Investigation of novel analytical tools and experimental methods for the development of intermediate temperature heat pipes

Heat pipes have been a large part of the thermal management market for the past four decades and have contributed to the development and optimisation of countless components in the design of satellites, spaceships, formula racing cars, power plants and electronics cooling. These thermal management systems span a wide range of temperatures, which in turn, requires the heat pipe fluid 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°C to 600°C temperature range – a range which is still severely underdeveloped in the heat pipe marketplace due to the lack of conventional fluids which can adequately operate at these temperatures. This range is dubbed the ‘medium’ or ‘intermediate’ temperature range. Though there has been some development in this temperature range with the aim of testing particular fluids and metals 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 little effort to catalogue successful and unsuccessful tests. The author has identified from literature review of the topic that the lack of a framework to follow which would aid the researcher to advance more rapidly in identification, modelling and experimentation of potential fluids may be a contributing factor. 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 without a well-rounded scientific process beforehand which often lead to incompatible results. This in some ways has ‘stalled’ the assessment of new potential fluids due to the long-winded nature of the approach. The following work intends to progress the research capabilities in this temperature range and set the foundations for rapid directed research effort in this area by developing the necessary equipment, techniques, databases and modelling tools. While the means to develop novel fluids themselves is beyond the scope of this work, the intention of this work is to advance the capabilities of the participating organisations to reach a point in which they are able to focus solely on novel fluids following the framework laid out in this thesis. The work also provides a comprehensive analysis of all currently available fluids and, through the developed ‘fluid selection process’, has identified a range of fluids which have the best potential for further development. The most cost-effective solutions were found to be Bismuth Trichloride and Antimony Trichloride, while other fluids such as Ruthenium Pentafluoride, Rhenium Heptoxide and Rhenium Heptafluoride have excellent thermal transport capability in the intermediate temperature range but are substantially more expensive. The fluid selection process has also proven to work universally in any temperature range through application in commercial projects. This has led to the identification of alternative fluids in what was previously thought as well-established temperature ranges which could provide better cost-benefit while maintaining high thermal conduction capabilities. The focal fluid selected to prove the processes was Antimony Trichloride due to the low cost and greater ease of handling. Theoretical analysis concluded that refractory metals such as Tungsten, Molybdenum, Zirconium and Tantalum are most likely to be compatible with halides in general. Preliminary contact angle results showed a Molybdenum has a superior wetting ability than the conventionally used Stainless Steel and the Molybdenum/Zirconium alloy TZM when testing with Antimony Trichloride. The wetting ability of Antimony Trichloride is also superior to that of water on the same surfaces. Compatibility tests were in agreement with predictions that refractory metals have the highest resistance to reaction with Halides. Molybdenum was shown to have the greatest resistance to reaction form analysing surface changes through SEM and EDX techniques. Molybdenum is selected to be focus of the future work due to its superior corrosion resistance, reasonable cost and wide application in targeted industries such as nuclear and aerospace. Water heat pipe tests were conducted to prove the testing ability of medium temperature heat pipe test rig. The study focused on the upper limitation of mesh wick heat pipes in the horizontal position. Experimental results show good agreement with numerical predictions of power handling capacity surrounding the boiling limit of the heat pipe, but they underestimated the temperature drop experienced across the heat pipe possibly due to there being a higher thermal resistance across multiple mesh layer than expected. The attempted fabrication of Molybdenum/Antimony Trichloride heat pipes was unsuccessful using conventional joining methods, future work will aim to create heat pipe structures using Electron Beam or Laser welding. This work has developed numerous novel concepts and analyses including: (1) Comprehensive fluid property, metal property and fluid/metal compatibility databases (2) Development of a heat pipe modelling tool incorporating these databases (3) Construction of a test rig able to test heat pipes in the medium temperature range (4) Development of short term compatibility and wettability tests for air sensitive fluids (5) Analysis of the performance of water heat pipes around their boiling limit, within the ‘intermediate’ temperature range