Study on bubble behaviors and heat transfer at shallow liquid boiling over open rectangular microchannels

The heat transfer performance of shallow liquid boiling differs from that of deep liquid boiling owing to distinct bubble venting mechanisms. Since the liquid level is of the same order of magnitude as the bubble departure diameter, bubble detachment is restricted by the surface tension of the free interface, which causes an untimely liquid reflux toward the nucleate site. Consequently, shallow liquid boiling has a high heat transfer coefficient (HTC) at a liquid height below the critical liquid level but at the expense of limited critical heat flux (CHF). This study aims to improve the CHF of shallow liquid boiling by utilizing open rectangular microchannels surface (ORMS), which can provide separated vapor–liquid pathways to facilitate the liquid return. The boiling heat transfer curves and bubble behavior of distilled water at shallow liquid levels are investigated using the ORMS. A critical liquid level of 5 mm was achieved with heat fluxes of approximately 15 W/cm², 25 W/cm², and 35 W/cm² over ORMS. Compared to a plain copper surface (PCS), ORMS improves the upper limit of the heat flux for the critical liquid level. A simplified and efficient analytical model with two-sphere bubble is established to predict the critical liquid level for the ORMS by considering the dynamic and static force balance. The critical liquid level is approximately twice the bubble departure diameter, which may aid in designing the liquid filling rates for phase-change heat sinks for HTC improvement.