along a tube length to determine the drop in fluid temperature (Figure 4-48). A summation of the results for a single tube enabled the calculation of total radiator performance. Fig. 4-50. Optimum Radiator Panel Dimensions — Low Temperature Helium Radiator However, it appears that substantial advantages in the Brayton cycle turbomachinery loop resulted if heat were transferred from the Brayton gas loop to a radiator NaK loop. NaK radiator fluid was consequently used as baseline. Optimum configurations of three types of radiator are shown in Figure 4-50. All take advantage of the anisotropic meteoroid flux and preferential panel orientation. Configuration A uses solid armor around the tubes and radiates heat from both sides of the fin. Configurations B and C use meteoroid bumpers, the outer sheet breaks up the meteoroids so that dispersion occurs before the tube is reached. Each candidate was designed to provide protection against particles of at least .001 gm (.0000022 Ibm). Tubes were sized by the 30-year creep rupture strength with a minimum factor of safety of 2.0. For equivalent thermal and meteoroid protection, Configuration C yields the lightest radiator. Figure 4-51 shows radiator heat rejection on an area basis. It is relatively insensitive to tube diameter. Figure 4-52 shows the specific heat rejection (kW/kg or BTU/hr Ibm) of radiator tube/fin panels with various tube diameters. Table 4-27 shows optimum dimensional and performance data for the three configurations analyzed. Configuration 3 provided the best performance with year "A" materials and fluid temperatures. Configuration 4 shows material and dimensional modifications providing optimum performance with "B" type radiator requirements. Fig. 4-48. Radiator Thermal Mode! A comparison was made of radiator performance when tube pitch, tube diameter, and fin thickness were systematically varied to achieve an optimum configuration. Two radiator concepts (Figure 4-49) were base- lined as a result of an optimization exercise which selected the ratio of radiator temperature to Brayton cycle turbine inlet temperature. For minimum system weight this ratio is approximately 0.35. For Type A the maximum turbine inlet temperatures with superalloys (e.g., columbium) is 1300 K (1880°F); for Type B a turbine inlet temperature of 1750 K (2690°F) is baselined for refractory metals or ceramics. The above turbine inlet temperatures were used in a preliminary cycle design to evaluate radiator concepts. Fig. 4-49. Baseline Radiators Many early studies were based on the use of helium as a radiator fluid because a trade study comparing helium with NaK showed helium provided a lighter system. Hence, the results shown in Figures 4-50 to 4-53 are based on helium as the working fluid.
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