Flow Boiling Heat Transfer from Downward-Facing Thick Heater Block in an Inclined Channel with Plain and Microporous Coated Surfaces

Flow boiling experiments were carried out with water on a 300 mm x 120 mm area and 85 mm thick downward-facing heated copper block at 1.4 bar pressure in a channel inclined 10° upwards with respect to horizontal. Heat transfer studies were conducted on a plain and microporous coated surfaces with heat fluxes up to 550 kW/m² for various flow rates and subcoolings. The results showed enhanced heat transfer coefficient on the microporous coated surface by a factor of two compared to the plain surface. Flow visualization study indicated that this heat transfer enhancement is due to enhanced microbubble generation on the microporous coated surface. Boiling curves were compared with the ones obtained from the small size heaters from previous researches. The local dryout or local critical heat flux was observed on the plain surface at around 450 kW/m². When heat flux was increased beyond the critical heat flux level, transition boiling occurred instead of direct excursion to the film boiling. Visualization studies of transition and film boiling regions were also carried out and were analyzed using transient temperature data.