Fig. 4-54. Use of Liquid Radiator With Brayton Cycle Requires Additional Heat Exchanger All liquid radiator working fluid candidates for the inlet temperature range of interest of 657K to 986K (723°F to 1315°F) are alkali metals. Selection was based on compatibility with the tubing material, stability over the temperature range and the fusion point. A near-eutectic of sodium and potassium (NaK) was selected; the boiling point is 1057K (1443°F), the fusion point is 262K (+12°F). Compatibility with columbium for exposure times up to three years has been demonstrated. Liquids provide high transfer rates and, due to their density, small header dimensions relative to helium. However, a separate gas-to- liquid heat exchanger is required for the Brayton cycle variants, and pump power and weight must be considered. Use of a separate gas-to-liquid heat exchanger can significantly reduce the pressure drop in the gas cycle. Table 4-29 shows masses for Table 4-29. Masses of Gas and Liquid Radiators helium and NaK radiators (high temperature variant) which reject heat appropriate to the generation of 16 GW by a helium Brayton cycle. Each of these systems was optimized for minimum total weight. One factor contributing to the higher mass of the NaK system is the temperature drop across the gas-to-liquid heat exchanger of 30K (54°F) which reduces the radiator effectiveness. The "Brayton cycle efficiency factor" is the mass of solar concentrator and absorber system necessary to counter the efficiency loss resulting from the higher pressure drops in the gas system. The optimum radiator panel configuration for the baseline Brayton cycle is shown in Figure 4-55. Liquid NaK is circulated through thin wall Haynes 188 alloy tubing. Fig. 4-55. Optimum Radiator Panel Dimensions Low Temperature NaK Radiator Aluminum radiating fins are bonded to the tubing and provide a bumper for protection against meteoroids. Segmented construction is used to minimize thermal stresses. 4.10.3 Radiator Configuration Various configurations were analyzed to obtain a radiator panel design of minimum mass as described in section 4.7.1. Arrangements of these panels with different configurations of header and feeder manifolds were analyzed to provide a suitable radiator conceptual design of minimum mass. Concept No. 1 is shown in Figure 4-56. This concept consists of input and output headers with a row of radiator panels between them. The headers are fixed in relation to each other at the feeder end and are free to expand at the other end. Due to the temperature difference between them
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