Table 8. Enclosure dimensions and effective gap widths.
Outside radius of
Effective radius
Configuration
Pipe description
pipe/insulation
of enclosure
Effective gap
0.583568 fta
1
4-in. bare
0.18750 ft
0.396068 ft
2
4-in. insulated
0.27083
0.583568
0.312738
3
2-in. bare
0.09896
0.583568
0.484608
4
2-in. insulated
0.18229
0.583568
0.401278
4-in. insulatedb
0.755456c
0.35416
0.401296
5
1.220187d
6
2-in. insulated
0.18229
1.037897
1.220187d
7
2- and 2-in. insulated
0.364583
0.855605
0.713542e
1.856808f
1.143266e
8
4- and 8-in. insulated
1-ft 1-ft enclosure
a
b
2 in. of insulation
c 1.27-ft 1.27-ft enclosure
d 2-ft 2-ft enclosure
e 0.442708 and 1.4140, if only 8-in. pipe heated
f 2-ft 4-ft enclosure
Table 9. Configurations for the numerical experiments.
Enclosure outside
Number of
Pipe
Pipe insulation
Emissivity values
Pipe1
dimensions
pipes
diameter(s)
thickness
Insulation
1 ft 1 ft
1
4.5 in.
0.0 in.
0.9
0.9
1
4.5
0.0
0.9
0.6
1
4.5
0.0
0.5
0.9
1
4.5
0.0
No radiation
1
4.5
1.0
0.9
0.9
1
4.5
1.0
0.9
0.6
1
4.5
1.0
0.5
0.9
1
2.375
0.0
0.9
0.9
1
2.375
0.0
0.9
0.6
1
2.375
0.0
0.5
0.9
1
2.375
1.0
0.9
0.9
1
2.375
1.0
0.9
0.6
1
2.375
1.0
0.5
0.9
1
2.375
1.0
No radiation
1.27 ft 1.27 ft
1
4.5
2.0
0.9
0.9
2 ft 2 ft
1
2.375
1.0
0.9
0.9
2
2.375, 2.375
1.0, 1.0
0.9
0.9
2 ft 4 ft
2
4.5, 8.625
1.0, 1.0
0.9
0.9
1 or
pipe insulation
Experimental data
The complete set of temperature and power measurements is reported in Rich-
mond et al. (1997). The average Nusselt and Rayleigh numbers obtained for the
physical experiments conducted with the 1-ft 1-ft enclosure are plotted in Fig-
ures 2124. Vertical scatter is due to the radiation effect on the surface conductance;
a similar effect is seen in the numerical data, shown in the next section. In Figure
21, the 1991 data result in slightly higher Nusselt numbers. Some of this difference
is due to the temperature effect on the conductance, but some may also be due to
the larger thermocouples used to measure surface temperatures. The effect of paint-
ing the pipe with the aluminum paint, with a subsequent lower emissivity, is clearly
seen, resulting in lower Nusselt numbers compared to the unpainted pipe. Figure
22 shows similar scatter with apparently one significant outlier, which occurred in
the data obtained on 28 September 1996 (see App. A). By plotting the calculated
32
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