Theoretical critical velocity of individual bubble forming vapor plug in oscillating heat pipes exceeding the critical diameter

The motion behavior of bubbles is of great significance for the formation of vapor plugs in oscillating heat pipes exceeding the critical diameter (Ecd-OHPs). By analyzing bubble dynamics and introducing the conditions for vapor plug formation in Ecd-OHPs, a theoretical critical velocity uc model for individual bubbles forming a vapor-liquid plug under vertical and large inclination angles was constructed. The variable parameters of the model include the structure and working parameters of oscillating heat pipes, as well as the physical properties of the working fluid. The theoretical model matches the validation experiment well. The established model indicates that with the increase of pipe diameter and inclination angle, uc increases, while with the increase of evaporation section length and drag coefficient, uc decreases. The drag coefficient is a key parameter affecting the model's drag term, and the drag coefficient of bubbles in working fluid water is about 1.09. The influence of working fluid properties is diverse, mainly manifested as the difference of (rho l-rho g)/rho l, rho l/rho g, and d0 (bubble detachment diameter).