New insights into the back-attack phenomenon in submerged massive gas injection: Complementary experimental and numerical investigations

This study employs experimental and numerical techniques to investigate compressible two-phase flow behavior during submerged massive gas injection into a liquid. It focuses on the back-attack phenomenon, which is the energetic backward motion of gas toward the injection region and causes refractory wear in industrial processes. To evaluate how the nozzle diameter and injection conditions influence the back-attack frequency, a series of in-house water-based experiments were carried out. Based on these findings, a hypothesis was formulated that connects the occurrence of back-attack and its frequency reduction to the downstream flow structure of the compressible gas jet, including shock-cell formation, pressure wave propagation, and their interactions with the gas-liquid interface. Due to the limitations in optical access to the downstream flow during the experiment, complementary numerical simulations were performed for a shorter period near the nozzle using a compressible large eddy simulation-volume of fluid approach. These simulations shed light on important aspects of high-speed submerged gas jets, including the growth of the interfacial area and compressibility effects. Furthermore, the hypothesized mechanism was evaluated using Fourier analysis of high-resolution pressure fluctuation data downstream of the nozzle for different injection conditions. The analysis shows that higher injection flow rates stabilize the shock-cell structure, reduce the probability of backward motions near the nozzle, and shift the energy peak downstream, all of which result in the back-attack frequency reduction. The results have significant implications for industrial processes that involve submerged gas injection and provide a theoretical and experimental basis for further validation of numerical simulations.