Nanofluid pool boiling can modify the morphology of the heating surface and the physical properties of the base fluids, interfering directly on the vapor bubbles dynamics and on the heat transfer mechanisms. This paper concerns an experimental investigation of the effects of surface roughness and nanoparticle deposition on the contact angle, surface wettability and pool boiling heat transfer coefficient (HTC). Experiments were carried out using copper surfaces with different roughnesses, and deionized water as the working fluid at a pressure of 98 kPa and under saturated conditions. The nanostructured surfaces were produced by maghemite nanoparticle deposition, which is achieved by boiling selected mass concentrations of a Fe 2 O 3 -deionized water nanofluid (0.029 g/l and 0.29 g/l, corresponding to low and high nanofluid concentration, respectively). The highest heat transfer coefficients were obtained for the smooth surface with deposition of nanoparticles at low mass concentrations. In addition, as the nanofluid concentration increases the surface roughness also increases, and the higher the nanofluid concentration, the lower the contact angle of water on the coated surface.

This study presents experimental results for the heat transfer coefficient during pool boiling of DI water and Al 2 O 3-water based nanofluid at saturation conditions. Based on these data, an analysis of the heat transfer enhancement due to the nanofluids was performed. The experiments were performed for Al 2 O 3-water based nanofluid with different volume concentrations (0.0007 vol.% and 0.007 vol.%, corresponding to low and high nanofluid concentration, respectively). A copper surface, with different roughness values (R a = 0.05 lm, corresponding to a smooth surface, and R a = 0.23 lm, corresponding to a rough surface), was used as test section. The nanoparticle average size was 10 nm and the applied heat flux ranged from 100 to 800 kW/m 2. For nanofluid pool boiling, it was observed an increase in the heat transfer coefficient up to 75 %, and 15% for the smooth and rough surfaces, respectively, in comparison to that of DI water. According to results, the surface roughness is strongly affected by nanofluid concentration due to the nanoparticle deposition on the heating surface. The results indicate that the use of nanofluids is effective on pool boiling heat transfer, for moderate heat flux and low volumetric concentration.

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h i g h l i g h t s An analysis of the influences of different parameters on the boiling phenomenon was performed. A new correlation to predict the pool boiling heat transfer coefficient is presented. The effect of different working fluids, gap size and surface roughness were considered. The proposed correlation predicts accurately the boiling heat transfer of fluids under confined and unconfined conditions.

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