Convection enclosed spaces

Heat transfer by natural convection across an enclosed space, called an enclosure or, sometimes, a cavity, occurs in many real situations, see [34] to [67]. For example, the heat transfet between the panes of glass in a double pane window, the heat transfer between the collector plate and the glass cover in a solar collector and in many electronic and electrical systems basically involves natural convective flow across an enclosure. 

Buchberg, H., Catton, I., and Edwards, D.K., Natural Convection in Enclosed Spaces— A Review of Application to Solar Energy , J. Heat Transfer, 98, p. 182,1976. 

The free-convection flow phenomena inside an enclosed space are interesting examples of very complex fluid systems that may yield to analytical, empirical, and numerical solutions. Consider the system shown in Fig. 7-10, where a fluid is contained between two vertical plates separated by the distance 5. As a temperature difference AT,. = T - T> is impressed on the fluid, a heat transfer will be experienced with the approximate flow regions shown in Fig. 7-11, according to MacGregor and Emery [18]. In this figure, the Grashof number is calculated as... 

Heat transfer in horizontal enclosed spaces involves two distinct situations. If the upper plate is maintained at a higher temperature than the lower plate, the lower-density fluid is above the higher-density fluid and no convection currents will be experienced. In this case the heat transfer across the space will be by conduction alone and Nus = 1.0, where 8 is still the separation distance between the plates. The second, and more interesting, case is experienced when the lower plate has a higher temperature than the upper plate. For values of Grs below about 1700, pure conduction is still observed and Nu = 1.0. As convection begins, a pattern of hexagonal cells is formed as shown in Fig. 7-12. These patterns are called Benard cells [33]. Turbulence begins at about Gr6 = 50,000 and destroys the cellular pattern. 

What is the approximate criterion dividing pure conduction and free convection in an enclosed space between vertical walls ... 

Natural convection heat transfer in enclosed spaces has been the subjec of many experimental and numerical studies, and numerous correlations fo the Nusselt number exist. Simple power-law type relations in the form o... 

Natural convection in enclosed spaces. Free convection in enclosed spaces occurs in a number of processing applications. One example is in an enclosed double window in which two layers of glass are separated by a layer of air for energy conservation. The flow phenomena inside these enclosed spaces are complex since a number of diflerent types of flow patterns can occur. At low Grashof numbers the heat transfer is mainly by conduction across the fluid layer. As the Grashof number is increased, different flow regimes are encountered. 

Cases involving free convection in enclosed spaces are less well defined. The usual method is to determine an effective thermal conductivity kg. Discussion of this approach is given elsewhere (6, 8, 18). 

R. Siegel and R. H. Norris, "Tests of free convection in a partially enclosed space between two heated vertical plates", Trans. ASME, Vol. 79, pp. 663-670 (1957). 

A solar collector consists of a horizontal copper tube of outer diameter 5 cm enclosed in a concentric thin glass tube of 9 cm diameter. Water is heated as it flows through the tube, and the annular space between the copper and glass lube is filled with air at 1 atm pressure. During a clear day, the temperatures of the tube surface and the glass cover are measured to be bO C and 32°C, respectively. Determine the rale of heat loss from the collector by natural convection per meter length of the tube. Answer 17.4 W... 

Figure 4.7-2. Natural convection in enclosed vertical space.

Natural convection from other shapes. For spheres, blocks, and other types of enclosed air spaces, references elsewhere (HI, Kl, Ml, PI, P3) should be consulted. In some cases when a fluid is forced over a heated surface at low velocity in the laminar region, combined forced-convection plus natural-convection heat transfer occurs. For further discussion of this, see (HI, Kl, Ml). 

Natural Convection in Enclosed Horizontal Space. Repeat Example 4.7-3 but for... 

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