十单元传热学综合传热12节.pptVIP

Chapter Twelve Multimode Heat And Mass Transfer That most technically important problems involve multimode heat transfer effects. qcond qrad qconv qrad qcond qconv To solve such problems, it may be necessary to simultaneously work with heat equation, surface energy balance, overall energy balances, and approprite rate rate equations for conduction, convection and radiation heat transfer. Example 12.1 The rear window of an automobile is of thickness L=8mm and height H=0.5m and contains fine-meshed heating wires that can induce nearly volumetric heating. Consider steady-state conditions for which the interior surface of the window is exposed to air at 10 ℃, while the exterior surface is exposed to air at -10 ℃ and a very cold sky with an effective temperature of -33 ℃. The air moves in parallel flow over the surface with a velocity of 20m/s. Determine the volumetric heating rate needed maintain the interior window surface at Ts,i=15℃. solution Known: Boundary conditions associated with a rear window experiencing uniform volumetric heating. Find: Volumetric heating rate needed to maintain inner surface temperature at Ts,i=15℃. Schematic: Tsky=240K Glass Assumptions: steady-state, one-dimensional conditions. Constant properties. Uniform volumetric heating in window. Convection heat transfer from interior surface of window to interior air may be approximated as free convection from a vertical plate. Radiation exchange with automobile interior is negligible. Heat transfer from outer surface is due to forced convection over a flat plate and radiation exchange with the sky. Boundary layer separation on the outer surface in negligible. Outer surface is diffuse and gray. Properties: Glass; Air; k, v, Pr, ?, ?, ? ( look at tables) Analysis: The temperature distribution in the glass is governed by the appropriate form of the heat equation. Boundary conditions Applying an energy balance to the inner surface Hence Applying an energy balance to the outer surface. (1) (2) (3) The