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 (Al 2 O 3) 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).

One promising way to enhance the heat transfer coefficient (HTC) and the critical heat flux (CHF) is modifying the heating surface morphology by using machining techniques, coating, and chemical processes. Microstructured surfaces, i.e., surfaces with the presence of micropillars on the surface, provide small perturbations in the liquid, affecting the vapor bubbles dynamic. These structures increase the heating surface area and change the fluid flow. Microfins can have different shapes and sizes and can be arranged in different patterns to improve heat transfer. This study aims to evaluate experimentally the thermal performance of different microfin surfaces by using HFE-7100 as working fluid. Square micro-pillar arrays were etched on a plain copper surface through the micro-milling process. Square microfins of different length scales (i.e., height and side length) were uniformly spaced on the plain copper surface. The inter-fin space had the same value, 250 μm, for all surfaces in order to control the effective roughness, R eff , defined as the ratio of the area in contact with the liquid to the projected area. Microfin surfaces intensify the HTC as compared to plain surfaces and the number of fins is the main factor for the HTC enhancement; if the number of microfins is constant, the larger the effective roughness, the higher the heat transfer performance. Additionally, the capillary-wicking ability increases and it also improves the HTC and the dryout heat flux due to the prevention of hotspots in the microfin surface. Thus, the surface thermal behavior is a function of the surface morphology and its surface capillary wicking.

No abstract

No abstract

No abstract