350- 400 cc per minute.

4. Bar temperature will start rising. Go on checking the temperatures

at time intervals of 5 minutes.

5. When all the temperatures remain steady, note down all the

observations and complete the observation table.

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Thermal conductivity of metal rod 15

OBSERVATION TABLE

Sl. No.

TEST BAR TEMPERATURE OC Shell Temp.oC

Water Temp.oC

Water flow

rate Lit/Sec.

T1 T2 T3 T4 T5 T6 T8 T9 T10 T11 T12

Using the temperatures of the bar at various points, plot the temperature

distribution along the length of the bar and determine the slopes of the

graph (i.e. temperature drop per unit length) dT/dx at the sections AA,

BB and CC as shown in figure.

(Note: As the value of temperature goes on decreasing along the length of

the bar, the value of the slope dT/dx is negative)

CALCULATIONS

Heat is flowing through the bar from heater end to water heat sink.

When steady state is reached, heat passing through the section CC of the

bar is heat taken by water.

1) Heat passing through Section CC

Qcc = m. CP T Watts.

Where,

m = mass flow rate of cooling water, kg/s.

CP = Specific heat of water= 4180 J / kgo C

T = (Water outlet temperature) – (Water inlet temperature)

Now, Qcc = -kcc

ccdx

dT

.A

A = Cross sectional area of the bar = 2

4D

Kcc = W / moC

2) Heat passing through section BB

Qbb = Qcc + Radial heat loss between CC & BB.

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Thermal conductivity of metal rod 16

= Qcc +)/(log

)(.2

0

1061

ie rr

TTLk

Where k = Thermal conductivity of insulation = 0.35 W / m oC

L1 = Length of insulation cylinder = 0.060 m

ro = outer radius =0.105 m

ri = inner radius = 0.0125 m

Qbb = -kbb.bbdx

dT

.A

Kbb = W/mo C

3) Heat passing through section AA

Qaa = Qbb + Radial heat loss between BB & AA.

= Qbb +)/(log

)(.2

0

932

ie rr

TTLk

Where,L2 = 0.090 m

Qaa = -kaa.aadx

dT

.A

Kaa = W/moC

RESULTS

1) Temperature of the bar decreases from hot end to cool end, which

satisfies the

Fourier law heat conduction.

2) Thermal conductivity of bar at three different sections.

Thermal conductivity at section AA = kaa =

Thermal conductivity at section BB = kbb =

Thermal conductivity at section CC = kcc =

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Thermal conductivity of metal rod 17

Fig.1 Apparatus of thermal conductivity of metal rod

1. Shell 2. Heater 3. Voltmeter 4. Ammeter 5. Temperature indicator 6. Main switch 7. Heater control

Fig.3 Thermocouple settings in metal rod Distance between two thermocouple = 0.032 m

Fig.3 Graph for temperature gradient in metal rod

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Heat transfer in pin fin

18

HHEEAATT TTRRAANNSSFFEERR IINN PPIINN FFIINN

AIM

To study the temperature distribution, heat transfer coefficient and

efficiency of a pin fin in natural and forced convection heat transfer.

INTRODUCTION

Extended surfaces or fins are used to increase the heat transfer rates

from a surface to the surrounding fluid wherever it is not possible to

increase the value of the surface heat transfer coefficient or the

temperature difference between the surface and the fluid. Fins are

fabricated in variety of forms. Fins around the air cooled engines are a

common example. As the fins extend from primary heat transfer surface,

the temperature difference with the surrounding fluid diminishes towards

the tip of the fin.

APPARATUS

The apparatus consists of a simple pin fin which is fitted in a rectangular

duct. The duct is attached to suction end of a blower. One end of fin is

heated by an electrical heater. Thermocouples are mounted along the

length of fin and a thermocouple notes the duct fluid temperature. When

top cover over the fin is opened and heating started, performance of fin

with natural convection can be evaluated and with top cover closed and

blower started, fin can be tested in forced convection.

SPECIFICATIONS

1) Fins – 12 mm O. D., Effective length 102 mm with 5 nos of

thermocouple positions along the length, made of brass, mild steel and

aluminum - one each.

Fin is screwed in heater block which is heated by a band heater.

2) Duct- 150 x 100mm cross-section, 1000mm long connected to suction

side of blower

3) FHP centrifugal blower with orifice and flow control valve on discharge

side.

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Heat transfer in pin fin

19

4) Orifice – dia 22mm, coefficient of discharge Cd = 0.64

5) Water manometer connected to orifice meter

THEORY

Let A= Cross sectional area of the fin, m2

P= Perimeter (circumference) of the fin, m

L= Length of the fin = 0.102 m

T1= Base temperature of fin

Tf= Duct fluid temperature (Channel No. 6 of temperature indicator)

θ = Temperature difference of fin and fluid temperature =T- Tf

h = Heat transfer coefficient, W / m2 oC

Kf = Thermal conductivity of fin material

= 110 W / m oC for brass

= 46 W / m oC for mild steel

= 232 W / m oC for aluminum

Heat is conducted along the length of fin and also lost to surroundings.

Applying first law of thermodynamics to a control volume along the

length of fin at a station which is at length ‘x’ from the base

0.

.2

2

Ak

Ph

dX

Td

f

(1)

).().( 21

mxeCeC mx (2)

Ak

PhmWhere

f .

. (3)

With the boundary conditions of θ = θ1 at x = 0, θ1 = T1-Tf

Assuming tip is to be insulated, 0dx

dat x = L,

Results in obtaining equation (2) in the form

]..[

)]([

11 LmCosh

XLmCosh

TT

TT

f

f

(4)

This is the equation for temperature distribution along the length of the

fin. Temperatures T1 and Tf will be known for the given situation and the

value of ‘h’ depend upon mode of convection i.e. natural or forced.

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Heat transfer in pin fin

20

EXPERIMENTAL PROCEDURE

A) NATURAL CONVECTION

Open the duct cover over the fin. Ensure proper earthing to the unit and

switch on the main supply. Adjust dimmerstat so that about 80 V are

supplied to the heater. The fin will start heating. When the

temperatures remain steady, note down the temperatures of the fin and

duct fluid temperature.

Sl.

NO. INPUT Fin temperatures oC

Duct fluid temperature

oC

V I T1 T2 T3 T4 T5 T6 (Tf)

B) FORCED CONVECTION

Close the duct cover over the fin. Start the blower. Adjust the

dimmerstat so that about 100 – 110v are supplied to the heater. When

the temperatures become steady, note down all the temperatures and

manometer difference

CALCULATIONS

Nomenclature:

Tm = Average fin temperature = (T1 + T2 + T3 + T4 +T5) /5

fm TTT

Tmf = Mean film temperature = (Tm+ Tf) / 2

ρa = Density of air, kg / m3

ρw = Density of water, kg / m3 = 1000 kg / m3

D = Diameter of pin fin = 12 x 10-3 m

Sl.

No.

INPUT Manometer

difference

Fin Temperature

oC

Duct fluid

temp. oC

V I H (m of water) T1 T2 T3 T4 T5 T6 (Tf)

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Heat transfer in pin fin

21

d = Diameter of orifice = 22 x 10-3 m

Cd = coefficient of discharge of orifice = 0.64

μ = Dynamic viscosity of air, N-s/m2

Cp = Specific heat of air, kJ/kg.K

Kinematic viscosity, m2/s =ט

kair = Thermal conductivity of air, W/m K

β = volume expansion coefficient = 1 / (Tmf+273.15)

H = Manometer difference, m of water

V = velocity of air in duct, m/s

Q = volume flow rate of air, m3/s

Vtmf = velocity of air at mean film temperature

All properties are to be evaluated at mean film temperature.

NATURAL CONVECTION

The fin under consideration is horizontal cylinder losing heat by natural

convection. For horizontal cylinder, Nusselt number, from data book,

page number 122.

Nu= 1.02 (Gr.Pr)0.148 ----------for 10-2 < Gr.Pr < 102

Nu = 0.85 (Gr.Pr)0.188 --------- for 102 < Gr.Pr < 104.

Nu= 0.48 (Gr.Pr)0.25 ----------for 104 < Gr.Pr < 107

Nu = 0.125 (Gr.Pr)0.333 --------- for 107 < Gr.Pr < 1012.

Where Gr = Grashof number. 2

3 TD..g

Pr = Prandtl number = airk

Cp . (take from data book.)

Determine Nusselt number.

Now, Nu = (hD)/kair

Therefore, h = Nu. Kair /D

From h determine m from equation (3)

Using h and m, determine temperature distribution in the fin from

equation (4)

The rate of heat transfer from the fin and efficiency can be calculated as,

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Heat transfer in pin fin

22

)(... 1 fffin TTAkPhQ and mL

mL][tanh

FORCED CONVECTION

For flow across Horizontal cylinder loosing heat by forced convection,

from data book, page number 100.

