Critical heat flux degradation due to flow disturbances and pressure oscillations under confined and submerged two-phase water jet impingement

Confined and submerged two-phase jet impingement offers a compact and effective heat transfer technology for thermal management. In contrast to free-surface jets, few studies have investigated the critical heat flux phenomenon during confined and submerged jet impingement. In this study, the impact of jet velocity on critical heat flux is examined during confined and submerged impingement of a water jet. A single jet issues through a 3.75 mm-diameter orifice and impinges on a circular 25.4 mm-diameter heated surface. The jet velocity is varied between 0.30–1.62 m/s (Re ≈ 3500–20,000) with the height of the confinement gap held equal to the jet diameter at 3.75 mm. The two-phase flow morphology is observed via high-speed visualization from the top of the confinement gap, and the transient pressure drop across the jet orifice and flow gap is measured simultaneously. Critical heat flux increases monotonically with velocity from 104 W/cm2 to 275 W/cm2 over the tested velocity range. As with free-surface jet impingement, two regimes can be identified: a low-velocity regime and a velocity-dominated regime. However, the observed critical heat fluxes for the confined and submerged jet are 32 % to 16 % lower than those predicted for a free-surface jet configuration. This curtailment of critical heat flux is related to disturbances in the incoming liquid flow induced by dynamics of the two-phase flow in the confinement gap, as indicated by large, recorded pressure drop oscillations. These flow and pressure oscillations are attenuated by the jet momentum with increasing velocity, as corroborated by high-speed flow visualizations. In the low-velocity regime, there are significant effects of these two-phase interactions in the confinement gap. Close to critical heat flux, brief periods occur when vapor completely obstructs the jet orifice, during which a significant portion of the heated surface dries out. In the velocity-dominated regime, at higher jet velocities, the two-phase flow disturbances cause observable instances of mild reduction in the flow rate of the incoming liquid jet, which are followed by dry out of small regions on the periphery of the heated surface. Flow visualizations during the transition to film boiling indicate that premature dry out of the heated surface is induced by these flow and pressure oscillations.