Effect of flow rate and subcooling on spray heat transfer on microporous copper surfaces

In this work, we experimentally investigated spray boiling heat transfer performance with degassed HFE-7100 as the coolant on a conductive microporous copper surface, and observed enhanced heat transfer performance compared to that on a plain surface. Spray heat transfer data were measured using two full-cone spray nozzles spanning a range of volumetric flow rate from 1.1 cm3/s to 15.8 cm3/s. We also investigated the effect of different liquid subcooling levels ranging from 30 °C to 0 °C on the heat transfer data. Spray impingement on the microporous surface showed an enhancement of 300-600% in the heat transfer coefficient at a given wall superheat compared to spray impingement on a plain surface. The critical heat flux also increased by up to 80% for the case of spray impingement on a microporous coated surface as compared to impingement on a plain surface, depending on the flow rates and the subcooling levels. Contrary to the results in the literature, for a given nozzle we observed that the liquid spray at near-saturated temperature (0 °C subcooling) had higher heat transfer performance and critical heat flux than the subcooled spray on both plain and microporous surfaces except at the lowest flow rates. This likely results from the limited residence time of the liquid droplets in contact with the heater surface and the much higher efficiency of phase change heat transfer. The near-saturated spray undergoes phase change much faster than the subcooled liquid, removing heat more efficiently than the subcooled liquid. A modified correlation, based on the Estes-Mudawar correlation (1995) [22], utilizing the experimental data from the present study and literature is proposed for the critical heat flux for spray impingement on both plain and microporous surfaces.