Mechanism study of low-ambient-pressure effects on the efficiency of cavitation erosion mitigation by air bubbles in gas-liquid two-phase flow.

Ambient pressure variations alter cavitation bubble dynamics, thereby affecting cavitation erosion on solid boundaries and the efficiency of aeration-based mitigation. In this study, the cavitation erosion mitigation efficiency and underlying mechanisms of entrained air bubbles in gas-liquid two-phase flow under varying ambient pressures were systematically investigated by combining macroscopic investigation and mesoscopic analysis. Ultrasonic cavitation experiments were conducted at ambient pressures of 55-95 kPa to quantify the erosion mitigation characteristics of aeration. The results showed that, under aerated conditions, the eroded-area ratio, pit depth, mean pit volume, cumulative mass loss and the erosion rate all decrease monotonically with decreasing ambient pressure, while the mitigation efficiency of aeration increases correspondingly. To elucidate the underlying mechanism, corona discharge system was employed to generate cavitation bubbles, enabling direct observation of the air bubbles interaction with cavitation bubbles. It was found that air bubbles significantly reduced the maximum velocity of microjet and attenuating the intensity of shockwaves impinging on the wall. Furthermore, both the maximum microjet velocity and the shockwave intensity decreased with decreasing ambient pressure under aerated conditions. The macroscopic trends in cavitation erosion mitigation of aeration and the mesoscopic effects of air bubbles on cavitation bubble dynamics together reveal the underlying mitigation mechanisms. They explain how aeration reduces cavitation erosion on solid boundaries exposed to high-speed water flow under high-altitude, low atmospheric pressure conditions. These findings offer theoretical support for applying aeration-based cavitation erosion mitigation in such conditions.