The influence of liquid height to bubble size ratio on the boiling crisis in pool boiling

Abstract

We conducted experiments to analyze water boiling under 10 K subcooling and atmospheric conditions in a horizontal pool boiling test setup, focusing on the effect of liquid height above the heating surface. Our main interest was the transition in boiling crisis mechanisms at liquid heights comparable to bubble size. For pool heights above 4.0 mm, we observed a relatively constant critical heat flux (CHF) of approximately 1.21 MW/m^2. Conversely, CHF significantly dropped to around 0.19 MW/m^2 for liquid heights below 2.5 mm. For depths between 2.5 – 4.0 mm the measured CHF values varied within these limits. Our pool boiling facility featured a stainless-steel chamber filled with deionized water heated by external circulating oil. A ceramic cartridge supported a transparent heater at the cell’s center-bottom, which utilized an indium tin oxide layer on a sapphire substrate for heat dissipation. We monitored bubbles’ footprint dynamics during boiling using infrared and high-speed video cameras. Preliminary high-speed data analysis revealed different hydrodynamic mechanisms corresponding to the obtained CHFs. For heights above 4.0 mm, the formation of an irreversible dry patch is triggered by bubble interactions. Conversely, at heights below 2.5 mm, one of the bubbles nucleating shortly after the onset of boiling failed to detach from the surface shortly. Instead, it grew in size and eventually spread over the entire surface, prematurely triggering a boiling crisis. We attributed this behavior to a lack of buoyancy forces to detach the bubble from the surface as it grew above the liquid level. Pool heights between 2.5 – 4.0 mm showed both behaviors. Further investigations will focus on exploring the transition in boiling mechanisms shifting from buoyancy- to surface-tension-dominated processes by varying bubble dimensions through the introduction of surfactants. Ultimately, we aim to analyze fundamental boiling parameters and heat flux to characterize how boiling dynamics, boiling crisis and CHF value vary with the bubble-to-liquid height ratio.