Pal et al., International Journal of Advanced Engineering Technology E-ISSN 0976-3945

Int J Adv Engg Tech/IV/IV/Oct-Dec.,2013/86-89

Research Paper

EFFECT OF RELATIVE LONG-WAY LENGTH ON NUSSELT

NUMBER AND FRICTION FACTOR OF CONCAVE AND

CONVEX STREAMLINED PROTRUDED ARTIFICIALLY

ROUGHENED DUCT OF SOLAR AIR HEATERS Singh Sukhbir Pal

a, Mahajan Tarun

b, Kumar Pardeep

c

Address for Correspondence a,b,c

Sri Sai College of Engineering and Technology, Badhani, Punjab -145001

ABSTRACT: This paper presents result of an experimental investigation of heat transfer and friction loss for a range of system and

operating parameters, to study Nusselt number and friction factor of concave streamlined protruded artificially roughened

duct of solar air heaters. Enhancement of heat transfer and pressure drop of roughened absorber plate has been observed.

Experimental data may be used for designing such collectors for practical applications having investigated type of roughness

geometry.

KEY WORDS: Solar air heater, artificial roughness, Heat transfer coefficient, Nusselt number and Friction factor.

INTRODUCTION

Energy is a basic need for human being for generation

and economic development of nation. Energy

resources may be classified as conventional and non-

conventional energy resources. Global energy crisis,

environmental threats and to meet increasing energy

demand, need of alternative and clean energy sources

have to be increased. Solar energy is available in

abundance on earth in the form of radiation may be

converted into thermal energy and used for heating

and other applications. Solar air heater is a cheapest

way for converting solar energy into thermal energy

absorbs incoming solar radiation, converting it into

thermal energy at absorbing surface, and transferring

the energy to a fluid flowing through the collector.

These have found several applications including space

heating, crop drying, etc. The thermal efficiency of flat

plate solar air heater has been found to be low because

of low convective heat transfer coefficient between

absorber plate and flowing air which increases

absorber plate temperature leading to higher heat

losses to the environment resulting in low thermal

performance. Thermal Performance of solar air heaters

may be increased by increasing convective heat

transfer coefficient, by using fins or creating the

turbulence on the heat transferring surfaces [1].

Artificial roughness is basically a passive heat transfer

enhancement technique by which thermo hydraulic

performance of a solar air heater can be improved. The

artificial roughness has been used extensively for the

enhancement of forced convective heat transfer, which

further requires flow at the heat transferring surface to

be turbulent. However, energy for creating such

turbulence has to come from the fan or blower and

excessive power is required to flow air through the

duct, which may further be creating turbulence only in

the region very close to the heat transferring surface.

Several investigators reported heat transfer coefficient

and friction loss for roughened duct of solar air heaters

and developed the correlations for Nusselt number

and friction factor of various types of roughness

geometries. Bhushan and Singh [2] reported various

artificial roughness geometries produced by several

methods such as by Casting, welding and wire fixation

in the form of transverse continuous ribs, transverse

broken ribs, inclined and V-shaped or staggered ribs;

rib formation by machining process in the form of

chamfered ribs, wedge shaped ribs, combination of

different integral rib roughness elements and by using

expanded metal mesh ribs etc. Prasad and Mullick [3]

utilized artificial roughness in the duct used in solar

heater in the form of small diameter wires to increase

heat transfer coefficient. Prasad and Saini [4]

investigated fully developed turbulent flow in the duct

with a small diameter protrusion wire on the absorber

plate. Muluwork et al. [5] compared thermal

performance of staggered discrete V-apex up and

down with corresponding transverse staggered discrete

ribs. Momin et al. [6] experimentally investigated

effect of geometrical parameters of V-shaped ribs on

heat transfer and fluid flow characteristics in

rectangular duct used in solar air heaters. Jaurker et al.

[7] experimentally investigated heat transfer and

friction characteristics of rib-grooved artificial

roughness. Saini and Saini [8] used expanded metal

mesh to create artificial roughness on absorber plate

and investigated effect of system and operating

parameters on heat transfer and friction loss. Saini and

Verma [9] investigated heat transfer and friction

characteristics of dimple shaped artificial roughness

geometry for Reynolds number 2000–12,000. Bhushan

and Singh [10] analyzed the effect of artificial

roughness on heat transfer and friction in solar air

heater duct having protrusions as roughness geometry.

