Modelling of the thermal behaviour of a two-phase closed thermosyphon

Abstract

Interest in the use of heat pipe technology for heat recovery and energy saving in a vast range of engineering applications has been on the rise in recent years. Heat pipes are playing a more important role in many industrial applications, especially in increasing energy savings in commercial applications and improving the thermal performance of heat exchangers. Computational techniques play an important role in solving complex flow problems for a large number of engineering applications due to their universality, flexibility, accuracy and efficiency. However, up to now, computational studies on heat pipes are still at an early stage due to the complexity of multiphase flow characteristics and heat and mass transfer phase changes. Therefore, the main objective of this study is to develop a CFD modelling that includes the complex physical phenomena of both the heat transfer processes of evaporation and condensation and the mass transfer process of phase change during the pool boiling and film condensation. In this thesis, two novel numerical models were developed in ANSYS FLUENT. In the first, a two-dimensional CFD model was developed to visualise the two-phase flow and the evaporation, condensation and heat transfer phenomena during the operation of a wickless heat pipe, that otherwise could not be visualised by empirical or experimental work. An in-house code was developed using user-defined functions (UDFs) to enhance the ability of FLUENT to simulate the phase change occurring inside the heat pipe. Three different fluids, water, R134a and R404a, were selected as the working fluids of the investigated wickless heat pipe. The cooling system of the condenser section was simulated separately as a three-dimensional CFD model of a parallel-flow double pipe heat exchanger to model the heat transfer across the condenser section's heat exchanger and predict the heat transfer coefficients. The overall effective thermal resistance along with the temperature profile along the wickless heat pipe have been investigated. An experimental apparatus was built to carry out a thermal performance investigation on a typical wickless heat pipe for the purpose of validating the CFD simulation. A theoretical model based on empirical correlations was developed to predict the heat transfer thermal resistances in the evaporator and the condenser section. The second model was developed to combine the two-dimensional CFD simulation of the wickless heat pipe and the three-dimensional CFD simulation of the condenser section's heat exchanger to simulate the two-phase flow phenomena of boiling and condensation and the cooling system of the condenser section through a comprehensive three-dimensional CFD model of a wickless heat pipe. Two fluids, water and R134a, were selected as the working fluids of the investigated wickless heat pipe. This model was validated using a transparent glass wickless heat pipe to visualise the phenomena of pool boiling and comparing the results with the three-dimensional CFD flow visualisation. This study demonstrated that the proposed CFD models of a wickless heat pipe can successfully reproduce the complex physical phenomena of both the heat transfer process of evaporation and condensation and the mass transfer process of phase change during the pool boiling that takes place in the evaporator section and the filmwise condensation that takes place in the condenser section. The CFD simulation was successful in modelling and visualising the multiphase flow characteristics for water, R134a and R404a, emphasising the difference in pool boiling behaviour between these working fluids. The CFD simulation results were compared with experimental measurements, with good agreement obtained between predicted temperature profiles and experimental temperature data.