A comprehensive mathematical model including the effects of vapor-liquid interface and surface tension was proposed to describe the vapor-liquid two-phase flow, heat and mass transfer and the phase change process in a closed loop oscillating heat pipe (CLOHP). The vapor-liquid two-phase flow in a typical CLOHP was numerically investigated using the proposed mathematical model and the VOF method. The comparisons between the computational and experimental results indicated that the proposed model could successfully simulate the initial distribution of working fluid, the complex flow patterns during different operation conditions, such as bubbly flow, slug flow, semi-annular/annular flow, back flow, and the flow pattern transitions in the CLOHP. The phenomenon that semi-annular/annular flow and slug flow formed in alternating vertical tubes at the initial stage of working fluid circulation was also simulated successfully. Those results were in good agreement with the experimental observations. The flow and heat transfer of a working fluid in two transition sections, and the effects of heating power on the interval flow patterns, were analyzed based on the numerical simulation. The results showed that the changes of temperature, pressure and flow pattern were obvious in the transition section between adiabatic section and condenser section, where the transition of heat transfer condition occurred. The violent boiling might occur in the evaporator section under the high heating power of 100 W and 120 W. The preliminary results indicated that the mathematical model proposed in present paper could effectively reveal the complex vapor-liquid two-phase flow in CLOHP, which established a basis for the further study of complex working mechanisms of CLOHP and effects of operation parameters.
Visual study on two-phase flow in a horizontal closed-loop oscillating heat pipe
