Critical heat flux maxima resulting from the controlled morphology of nanoporous hydrophilic surface layers

Porous hydrophilic surfaces have been shown to enhance the critical heat flux (CHF) in boiling heat transfer. In this work, the separate effects of pore size and porous layer thickness on the CHF of saturated water at atmospheric pressure were experimentally investigated using carefully engineered surfaces. It was shown that, for a fixed pore diameter (∼20 nm), there is an optimum layer thickness (∼2 μm), for which the CHF value is maximum, corresponding to ∼115% enhancement over the value for uncoated surfaces. Similarly, a maximum CHF value (∼100% above the uncoated surface CHF) was observed while changing the pore size at a constant layer thickness (∼1 μm). To explain these CHF maxima, we propose a mechanistic model that can capture the effect of pore size and pore thickness on CHF. The good agreement found between the model and experimental data supports the hypothesis that CHF is governed by the competition between capillary wicking, viscous pressure drop and evaporation, as well as conduction heat transfer within the porous layer. The model can be used to guide the development of engineered surfaces with superior boiling performance.