Medium temperature heat pipes - Applications, challenges and future direction

Heat pipes have played a large part in the thermal management market for the past five decades and have contributed to the development and optimisation of countless components in a wide range of high-level applications, most notably in the aerospace, electronics, automotive and power generation industries. These thermal management systems span a wide range of temperatures, which in turn requires the heat pipe fluid and casing material to be specially selected to meet the application requirements. Recently, there has been an increasing demand for heat pipes which can operate in the 300-600 degrees C temperature range - a range which is still under-developed in the heat pipe marketplace due to the lack of conventional fluids which can adequately operate at these temperatures. This range is referred to as the 'medium' or 'intermediate' temperature range. The analysis and exploration of novel fluids, which could potentially be used in this range, will cater for a huge market potential. Although there has been mild development in this temperature range with the aim of testing particular fluid/metal combinations which may be suitable, there appears to currently be a severe lack of continuity in the work with little progression towards a definitive solution and no central reference catalogue of successful and unsuccessful tests. Previous works on the topic tends to follow a 'patchwork' process, often with overlaps in testing and with a focus only on long-term compatibility tests with a limited analytical approach which often lead to incompatible results. This paper intends to summarise all major and stand out efforts in developing medium temperature heat pipes and highlight the most promising fluids and wall materials which have been tested to date. To summarise the content, this review will explore (a) current applications which could benefit from the use of medium temperature heat pipes, (b) the work that has been done on investigating medium temperature fluids, (c) highlight some of the principles behind heat pipe performance prediction, fluid analysis, fluid/metal compatibility and fluid selection and (d) suggest the potential future direction of research in this area, particularly focusing on the development of novel heat pipe fluids. Additionally, a standardised fluid assessment framework is also proposed aiming to aid the identification and analysis of both existing and newly developed heat pipe fluids.