Numerical data

50

Both Pipes Heated, approx. 150F

40

Both Pipes Heated, approx. 230F

8 in. Pipe Heated, approx. 150F

8 in. Pipe Heated, approx. 230F

Figure 25. Nusselt and Rayleigh number plot for the 2-ft 4-ft enclosure

(experimental data).

There is significant difference in the curve shapes between the two-pipe heat-

ing conditions. This may be due to more stratification of the air in the bottom of

the enclosure combined with a larger temperature difference compared to the single

heated pipe configuration; this affects the heat conductance (h) in the Nusselt num-

ber. Comparing Nusselt numbers for the bottom surfaces (App. A), it can be seen

that there is a greater change for two heated pipes compared to the single heated

pipe. This observation also explains the curved shape of the Nu-Ra number data

obtained with numerical data discussed in the next section.

Numerical data

The finite-element computer program FECOME, described earlier, was used to

obtain additional heat transfer data from numerical experiments. The objective of

the numerical experiments was to extend the database of enclosure configurations

and boundary conditions, and to make comparisons with the physical experiments.

The numerical experiments allowed calculations to be made without radiation

boundaries and with different combinations of emissivity values.

Figure 26 shows one of the meshes used

for the uninsulated 4-in. pipe in the 1-ft 1-

ft enclosure. It has the same internal dimen-

sions as the experimental apparatus, includ-

ing the 0.5-in. layer of EPS insulation

enclosing the cavity of air. Temperatures

around the outside of the insulation were

held constant and the surface representing

the outer diameter of the pipe was held at a

series of temperatures. Increasing Rayleigh

number values were obtained by decreasing

the outside boundary temperatures in 5- or

10-degree increments until FECOME was no

Figure 26. Mesh for the 1-ft 1-ft

longer able to converge to a solution. A small

temperature change in boundary condi-

enclosure with a 4-in. pipe (3,552

tions, and the use of a previous solution as

nodes, 1,152 elements, 10,848 d.o.f.).

35

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