Introductory Definitions

• Heat

– Form of energy that can be transferred from one system to another as a result of a temperature difference.

• Heat Transfer

– Science that deals with the determination of rates of energy transfer.

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MEC551

THERMAL ENGINEERING

1.0 Introduction

Why Study Heat Transfer?

• Thermodynamics is concerned with the amount of total heat transfer as a system undergoes a process from one equilibrium state to another.

• However, the study of thermodynamics gives no indication of how long it takes.

2

Why Study Heat Transfer?

• Thermodynamics

– Deals with equilibrium states and changes from one system to another

• Heat Transfer

– Deals with systems that lack thermal equilibrium (e.g. non-equilibrium phenomenon).

3

Foundational Laws

• However, the laws of thermodynamics lay out the framework for studying heat transfer.

• 1st Law – Energy Equation

– Rate of energy transfer into a system equal the rate of increase of energy in the system

• 2nd Law

– Heat is transferred in the direction of decreasing temperature.

4

Heat Transfer Direction

5

HOT COLD

Heat Transfer Mechanisms

6 http://www.wisc-online.com/Objects/ViewObject.aspx?ID=sce304

Types of Heat Transfer

Conduction

• Transfer of energy from the more energetic particles of a substance to an adjacent substance with less energetic particles.

• Can take place in liquids, solids, or gases. – In a gas, conduction is due to the collisions

and diffusion of the molecules due to their random motion.

– In solids, it is due to the combination of vibrations of the molecules in their lattice and the energy transport of free electrons.

Conduction Equation (Fourier’s Law of Heat Conduction)

Thickness

DifferenceeTemperaturAreak

x

TAkQ

cond

Conduction Equation (Fourier’s Law of Heat Conduction)

• Fourier’s Law of Heat Conduction (1822) is:

WdirectionxinconductionofRateQx

~

2

sec~

mflowheatthe

ofdirectionthetonormalareationalCrossA

Wattsdx

dTkAQ

x~

m

CflowheatofdirectiontheingradienteTemperatur

dx

dT~

Cm

WmaterialtheoftyconductiviThermalk ~

Thermal conductivity

• The thermal conductivity (k) of a material is defined as the rate of heat transfer through a unit thickness of a material per unit area per unit temperature difference.

• Symbol: k

• Units: W/(m·ºC)

• Tables in text book

100 cm x

50 cm

Conduction (Example 1.1)

• The inside and outside of the surface of a window glass are at 20ºC and -5ºC respectively. If the glass is 100 cm x 50 cm in size and 1.5 cm thick, with a thermal conductivity of 0.78 W/(m·ºC).

• Determine the heat loss through the glass over a period of 2 hours.

20ºC -5ºC

1.5 cm

Conduction (Example 1.1)

T1 = 20 ºC

T2 = -5 ºC

A = (100x50)= 5,000 cm2 = 0.5 m2

k = 0.78 W/(m·ºC)

dx= 0.015 m

hrkWhourskW

hoursoverLossHeatTotal

3.1265.0

:2

100 cm x

50 cm

20ºC -5ºC

1.5 cm

W

m

CCm

x

TTAkQ

CmW 650

015.0

5205.078.0 2

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Convection

• Is the mode of energy transfer between a solid surface and the adjacent liquid or gas that is in motion.

• Convection involves the combined

effects of conduction and fluid motion.

• Forced convection- fluid is forced to flow over the surface by external means such as fan.

• Natural/free convection-fluid motion is caused by buoyancy forces that are induced by density differences due to the variation of temperature in fluid

Convection Equation (Newton’s Law of Cooling)

• Newton’s Law of Cooling (1701) is:

WattsTTAhQ fluidwallconv ~

u∞ y

x

T∞

Heated Surface

Convection Equation (Newton’s Law of Cooling)

WattsTTAhQ fluidwallconv ~

WdirectionyinconvectionofRateQconv

~

Cm

WtcoefficienConvectionh

2~

2~ mareaSurfaceA

CetemperatursurfaceWallTwall ~

CetemperaturFluidTTfluid ~

Convection Heat Transfer Coefficient

• The convection heat transfer coefficient (h) is not a property of a fluid (unlike k). It is an experimentally determined parameter whose value depends on surface geometry, fluid motion, fluid properties, and bulk fluid velocity.

• Symbol: h

• Units: W/(m2·ºC)

Convection (Example 1.2)

• Atmospheric air at a temperature of 10ºC flows with a velocity 5 m/s across a tube with an outer diameter (OD) of 1 cm and a length of 5 m. The surface is maintained at 110ºC.