Varun et al. [11] presented a review on artificial

roughness investigations reported in literature.

Belenkiy et al. [12] used staggered array of concave

dimples in annular passages on the interior cylindrical

surfaces. Heat transfer augmentation as high as 150%,

compared to smooth surfaces were reported with

appreciable pressure losses. Kesarev and Kozlov [13]

present distributions of local heat transfer coefficients

inside a single hemispherical cavity, and indicate that

the convective heat transfer from the cavity is higher,

especially on the downstream portion, than that from

the surface of a plane circle of the same diameter as

the cavity diameter. Literature review revealed that

protrusions as artificial roughness is a good

technique for enhancing heat transfer coefficient of

air flowing through the duct and increases in friction

losses. Further, heat transfer coefficient of air

passing underside of the absorbing plate could be

enhanced by protruded concave streamlined

dimples. An experimental investigation has been

carried out to study the Nusselt number and friction

factor of roughened duct with concave streamlined

dimples protruded on the underside of absorber plate

of a solar air heater. The resulted increase in friction

loss can be further minimize by creating turbulence

only in the region very close to the absorber surface

i.e. in laminar sub-layer only [9].

EXPERIMENTAL SET-UP AND PROCEDURE To generate experimental data, a test rig has been

designed and fabricated; schematic and photographic

Pal et al., International Journal of Advanced Engineering Technology

Int J Adv Engg Tech/IV/IV/Oct-Dec.,2013/86

views of experimental set-up are shown in Fig. (

and Fig. (2) respectively. It consists of a

mm x 840 mm x 70 mm) is made up of wooden ply

board of 20 mm thickness with length of entry, test

and exit sections are 900 mm, 1000 mm and 500 mm

respectively. Temperature and pressure drop

measuring instruments are provided in the

with surmounted electric heater assembly and

converging plenum (500 mm) was provided on exit

side of the air duct. To get uniform heating flux,

insulated electric heater (2200 mm x 82

been fabricated by combining series and parallel loops

of heating wire on mica sheet. Electric supply of

heater is being varied with help of Variac

U-tube manometer on calibrated orifice-meter is used

to measure the mass flow rate of air.

controlling of air flow rate, two gate valves one at

entry and other at exit of centrifugal blower

installed. To measure temperature of air and absorber

plate (22 SWG, GI sheet, 2400 mm x 880 mm),

copper-constantan thermocouples were used at

different locations as shown in Figs. (

Micro-manometer with kerosene oil as fluid was used

to measure the pressure drop across test section of the

duct. Photographic and geometrical views of

experimentally investigated protruded plate have

shown in Fig. (5) and Fig. (6).

Fig. 1: (a) Schematic of experiment set-up (b) Sectional

view of air duct at entry section.

Fig.2: Photographic view of experimental set

Table 1 shows range/value of parameters used in

present experimental investigation.

streamlined dimples were punched on the absorber

plate. Before starting the experimentation,

instruments/equipments like orifice meter, temperature

indicator, and temperature selector switch, micro

manometer and U-tube manometer were properly

inspected. All joints of the test rig were properly filled

with glue, to avoid leakage in any joint. Five values of

flow rate were used for each set and following data

were collected at an interval of one hour in each set of

experimentation:

• Pressure drop across orifice meter

flow rate of air.

• Pressure drop across test section of duct.

• Temperature of absorber plate at various

locations in test section of the duct.

• Temperature of air at various locations in test

section of the duct.

