Natural convection radiation, combined with

Consider a l.2-m-high and 2-m-wide glass window with a thickness of 6 nun, thermal conductivity k = 0.78 W/m C, and emissivity e = 0.9. The room and the walls that face the window are maintained at 25°C, and the average temperature of the inner surface of the window is measured to be 5°C. If the temperature of Ihe outdoors is -5 C, determine (a) the convection heat transfer coefficient on Ihe inner surface of the window, (b) the rate of total heat transfer through the window, and (c) the combined natural convection and radiation beat transfer coefficient on the outer... 

Coefficients of heat transfer by natural convection from bodies of various shapes, chiefly plates and cylinders, are correlated in terms of Grashof, Prandtl, and Nusselt numbers. Table 8.9 covers the most usual situations, of which heat losses to ambient air are the most common process. Simplified equations are shown for air. Transfer of heat by radiation is appreciable even at modest temperatures such data are presented in combination with convective coefficients in item 16 of this table. 

Z.-Q Tan and J. R. Howell, Combined Radiation and Natural Convection in a Square Enclosure with Participating Medium, International Journal of Heat Mass Transfer, 34(3), pp. 785-793,1991. 

Gases— Natural Convection and Radiation Combined. Convection is always accompanied by radiation, and hence the most convenient method of computing heat losses from an exposed surface to air is by a coefficient that represents both the convection and radiation transfer of heat. A study of the literature on this subject shows that the coefficient is substantially the same for vertical brick, asbestos, metal, canvas, and wood surfaces. These coefficients must not be confused with the pure convection coefficients found in chemical engineering textbooks. Figure 17-10 indicates the magnitude of these coefficients and the effect of wind velocity. The position of the surface alters the coefficients somewhat as follows ... 

A 2-m X t,5-in section of wall of an industrial furnace burning natural gas is not insulated, and the temperatuce at the outer surface of this section is iiieasuied to be 80°C. The tem-peralure of the furnace room is 30 C, and the combined convection and radiation heat transfer coefficient at the surface of the outer furnace is 10 W/m °C, It is proposed to insulate this section of tiie furnace wall with glass wool insulation (A = 0,038 W/m -" C) in order to reduce the heat loss by 90 percent, Assuming the outer surface temperature of the metal section still remains at about 80°C, determine the thickness of the insulation that needs lobe used. 

The present section deals with a number of examples combining radiation with conduction and/or convection. Most problems involving more than one mode of heat transfer are relatively involved, as they yield nonlinear differential equations and/or boundary conditions whenever radiation is included. They are usually solved after a linearization of the Stefan-Boltzmann law. During this process, however, the quantitative nature of a problem gets lost. 

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