1. MECHANISMS OF HEAT TRANSFER
Dr. Şaziye Balku

2. BASIC MECHANISMS OF HEAT TRANSFER CONDUCTION CONVECTION RADIATION
HEAT TRANSFER form of energy transfer due to temperature difference
higher temperature lower temperature
stops when two mediums reach the same temperature
BASIC MECHANISMS OF HEAT TRANSFER
CONDUCTION
CONVECTION
RADIATION
Dr. Şaziye Balku

3. Mechanism of heat conduction
CONDUCTION: Transfer of energy from more energetic particles of a substance to the adjacent less energetic ones as a result of interaction between the particles
Mechanism of heat conduction
In gases and liquids: due to collisions and diffusions of the molecules during their random motion
In solids: combination of vibrations of the molecules in a lattice and the energy transport by free electrons.
Dr. Şaziye Balku

4. RATE OF CONDUCTION Geometry of medium Thickness Material of the medium
Temperature difference
The rate of heat conduction through a plane layer is proportional to the temperature difference across the layer and the heat transfer area, but is inversely proportional to the thickness of the layer
Rate of heat conduction (area)(temperature difference)
Thickness
Dr. Şaziye Balku

5. Fourier’s Law of Heat Conduction
k: thermal conductivity of the material, measure of the ability of a material to conduct heat
Fourier’s Law of Heat Conduction
: Temperature gradient
(Negative when temperature decreases with increasing x)
Heat is conducted in the direction of decreasing temperature. Heat transfer area A is always normal to the direction of heat transfer
Dr. Şaziye Balku

6. For copper
Note that: the negative sign in Fourier’s law ensures that heat transfer in the positive x direction is a positive quantity
Dr. Şaziye Balku

7. THERMAL CONDUCTIVITY
The rate of heat transfer through a unit thickness of the material per unit area per unit temperature difference
Kinetic theory: thermal conductivity of gases is proportional to the square root of the absolute temperature and inversely proportional to the square root of the molar mass.
independent of pressure in a wide range
In liquids decreases with increasing temperature and also with increasing molar mass
In solids depends upon lattice vibrational waves and the free flow electrons
Dr. Şaziye Balku

8. The range of thermal conductivity of various materials at room temperature
ability to conduct heat
different for each material
At room temperature;
k= W/m 0 C for water
k= 80.2 W/m 0 C for iron
Which one conducts heat better ?
The ones having low thermal conductivities are insulators
Dr. Şaziye Balku

9. The thermal conductivities of materials vary with temperatures
The thermal conductivities of certain solids exhibit dramatic increases at temperatures near absolute zero, when these solids become super conductors.
For example: Copper
T ( K) k (W/m. 0 C)
Dr. Şaziye Balku

10. THERMAL DIFFUSIVITY
Thermal diffusivity is a material property which represents how fast heat diffuses through a material.
The ratio of heat conducted through a material to the heat stored per unit volume
Thermal diffusivity
Specific heat
High k or low CP large thermal diffusivity
The larger the thermal diffusivity, the faster the propagation of heat into the medium. A small value of thermal diffusivity means that heat is mostly absorbed by the material and a small amount of heat will be conducted further.
Dr. Şaziye Balku

11. CONVECTION
Heat transfer between a solid surface and the adjacent fluid that is in motion and it involves the combined effects of conduction and fluid motion
Heat is first transferred from hot block to the adjacent layer of air by conduction and then carried away from the surface by convection
Dr. Şaziye Balku

12. If the fluid is forced to flow over the surface by external means such as a fan, pump or the wind, heat is transferred by forced convection whereas if the fluid is caused by buoyancy forces that are induced by density differences due to the variation of temperature in fluid it is called natural or free convection
Dr. Şaziye Balku

13. NEWTON’S LAW OF COOLING
Rate of convection heat transfer temperature difference
Convection heat transfer coefficient in W/m2.0 C
Surface area through which convection heat transfer takes place
Surface temperature
Temperature of the fluid sufficiently far from the surface
h is not a property of the fluid, experimentally determined, depends on
surface geometry
nature of fluid motion
Properties of fluid
Bulk fluid velocity
Note: Fluid temperature at the surface equals the surface temperature of the solid
Dr. Şaziye Balku

14. RADIATION
Energy emitted by matter in the form of electromagnetic waves(or photons) as a result of the changes in the electronic configurations of the atoms or molecules
All bodies at a temperature above absolute zero emit thermal radiation
Does not require an intervening medium
Fastest (at the speed of light)
Possible also in vacuum
Example: energy of sun reaching the earth
Thermal radiation: form of radiation emitted by bodies because of their temperature
different from other forms of electromagnetic radiation; X-rays, gamma rays, microwaves, and television waves that are not related with temperature
Dr. Şaziye Balku

15. STEFAN-BOLTZMANN LAW
The maximum rate of radiation that can be emitted from a surface at an absolute temperature is;
Stefan-Boltzman constant
=5.67×10-8 W/m2.K4
Black body: an idealized surface that emits radiation at this maximum rate
Black body radiation: radiation emitted by blackbodies
Real surfaces emit less radiation
For black bodies
For real bodies
Emissivity of the surface
Dr. Şaziye Balku

16. Radiation heat transfer between a surface and the surfaces around it
Dr. Şaziye Balku

17. Combined heat transfer coefficient includes effects of both convection and radiation in such an example and conduction heat transfer may be neglected.
Dr. Şaziye Balku

18. Solids: conduction and radiation Fluids:
Although there are three mechanisms of heat transfer, a medium may involve only two of them simultaneously
Solids: conduction and radiation
Fluids:
conduction and radiation (no motion)
convection and radiation (in motion)
conduction and convection (no radiation)
Heat transfer through a vacuum is by radiation only
Dr. Şaziye Balku

19. HEAT TRANSFER BETWEEN TWO PLATES
Dr. Şaziye Balku