Nu = 0.911 (Re)0.385. Pr.0.333 ---------- for 4 < Re < 40

Nu = 0.683 (Re)0.466 . Pr.0.333 ---------- for 40 < Re < 4000

Nu= 0.193 (Re)0.618 . Pr.0.333 ----------for 4000 < Re < 40,000

Where,

DVR

tmf

e

.

)273(

)273(.

f

mf

tmfT

TVV

Velocity of air is determined from air volume flow.

smXQareationalcrossDuctQV

smHgdCdQ aw

/)1.015.0/(sec/

/)/(..24

32

From Nusselt Number, find out ‘h’ and from ‘h’, find out ‘m’

Now temperature distribution, heat transfer rate and effectiveness of the

fin can be calculated using equations 4, 5 and 6 respectively.

CONCLUSION

1. Comment on the observed temperature distribution and calculation

by theory, it is expected that observed temperatures should be

slightly less than their calculated values because of radiation and

non- insulated tip.

2. Plot the graphs of temperature distribution in both natural and

forced convection.

PRECAUTIONS

1. Operate all the switches and controls gently

2. Do not obstruct the suction of the duct or discharge pipe

3. Open the duct cover over the fin for natural convection experiment

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Heat transfer in pin fin

23

4. Fill up water in the manometer and close duct cover for forced

convection experiment

5. Proper earthing to the unit is necessary

6. While replacing the fins, be careful for fixing the thermocouples.

Incorrectly fixed thermocouples may show erratic readings

Fig.1: Thermocouple position on fin

GRAPH

Fig 2: Variation of fin temperature along the length of fin with natural

convection and forced convection.

RESULTS

Natural convection:

Heat transfer coefficient =

Efficiency of pin fin =

Forced convection:

Heat transfer coefficient =

Efficiency of pin fin =

10 23 23 23 23

T1 T2 T3 T4 T5

Natural Convection

Forced Convection

Fin

Tem

pe

ratu

re,

0C

Thermo couple distance, x

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Heat transfer in pin fin

24

Fig. 3 Pin fin apparatus

1. Manometer

2. Ammeter 3. Voltmeter 4. Temperature indicator

5. Selector switch 6. Blower switch

7. Heater control 8. Main switch 9. Suction duct

10. Orifice meter

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Heat transfer in natural convection 25

HHEEAATT TTRRAANNSSFFEERR IINN NNAATTUURRAALL CCOONNVVEECCTTIIOONN

AIM

To determine the experimental and theoretical heat transfer coefficient for

vertical tube losing heat by natural convection.

INTRODUCTION

In contrast to the forced convection, natural convection phenomenon is

due to the temperature difference between the surface and the fluid and

is not created by any external agency. The present experimental set up is

designed and fabricated to study the natural convection phenomenon

from a vertical cylinder in terms of the variation of local heat transfer

coefficient along the length and also the average heat transfer coefficient

and its comparison with the value obtained by using and appropriate

correlation.

APPARATUS

The apparatus consists of a brass tube fitted in a rectangular vertical

duct. The duct is open at the top and bottom and forms an enclosure

and serves the purpose of undisturbed surrounding. One side of the

duct is made up of perspex for visualization. An electric heating element

is kept in the vertical tube which in turn heats the tube surface. The

heat is lost from the tube to the surrounding air by natural convection.

The temperature of the vertical tube is measured by seven

thermocouples. The heat input to the heater is measured by an ammeter

and a voltmeter and is varied by a dimmerstat. The vertical cylinder with

the thermocouple positions is shown in figure. The tube surface is

polished to minimize the radiation losses.

SPECIFICATIONS

1. Diameter of the tube (d)= 38mm

2. Length of tube (L) = 500mm

3. Duct size 200mm x 200mm x 800mm Length

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Heat transfer in natural convection 26

THEORY

When a hot body is kept in still atmosphere, heat is transferred to the

surrounding fluid by natural convection. The fluid layer in contact with

the hot body gets heated, rises up due to the decrease in its density and

the cold fluid rushes in to take place. The process is continuous and the

heat transfer takes place due to the relative motion of hot and cold fluid

particles.

The heat transfer coefficient is given by:

)TT(A

QQh

ass

R

(1)

Where h = average surface heat transfer coefficient (W/m2 oC)

Q = Heat transfer rate V. I (watts)

As = Area of the heat transferring surface =∏ .d L (m2)

Ts = Average surface temperature 7

)TTTTTTT( 7654321 0 C

Ta = Ambient temperature in the duct = T8 0C

QR = Heat loss by radiation = )(.. 44

as TTA

Where

. = Stefan Boltzmann constant = 5.667x 10-8 W/m2 K4

= Emissivity of pipe material = 0.06

Ts & Ta = Surface and ambient temperatures in o K respectively.

The surface heat transfer coefficient, of a system transferring heat by

natural convection depends on the shape, dimensions and orientation of

the fluid and the temperature difference between heat transferring

surface and the fluid. The dependence of h on all the above mentioned

parameters is generally expressed in terms of non-dimensional groups as

follows:

n

K

Cx

TLgA

k

hxL

2

3 .. (2)

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Heat transfer in natural convection 27

Where k

hxL is called the Nusselt number.

2

3..

v

TLg = is called the Grashof Number and

k

C . = is the Prandtl Number.

A and n are constants depending on the shape and orientation of the

heat transferring surface.

Where L = A characteristic dimension of the surface.

K= Thermal conductivity of fluid

Kinematic viscosity of fluid =ט

μ = Dynamic viscosity of fluid

Cp = Specific heat of fluid

β = Coefficient of volumetric expansion for the fluid

g = Acceleration due to gravity.

T = [Ts – Ta]

For gases

)273(

1

fT K-1, Tf = (Ts + Ta)/2

For a vertical cylinder losing heat by natural convection, the constants A

and n of equation (2) have been determined and the following empirical

correlation’s obtained from data book, page number 120.

k

hL = 0.59 (Gr.Pr.)0.25 for 104 < Gr.Pr. < 109 (3)

k

hL = 0.10 (Gr.Pr.)1/3 for 109 < Gr.Pr. < 1013 (4)

L = Length of the cylinder.

All the properties of the fluid are determined at the mean film

temperature (Tf)

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Heat transfer in natural convection 28

PROCEDURE

1. Put ON the supply and adjust the dimmerstat to obtain the

required heat input (say 40 W, 60 W, 70 W etc).

2. Wait till the steady state is reached, which is confirmed from

temperature readings ( T1- T7)

3. Measure surface temperature at the various points i.e. T1 to T7

4. Note the ambient temperature i.e. T8

OBSERVATIONS

1) O.D. Cylinder = 38 mm

2) Length of cylinder = 500 mm

3) Input to heater = V. I Watts

Sl.No. Volt Amp Temperature, 0C

T1 T2 T3 T4 T5 T6 T7 T8

CALCULATIONS

1) Calculate the value of average surface heat transfer coefficient

neglecting end losses using equation (1)

2) Calculate and plot (Fig.4) the variation of local heat transfer

coefficient along the length of the tube using:

T = T1 to T7 and h = Q/[As(T-Ta)]

3) Compare the experimentally obtained value with the predictions of

the correlation equation (3) or (4).

PRECAUTIONS

1. Proper earthing is necessary for the equipment.

2. Keep dimmerstat to ZERO volt position before putting on main

switch and increase it slowly.

3. Keep at least 200 mm space behind the equipment.

4. Operate the change-over switch of temperature indicator gently

from one position to other, i.e. from 1 to 8 positions.

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Heat transfer in natural convection 29

5. Never exceed input above 80 Watts.

RESULTS AND DISCUSSIONS

The heat transfer coefficient is having a maximum value at the beginning

as expected because of the just staring of the boundary layer and it

decreases as expected in the upward direction due to thickening of layer

and which is laminar one. This trend is maintained up to half of the

lengths (approx) and beyond that there is little variation in the value of

local heat transfer coefficient because of the transition and turbulent

boundary layers. The last point shows some what increase in the value

of heat transfer coefficient which is attributed to end loss causing a

temperature drop. The comparison of average heat transfer coefficient is

also made with predicted values are some what less than experimental

values due to the heat loss by radiation.

RESULTS

Experimental heat transfer coefficient =

Theoretical heat transfer coefficient =

Fig. 1 Natural convection apparatus

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Heat transfer in natural convection 30

Fig. 2 Thermocouple locations

Fig.3 Variation of local heat transfer coefficient

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Heat transfer in forced convection 31

HHEEAATT TTRRAANNSSFFEERR IINN FFOORRCCEEDD CCOONNVVEECCTTIIOONN

AIM

To determine the experimental and theoretical heat transfer coefficient in

forced convection heat transfer for internal flow.

