Intermediate Temperature Heat Pipe Life Tests and Analyses

Abstract

There are a number of applications that could use heat pipes or loop heat pipes (LHPs) in the intermediate temperature range of 450 to 750 K, including space nuclear power system radiators, fuel cells, geothermal power, waste heat recovery systems, and high temperature electronics cooling. Since 2004, we have been conducting life tests at temperatures up to 550 K with water and Commercially Pure Titanium Grade 2 (CP-Ti), titanium alloys, Monel 400, and Monel K500 heat pipes. Since 2006, life tests have been conducted at temperatures up to 673 K with titanium and Hastelloy B-3, C-22, and C-2000 envelopes paired with AlBr3, GaCl3, SnCl4, TiCl4, and TiBr4 halide working fluids. Recently, roughly half of these heat pipes were selected for destructive evaluation. The working fluids were analyzed, and sections of the heat pipes were examined to determine the type and amount of corrosion in the wicks and heat pipes. The results showed that Titanium/water and Monel/water heat pipes are suitable for temperatures up to 550 K. Analysis of titanium/water heat pipe crosssections using optical and electron microscopy revealed little if any corrosion even when observed at high magnifications. Copper depleted zones, as well as copper surface nodules formed on the Monel 400 screen wick, but not on the Monel K500 envelopes. An analysis of the water working fluids showed minimal pickup of metals. The long terms tests also established that Titanium/TiBr4 at 653 K, and Hastelloy B-3, C-22 and C-2000/AlBr3 at 673 K were compatible. Hastelloy C-2000 underwent little corrosion when used with TiCl4 working fluid. Hastelloy C-22 exhibited a 5-10 micrometer thick dual corrosion layer when tested with AlBr3 working fluid. The results indicate that the tested envelope materials and working fluids can form viable material/working fluid combinations.