S

ERT 216

INTRODUCTION TO HEAT TRANSFER

MISS HANNA ILYANI ZULHAIMI

WHAT IS HEAT TRANSFER?

Heat transfer is thermal energy transfer that is induced by a temperature difference (or gradient)

Mode of heat transfer

• Conduction : Occurs when a temperature gradient exists through a solid or a stationary fluid (liquid or gas). (Surface to surface)

• Convection : Occurs within a moving fluid, or between a solid surface and a moving fluid, when they are at different temperatures. (Surface to air)

• Thermal radiation: Heat transfer between two surfaces (that are not in contact), often in the absence of an intervening medium. (Direct exchange across space)

HEAT TRANSFER VS THERMODYNAMICS

Example: Cooling of hot steel bar placed in a pail water

THERMODYNAMICà predict the final equilibrium temperature of the steel bar-water combination.

HEAT TRANSFER à predict the temperature of both the bar and the water as a function of time also the temperature of the bar will be after a certain length of time.

How  is  Heat  Transferred?  

There are THREE ways heat can move.

Ø  Conduction

Ø  Convection

Ø  Radiation

CONDUCTION

S  Transfer of energy from the more energetic to less energetic particles of a substance by collisions between atoms and/or molecules.

S  Conduction is the transfer of heat from a hotter area to a cooler area .

S  Heat  is  transferred  from  one  par/cle  of  ma2er  to  another  in  an  object  without  the  movement  of  the  object.  

S  Conduc3on  =  CONTACT  

Conduction

•  Temperature different: T1 and T2 [K]

•  Thermal conductivity: k [W m-1 K-1]

•  Heat transfer area: A [m2]

•  Thickness of the material: x [m]

•  Heat transfer rate by conduction: qc •  Heat flux: q/A

Since T2>T1, the heat flows from right to left

Fourier’s Law à

Fourier’s Law

•  Heat transfer by conduction from a high temperature region to the low temperature region

•  The driving force of this heat transfer is the temperature gradient

Have  you  ever…  

S  Touched  a  metal  spoon  si=ng  in  a  pan  of  boiling  water  only  to  be  surprised  by  HOW  hot  it  is?  

Think back to what you know about metals and nonmetals. What conducts heat better, metal or nonmetal? Why?

THERMAL CONDUCTIVITY •  Experiment measurement made to determine the value

of k (different material, different k)

•  The k value is a physical property of each solid, liquid and gas material

•  The numerical value of k indicates à how fast heat will flow in a given material.

•  If molecules move fasters (gas), the faster they will transport energy.

•  So, value of k depends on the molecules structure (gas, liquid or solid)

•  k is strongly temp-dependent •  Unit W m-1 K-1

K of various materials at

0 °C à

Calculate the heat loss per m2 of surface area for an insulating wall composed of 25.4 mm thick fiber insulating board, where inside Temperature is 352.7L and outside temperature is 297.1 K.

From Appendix A.3, thermal conductivity for fiber insulating board is 0.048 W/m.K. The thickness is 25.4 mm. Thus:

1)  Gas

•  If temp. higher, the molecules have higher velocities

•  Molecules are in continuous random motion and will colliding with one another and exchanging the energy and momentum

•  The random motion of gas molecules happen whether/ not a temp. gradient exist in gas

•  If a molecules move from higher temp. region to low temp. region, it transport kinetic energy to lower temp. part of system (through collisions molecules)

•  The faster molecules moveà faster they will transport energy

THERMAL CONDUCTIVITY

2) Liquid

•  The molecules are move closely space •  Exchange energy in collision process

3) Solid

•  Thermal energy conducted in solid by 2 modes: lattice vibration and transport by free electron

THERMAL CONDUCTIVITY

CONVECTION •  Transfer of heat between an object and fluid (Surface to fluid).

•  Convection = VENTS (through air and liquid particles) •  Transfer of heat by bulk transport and mixing of macroscopic elements

of warmer portions with cooler portion in a gas or liquid.

# Transfer is affected by

i) the speed with which the air is moving ii) the ability of the object to conduct heat

•  Temperature different between wall and fluid: Tw and Tf [K]

•  Convective heat transfer coefficient: hcv [W m-2 K-1] •  Heat transfer area: A [m2]

•  Heat transfer rate by convection: qcv

•  Heat flux: q/A

Also referred to as the “Newton rate equation” or “Newton’s law of cooling”

Convection

Appproximate value of h à

Two types: (i)   Forced convection:

# Fluid is forced to flow past a solid surface by a mechanical means

(ii) Natural (free) convection: # Heat circulation due to fluid density difference resulting from the temperature variation throughout a region of the fluid

Convection

Convection

•  The temperature of the plate is Tw and the temperature of the fluid is T∞. •  The velocity, u reduce to 0 at the plate as a result of viscous

action. Because of that, heat must be transferred only by conduction at that point (wall).

•  convection heat transfer, hcv will have a dependence on the viscosity of the fluid in addition to its dependence on the thermal properties of the fluid (thermal conductivity, specific heat, density).

Radiation •  Transfer of energy across the space by means of

electromagnetic waves- in the same way as the electromagnetic light waves transmitting light (Direct exchange across space)

•  Radia/on  does  NOT  require  ma2er  to  transfer  thermal  energy  •  Radia/on  =  Radiates  (heat  escaping  the  sun)  

Examples  of  RADIATION  

Fire      Heat  Lamps    Sun    

•  Electromagnetic radiation which is propagated because of temp. different called à thermal radiation

•  Ideal thermal radiation/ black body radiation, will emit energy at a rate proportional to the fourth power of the absolute temp. of the body and directly proportional to it’s surface area.

•  Stefan-Boltzmann Law;

•  Black-body = Black surface (a piece of metal recovered with carbon black)

q emitted = σ A T4

Radiation

σ = 5.669 x 10-8 W/m2.K4

•  Stefan-Boltzmann Lawà thermal radiation, it govern only radiation emitted by black-body.

•  The net radiant exchange between 2 surface proportional to the different in absolute temperature to the fourth power

q emitted = σ A T4

q emitted = σ A (T14 – T2

4)

Radiation

•  Emissivity, ϵ à relates the radiation on the ‘gray surface’ •  Not all the radiation leaving one surface will reach the

other surface since electromagnetic radiation travels in straight line & some will be lost to the surroundings.

•  Because of that, 2 new factors will be considered ;

Fϵ: Emissivity function and

FG: Geometric “view factor’ function

Radiation

Radiation in an enclosure

Simple radiation problem is encounter when a heat transfer surface at temp., T1 completely enclosed by a much large surface maintain at temp. T2. So that, the net radiant exchange:

Radiation