In-nozzle flow of superheated liquid release and the influence on the external flashing jet using transparent nozzles

Accident release of superheated liquid could produce a violent phase change and trigger a highly destructive flashing jet. In this work, release experiments are performed using a 20 L tank and transparent nozzles, with water as the fluid throughout. The experimental conditions included storage temperature (T_st = 100-160 °C), storage pressure (P_st = 6-16 bar), and nozzle sizes (D = 1-6 mm). High-speed image processing and capacitive void fraction sensor are used to quantify the two-phase flow of superheated liquid inside the nozzle and the downstream jet break-up regimes. The experimental results show that void fraction (α) goes through three phases during the release process: growth, stabilization and decay. It is also demonstrated that bubble nucleation, growth and burst determine the breakup of the downstream jet. The T_st and D are the main factors affecting the α, while the P_st only change the release rate. Moreover, it is verified that none of the existing flashing criteria (Ja-We, χ*) can accurately determine the boundary conditions of the flashing regimes. Instead, the parameters characterizing bubbles nucleation and growth, α, can unify the effects of (T_st, P_st, and D) to determine the physical mechanism of the flashing process. In this work, a novel method to distinguish between external flashing (α = 0), internal flashing (0 < α ≤ 25) and full flashing (α > 25), and a steady-state model k = f (Ja, We, α) is developed for the flashing jet.