Influence of Liquid Height on Bubble Coalescence, Vapor Venting, Liquid Return, and Heat Transfer in Pool Boiling

Abstract

The maximum heat extraction rate from a pool boiling process is limited by the Critical Heat Flux (CHF). In most pool boiling applications, the liquid height over the boiling surface is several times greater than the departing bubble diameter and has little influence on bubble behavior, vapor removal, or liquid return. However, limited research is available in the literature to identify the pool boiling behavior at different liquid heights that are comparable to the departure bubble diameter. It is hypothesized in this work that removal of bubbles from the heater surface is critical to enhancing performance, and implementing liquid heights comparable to vapor bubble diameters may have a beneficial effect on heat transfer, especially on the heat transfer coefficient (HTC). Pool boiling studies are conducted with water boiling on a plain copper surface at 2 mm, 3 mm, 6 mm, 9 mm, 12 mm, 15 mm, 20 mm, 30 mm, and 50 mm liquid heights. CHF and HTC are calculated, and bubble interactions are captured using a high-speed camera. It is observed that CHF increases as the liquid height is increased initially but reaches a plateau beyond which no further enhancement in pool boiling performance was observed. Upon analyzing the high-speed videos of escaping bubbles, the liquid height to departure bubble diameter ratio is identified as a key parameter distinguishing between two bubble venting mechanisms. The findings of this work can be used with liquid height as a control variable in pool boiling equipment design, especially in applications such as electronics cooling.