Visualizing near-wall two-phase flow morphology during confined and submerged jet impingement boiling to the point of critical heat flux

Confined and submerged two-phase jet impingement is a compact, low-pressure-drop solution for high heat flux dissipation from electronic components. Nucleate boiling can be sustained up to significantly higher heat fluxes during two-phase jet impingement as compared to pool boiling. The increases in critical heat flux are explained via hydrodynamic mechanisms that have been debated in the literature. In this study, the two-phase flow morphology of a single subcooled jet of water that impinges on a circular heat source is visualized at high speed with synchronized top and side views of the confinement gap. The impinging jet issues from a 3.75-mm-diameter orifice that is held at a height of 2 orifice diameters above a 25.4-mm-diameter heat source. The experiments are conducted at a jet Reynolds number of 15,000 and a jet inlet subcooling of 10 degrees C across a range of heat fluxes up to the critical heat flux. When boiling occurs under subcooled exit flow conditions and at moderate heat fluxes, a regular cycle is observed of formation and collapse of vapor structures that bridge the heated surface and the orifice plate, which causes significant oscillations in the pressure drop. Under saturated exit flow conditions, the vapor agglomerates in the confinement gap into a bowl-like vapor structure that recurrently shrinks, due to vapor break-off at the edge of the orifice plate, and is again replenished due to vapor generation at the heater surface. The optical visualizations from the top of the confinement gap provide a unique perspective and indicate that the liquid jet flows downwards through the vapor structure, impinges on the heated surface, and then flows underneath the vapor structure as a fluid wall jet that wets the heated surface upon which discrete bubbles are generated due to boiling. At high heat fluxes, intense vapor generation causes the fluid wall jet to transition from a bubbly to a churn-like regime, shearing off some liquid droplets into the vapor structure. The origin of critical heat flux appears to result from a significant portion of the liquid in the wall jet being deflected off the surface, and the remaining liquid film on the surface drying out before reaching the edge of the heater. (C) 2019 Elsevier Ltd. All rights reserved.