Heat Transfer CONVECTION HEAT TRANSFERThis results in a much simpler expression that is similar to the one developed for a flat plate heatexchanger in Figure 6.(2-11)Uo11h1Drk1h2The convection heat transfer process is strongly dependent upon the properties of the fluid beingconsidered. Correspondingly, the convective heat transfer coefficient (h), the overall coefficient(Uo), and the other fluid properties may vary substantially for the fluid if it experiences a largetemperature change during its path through the convective heat transfer device. This is especiallytrue if the fluid’s properties are strongly temperature dependent. Under such circumstances, thetemperature at which the properties are "looked-up" must be some type of average value, ratherthan using either the inlet or outlet temperature value.For internal flow, the bulk or average value of temperature is obtained analytically through theuse of conservation of energy. For external flow, an average film temperature is normallycalculated, which is an average of the free stream temperature and the solid surface temperature.In any case, an average value of temperature is used to obtain the fluid properties to be used inthe heat transfer problem. The following example shows the use of such principles by solvinga convective heat transfer problem in which the bulk temperature is calculated.ConvectionHeatTransferExample:A flat wall is exposed to the environment. The wall is covered with a layer of insulation1 in. thick whose thermal conductivity is 0.8 Btu/hr-ft-°F. The temperature of the wallon the inside of the insulation is 600°F. The wall loses heat to the environment byconvection on the surface of the insulation. The average value of the convection heattransfer coefficient on the insulation surface is 950 Btu/hr-ft2-°F. Compute the bulktemperature of the environment (Tb) if the outer surface of the insulation does not exceed105°F.Rev. 0 Page 23 HT-02
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