Pressure characteristics of multiphase flow in an underwater pressure vessel subject to gas injection and water discharge

This study investigates the multiphase coupling characteristics and evolution mechanisms of the flow field induced by high-pressure gas release in a confined space. The gas supply and drainage mechanisms that occur during the gas–liquid displacement are systematically clarified through a combined experimental–numerical methodology that accounts for the pressure decay and air compressibility effects of the air tank on the water drainage process. The results demonstrate that the transient pressure characteristics of the gas supply system play a dominant role in determining the drainage dynamics. On this basis, the ascent time of pressure-driven drainage in the water tank can be predicted with a maximum error of 1.4%. Furthermore, the concept of an effective pressure-driven drainage interval is proposed. Theoretical derivation and dimensional analysis reveal that this effective pressure-driven drainage interval is inversely correlated with the drainage duration. The results presented herein allow us to establish a framework for understanding pressure-driven ballast systems, and provide references for pressure-based surfacing design and strategy optimization.