CFD ANALYSIS OF A WICKLESS HEAT PIPE
Heat pipes are the devices capable of transporting heat efficiently without the use of any external power supply and are used in a variety of thermal management applications, such as satellite cooling systems, electronic cooling systems, concentrated solar power plants, etc. A conventional heat pipe uses wick structure to circulate the working fluid within it. Wickless heat pipe, also referred as thermosyphon, uses gravity to circulate the working fluid within it. The advancements in this technology have led to the cost efficient thermal systems and hence a number of experimental studies have been done in this area to further improve these systems.
The present study investigates the thermal characteristics of a copper thermosyphon using Computational Fluid Dynamics (CFD). Numerical simulations are performed for transient, two-phase flow and heat transfer using ANSYS Fluent CFD solver. Volume-of-Fluid (VOF) method is used to track the interfaces between the water vapor and liquid water. In a heat pipe, heat is supplied to an evaporator section resulting in vaporization of water. The water vapor moves towards the condensor section, where it condenses due to heat removal from the heat pipe and finally the liquid water circulates back to evaporator due to capillary action. In a thermosyphon, this capillary action is provided by gravity. The evaporation-condensation phenomenon in thermosyphon has been modelled using Lee model, available in ANSYS Fluent CFD software version 16.2. The temperature dependent density and surface tension coefficient of water are given as inputs through user defined functions. The simulations are performed for about 60 seconds with a time step of 10-4 seconds. The predicted temperatures across the thermosyphon for a fill ratio (portion of evaporator section filled with water) of 0.5 are validated with the available experimental data and found a good match. Susbequently, the effect of fill ratios (0.5-0.8) on the thermal characteristics of the thermosyphon has been studied. An optimum fill ratio for a given heat flux is determined by comparing the thermal resistance values and are discussed in detail.