Influence of Cavity Geometry on the Bubble Dynamics of Nucleate Pool Boiling

Abstract

Nucleate pool boiling is known for its exceptional heat transfer coefficients, with the use of cavities further improving bubble nucleation and heat transfer rate. To promote this heat transfer enhancement technique, a thorough understanding of the influence of cavity geometry on single bubble dynamics is required. The influence of depth and radius of cylindrical and conical cavities on the bubble dynamics of nucleate pool boiling of R1234yf were numerically investigated. The cavity radius was varied between 50 {\mu}m and 400 {\mu}m and the cavity depth between 100 {\mu}m and 1000 {\mu}m at a fixed heat flux of 28 kW/m$^2$. It was found that the maximum equivalent diameter prior to departure was constant for cavities with radii smaller than 120 {\mu}m, while it increased linearly when increasing the cavity radius further. Cylindrical cavities exhibited high stability regardless of cavity radius or depth whereas conical cavities showed a decrease in vapour retention with increasing cavity angle. During the necking phase, the bubble interface became pinned at the cavity edge, depending on conical cavity angle, implying that smaller radii allowed for enhanced surface rewetting. Conical cavities could be considered as cylindrical cavities when the cavity angle was less than a quarter of the interface contact angle. When translating the single cavity findings to cavity array design, cylindrical cavities were recommended as they allowed for stable bubble behaviour. For increased nucleation zones and rewetting, a sub-critical radius was recommended. Wider cavities were recommended for high superheat conditions as larger bubbles could enhance bubble growth.