Heat Transfer Modes (Chapter 2)

Modes of Heat Transfer ►For any particular application, energy transfer by heat can occur by one or more of three modes: ►conduction ►radiation ►convection 35  Conduction ►Conduction is the transfer of energy from more energetic p...

Modes of Heat Transfer ►For any particular application, energy transfer by heat can occur by one or more of three modes: ►conduction ►radiation ►convection 35  Conduction ►Conduction is the transfer of energy from more energetic particles of a substance to less energetic adjacent particles due to interactions between them. ►The time rate of energy transfer by conduction is quantified by Fourier’s law. ►An application of Fourier’s law to a plane wall at steady state is shown at right. 36  Conduction ►By Fourier’s law, the rate of heat transfer across any plane normal to the x direction, Q x, is proportional to the wall area, A, and the temperature gradient in the x direction, dT/dx, dT  Qx = −A (Eq. 2.31) dx where ► is a proportionality constant, a property of the wall material called the thermal conductivity. ►The minus sign is a consequence of energy transfer in the direction of decreasing temperature. ►In this case, temperature varies linearly with x, and thus dT T2 − T1   T2 − T1  = ( 0) and Eq. 2.31 gives Qx = −A   dx L  L 37  Thermal Radiation ►Thermal radiation is energy transported by electromagnetic waves (or photons). Unlike conduction, thermal radiation requires no intervening medium and can take place in a vacuum. ►The time rate of energy transfer by radiation is quantified by expressions developed from the Stefan-Boltzman law. 38  Thermal Radiation ►An application involving net radiation exchange between a surface at temperature Tb and a much larger surface at Ts (< Tb) is shown at right. ►Net energy is transferred in the direction of the arrow and quantified by Q =  A[T 4 − T 4 ] e b s (Eq. 2.33) where ►A is the area of the smaller surface, ► is a property of the surface called its emissivity, ► is the Stefan-Boltzman constant. 39  Convection ►Convection is energy transfer between a solid surface and an adjacent gas or liquid by the combined effects of conduction and bulk flow within the gas or liquid. ►The rate of energy transfer by convection is quantified by Newton’s law of cooling. 40  Convection ►An application involving energy transfer by convection from a transistor to air passing over it is shown at right. ►Energy is transferred in the direction of the arrow and quantified by Q c = hA[Tb − Tf ] (Eq. 2.34) where ►A is the area of the transistor’s surface and ►h is an empirical parameter called the convection heat transfer coefficient. 41