International Journal of Advanced Engineering Technology E-ISSN 0976

86-89

shown in Fig. (1)

consists of air duct (2400

mm x 840 mm x 70 mm) is made up of wooden ply

board of 20 mm thickness with length of entry, test

and exit sections are 900 mm, 1000 mm and 500 mm

Temperature and pressure drop

are provided in the air duct,

surmounted electric heater assembly and

was provided on exit

To get uniform heating flux, well

20 mm) have

ing series and parallel loops

Electric supply of

of Variac (0-250V). A

meter is used

to measure the mass flow rate of air. For smooth

wo gate valves one at

entry and other at exit of centrifugal blower was

To measure temperature of air and absorber

plate (22 SWG, GI sheet, 2400 mm x 880 mm),

constantan thermocouples were used at

in Figs. (3) and (4).

manometer with kerosene oil as fluid was used

ressure drop across test section of the

. Photographic and geometrical views of

experimentally investigated protruded plate have been

up (b) Sectional

view of air duct at entry section.

Photographic view of experimental set-up.

shows range/value of parameters used in

present experimental investigation. Concave

on the absorber

Before starting the experimentation,

equipments like orifice meter, temperature

indicator, and temperature selector switch, micro

tube manometer were properly

the test rig were properly filled

Five values of

flow rate were used for each set and following data

were collected at an interval of one hour in each set of

Pressure drop across orifice meter to measure

Pressure drop across test section of duct.

Temperature of absorber plate at various

Temperature of air at various locations in test

• Voltage and current supplied to electric heater.Table 1: Range/Value of parameters.

Accuracy of experimental data was verified by

conducting experiments for a conventional smooth

duct. Experimental and predicted values of

number and friction factor were

rectangular smooth duct as reported by Momin et al.

[6].

Dittus-Boelter correlation for Nusselt number is

Nus=0.023Re0.8Pr

0.4

Blasius equation for friction factor is

25.0Re085.0 −=sf

Fig. (7) and Fig. (8) shows experimental and predicted

values of Nusselt number and friction factor for

smooth absorber plate. Error analysis based on the

procedure described by Holman [14] has been carried

out to find out uncertainties in measured/calculated

values of experimental data. Uncertainty in Reynolds

number, Nusselt number and friction factor values has

been estimated as 2.63%, 2.55%, and 4.04%

respectively. Good agreement in experimental and

predicted data ensures accuracy of collected data from

self designed and fabricated experimental set

Fig. 3: Different locations of thermo

measure absorber plate temperature.

Fig.4: Different locations of thermocouples used to

measure air temperature in the duct.

Fig. 5: Photographic view of protruded metallic plate

Fig. 6: Geometrical views of protruded metallic plate

ISSN 0976-3945

electric heater. Range/Value of parameters.

Accuracy of experimental data was verified by

conducting experiments for a conventional smooth

Experimental and predicted values of Nusselt

were compared for

as reported by Momin et al.

Boelter correlation for Nusselt number is

(1)

(2)

experimental and predicted

of Nusselt number and friction factor for

smooth absorber plate. Error analysis based on the

] has been carried

t uncertainties in measured/calculated

values of experimental data. Uncertainty in Reynolds

number, Nusselt number and friction factor values has

been estimated as 2.63%, 2.55%, and 4.04%

Good agreement in experimental and

res accuracy of collected data from

self designed and fabricated experimental set-up.

couples used to

temperature.

Different locations of thermocouples used to

measure air temperature in the duct.

Fig. 5: Photographic view of protruded metallic plate

Fig. 6: Geometrical views of protruded metallic plate

Pal et al., International Journal of Advanced Engineering Technology

Int J Adv Engg Tech/IV/IV/Oct-Dec.,2013/86

Fig.7:Experimental and predicted Friction factor and

Reynolds number

Fig. 8: Experimental and predicted Nusselt number and

Reynolds number.

Data Reduction

Following equations were used for calculating

pressure drop across test section (∆Pt) and orifice plate

(∆Po), mass flow rate of air (m), velocity of

heat transfer rate (q), heat transfer coefficient (h),

Nusselt number (Nu) and friction factor (f):

( )t k tP g hρ∆ = ∆

( )o w wP g hρ∆ = ∆

0.5

4

2

1

od o

P Sinm C A

ρ θβ

∆=

− &

c

mV

Aρ=

&

( )p o iq mC T T= −&

Also,

( )ampmc TThAq −=

Therefore, from eqs. (7) and (8)