APPARATUS

The apparatus consists of a circular pipe, through which cold fluid, i.e.

air is being forced. Pipe is heated by a band heater outside the pipe.

Temperature of pipe is measured with thermocouples attached to pipe

surface. Heater input is measured by a voltmeter and ammeter. Thus,

heat transfer rate and heat transfer coefficient can be calculated.

SPECIFICATIONS

Test pipe – 33mm I.D. 500 mm long

Band heater for pipe.

Blower to force the air through test pipe

Orifice meter with water manometer.

EXPERIMENTAL PROCEDURE

1. Put ON main supply.

2. Adjust the heater input with the help of dimmerstat.

3. Start the blower and adjust the air flow with valve

4. Wait till steady state is reached and note down the reading in

the observation table

Sl.

No

Vol

t

Amp. Temperatures O C Manomete

r

Difference

V I T1 T2 T3 T4 T5 T6 T7

hw

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Heat transfer in forced convection 32

CALCULATIONS

1. Air inlet temp T1 = 0 C

2. Air outlet temp. T7 = 0 C

3. Density of air, 1273

273293.1

T

xa

kg/m3

4. Diameter of orifice = 22 mm

Manometer difference = Water head = hw meters

Air head, ha = hw (ρw / ρa)

Where ρw = density of water = 1000 Kg/m3

Air volume flow rate, i.e., discharge

ad ghaCQ 20 m3 /sec.

Where Cd = 0.64

ao = cross section area of orifice

5. Mass flow rate of air, ma = Q x ρa kg/sec.

Velocity of air, a

QV m/sec.

Where, a = Cross Sectional area of pipe = m2

6. Heat gained by air, Qair = ma x Cpa x (T7-T1)

Where Cpa = Specific heat of air = 1KJ/Kg.K. or 103 J/Kg K

7. Average inside surface temperature, Ts 5

)( 65432 TTTTT 0C

8. Bulk mean temperature of air, Tm 2

)( 71 TT 0C

9. Average heat transfer coefficient

Actual Heat Loss Due to Forced Convection= Qair– QR

Heat Loss due to radiation, QR = σ 0.4 x A x (Ts4- Ta

4)

Ta = atmospheric temperature = T1

hexpt )( ms

Rair

TTA

QQ

W/m2 K

Where A = Inside surface are of the pipe =π x di x L

10. Reynolds number, ReD

VxD

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Heat transfer in forced convection 33

= Kinematic Viscosity at Tm

D = 0.033 m

If ReD < 2300, flow is laminar.

For laminar flow, NuD = airk

Dh. = 4.36, from data book, page number 109.

If Reynolds number exceeds 2300, flow is turbulent.

For turbulent flow, NuD = 0.023 (ReD)0.8 (Pr.)n from data book, page

number 110.

Where n = 0.4 when fluid is being heated.

n = 0.3 When fluid is being cooled.

Determine htheo from NuD.

Note: The calculated values and actual values may differ appreciably

because of heat losses. The heat loss through natural convection,

conduction and heat loss through insulation over the heater is not

considered, but they are present. Also, the heat flux is not uniform

practically, as assumed in theory, which gives difference between actual

and theoretical value.

PRECAUTIONS

1. While putting ON the supply, keep dimmerstat at zero position and

blower switch OFF

2. Operate all the switches and controls gently.

3. Do not obstruct the flow of air while experiment is going on.

RESULT

Experimental heat transfer coefficient, hexpt =

Theoretical heat transfer coefficient = htheo =

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Heat transfer in forced convection 34

Fig.1 Forced convection apparatus

1. Manometer

2. Voltmeter 3. Ammeter 4. Temperature indicator

5. Selector switch 6. Blower switch

7. Heater control 8. Main switch 9. Blower

10. Orifice meter 11. Test section

12. Thermocouple setting

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Stefan Boltzmann apparatus

34

SSTTEEFFAANN BBOOLLTTZZMMAANNNN AAPPPPAARRAATTUUSS

AIM

To determine the Stefan- Boltzmann’s constant in the radiation heat

transfer.

INTRODUCTION

All the substances emit thermal radiation. When heat radiation is

incident over a body, part of radiation is absorbed, transmitted through

and reflected by the body. A surface which absorbs all thermal radiation

incidents over it is called black surface. For black surface, transmissivity

and reflectivity are zero and absorptivity is unity. Stefan Boltzmann Law

states that emissivity of a surface is proportional to fourth power of

absolute surface temperature i.e.

E α T4

or E = σ ε T4

Where E = emissive power of surface, W / m2

T = absolute temperature

σ = Stefan Boltzmann constant

ε = emissivity of the surface

Value of Stefan Boltzmann constant is taken as

σ = 5.667 x 10-8 W / m2 K4

For black surface ε = 1, hence above equation reduces to

E = σ. T4

APPARATUS

The Apparatus consists of a water-heated jacket of hemispherical shape.

A copper test disc is fitted at the center of jacket. The hot water is

obtained from a hot water tank, fitted to the panel, in which water is

heated by an electric immersion heater. The hot water is taken around

the hemisphere, so that hemisphere temperature rises. The test disc is

then inserted at the center. Thermocouples are fitted inside hemisphere

to average out hemisphere temperature. Another thermocouple fitted at

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Stefan Boltzmann apparatus

35

the center of test disc measures the temperature of test disc. A timer

with a small buzzer is provided to note down the disc temperatures at the

time intervals of 5 seconds.

EXPERIMENTAL PROCEDURE

1. See that water inlet cock of water jacket is closed and fill up

sufficient water in the heater tank.

2. Put ‘ON’ the heater.

3. Blacken the test disc with the help of lamp black and let it cool.

4. Put the thermometer and check water temperature.

5. Boil the water and switch ‘OFF’ the heater

6. See that drain cock of water jacket is closed and open water inlet

cock.

7. See that there is sufficient water above the top of hemisphere ( A

piezometer tube is fitted to indicate water level)

8. Note down the hemisphere temperatures (up to channel 1 to 4)

9. Note down the test disc temperature (i.e.. channel 5)

10. Start the timer. Buzzer will start ringing. At the start of

timer cycle, insert test disc into the hole at the bottom of

hemisphere.

11. Note down the temperatures of disc, every five times of the

buzzer rings. Take at least 8-10 readings

OBSERVATIONS

Hemisphere Temperature (oC)

T1 =

T2 =

T3 =

T4 =

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Stefan Boltzmann apparatus

36

Time Interval

(Sec)

Test disc Temperature

(oC)

0

25

50

75

100

125

150

175

CALCULATIONS

1) Area of test disc, A = (π/4)d2 = m2 (d = 20 mm)

2) mass of test disc, m = 7.6 gr = 7.6 x 10-3 kg.

3) Plot a graph of temperature rise of test disc with time as base and

find out its slope at origin. i.e. 0

attdt

dT K / sec

Hemisphere temperature, TH = KTTTT

15.2734

4321

4) Initial Test disc temperature

TD = T5 + 273.15 k

As area of hemisphere is very large as compared to that disc, we can put

Q = σ є.A (TH4 – TD

4)

Where Q = heat gained by disc/sec.

Q = m. cP. (dT/dt)t=0

σ = Stefan Boltzmann Constant

m = Mass of test disk = 7.6 x 10-3 kg.

є = Emissivity of test disc = 1

A = Area of Test disc

cP= Specific heat of copper = 381 J/Kg 0 C

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Stefan Boltzmann apparatus

37

σ = ).(

)/.(.44

0

DH

tP

TTA

dtdTcm

W/ m2 K4

Theoretical value of σ is 5.667x 10-8 W/m2 K4.

In the experiment this value may deviate due to reasons like convection,

temperature drop of hemisphere, heat losses etc.

PRECAUTIONS

1) Never put ‘ON” the heater before putting water in the tank.

2) Put ‘OFF’ the heater before draining the water from heater tank.

3) Drain the water after completion of experiment.

4) Operate all the switches and controls gently

RESULT

Stefan – Boltzmann’s constant, σ = ________W / m2 K4

Fig.1 Stefan Boltzmann apparatus

1. Water tank

2. Main switch 3. Temperature indicator

4. Temperature selector switch 5. Buzzer switch 6. Heater switch

7. Shell

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Stefan Boltzmann apparatus

38

Fig.2 Thermocouple setting

Fig.3 Variation of temperature of disc with time

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Emissivity measurement 39

EEMMIISSSSIIVVIITTYY MMEEAASSUURREEMMEENNTT

AIM

To determine the emissivity of the test plate.

