Influence of liquid height on pool boiling heat transfer over open rectangular microchannels

Open rectangular microchannels surface (ORMS) has been extensively studied in pool boiling heat transfer because of lower cost of fabrication, ease of application and better heat transfer performance. Researchers have examined liquid height effect on heat transfer performance at various heat fluxes for copper plain surface (CPS). However, few studies have revealed the underlying mechanisms of liquid height for ORMS. This study tested different liquid heights of 1 mm, 3 mm, 5 mm, 7 mm, 9 mm, 12 mm, 15 mm, 20 mm and 50 mm over both ORMS and CPS with deionized water at atmospheric pressure. Critical heat flux (CHF) and heat transfer coefficient (HTC) were determined and considered as functions of liquid height for each liquid level. For ORMS, three different HTC variation trends versus liquid height were evaluated. The results show that the distinction is primarily determined by heat flux, as follows: Type-I has critical liquid level, Type-II has both critical liquid level and optimum liquid level, and Type-III has optimum liquid level. Type-II, however, did not appear for CPS. This research also showed that ORMS and CPS have a critical liquid level of 5 mm, whereas ORMS may be able to achieve a higher heat flux in shallow liquid levels of 1 mm and 3 mm due to its improved liquid replenishment capability.