Fig. 4-79. Heat Pipe Fluids: Heat Transport Capability and Temperature Range 4.10.6.4 Heat Pipe Performance Heat pipe performance used in the radiator system mass analysis was obtained from the design and performance data in Reference (5). A maximum internal pressure of 3 x 10$ Pa (450 psi), including safety factor, was assumed to size the heat pipes. Diameters of 12.7 mm (0.5") and 25.4 mm (1") have been considered to date. The 12.7 mm (0.5") diameter heat pipe was, however, discarded because of its higher specific mass per QL. Diameters larger than 25.4 mm (1") have not been analyzed yet, because of the difficulty of wrapping the evaporator section of the heat pipe around the manifold. Table 4-30 gives details of the heat pipe designs used in the radiator mass analysis. It should be.noted that it may not be possible to obtain the QL in all cases if L is greater than approximately 1.5 m (4.92 ft). If the vapor flow is turbulent (Reynolds number > 3000) the Q x L relationship does not always hold. However the QL given in the table is always obtainable with L = 1 m (3.28 ft) or less. When the radiator has been mass optimized the heat pipe can be designed to suit the conditions. Its requirements should fall within the QL range shown in Table 4-30. 4.10.6.5 Candidate Materials: Heat Pipe/Fin Radiator The temperature range of interest for heat pipe radiators is 300K (80°F) (for solar cell cooling) to 1000K (1340°F) (for Brayton cycle cooling). Because of their light weight and relatively high thermal conductivity, candidate radiator fin materials are aluminum and beryllium. Aluminum is suitable for low temperature fins with operating temperatures to 460K (368°F); beryllium, with a melting temperature of 1555K (2340°F), would be suitable for higher temperatures. For the heat pipes aluminum is not a suitable material since it is not compatible with water or mercury as a working fluid. Water is compatible with stainless steel, copper, nickel and titanium. Stainless steel is the most likely candidate. For mercury, non-austenitic steel, such as 304SST or 347SST can be used. Heat pipes using sodium as a transport medium may be fabricated from stainless steel, nickel or niobium. Manifolds, headers and feeders for heat pipe radiators can be fabricated from materials suitable for the tube/fin radiators, such as Haynes 188 for temperatures up to 800K (1000°F) and B-66 for greater temperatures. 4.10.6.6 Heat Pipe Radiator Configuration Physical limitations on the maximum length of heat pipes are imposed by the heat pipe geometry and required heat flux. Heat pipes should be made as long as possible within launch vehicle capabilities to reduce radiator complexity and the total number of panels per radiator. Capillary pumping forces must equal the sum of the liquid and vapor pressures since the vapor must flow from the evaporator to the condenser and return in liquid form. This imposes a limitation on heat pipe length relative to vapor and liquid passage cross sectional areas. The evaporator section is approximately one fifth the length of the heat pipe and must be in contact with the heat source. In the radiator design the heat source is liquid NaK flowing in a small diameter manifold or Table 4-30. Heat Pipe Data
RkJQdWJsaXNoZXIy MTU5NjU0Mg==