INTRODUCTION

All the bodies emit and absorb the thermal radiation to and from

surroundings. The rate of thermal radiation depends upon the

temperature of body. Thermal radiations are electromagnetic waves and

they do not require any medium for propagation. When thermal radiation

strikes a body, part of it is reflected, part of it is absorbed and part of it is

transmitted through body. The fraction of incident energy, reflected by

the surface is called reflectivity (ρ). The fraction of incident energy,

absorbed by the surface is called absorptivity (α) and the fraction of

incident energy transmitted through body is called transmissivity (τ). The

surface which absorbs all the incident radiation is called a black surface.

For a black surface, ρ+α+τ = 1.

The radiant flux, emitted from the surface is called emissive power (E).

The emissivity of a surface is ratio of emissive power of a surface to that

of black surface at the same temperature. Thus,

ε = E / Eb

APPARATUS

The apparatus uses comparator method for determining the emissivity of

test plate. It consists of two aluminum plates, of equal physical

dimensions. Mica heaters are provided inside the plates. The plates are

mounted in an enclosure to provide undisturbed surroundings. One of

the plates is blackened outside for use as a comparator (because black

surface has ε = 1). Another plate is having natural surface finish. Input

to heaters can be controlled by separator dimmer stats. Heater input is

measured on common ammeter and voltmeter. One thermocouple is

fitted on surface of each plate to measure the surface temperature with

digital temperature indicator. By adjusting input to the heaters, both the

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Emissivity measurement 40

plates are brought to same temperature, so that conduction and

convection losses form both the plates are equal and difference in input

is due to different emissivities. Holes are provided at backside bottom

and at the top of enclosure for natural circulation of air over the plates.

The plate enclosure is provided with Perspex acrylic sheet at the front.

EXPERIMENTAL PROCEDURE

1. Blacken one of the plates with the help of lamp black (Normally

this is blackened at the works, but if blackening is wiped out, then

blackening is necessary)

2. Keep both the dimmer knobs at ZERO position.

3. Insert the supply pin-top in the socket (which is properly earthed)

and switch ‘ON’ the mains supply.

4. Switch ON the mains switch on the panel.

5. Keep the meter selector switch (toggle switch) at the black plate

side position.

6. Adjust dimmer of black plate, so that around 110-120 volts are

supplied to black plate.

7. Now, switch the meter selector switch on other side.

8. Adjust test plate voltage slightly less than that of black plate (say

100- 110 volts)

9. Check the temperatures (after, say 10 min) and adjust the dimmers

so that temperatures of both the plates are equal and steady.

Normally, very minor adjustments are required for this.

10. Note down the readings after the plate temperatures reach

steady state.

OBSERVATIONS

Plate Input Surface temperature,

0C V I

Test plate T1 =

Black plate T2 =

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Emissivity measurement 41

Enclose temperature, T3 = 0C

CALCULATIONS

1. Enclose temperature:

TE =T3 = 0C = (T3 + 273.15) 0K

2. Plate surface temp.

T = T1 =T2 = 0 C

TS = (T+273.15) 0K

3. Heat input to black plate, Wb = V x I Watts

4. Heat input to test plate, WT = V x I Watts

5. Surface area of test plates, A = 2 x (π/4) D2 + (π D t) =

Where, D = dia. Of plates = 0.16 m.

And t = thickness of plates = 0.009 m.

6. For black plate,

Wb = WCVb + WCdb + WRb (i)

Where, WCvb = Convection losses

WCdb = Conduction losses

WRb = Radiation losses

Similarly, for test plate,

WT = WCvT + WCdT + WRT (ii)

As both plates are of same physical dimensions, same material and

at same temperatures,

WCvb = WCvT and WCdb = WCdT

Subtracting equation (ii) from (i) we get,

Wb - WT = WRb - WRT

= [σ A (Ts4 – TE

4)] - [σ A εT (Ts4 – TE

4)]

= σ A (Ts4 – TE

4) (εb – ε)

As emissivity of black plate is 1,

Wb - WT = σ A (Ts4 – TE

4) (1 – ε)

Where, ε = Emissivity of test plate

σ = Stefan Boltzman constant = 5.667 x 10-8 W/m2K4

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Emissivity measurement 42

PRECAUTIONS

1. Black plate should be perfectly blackened.

2. Never put your hand or papers over the holes provided at the top of

enclosure.

3. Keep at least 200 mm distance between the backside of unit and

the wall.

4. Operate all the switches and knobs gently.

Note: Emissivity of oxidized aluminum plate i.e. test plate is normally

with in the range of 0.3 to 0.7.

RESULT

Emisvity of the test plate surface = ________at temperature of _____

Fig.1 Emissivity measurement apparatus

1. Voltmeter 2. Ammeter

3. Temperature indicator 4. Meter selector switch

5. Heater control 6. Heater control 7. Black plate

8 Test plate

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Concentric tube heat exchanger 43

CCOONNCCEENNTTRRIICC TTUUBBEE HHEEAATT EEXXCCHHAANNGGEERR

AIM

To determine the heat transfer rate, LMTD, over all heat transfer

coefficient and effectiveness of heat exchangers in parallel flow and

counter flow concentric tube heat exchanger.

INTRODUCTION

Heat exchangers are the devices in which the heat is transferred from one

fluid to another. Exchange of heat is required at many industrial

operations as well as chemical process Common examples of heat

exchangers are radiator of a car, condenser of a refrigeration unit or

cooling coil of an air conditioner.

Heat exchanger are of basically three types – i) Transfer type- in which

both fluids pass through the exchanger and heat gets transferred

through the separating walls between the fluids, ii) Storage type – in this,

firstly the hot fluid passes through a medium having high heat capacity

and then cold fluid is passed through the medium to collect the heat.

Thus hot and cold fluids are alternately passed through the medium. iii)

Direct contact type – in this type, the fluids are not separated but they

mix with each other and heat passes directly from one fluid to the other.

Transfer type heat exchangers are the type most widely used. In transfer

type heat exchangers, three types of flow arrangements are used, viz.

parallel, counter or cross flow. In parallel flow, the fluids flow in the same

direction while in counter flow, they flow in the opposite direction. In

cross flow, they flow at right angles to each other.

APPARATUS

The apparatus consists of two concentric tubes in which fluids pass. The

hot fluid is hot water, which is obtained from an electric geyser. Hot

water flows through the inner tube in one direction. Cold fluid is cold

water, which flows through annulus. Control valves are provided so that

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Concentric tube heat exchanger 44

direction of cold water can be kept parallel or opposite to that of hot

water. Thus, the heat exchanger can be operated either as parallel or

counter flow heat exchanger. The temperatures are measured with

thermometers. Thus, the heat transfer rate, heat transfer coefficient,

LMTD and effectiveness of heat exchanger can be calculated for both

parallel and counter flow.

SPECIFICATIONS

1. Heat exchanger- a) Inner tube - 12.7 mm O.D., 11.7 mm I.D.

copper tube

b) Outer tube 25 mm G.I. Pipe.

c) Length of heat exchanger – 1 m.

2. Valves for flow and direction control – 5nos

3. Thermometers to measure temperatures – 10 to 1100 C – 4nos

4. Measuring flask and stop clock for flow measurement.

EXPERIMENTAL PROCEDURE

1. Start the water supply. Adjust the water supply on hot and cold

sides. Firstly, keep the valves V2 and V3 closed and V1 and V4

opened so that arrangement is parallel flow.

2. Put few drops of oil in thermometer pockets. Put the thermometer

in the thermometer pockets.

3. Switch ‘ON’ the geyser. Temperature of water will start rising.

After temperatures become steady, note down the readings and fill

up the observation table.

4. Repeat the experiment by changing the flow.

5. Now open the valvesV2 and V3 and then close the valves V1 and V4.

The arrangement is in now counter flow.

6. Wait until the steady state is reached and note down the readings.

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Concentric tube heat exchanger 45

OBSERVATION

TYPE OF

FLOW

HOT WATER COLD WATER

Temperatures Time for 1

Lit. Water Temperatures

Time for 1

Lit. Water

Inlet

0C

Outlet

0C xh sec

Inlet

0C

Outlet

0C xc sec

Parallel

Flow

Counter

Flow

CALCULATIONS

1. Hot water inlet temperature, thi = 0C

Hot water outlet temperature, tho = 0C

2. Hot water flow rate, mh

Let time required for 1lit of water be xh sec

Mass of 1lit water = 1kg

There fore, mh = 1/ xh kg/s

3. Heat given by hot water (inside heat transfer rate)

Qh = mh cp (thi – tho) Watts

where cp =specific heat of water = 4200 J/kgK

4. Similarly, for cold water

Heat collected by cold water (out side heat transfer rate)

Qc = mc cp (tco – tci) Watts

5. Logarithmic mean temperature difference (LMTD)

LMTD = Tm = (Ti – To) / ln (Ti / To)

Where for parallel flow, for counter flow

Ti = thi - tci Ti = thi - tco

To = tho - tco To = tho - tci

6. Overall heat transfer coefficient, U

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Concentric tube heat exchanger 46

a) Inside overall heat transfer coefficient, Ui

Inside diameter of tube = 0.011 m

Inside surface area of the tube, Ai = x 0.011 x L

Now Qh = Ui Tm Ai

Therefore Ui = Qh / (Tm Ai) W/m2 0C

b) Outside overall heat transfer coefficient, Uo

Outside diameter of tube = 0.012 m

Outside surface area of the tube, Ao = x 0.012 x L

Similarly Qc = Uo Tm Ao

Therefore Uo = Qc / (Tm Ao) W/m2 0C

7. Effectiveness of heat exchanger

Є = Rate of heat transfer in heat exchanger / Max. possible heat

transfer rate

)(][

)(

min cihip

hohiph

ttmc

ttcm

Where [mcp]min is smaller of two capacity rates of mh or mc

PRECAUTIONS

1. Never switch on the geyser unless there is water supply through it.

2. If the red indicator on geyser goes off during operation, increase the

water supply, because it indicates that water temperature exceeds

the set limit.

