Ceramic High-Temperature Heat-Pipes

Abstract

Ceramic heat pipes and heat pipe based heat exchangers are tailored for automatically heat removal and heat distribution in thermally, chemically and abrasive high stressed systems. The manufacture of silicon carbide heat pipes was carried out. These were filled with sodium or zinc and sealed by laser brazing using metallic and glassy solder materials. High-temperature performance tests revealed a stable operating regime for both ceramic heat pipes with sodium and zinc as working fluid, respectively. Specifically the heat transferred by a zinc filled heat pipe of 22 mm in diameter and 750 mm in length accounted for 600 W at a temperature difference of 400 K. Notably the internal heat transfer capacity of the working fluid was even higher however, the total heat transfer was limited by the external active heat transfer area of the heat pipe. In order to evaluate the long-term stability of the heat pipes, particularly with respect to the joining seam, manufactured heat pipes are currently being tested in long-term annealing experiments at a temperature of 1000 °C under a variety of corrosive atmospheres.

Analysis of a Vertical Flat Heat Pipe Using Potassium Working Fluid and a Wick of Compressed Nickel Foam

“…Compatibility between the heat pipe and its surroundings is, therefore, equally important to the internal compatibility between the envelope, wick, and working fluid. Metals are the dominating envelope and wick materials, but high temperature wickless heat pipes (thermosyphons) of ceramic material are also being developed; results from promising experiments with sodium and zinc as working fluids were reported recently [20]. Nickel is compatible with potassium [4], but starts oxidizing rapidly at about 350˝C [21] and a protective/reducing gas atmosphere of 5 %(v) H 2 , 95 %(v) Ar is therefore often used during the heat treatments and preparations of the nickel wicks and heat pipe envelopes.…”