Free Convection from Inclined Surfaces

A cube, 20 cm on a side, is maintained at 60°C and exposed to atmospheric air at 20°C. Calculate the heat transfer. 

This is an irregular solid so we use the information in the last entry of Table 7-1 in the absence of a specific correlation for this geometry. The properties were evaluated in Ex. 7-2 as 

The characteristic length is the distance a particle travels in the boundary layer, which is LI2 along the bottom plus L along the side plus LI2 on the top, or 2L = 40 cm. The Gr Pr product is thus  

From the last entry in Table 7-1 we find C - 0.52 and n - 1/4 and calculate the Nusselt number as 

Extensive experiments have been conducted by Fujii and Imura [44] for heated plates in water at various angels of inclination. The angle which the plate makes with the vertical is designated 6, with positive angles indicating that the heater 

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Uncertainties still remain in the prediction of free convection from inclined surfaces, and an experimental-data scatter of 20 percent is not unusual for the empirical relations presented above. 

Extensive experiments have been reported in Refs. 25, 26, and 39 for free convection from vertical and inclined surfaces to water under constant-heat-... 

A flat-plate solar collector is 1 m square and is inclined at an angle of 200 with the horizontal. The hot surface at 160°C is placed in an enclosure which is evacuated to a pressure of 0.1 atm. Above the hot surface, and parallel to it, is the transparent window which admits the radiant energy from the sun. The hot surface and window are separated by a distance of 8 cm. Because of convection to the surroundings, the window temperature is maintained at 40°C. Calculate the free-convection heat transfer between the hot surface and the transparent window. 

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