3. Ensure steady water flow rate and temperatures before noting

down the readings, as fluctuating water supply can give erratic

results.

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Concentric tube heat exchanger 47

RESULTS

TYPE

OF

FLOW

HEAT TRANSFER

RATE LMTD

Heat Transfer

Coefficient

Effective

- ness

Inside

(Watts)

Outside

(Watts)

0C

Ui

W/m2k

UO

W/m2k

Parallel

Flow

Counter

Flow

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Concentric tube heat exchanger 48

Fig.1 Concentric tube heat exchanger (plain tube type)

1. Tci

2. Tho

3. V4

4. V3

5. Thi

6. V1

7. Tco

8. V2

Parallel flow Cross flow

V1 & V4 open V2 & V3 open

V2 & V3 close V1 & V4 close

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Heat transfer in drop & film wise condensation 49

HHEEAATT TTRRAANNSSFFEERR IINN DDRROOPP AANNDD FFIILLMM WWIISSEE CCOONNDDEENNSSAATTIIOONN

AIM

To determine the experimental and theoretical heat transfer coefficient for

drop wise and film wise condensation.

INTRODUCTION

Condensation of vapor is needed in many of the processes, like steam

condensers, refrigeration etc. When vapor comes in contact with surface

having temperature lower than saturation temperature, condensation

occurs. When the condensate formed wets the surface, a film is formed

over surface and the condensation is film wise condensation. When

condensate does not wet the surface, drops are formed over the surface

and condensation is drop wise condensation

APPARATUS

The apparatus consists of two condensers, which are fitted inside a glass

cylinder, which is clamped between two flanges. Steam from steam

generator enters the cylinder through a separator. Water is circulated

through the condensers. One of the condensers is with natural surface

finish to promote film wise condensation and the other is chrome plated

to create drop wise condensation. Water flow is measured by a Rota

meter. A digital temperature indicator measures various temperatures.

Steam pressure is measured by a pressure gauge. Thus heat transfer

coefficients in drop wise and film wise condensation cab be calculated.

SPECIFICATION

1. Condensers: Made of copper, 19 mm O.D., 150 mm long, one with

natural surface and one with chrome-plated surface.

2. Pressure gauge to measure steam pressure

3. Necessary valves for water and steam control.

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Heat transfer in drop & film wise condensation 50

EXPERIMENTAL PROCEDURE

Fill up the water in the steam generator and close the water-filling

valve.

Start water supply through the condensers.

Close the steam control valve, switch on the supply and start the

heater.

After some time, steam will be generated. Close water flow through

one of the condensers.

Open steam control valve and allow steam to enter the cylinder and

pressure gauge will show some reading.

Open drain valve and ensure that air in the cylinder is expelled

out.

Close the drain valve and observe the condensers.

Depending upon the condenser in operation, drop wise or film wise

condensation will be observed.

Wait for some time for steady state, and note down all the readings.

Repeat the procedure for the other condenser.

OBSERVATIONS

Drop wise Condensation

Steam Pressure, kg/cm2

Water flow rate, LPH

Steam temperature, T1, 0C

Drop wise condensation surface temperature, T2, 0C

Water inlet temperature , T4, 0C

Water outlet temperature , T5, 0C

Film wise Condensation

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Heat transfer in drop & film wise condensation 51

Steam Pressure, kg/cm2

Water flow rate, LPH

Steam temperature, T1, 0C

Film wise condensation surface temperature, T3, 0C

Water inlet temperature , T4, 0C

Water outlet temperature , T6, 0C

CALCULATIONS

(Film wise condensation)

Water flow mw= LPH = kg/sec

Water inlet temperature T4= oC

Water outlet temperature = oC

(T5 for drop-wise condensation and T6 for film-wise condensation)

Heat transfer rate at the condenser wall,

Q = mw.cP. (T6-T4) Watts

Where cp = Specific heat of water = 4.2 x 103 J / Kg K

Surface area of the condenser, A =πdL m3

Experimental heat transfer coefficient, CmWTTA

Qh

o

ws

2/)(

(for both film

wise and drop wise condensation)

Where Ts = Temperature of steam (T1)

TW = Condenser wall temperature (T2 or T3)

Theoretically, for film wise condensation

25.032

.).(

...943.0

LTT

kghh

ws

fg

Where

hfg = Latent heat of steam at TS J/kg (take from temperature tables

in steam tables)

ρ = Density of water, Kg / m3

g = Gravitational acceleration, m / sec2

k = Thermal conductivity of water W / mo C

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Heat transfer in drop & film wise condensation 52

μ = Viscosity of water, N.s/m2

and, L = Length of condenser = 0.15 m

Above values at mean temperature, CTT

T oWs

m2

)( (from data book)

(For drop wise condensation, determine experimental heat transfer

coefficient only)

In film wise condensation, film of water acts as barrier to heat transfer

where as, in case of drop formation, there is no barrier to heat transfer,

Hence heat transfer coefficient in drop wise condensation is much greater

than film wise condensation, and is preferred for condensation. But

practically, it is difficult to prolong the drop wise condensation and after

a period of condensation the surface becomes wetted by the liquid.

Hence slowly film wise condensation starts.

PRECAUTIONS

1. Operate all the switches and controls gently

2. Never allow steam to enter the cylinder unless the water is flowing

through condenser.

3. Always ensure that the equipment is earthed properly before

switching on the supply.

RESULTS

Film wise condensation

Experimental average heat transfer coefficient =

Theoretical average heat transfer coefficient =

Drop wise condensation

Experimental average heat transfer coefficient =

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Heat transfer in drop & film wise condensation 53

Fig. 1 Condensation in drop and film forms

1. Steam generator

2. Water level

3. Rota meter

4. Steam pressure

5. Condensers

6. Temperature indicator

7. Selector switch

8. Heater control

9. Main switch

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Critical heat flux apparatus

54

CCRRIITTIICCAALL HHEEAATT FFLLUUXX AAPPPPAARRAATTUUSS

AIM

To determine the experimental and theoretical value of critical heat flux

in pool boiling of water.

INTRODUCTION

When heat is added to a liquid from a submerged solid surface which is

at a temperature higher than the saturation temperature of the liquid, it

is usual for a part of the liquid to change phase. This change of phase is

called boiling. Boiling is of various types, the type depending upon the

temperature difference between the surface and the liquid. The different

types are indicated in figure, in which a typical experimental boiling

curve obtained in a saturated pool of liquid is drawn. The heat flux

supplied to the surface is plotted against (Tw – Ts) the difference between

the temperature of the surface and the saturation temperature of the

liquid. It is seen that the boiling curve can be divided into three regions.

I) Natural convection region

II) Nucleate boiling region and

III) Film boiling region

Heat flux, W/m

2

106

107

C

B

A

Stable film

II b

Bubbles

rise

Bubbles condense

Nucleate boiling

Unstable

film

Excess temperature (Tw - Ts)

Radiation coming in to

play

Film

boiling I II III

II a III a III b

1

10

1000

1000

0

100

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Critical heat flux apparatus

55

The region of natural convection occurs at low temperature differences (of

the order of 100C or less). Heat transfer from the heated surface to the

liquid in its vicinity causes the liquid to be superheated. This

superheated liquid rises to the free liquid surface by natural convection,

where vapour is produced by evaporation. As the temperature difference

(Tw – Ts) is increased, nucleate boiling starts. In this region, it is observed

that bubbles start to form at certain locations on the heated surface

region (II) consists of two parts. In the first part (II-a) the bubbles formed

are very few in number. They condense in the liquid and do not reach the

free surface. In the second part (II-b) the rate of bubble formation as well

as the number of locations where they are formed increase. Some of the

bubbles now rise all the way to the free surface.

