chapter 1 introduction-oke

Chapter 1 Perpindahan Panas/Heat Transfer 1

Chapter 1

Introduction to Heat Transfer


IntroductionThermodynamics: Energy can be transferred between a system and its

surroundings. A system interacts with its surroundings by exchanging work

and heat Deals with equilibrium states Does not give information about:

– Rates at which energy is transferred– Mechanisms through with energy is transferred

In this chapter we will learn: What is heat transfer

How is heat transferred Relevance and importance


Definitions• Heat transfer or Heat is thermal energy transfer that is

induced by a temperature difference (or gradient)

Modes of heat transfer Conduction heat transfer : Occurs when a temperature

gradient exists through a solid or a stationary fluid (liquid or gas).

Convection heat transfer: Occurs within a moving fluid, or between a solid surface and a moving fluid, when they are at different temperatures

Thermal radiation: All surfaces at finite temperature emit energy in the form of electromagnetic waves. Heat transfer between two surfaces (that are not in contact), often in the absence of an intervening medium.


Example: Design of a containerA closed container filled with hot coffee is in a room whose air and walls are at a fixed temperature. Identify all heat transfer processes that contribute to cooling of the coffee. Comment on features that would contribute to a superior container design.


1. ConductionTransfer of energy from the more energetic to less energetic particles of a substance by collisions between atoms and/or molecules. Atomic and molecular activity – random molecular motion (diffusion)

T1>T2

T2

T1

x

xo

T2

qx”


1. ConductionConsider a brick wall, of thickness L=0.3 m which in a cold winter day is exposed to a constant inside temperature, T1=20°C and a constant outside temperature, T2= -20°C.

Under steady-state conditions the temperature varies linearly as a function of x.

The rate of conductive heat transfer in the x-direction depends on

T1=20°C

T2= -20°C

L=0.3 mx

T

qx”

LTTqx 21"

Wall Area, A


1. Conduction• The proportionality constant is a transport property, known as thermal

conductivity k (units W/m.K)

LTk

LTTkqx

21"

• For the brick wall, k=0.72 W/m.K (assumed constant), therefore qx”= 96 W/m2

How would this value change if instead of the brick wall we had a piece of polyurethane insulating foam of the same dimensions? (k=0.026 W/m.K)

qx” is the heat flux (units W/m2 or (J/s)/m2), which is the heat transfer rate in the x-direction per unit area perpendicular to the direction of transfer.

The heat rate, qx (units W=J/s) through a plane wall of area A is the

product of the flux and the area: qx= qx”. A


1. Conduction

• In the general case the rate of heat transfer in the x-direction is expressed in terms of the Fourier law:

dxdTkqx " T1 (high)

T2 (low)

x

qx”

• Minus sign because heat flows from high to low TFor a linear profile

0)()(

12

12

xxTT

dxdT

x1 x2


2. ConvectionEnergy transfer by random molecular motion (as in conduction) plus bulk (macroscopic) motion of the fluid.– Convection: transport by random motion of molecules and by bulk

motion of fluid.– Advection: transport due solely to bulk fluid motion.

Forced convection: Caused by external means Natural (free) convection: flow induced by buoyancy forces, arising

from density differences arising from temperature variations in the fluid

The above cases involve sensible heat (internal energy) of the fluid Latent heat exchange is associated with phase changes

boiling and condensation. Hydrodynamic or velocity and Thermal boundary layers


2. ConvectionAir at 20°C blows over a hot plate, which is maintained at a temperature Ts=300°C and has dimensions of 20x40 cm.

CT 20

q”

CTS300

Air

The convective heat flux is proportional to the temperature difference

TTq Sx"


2. Convection• The proportionality constant is the convection heat transfer coefficient,

h (W/m2.K)

)(" TThq Sx Newton’s law of Cooling

• For air h=25 W/m2.K, therefore the heat flux is qx”= 7,000 W/m2

How would this value change if instead of blowing air we had still air (h=5 W/m2.K) or flowing water (h=50 W/m2.K)

• The heat rate, is qx= qx”. A = qx”. (0.2 x 0.4) = 560 W.

• The heat transfer coefficient depends on surface geometry, nature of the fluid motion, as well as fluid properties.


3. Radiation Thermal radiation is energy emitted by matter Energy is transported by electromagnetic waves (or photons). Can occur from solid surfaces, liquids and gases. Does not require presence of a medium


Emissive Power E is the radiation emitted by the surfaceThe upper limit of the emissive power Stefan Boltzmann Emissive Power of Black body

Stefan Boltzmann constant

Ts Absolute surface temperature [K]

Irradiation G is the rate of incident radiation per unit area of the surface originating from its surroundingsA portion or all of G is absorbed by the surface increase thermal energy Absorbed irradiation for real surface 0 ≤ ≤ 1

Emissive power for real surface 0 ≤ ≤ 1


3. Radiation is the absorptivity For a “grey” surface, =

10 4surTG

Net radiation heat transfer of the surface per unit area


Thermal Resistance Concept


Conservation of Energy

• Energy conservation on a rate basis:

Control Volume (CV)

Surroundings, S

Boundary, B (Control Surface, CS)

-Accumulation (Storage)

-GenerationAdditionthrough inlet

Lossthrough outlet

stst

outgin EdtdEEEE

inE outEgE

stE

Inflow and outflow are surface phenomena Generation and accumulation are volumetric phenomena

Units W=J/s


0

2

2

2

i

2

Wq

zgVpumzgVpumout

outi

Enthalpy i = u + pvIdeal gas of constant specific heat (iin-iout) = cp(Tin-Tout)Simplified Steady-flow thermal energy equation


ExampleIn an orbiting space station, an electronic package is housed in a compartment having a surface area As=1 m2, which is exposed to space. Under normal operating conditions, the electronics dissipate 1kW, all of which must be transferred from the exposed surface to space.(a) If the surface emissivity is 1.0 and the surface is not exposed to the sun, what is its steady-state temperature? (b) If the surface is exposed to a solar flux of 750 W/m2 and its absorptivity to solar radiation is 0.25, what is its steady-state temperature?


Surface Energy Balance

For a control surface:

0

0

"""

radconvcond

outin

qqq

orEE

T

x

T1

T2

T

qcond”qrad”

qconv”


ExampleThe roof of a car in a parking lot absorbs a solar radiant flux of 800 W/m2, while the underside is perfectly insulated. The convection coefficient between the roof and the ambient air is 12 W/m2.K.

a) Neglecting radiation exchange with the surroundings, calculate the temperature of the roof under steady-state conditions, if the ambient air temperature is 20°C.

b) For the same ambient air temperature, calculate the temperature of the roof if its surface emissivity is 0.8


Satellites and space-crafts are exposed to extremely high radiant energy from the sun. Propose a method to dissipate the heat, so that the surface temperature of a spacecraft in orbit can be maintained to 300 K.


Thank you for your kind attention and participation

chapter 1 introduction-oke

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