MODELING OF THE TRANSIENT BEHAVIOR OF HEAT PIPES WITH ROOM-TEMPERATURE WORKING FLUIDS

The heat pipe is a capillary-driven and two-phase flow device, capable of transporting and converting large amounts of energy with minimal losses. As a means of thermal management, uses of heat pipe technology not only include thermal control of satellites and spacecrafts in aerospace applications, but also the cooling of electronic components for ground applications. Recently, there has been a flourishing interest in exploring the use of heat pipe technology in the automotive field. However, in many thermal control applications, heat pipes using room-temperature working fluids, such as water or ammonia, with operating temperatures between 200 K (-73ºC) and 550 K (277ºC), can hardly operate at steady state conditions. The study of transient heat pipe phenomena becomes a significant area of research interests including not only startup and shutdown phases, but also heat redistribution, changes of thermal loading and heat removal. The transient performance is affected by thermal capacity and conductance of the heat pipe, capillary pumping forces, heating and cooling conditions. In the present study, the transient operations of different conventional room-temperature heat pipes were investigated analytically, including the capillary dryout and rewetting behaviors occurring at the evaporator section during startups. The physical model is based on the displacement of a leading-edge front of a thin liquid layer flowing on finite groove uniformly heated with a constant heat flux. A one-dimensional transient heat conduction model along the evaporator wall is coupled with the movement of the fluid layer during startup. Numerical solutions were obtained by a fully implicit Finite Difference Method, accounting for the movement of the liquid and a known time-variable temperature boundary condition at the liquid front. The velocity and position of the liquid front were found to vary with the applied heat flux, the initial conditions, and the thermophysical properties of the working fluid. The wall temperature distribution in the dried region was also predicted. The working fluid temperature distribution compared well with the experimental results from the literature and provided good insight of the room-temperature startup phenomena. The analysis calculated the fluid depletion in the evaporator during startup which dictated the maximum limit on startup power. Combined effects of evaporator wall superheat, uniform longitudinal accelerations, and formation of a liquid slug at the condenser end also were examined and implemented in the current model. Successful comparisons were found and demonstrated the ability of predicting complex transient heat and mass transfer phenomena from a meaningful analytically-based solution. Keywords: Dryout, Rewetting, Heat Pipe Transients, Room-Temperature Startups, Thermal Mathematical Model