50

eq 12

eq 13

eq 14

10

eq 15

5

1

0.5

3

4

5

6

10

10

10

10

Grashof Number

(16)

*W *

where *Y/W *is the height/width ratio, and *A*, *B*, and *C *are the constants for air (see

table below).

Reference

eq

Newell and Schmidt (1969)

0.155

0.315

0.265

(17)

Eckert and Carlson (1961)

0.119

0.3

0.1

(18)

Jakob (1949)

0.18

0.25

0.111

(19)

MacGregor and Emery (1969)

0.25

0.25

0.25

(20)

Horizontal rectangular enclosures are described as cavities in which the lower

horizontal surface is heated while the upper surface is cooled; the sides are insu-

lated. The correlations obtained by several researchers can be presented in the form

of eq 10, when the Prandtl number for air is taken as 0.72. The characteristic length

lations are shown in the following table (Gebhart et al. 1988).

Reference

eq

Dropkin and Somerscales (1965)

0.0673

0.3333

(21)

Silveston (1958)

0.0877

0.31

(22)

Kraichnan (1962)

0.1524

0.3333

(23)

Probably the most investigated enclosure containing an interior heat source is

a concentric pipe system. Gebhart et al. (1988) reviewed the significant number of

nondimensional systems have been used in most studies. For correlations based

on mean heat transfer rates, gap width (outer radius-inner radius) is often used as

the characteristic length (*L*). An example of this is the following equation by Grigull

and Hauf (1966):

5

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