With increasing temperature difference, a stage is finally reached when

the rate of formation of bubbles is so high, that they start to coalesce and

blanket the surface with a vapour film. This is the beginning of region (III)

viz, film boiling. In the first part of this region (III-a) the vapour film is

unstable, so that film boiling may be occurring on a portion of the heated

surface area, while nucleate boiling may be occurring on the remaining

area. In the second part (III-b) a stable film covers the entire surface. At

the end of region (II) the boiling curve reaches a peak (point A). Beyond

this, in region (III-A) in spite of increasing temperature difference, the

heat flow increases with the formation of a vapour film. The heat flux

passes through a minimum (point B) at the end of region (III-a). It starts

to increase again with (Tw – Ts) only when stable film boiling begins and

radiation becomes increasingly important.

It is of interest to note how the temperature of the heating surface

changes as the heat flux is steadily increased from zero. Up to the point

A, natural convection boiling and then nucleate boiling occur and the

temperature of the heating surface is obtained by reading off the value of

(Tw – Ts) from the boiling curve and adding to it the value of Ts. If the heat

flux is increased even a little beyond the value of A, the temperature of

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Critical heat flux apparatus

56

the surface will shoot up to the value corresponding to the point C. It is

obvious from figure that the surface temperature corresponding to point

C is high. For most surfaces it is high enough to cause the material to

melt. Thus in most practical situation, it is undesirable to exceed the

value of heat flux corresponding to point A. This value is therefore of

considerable engineering significance and is called the critical or peak

heat flux. The discussions so far has been concerned with the various

type of boiling which occurring saturated pool boiling. If the liquid is

below the saturation temperature we say that sub-cooled pool boiling is

taking place. Also in many practical situations, e.g. steam generators,

one is interested in boiling in a liquid flowing through tubes. This is

called forced convection boiling may also be saturated or sub cooled and

of the nucleate or film type. Thus in order to completely specify billing

occurring in any process, one must state that (i) whether it is forced

convection boiling or pool boiling, (ii) whether the liquid is saturated or

sub cooled and (iii) whether is in the natural convection nucleate of film

region.

APPARATUS

The apparatus consists of a cylindrical glass housing the test heater and

heater coil for heating of the water. This heater coil is direct connected to

the mains (Heater R1) and the test wire is also connected to mains via.

variac. An ammeter is connected in series while a voltmeter across it to

read the current and voltage respectively. The glass container is kept on a

stand. There is provision of observing the test heater wire with the help

of a lamp light from back and the heater wire can be view a lens.

SPECEIFICATIONS

1) Glass container – Diameter – 250 mm

Height – 100 cm

2) Heater for initial heating, Nichrome Heater (R–1) – 1 kW

3) Test Heater (R-2), Nichrome wire size - Φ mm

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Critical heat flux apparatus

57

(To be calculated according to wire used say 36 SWG to 40 SWG.)

3) Length of test Heater (R-2) = 100 mm

4) Thermometer – 0 to 100o C

EXPERIMENTS

This experimental set up is designed to study the pool boiling

phenomenon up to critical heat flux point. The pool boiling over the

heater wire can be visualized in the different regions up to the critical

heat flux point at which the wire melts. The heat flux from the wire is

slowly increased by gradually increasing the applied voltage across the

test wire the change over from natural convection to nucleate boiling can

be seen. The formation of bubbles and their growth in size and number

can be visualized followed by the vigorous bubbles formation and their

immediate carrying over to surface and ending this in the breaking of

wire indicating the occurrence of critical heat flux point.

PROCEDURE

1) Take sufficient amount of distilled water in the container.

2) See that both the heaters are completely submerged.

3) Connect the heater coil R-1 (1KW Nichrome coil) and test heater

wire across the studs and make the necessary electrical

connections.

4) Switch on the heater R-1(Let variac be at O position.)

5) Keep it ON till you get the required bulk temperature of water in

the container say 50O C, 60O C, 70O C temperature.

6) Switch off the heater R-1.

7) Very gradually increase the voltage across test heater by slowly

changing the variac position and stop a while at each position to

observe the boiling phenomenon on wire.

8) Go on increasing the voltage till wire brakes and carefully note the

voltage and current at this point.

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Critical heat flux apparatus

58

PRECAUTIONS

1) Keep the variac to zero voltage position before starting the

experiments.

2) Take sufficient amount of distilled water in the container so that

both the heaters are completely immersed.

3) Connect the test heater wire across the stud.

4) Do not touch the water or terminal points when the main switch

ON.

5) Operate the variac gently in steps and sufficient time in between.

6) After the attainment of critical heat flux decrease slowly the voltage

and bring it to zero position.

OBSERVATIONS

1) Diameter of test heater wire, d = m.

2) Length of the test heater, L = 0.1 m

3) Surface area A = π.d.L m2 =

Note: - The ammeter and voltmeter readings are to be note down when

wire melts.

CALCULATIONS

The critical heat flux at various bulk temperatures water can be

calculated by the following procedure.

1. Heat input Q = V. l Watts

2. Critical Heat flux 2texp m/W

A

I.V

A

Qq

3. Zubfer has given following equation for calculating peak heat flux in

saturated pool boiling.

Bulk

Temperature of

water OC

Ammeter Reading

(I Amps)

Voltmeter Reading

(V volts)

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Critical heat flux apparatus

59

4/1

2

(.18.0

v

vLLVvfgtheor

gh

A

Qq

Where Q/A = Heat Flux, W/m2

hfg = Latent Heat of vaporization J/kg ----------- (from steam table)

LV = Liquid vapour surface tension N/m ----------- (from Chart)

L = Density of Liquid

= Density of vapour = 1/vg kg/m3 (from steam table)

hfg, LV , L and are evaluated at the water temperature.

The experimental value of critical heat flux at the sat temperature is

comparable to that obtained by Zuber’s correlation.

RESULT

1. Experimental critical heat flux =

2. Theoretical critical heat flux =

Fig.1 Critical heat flux apparatus

1. Voltmeter 2. Ammeter

3. Heater switch 4. Lamp switch

5. Main switch 6. Heater control 7. Class container

8. Heater fitting

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Heat pipe demonstrator

60

HHEEAATT PPIIPPEE DDEEMMOONNSSTTRRAATTOORR

AIM

To study the variation of heat sink temperature and longitudinal

temperature distribution for heat pipe, stainless steel and copper pipe

with comparison.

INTRODUCTION

Heat pipe is an interesting device, which is used to transfer heat from one

location to another. It works with the help of evaporation and

condensation of liquid, which is filled inside heat pipe as a working

medium.

Heat pipe basically consist of a stainless steel pipe, sealed at both the

ends. It is evacuated and filled partially with distilled water. Stainless

steel mesh is provided at inside periphery of the pipe. When heat is

applied at the lower end of the heat pipe, water inside it evaporates and

vapor passes to upper end of the pipe. The heat is taken by the medium

surrounding upper portion of heat pipe. The vapor condenses giving its

latent heat of evaporation to the surrounding medium. The condensed

vapor returns to bottom through the mesh packing, thus because of

circulation of vapor, heat pipe operates at near to isothermal operation

and conducts much heat than conventional conductors.

APPARATUS

The apparatus consists of three pipes, viz., a heat pipe, copper pipe and a

stainless steel pipe. All the pipes have same physical dimensions.

Copper and Stainless steel pipes serve the purpose of comparison of heat

pipe performance with copper pipe as good conductor of heat and with

stainless steel pipe as same material of construction. All pipes are

mounted vertically with a band heater at lower end and a water filled

heat sink at upper end. When heaters start heating the pipes, begin to

transfer the heat to heat sinks. Rapid rise of temperature of water in the

heat pipe heat sink demonstrates high [apparent] thermal conductivity of

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Heat pipe demonstrator

61

heat pipe. Nearly isothermal operation of heat pipe is clearly visualized

from longitudinal temperature distribution of pipes.

SPECIFICATIONS

1. Heat pipe- Stainless steel pipe, φ 25 mm OD, 400 mm long at both

ends, evacuated & filled partially with distilled water – one no.

2. Copper and stainless steel pipes of same size as that of heat pipe –

one each

3. Equal capacity heaters at bottom end of each pipe.

4. Water filled heat sinks at upper end of each pipe.

5. Thermometers to note down water temperatures in heat sinks –

3nos.

6. Thermocouples for longitudinal temperatures and three

thermometers to sink temperatures.