( )ampmc TTA

qh

−=

Where Tpm and Tam are mean temperature of valu

recorded for absorber plate and air at different

locations along test section of the duct. Reynolds

number, Nusselt number and friction factor values

were calculated by using the following relationships:

µρVD

=Re

k

hDNu =

2

2

4

tPDfLVρ∆

=

RESULTS AND DISCUSSION

Variation of heat transfer coefficient as a fu

mass flow rate of air for smooth and roughened

absorber plate is shown in Fig. 9. It has been observed

that heat transfer coefficient increases consistently

with positive slope for entire range of mass flow rate

of air. It is investigated that concave roughened plate

(s/e=14, l/e=16, e/D=.03928655) have highest heat

transfer coefficient as compared all other

convex roughened plates as well as smooth plate.

Variation of Nusselt number as a function of Reynolds

number for roughened and smooth absorber plate

shown in Fig. 10. Concave Roughened plate

s/e=14, e/D=0.03925688) has shown highest

International Journal of Advanced Engineering Technology E-ISSN 0976

86-89

Experimental and predicted Friction factor and

Experimental and predicted Nusselt number and

Following equations were used for calculating

and orifice plate

), velocity of air (V),

heat transfer rate (q), heat transfer coefficient (h),

Nusselt number (Nu) and friction factor (f):

(3)

(4)

(5)

(6)

(7)

(8)

(9)

are mean temperature of values

recorded for absorber plate and air at different

locations along test section of the duct. Reynolds

Nusselt number and friction factor values

were calculated by using the following relationships:

(10)

(11)

(12)

as a function of

smooth and roughened

It has been observed

that heat transfer coefficient increases consistently

mass flow rate

oncave roughened plate

(s/e=14, l/e=16, e/D=.03928655) have highest heat

other concave and

roughened plates as well as smooth plate.

Variation of Nusselt number as a function of Reynolds

smooth absorber plate is

Roughened plate (l/e=16,

highest Nusselt

number as compared all other concave and convex

roughened plates as well as smooth plate

turbulence caused by roughness el

laminar sub-layer [4]. Pressure drop increases with

rise of mass flow rate of air for all roughened

compared to smooth plate as shown in Fig. 1

noted that for entire range of mass flow rate of air,

pressure drop is highest in case of convex plate

roughened with (s/e=14, l/e=16, e/D=.03928655)

compared all other convex and concave roughened

plates as well as smooth plate. Variation of friction

factor as a function of Reynolds number for smooth

and roughened plate is shown in Fig. 1

that friction factor for roughened plate (s/e=12, l/e=10,

e/D=.03928655) has highest value as compare to

smooth plate and other roughened plates for all values

of Reynolds number, due to mixing of flow an

generation of secondary flows in flow regimes similar

to as reported by Saini and Saini (10).

Fig. 9: Variation of heat transfer coefficient

of mass flow rate of air for smooth and roughened

absorber plate.

Fig. 10: Variation of Nusselt number as a function of

Reynolds number for roughened and smooth

plate.

Fig.11: Variation of pressure drop as a function of

flow rate of air for roughened smooth absorber plate

ISSN 0976-3945

(3)

(6)

(8)

(9)

(10)

(12)

other concave and convex

roughened plates as well as smooth plate, due to

turbulence caused by roughness elements in the

ressure drop increases with

roughened plate as

smooth plate as shown in Fig. 11. It is

for entire range of mass flow rate of air,

pressure drop is highest in case of convex plate

roughened with (s/e=14, l/e=16, e/D=.03928655) as

compared all other convex and concave roughened

Variation of friction

factor as a function of Reynolds number for smooth

roughened plate is shown in Fig. 12. It is observed

that friction factor for roughened plate (s/e=12, l/e=10,

e/D=.03928655) has highest value as compare to

smooth plate and other roughened plates for all values

, due to mixing of flow and

generation of secondary flows in flow regimes similar

heat transfer coefficient as a function

smooth and roughened

Variation of Nusselt number as a function of

and smooth absorber

s a function of mass

roughened smooth absorber plate.

