designs which provide superior performance during normal operation. In addition, the noncondensible gas filled in the heat pipe to facilitate startup from the frozen state changes the transient behavior significantly from that of the conventional (vacuum mode) heat pipe. In the present study, only the transient characteristics pertaining to the startup from the frozen state (Transient condition (3) in Fig. 1) were evaluated and presented while the other transient conditions would be dealt with in the future. The approach used was to test a specially designed and fabricated two meter long sodium heat pipe under simulated heat input and rejection conditions. Both vacuum and gas-filled mode performances were obtained for comparison purposes. The present results could provide valuable insight to the high temperature heat pipe space radiator designer. Experimental Work Test Article The experimental stainless steel-sodium heat pipe assembly consisted of an outer tube, an inner tube, two screen tubes, a reservoir screen plug, two end caps, a capillary insert plug and a process valve with a fill tube. The longitudinal and cross-sectional views of the heat pipe are shown in Fig. 2(a) and (b). The distinct cross-sectional details of the evaporator, adiabatic section and condenser are given in Fig. 2(b). The outer and inner tubes were stainless steel type 304 alloy tube stocks cut in straight and continuous lengths. The inner tube contained 24 equally spaced longitudinal small rectangular grooves each on the evaporator and condenser sections with a single large artery groove on the center section (adiabatic) connecting them. The screen tubes and reservoir screen wick were formed of 304 stainless steel wire mesh. The inner tube and screen composite fit tightly into the outer tube. The end caps, fill tube and valve were tungsten-inert-gas welded in a dry-box. A bellows-type high temperature all-metal
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