• Determine the rate of heat

flow from the tube surface to atmospheric air if h is 85 W/(m2·ºC).

AIR

1 cm

Tw=110ºC

5 m

T∞=10ºC

V = 5 m/s

h = 85 W/(m2·ºC)

Convection (Example 1.2)

Surface Area:

Heat Transfer per unit area:

Total Heat Flow:

2157.0501.0 mmm

LDA

22 500,81011085m

W

Cm

W

w

CC

TThA

Qq

WmAqQm

W 335,1157.0500,8 22

Radiation

• The energy emitted by matter in the form of electromagnetic waves (or photons) as a result of changes in electronic configurations of the atoms or molecules.

• Unlike conduction or convection, the

transfer of energy by radiation does not require the presence of an intervening medium.

• Energy transfer by radiation is the fastest (speed of light) and suffers no attenuation in a vacuum.

Radiation Equation (Stefan-Boltzmann Equation)

• Stefan-Boltzmann Equation:

WattsTAQemit

~4

KetemperatursurfaceabsoluteT ~

42

81067.5

tan~

Km

W

tconsBoltzmannStefan

0.10~ emissivity

Radiation constants

• Stefan-Boltzmann constant (σ = 5.67x10-8 W/(m2·K4)

– The maximum amount of radiation that can be emitted from a surface at absolute temperature.

• Blackbody – Idealized surface that emits radiation at this maximum rate

(σ).

• Emissivity (0 ε 1) – A measure of how closely the surface approximates a

blackbody.

• When a surface of emissitivity Ɛ and surface area AS at thermodynamic temperature Ts is completely enclosed by a much larger (or black) surface at thermodynamic temperature Tsurr separated by a gas (such as air), the net rate of radiation heat transfer between these two surfaces is given by;

23

)( 44

surrssrad TTAQ

Radiation (Example 1.3)

• A horizontal pipe, with a 50 mm outside diameter, is maintained at a temperature of 50ºC in a large room where the air and wall temperature are kept at 20ºC. The surface emissivity of the steel pipe may be taken as 0.8.

• Calculate the heat loss by radiation per unit length.

50 mm

T1=50ºC

ε = 0.8

L

T2=20ºC

Radiation Equation (Example 1.3)

Heat loss by radiation per unit length:

KCT

KCT

29327320

32327350

2

1

LLmLDA 157.005.0

mW

Km

W KKm

TTDL

Q

03.25

293323157.01067.58.0448

4

2

4

1

42

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Heat Transfer Mechanisms

• Now we have covered all 3 of the heat transfer mechanisms.

• Most real problems will involved combinations of these mechanisms.

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Conduction only

Radiation

+

(conduction/

convection)

Radiation only

Simultaneous Heat Transfer

Mechanism

Combined Example (Example 1.4)

Air blows (at 20ºC) over carbon steel

[k=43 W/(m·ºC] hot plate which is 0.5 m x 0.75 m and 20 mm thick maintained at 250ºC. The convection heat transfer coefficient is 25 W/(m2·ºC) and the heat loss from the plate surface by radiation is 300 W.

(a) Calculate the heat transfer by

convection.

(b) The inside plate temperature.

T1

k=43 W/(m·ºC)

Hot plate

Tw= 250ºC

Energy Loss by Radiation

(300 W)

Air (T∞=20ºC)

h= 25 W/(m2·ºC)

Combined Example (Example 1.4)

• Heat Transfer from Newton’s Law of Cooling:

• Energy balance:

W

CCm

TTAhQ

Cm

W

fw

25.156,2

2025075.050.025 22

kWkWkWx

TkA

QQQradconvcond

456.23.0156.2

Combined Example (Example 1.4)

• Solving for the inside plate temperature:

Cmm

mW

Ak

xWT

Cm

W

05.35.075.043

02.0456,2

456,2

2

CCC

TTT

05.25305.3250

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1.2(a) OZONE LAYER DEPLETION

• The ozone layer is a concentration of ozone molecules in the stratosphere. About 90% of the planet's ozone is in the ozone layer

• The ozone depletion process begins when CFCs and other ozone-depleting substances (ODS) are emitted into the atmosphere

• It is caused by the release of chlorofluorocarbons (CFCs), hydrofluorocarbons (HCFCs), and other ozone-depleting substances (ODS), which were used widely as refrigerants, insulating foams, and solvents.

