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 (Tst = 100-160 °C), storage pressure (Pst = 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 Tst and D are the main factors affecting the α, while the Pst 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 (Tst, Pst, 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.