Concerning the operating temperature range of commercially utilized working fluids for heat pipes and closed two-phase thermosyphons a certain gap exists between about 250 °C and about 450 °C to 500 °C, i.e. above the temperature range of water and below that of the alkali metals.
From a thermophysical point of view the so-called high-temperature organic working fluids are most attractive in a part of the mentioned temperature range, namely between about 250 °C and 400 °C. Candidate materials are, among others, diphyl, diphenyl and naphthalene. One major drawback of all these high-temperature organics is the fact that they rapidly decompose above a certain limiting temperature.
The present paper describes experimental work concerning the performance and long-term behaviour of stainless steel and boiler steel thermosyphons with diphenyl and naphthalene as working fluids. Results of maximum performance tests as well as life tests at different heat loads are presented.