• A diminished ozone layer allows more radiation to reach the Earth's surface. For people, over exposure to UV rays can lead to skin cancer, cataracts, and weakened immune systems. Increased UV can also lead to reduced crop yield and disruptions in the marine food chain .

OZONE LAYER DEPLETION

OZONE LAYER DEPLETION

1.2(b) Global Warming

• Green House Effect

• GHGs

GREEN HOUSE EFFECT

• Glass transmits over 90 percent of radiation in the visible range but not the longer-wavelength (infrared regions)

• Radiation emitted by surfaces at room temperature falls in the infrared region.

• Consequently glass allows the solar radiation to enter but does not allow the infrared radiation from the interior surfaces to escape.

• This causes a rise in the interior temperature as a result of the energy buildup known as the greenhouse effect,

GREEN HOUSE EFFECT

• The greenhouse effect is also experienced on a larger scale on earth.

• The surface of the earth, which warms up during the day as a result of the absorption of solar energy, cools down at night by radiating its energy into deep space as infrared radiation.

• The combustion gases such as CO2 and water vapor in the atmosphere transmit the bulk of the solar radiation but absorb the infrared radiation emitted by the surface of the earth.

• Thus, there is concern that the energy trapped on earth will eventually cause global warming and thus drastic changes in weather patterns.

GREEN HOUSE EFFECT

GHGs

• The major greenhouse gases in the atmosphere are carbon dioxide (CO2), methane, (CH4), nitrous oxide (N2O), chlorofluorocarbons (CFCs) and ozone (O3). Atmospheric water vapour (H2O) also makes a large contribution to the natural greenhouse

• Global atmospheric concentrations of CO2, CH4 and N2O have increased markedly as a result of human activities since 1750 and now far exceed pre-industrial values

• The global increases in CO2 concentration are due primarily to fossil fuel use and land-use change, while those of CH4 and N2O are primarily due to agricultural/industrial activities.

Global Warming

MAJOR STEP IN CO2 REDUCTION

• Improve Energy Management : New (non fossil) resources & Efficiency in utilization.

• Land & Forest usage: Sustainable Development Policy.

CARBON NEUTRAL TARGET

1.3 RENEWABLE ENERGY RESOURCES

RENEWABLE ENERGY RESOURCES

• Renewable energy is energy which comes from natural resources such as sunlight, wind, rain, tides, and geothermal heat, biomass etc. which are renewable (naturally replenished).

• In 2010, only about 18% of global final energy consumption came from renewables (Ref: )

WIND ENERGY FOR ELECTRICAL POWER GENERATION

• Airflows can be used to run wind turbines.

• Modern wind turbines range from around 600 kW to 5 MW of rated power. Turbines with rated output of 1.5–3 MW have become the most common for commercial use.

• In Malaysia, wind energy is not technically commercially viable resource due to low average wind speed. – may be used in micro application.

DIRECT SOLAR ENERGY

• Solar energy could be harnessed by: Actively -Photovoltaic (PV) cells, or Passively (absorbed by building materials etc)

• Although solar energy is sufficient to meet the entire energy needs of the world, currently it is not economical to do so because of the low concentration of solar energy on earth ( W/m2) and the high capital cost of harnessing it due to low conversion efficiency.

• High potential from emerging technologies

Biomass

• Biomass - (plant material, non-fossil), organic materials which can be burned to produce energy or converted into fuels or other products.

• Biomass is a renewable energy source because the energy it contains comes from the sun. Through the process of photosynthesis, plants capture the sun's energy.

BIOMASS & BIOFUEL

Two approaches to biomass

as fuel :

• growing plants specifically

for energy or using the

residue from plants used for

other things.

• as bio-fuel for petroleum

subtitute

GEO-THERMAL

Geothermal

• Geothermal energy is energy obtained by tapping the heat of the earth itself, either from kilometers deep into the Earth's crust, or in some places of the globe from some meters, in geothermal heat pump

HYDRO

Hydro

• Hydroelectric energy is a term usually reserved for large-scale hydroelectric dams .

• Micro hydro systems are hydroelectric power installations that typically produce up to 100 kW of power .

• Ocean energy describes all the technologies to harness energy from the ocean/sea. This includes marine current power, ocean thermal energy conversion (OTEC), and tidal power.

SUSTAINABLE DEVELOPMENT

Sustainable Development

• Sustainable development is a pattern of resource use that aims to meet human needs while preserving the environment so that these needs can be met not only in the present, but also for future generations.

• Sustainable development can be conceptually devided into three constituent parts: environmental sustainability, economic sustainability and sociopolitical sustainability

End of Chapter 1

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