MECHANISMS OF HEAT TRANSFER Dr. Şaziye Balku

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

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

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

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

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

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

The range of thermal conductivity of various materials at room temperature ability to conduct heat different for each material At room temperature; k= 0.608 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

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) 20 20 000 300 401 Dr. Şaziye Balku

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

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

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

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

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

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

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

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

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

HEAT TRANSFER BETWEEN TWO PLATES Dr. Şaziye Balku