EXPERIMENTAL PROCEDURE

Fill up sufficient water in heat sinks. Insure proper earthling to the

unit; put the thermometers in the grommets provided at the top of

heat sinks. Keep dimmer stat zero position and start electric supply

to unit. Slowly increase the dimmer so that power is supplied to

heaters. As same dimmer stat supplies power to all heaters and all

heaters are of same capacity, power input to all the heaters remains

same. This makes the comparison simpler. Go on noting down the

temperatures of water in heat sinks every 5 min (stir the water before

noting down the temperature. After around 30 minutes, note down the

longitudinal temperature of the pipes, from the temperatures

indicator. Replace the water when pipes become cool lower than 45oC

otherwise removing water at high temperature of pipe may burn the

seals at the bottom of heat sinks. If an experiment is conducted for

more time, it is merely to raise the water temperature and ultimately

evaporation of water. Hence it is not recommended to conduct the

experiment for more times than 30 minutes.

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Heat pipe demonstrator

62

OBSERVATIONS

I) Heat sink water temperatures

Time, minutes S.S Pipe

Heat sink

Copper Pipe

Heat sink

Heat Pipe

Heat sink

0 min

5 min

10 min

15 min

20 min

25 min

30 min

II) Longitudinal temperature distribution

S.S. Pipe Copper Pipe

(Switch at right hand

position)

Heat Pipe

T6 T1 T1

T7 T2 T2

T8 T3 T3

T9 T4 T4

T10 T5 T5

PRECAUTIONS

1. Proper earthing is necessary.

2. Stir the water before noting the water temperature in heat sink.

3. Do not remove water from heat sink till the pipes become cool.

4. Operate only one meter selector switch at a time in upward

position. Other two switches must be in downward position.

GRAPHS

1. Plot the graph of heat sink water temperature rise up to 30

minutes

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Heat pipe demonstrator

63

2. Plot longitudinal temperature distribution for pipes.

Fig.1: Thermocouple settings along the pipe length.

Fig.2: Variation of water temperature in three sinks with increase in time.

Fig.3: Variation of surface temperature along the length of the pipe after

30 min.

Lon

gitu

din

al

Te

mp

era

ture

, 0C

Height of pipe, mm

0 10 120 200 290 440 450

He

at

sin

k w

ate

r

tem

pera

ture

, 0C

Time, t, min

0 5 10 15 20 25 30

450

10 150 90 80 110 10

T1 T2

2

T3 T4 T5

GUDLAVALLERU ENGINEERING COLLEGE HEAT TRANSFER LAB

Heat pipe demonstrator

64

RESULT

Graphs are plotted and the performance of heat pipe is compared with

stainless steel pipe and copper pipe. Heat pipe conducts much heat than

conventional conductors. In longitudinal temperature distribution graph,

the heat pipe curve is almost straight line.

Fig.4 Heat pipe demonstrator

1. Heater 2. Water container

3. Thermometer 4. Heat pipe 5. S.S. pipe

6. Copper pipe 7. Voltmeter

8. Ammeter 9. Temperature indicator 10 selector switch

11. Temperature selector switch 12 Pipe toggle switches (only one switch is to be put in upward direction at time)

13. Main switch 14. Heater control

Prepared by B. V. S. RAGHU VAMSI, Asst. Professor, Department of Mechanical Engineering Page 1

HEAT TRANSFER LAB – VIVA QUESTIONS

CONDUCTION

1. State Fourier’s law of conduction.

ANS: The rate of flow of heat through a homogeneous body is directly proportional to the area of b

cross section at right angles to the direction of heat flow and temperature difference with

respect to the length of the path of the heat flow.

2. Difference between steady state and unsteady state.

ANS: A steady process is one which is not dependent on time, ie., rate of heat transfer does not

vary with time. There can be no change of internal energy of the system.

When the temperature at various points in a system do change with time, the process is

called an Unsteady or transient process.

3. State the significance of thermal conductivity and thermal diffusivity.

ANS: Thermal diffusivity tells us how fast the heat is propagated or it diffuses through a material

during changes of temperature with time. Larger the value of thermal diffusivity, the shorter

is the time required for the applied heat to penetrate deeper into the solid.

Thermal conductivity tells us the ability of a substance to conduct the heat.

4. Explain in brief the mechanism of heat transfer by conduction?

ANS: (i) By the drift of free electrons in the case of metallic solids.

(ii) By molecular interaction due to the existence of temperature gradient..

5. Give the one-dimensional steady state heat conduction equation with internal heat generation.

ANS: 𝜕2𝑡

𝜕𝑥 2 +𝑞

𝑘= 0

6. Give the one-dimensional unsteady steady state heat conduction equation with internal heat generation.

ANS: 𝜕2𝑡

𝜕𝑥 2 +𝑞

𝑘=

1

∝ .

𝜕𝑡

𝜕𝑇

7. Give the formula of thermal diffusivity.

ANS: α = 𝑘

𝜚𝐶𝑝

8. Write the equation for diffusion equation.

ANS: 𝜕2𝑇

𝜕𝑥 2+

𝜕2𝑇

𝜕𝑦 2+

𝜕2𝑇

𝜕𝑧 2=

1

∝ .

𝜕𝑇

𝜕𝑡

9. Give the increasing and decreasing order of thermal conductivity for solids, liquids and

gases.

ANS: Thermal conductivity of solids > liquids >gases.

Prepared by B. V. S. RAGHU VAMSI, Asst. Professor, Department of Mechanical Engineering Page 2

10. Which materials will have higher and lower thermal conductivity values?

ANS: Pure metals have high thermal conductivity and Gases, insulating materials have low thermal

conductivity.

For pure metals it decreases with increase in temperature whereas for gases and insulating

metals it increases with rise in temperature.

11. Write the formula for heat transfer through a hollow cylinder.

ANS: 𝑄 = 𝑇1− 𝑇2

ln𝑟2𝑟1

2𝜋𝑘𝑙

12. Write the formula for heat transfer through a hollow sphere.

ANS: 𝑄 = 𝑇1− 𝑇2

𝑟2 − 𝑟1

4𝜋𝑘 𝑟1𝑟2

13. Give the equations critical thickness of insulation for sphere and cylinder.

ANS: for cylinder 𝑟2 = 𝑟𝑐 = 𝑘

ℎ0

For sphere 𝑟2 = 𝑟𝑐 = 2𝑘

ℎ0

14. Write the formula for temperature distribution in a slab.

ANS: 𝑡− 𝑡1

𝑡2 − 𝑡1=

𝑥

𝑙

15. Write the formula for heat transfer in a composite slab.

ANS: Considering the composite wall has 3 slabs 𝑄 = 𝐴 𝑡1 −𝑡4

𝐿𝑎𝑘𝑎

+ 𝐿𝑏𝑘𝑏

+ 𝐿𝑐𝑘𝑐

16. How the thermal conductivity will vary with temperature gradient?

ANS: Thermal conductivity and temperature gradient are inversely proportional.

17. How the thermal conductivity will vary for solids, liquids & gases?

ANS: thermal conductivity of solids > liquids > gases

CONVECTION

1. State Newton’s law.

ANS: That the rate of heat loss of a body is proportional to the difference in temperatures between

the body and its surroundings.

Q = hA(ts –tf)

2. Explain in brief the mechanism of heat transfer by convection?

ANS: When a fluid flows over a duct or a solid body and the temperatures of the fluid and the solid

Prepared by B. V. S. RAGHU VAMSI, Asst. Professor, Department of Mechanical Engineering Page 3

surfaces are different, then the heat transfer will takes place between fluid and solid surface

due to the motion of fluid relative to the surface. This type of heat transfer is called

Convection.

If the fluid motion is set up by buoyancy forces resulting from the density variation

caused by the temperature difference in the fluid, heat transfer is said to be free or natural

convection.

If the fluid motion is set up by an external agency like fan or blower, heat transfer is

termed as forced convection.

3. Differences between natural convection and forced convection.

ANS: In natural convection the fluid circulates due to the difference in the densities of hot and cold fluids. In forced convection work is done to blow or pump the fluid.

4. Define the following with formulae and mention their significance.

i. Nusselt number: Ratio of heat flow rate by convection under unit temperature gradient to

the heat flow rate by conduction under unit temperature gradient through stationary

thickness of L meters.

𝑁𝑈 =ℎ𝐿

𝑘

ii. Grashoff number: Ratio of product of inertia force and buoyancy force to the square of

viscous force.

