Analysis of Thermal and Operating Performance of Thermosyphons and Heat Pipes

Thermal devices such as thermosyphons and heat pipes have a wide variety of applications from underground heat exchangers and temperature control to de-icing of civil engineering infrastructure to aerospace electronic devices. Understanding and optimization of their operating conditions is thus important. In this paper, a weighted decision matrix was used to determine what working fluid is the most appropriate for use in a closed two-phase thermosyphon and an artery wick heat pipe. The fluid is required to have a high merit number, compatibility with all materials that might be found in a heat pipe, high maximum permissible heat flux, and a broad operating temperature range. The primary objective of this study is to develop models to define optimum working fluids to be used for a given device design. The model for thermosyphons estimates the maximum heat flux using the flooding limit, boiling limit and dry-out limit; the model for a heat pipe uses the boiling limit, capillary limit, entrainment limit, sonic limit and viscous limit. The maximum heat flux is then determined by taking the minimum of the heat transfer limitations and dividing it by the radial area of the condenser section for each operating condition. This value was then used as a criterion for a weighted decision matrix, along with the merit number, the number of compatible pipe materials, and operating temperature range. The model was applied to low-temperature range working fluids such as water, ammonia, and heptane and allows evaluation of what fluid is optimal at any particular device geometry as well as operational conditions such as temperature and pressure.