Theoretical analysis of the vapor-liquid slug-train dynamic formation in an ultra-maximum hydraulic diameter oscillating heat pipe in horizontal orientation

The formation of liquid slug is critical to the pulsating flow of vapor-liquid slug-train (VLST) in an ultra-maximum hydraulic diameter oscillating heat pipe (UHDOHP) in horizontal orientation. The high-speed vapor causes the fluctuations in the vapor-liquid interface and forms local low pressure at the wave peak, which promotes the wave peak growth to create a liquid slug. Therefore, how to obtain a sufficient vapor velocity is important. The results show that the required heat to obtain sufficient vapor velocity cannot exceed the maximum load-bearing heat that maintains the effective evaporation of liquid and the unobstructed flow of vapor. And the effects of liquid reflux limit (RL), sonic limit (SL) and critical heat flux (CHF) appear sequentially as the diameter increases. In the effective range of RL, the optimal filling ratio decreases with diameter. And reducing the effective length can weaken the RL limitation and increase the effective range of SL. Moreover, increasing the diameter and filling ratio can weaken the limiting effect of CHF. This study reveals the start-up of UHDOHP in a horizontal orientation and provides a guideline for promoting the VLST formation.