Pal et al., International Journal of Advanced Engineering Technology

Int J Adv Engg Tech/IV/IV/Oct-Dec.,2013/86

Fig. 12: Variation of friction factor as a function of

Reynolds number for roughened smooth absorber plate

CONCLUSION

This paper presents an experimental investigation of

artificially roughened duct used in solar air heaters.

Variation of heat transfer and friction loss due to

artificial roughness (created by concave streamlined

dimples on absorber plate) has been investigated for

Reynolds numbers range of 8000–24000. It has been

observed that roughened absorber plate results into

higher heat transfer coefficient at the cost of

penalty. It is also noted that roughened plate (concave,

s/e=14, l/e=16, e/D=.03928655) has Nusselt number

and friction factor of the order of 2.87 times and

times as compared to smooth absorber plate

respectively.

ACKNOWLEDGEMENT

The author would like to express a deep sense of

gratitude and thanks to Er. Tarun Mahajan,

Professor and Head, Department of Mechanical

Engineering, Sri Sai College of Engineering

Technology (SSCET), Pathankot for their valuable

guidance, persistent encouragement and suggestions at

every stage of my dissertation. Without able guidance

and wise counseling, it would have been impossible to

complete the research work in this manner.

The constant encouragement received from Prof. J.S.

Kaler M.Tech Coordinator, Department of Mechanical

Engineering, and SSCET Pathankot has been of great

importance in carrying out the present research work.

The author also expresses gratitude to all other faculty

members of Mechanical Engineering Department,

SSCET Pathankot for their intellectual support

throughout the course of this work. The help rendered

by Sh. Balbir Singh, Workshop Superindent and other

workshop staff of SSCET Pathankot for conducting

experimentation is greatly acknowledged.

The author is highly grateful to Principal, Dr. Dayal

Chand SSCET Pathankot for providing moral support

and help in each aspect to carry out the present

research work. I am deeply indebted to my parents for

their best wishes and co-operation extended

throughout the work duration.

Finally, the author is indebted to all whosoever have

helped directly or indirectly in this dissertation

and for friendly stay at SSCET Pathankot.

REFERENCES 1. Kumar P., Singh S., Mahajan T., “Effect of Relative long

way length on Nusselt number and Friction factor of streamlined dimpled artificially roughened duct solar air

heater”, Int. J. of Adv. Engg. Tech., IV (2013) 57

2. Bhushan B., Singh R., “A review on methodology of artificial roughness used in duct of solar air heaters

Energy, 35 (2010) 202-212.

International Journal of Advanced Engineering Technology E-ISSN 0976

86-89

as a function of

roughened smooth absorber plate.

This paper presents an experimental investigation of

artificially roughened duct used in solar air heaters.

Variation of heat transfer and friction loss due to

concave streamlined

on absorber plate) has been investigated for

24000. It has been

observed that roughened absorber plate results into

higher heat transfer coefficient at the cost of frictional

It is also noted that roughened plate (concave,

s/e=14, l/e=16, e/D=.03928655) has Nusselt number

times and 2.13

imes as compared to smooth absorber plate

The author would like to express a deep sense of

gratitude and thanks to Er. Tarun Mahajan, Assistant

, Department of Mechanical

Engineering, Sri Sai College of Engineering

for their valuable

tent encouragement and suggestions at

every stage of my dissertation. Without able guidance

and wise counseling, it would have been impossible to

complete the research work in this manner.

The constant encouragement received from Prof. J.S.

ordinator, Department of Mechanical

Engineering, and SSCET Pathankot has been of great

importance in carrying out the present research work.

The author also expresses gratitude to all other faculty

members of Mechanical Engineering Department,

kot for their intellectual support

The help rendered

by Sh. Balbir Singh, Workshop Superindent and other

workshop staff of SSCET Pathankot for conducting

experimentation is greatly acknowledged.

eful to Principal, Dr. Dayal

Chand SSCET Pathankot for providing moral support

and help in each aspect to carry out the present

I am deeply indebted to my parents for

operation extended

Finally, the author is indebted to all whosoever have

dissertation work

and for friendly stay at SSCET Pathankot.

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Heat transfer characteristics of

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