Influence of coated surfaces and gap size on boiling heat transfer of deionized water
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Nanocoating techniques have been used to increase the heat transfer coefficient by changing the surface morphology, which could potentially increase the heat transfer in pool boiling systems. The present study aims to determine the influence of nanocoated surfaces and the gap size on the heat transfer coefficient and the critical heat flux during the pool boiling of deionized water, at saturation temperature in atmospheric pressure. Tests were performed on a copper heating bare surface with an average roughness of 0.330 μm. The nanocoated surfaces were produced by alumina (Al2O3) nanoparticle deposition with 0.007% of volumetric concentration by using nanofluid boiling process. A gap size of 1.0 mm, corresponding to a Bond number equal to 0.4, was analyzed, and the results were compared with the cases without confinement. Concerning the heat transfer coefficient, the coated surface showed deterioration in the heat transfer performance (approximately 29%) as compared with the uncoated surface mainly due to the fouling resistance formed on the heating surface, confirmed by the surface characterization (SEM images). However, for coated surfaces and for confined cases, enhancement of 28% in the dryout heat flux was observed; the coating process significantly increases the surface wettability, which, in turn, increases the re-wetting capacity during the confined boiling process. Moreover, the heat transfer coefficient is more influenced by the gap size effect than the coating process. The chemical analysis showed that changes in the surface morphology occurred due to the effects of the confinement as compared to the original coated layer (the morphological aspect and melting mechanism were similar to the named liquid phase sintering).