𝐺𝑟 = 𝜌2𝛽𝑔𝐿3∆𝑡

𝜇2

𝐴 critical value of Grashoff number is used to indicate transition from laminar to

turbulent in free convection.

iii. Prandtl number: Ratio of kinematic viscosity to the thermal diffusivity

𝑃𝑟 = 𝜇𝑐𝑝

𝑘

It is a measure of relative effectiveness of momentum and energy diffusion in the

velocity and thermal boundary layers, respectively.

iv. Reynolds number: Ratio of inertia force to viscous force

𝑅𝑒 =𝑉𝐿

𝜗

It determines the nature of flow whether it is laminar or turbulent.

v. Biot number: Ratio of internal conduction resistance to surface convection resistance

𝐵𝑖 =ℎ𝐿

𝑘=

ℎ𝑅

𝑘

𝐼𝑡 𝑑𝑒𝑐𝑖𝑑𝑒𝑠 𝑤ℎ𝑒𝑡ℎ𝑒𝑟 𝑡ℎ𝑒 𝑠𝑦𝑠𝑡𝑒𝑚 𝑖𝑠 𝑢𝑛𝑑𝑒𝑟 𝑙𝑢𝑚𝑝𝑒𝑑 𝑠𝑦𝑠𝑡𝑒𝑚 𝑎𝑛𝑎𝑙𝑦𝑠𝑖𝑠 𝑜𝑟 𝑛𝑜𝑡.

Prepared by B. V. S. RAGHU VAMSI, Asst. Professor, Department of Mechanical Engineering Page 4

vi. Fourier number: Ratio of characteristic body dimension to temperature wave penetration

depth in time,𝜏

𝐹𝑜 =𝛼𝜏

𝑅2

5. Write the units for heat transfer coefficient, thermal conductivity and thermal diffusivity.

ANS: Heat transfer coefficient - w/m2k

Thermal conductivity - w/mk

Thermal diffusivity - wm2/kJ

6. What is a heat exchanger?

ANS: A heat exchanger is equipment which transfers the energy from hot fluid to cold fluid with

maximum rate and minimum investment.

7. Differences between parallel flow, counter flow and cross flow heat exchangers.

ANS:

Parallel Flow Heat Exchanger Counter Flow heat Exchanger Cross Flow Heat Exchanger

Hot and cold fluids flow in

same direction.

Hot and cold fluids flow in

opposite direction

Hot and cold fluids flow in

perpendicular direction

When one of the fluids changes phase, it is immaterial whether it is a parallel or counter

or cross flow heat exchanger. Rate of heat transfer in all these modes is same.

8. What is the effectiveness of a heat exchanger?

ANS: Effectiveness of heat exchanger is the ratio of actual heat transfer to the maximum possible

heat transfer. 9. Write the formulae for LMTD of parallel flow and counter flow heat exchangers.

ANS: LMTD 𝑄 = 𝑈𝐴𝜃𝑚

𝜃𝑚 = 𝑡ℎ1−𝑡𝑐1 − 𝑡ℎ2−𝑡𝑐2

ln 𝑡ℎ1−𝑡𝑐1

𝑡ℎ2−𝑡𝑐2

for parallel flow

𝜃𝑚 = 𝑡ℎ1−𝑡𝑐2 − 𝑡ℎ2−𝑡𝑐1

ln 𝑡ℎ1−𝑡𝑐2

𝑡ℎ2−𝑡𝑐1

for counter flow

10. Define number of transfer units.

ANS: It is a dimensionless parameter. It is a measure of the heat transfer size of a heat exchanger. The Larger the value of the NTU, the closer the heat exchanger reaches its thermodynamic limit of operation.

NTU=𝑈𝐴

𝐶 𝑚𝑖𝑛 for counter flow

11. How heat exchanger is classified based on specific heat capacity, Cmax & Cmin?

ANS: First calculate Cc & Ch ie., heat capacity of cold and hot fluids respectively. Then the one which

has the maximum will be Cmax and the other will be Cmin.

Prepared by B. V. S. RAGHU VAMSI, Asst. Professor, Department of Mechanical Engineering Page 5

If Ch < Cc or Cc ∞, then the heat exchanger will be a Evaporator. The cold fluid

temperature will remain uniform and undergoes a phase change.

If Ch > Cc or Ch ∞, then the heat exchanger will be a condenser. The hot fluid will remain

at a constant temperature, while the temperature of a cold fluid increases.

12. What is pool boiling and flow boiling?

ANS: If the heat is added to a liquid from a submerged solid surface, the boiling process is referred

to as POOL BOILING. The liquid above the hot surface is essentially stagnant and its motion

near surface is due to free convection. Ex: Boiling of water in a kettle.

FLOW BOILING occurs in a flowing stream and the boiling surface may itself be a portion of

the flow passage. It is also referred to as Forced Convection Boiling.

13. What is the difference between critical heat flux and burnout point?

ANS: CRITICAL HEAT FLUX is the point of maximum heat flux on the boiling curve at which

transition from nucleate to film boiling initiates. It is also called as Boiling Crisis because the

boiling point beyond this point is unstable.

BURNOUT POINT is the peak heat flux point on the boiling curve which occurs at the end of

the radiation part of the film boiling. Equipment should be made to operate close to this point

but never beyond this.

14. In which systems design, the determination of critical heat flux is useful?

ANS: Design of equipment used for boiling of liquids.

15. What is the purpose of a Fin?

ANS: To increase the effective area of surface thereby increasing the heat transfer by convection.

16. What are the various conditions in Fins?

ANS: 1. Long Fin

2. Fin with insulated tip

3. Fins with Convection off the End

17. Out of parallel flow, counter flow and cross flow heat exchangers, which is more effective.

ANS: counter flow is more effective

RADIATION

1. State Stefan Boltzmann law.

ANS: Emissive power of black body is proportional to the fourth power of its absolute

temperature.

2. Define absorptivity, transmittivity and reflectivity. Give the relation between them.

ANS: Absorptivity (α): Fraction of incident radiation absorbed.

Prepared by B. V. S. RAGHU VAMSI, Asst. Professor, Department of Mechanical Engineering Page 6

Transmittivity (τ): Fraction of incident radiation transmitted.

Reflectivity (ρ): Fraction of incident radiation reflected.

α + ρ + τ = 1

3. Define emissivity.

ANS: It is a measure of how it emits radiant energy in comparison with a black surface at the same

temperature.

4. State the properties of a black body.

ANS:1. It absorbs all the incident radiation falling on it and does not transmit or reflect regard less

of wave length and direction.

2. It is a perfect emitter.

3. Total radiation emitted by a black body is a function of temperature only.

5. Give the materials for which absorptivity, transmittivity, reflectivity and emissivity value

will be zero and one.

Gases such as Hydrogen, Oxygen and Nitrogen have a transmittivity of practically unity.

For an opaque substance, τ = 0 and for a perfectly transparent surface, τ = 1.

For a non-reflecting surface, ρ = 0 and for a perfect reflector, ρ = 1

For a non-absorbing surface (white body), α = 0 & for perfect absorber (black body), α = 1

6. Define emissive power.

ANS: It is defined as energy emitted by a surface per unit time per unit area.

Units: w/m2

7. Define monochromatic emissive power.

ANS: It is defined as the rate of energy radiated per unit area of the surface per unit wavelength.

Units: w/m2μ

8. State Wein’s law of radiation.

ANS: it states that the product of 𝜆𝑚𝑎𝑥 and T is constant.

9. How the heat is transferred by radiation phenomenon?

ANS: If two bodies at different temperatures are placed in an evacuated adiabatic enclosure so that

they are not in contact through a solid or fluid medium, then the temperatures tend to

become equal. This mode of heat transfer is called radiation. This is maximum only when the

two bodies are separated by a perfect vacuum. It is an electromagnetic wave phenomenon

and no medium is needed.

10. Write the units for emissive power and Stefan Boltzmann constant.

ANS: Emissive power: w/m2.

Prepared by B. V. S. RAGHU VAMSI, Asst. Professor, Department of Mechanical Engineering Page 7

Stefan Boltzmann constant: w/m2K4.

11. Explain in brief about the transfer of heat in condenser, boiler & regenerator in two lines.

ANS: Heat transfer in condenser – by Conduction and Convection.

Heat transfer in Boiler and Regenerator – by Conduction, Convection and Radiation.

GENERAL

1. What are the various temperature measuring devices used in various Engineering

applications? Give their range of application?

ANS: Refer to ICS text book.

2. Define coefficient of discharge.

ANS: It is the ratio of actual discharge to theoretical discharge. Its value will vary with the type

of discharge measuring device.

3. In which applications Orifice, Venturimeter and simple U- tube manometer are used.

ANS: Orifice, Venturimeter – To measure the discharge of a fluid in a pipe.

Simple U-tube manometer – To measure the pressure head at a point

4. What is the purpose of a dimmerstat?

ANS: To control the voltage and current.

5. Define kinematic viscosity, dynamic viscosity & surface tension. Write the formulae and

mention their units.

ANS: Dynamic Viscosity – Ratio of shear stress to time rate of strain.

Units: N-sec / m², Poise

Kinematic Viscosity – Ratio of dynamic viscosity to density of the fluid.

Units: m²/sec, cm² / sec, stoke etc.,

Surface Tension – Energy expended per unit area of the surface.

Units: Nec / m², Poise


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