The present proposal concerns a doctoral research dealing with an experimental investigation on the effects of adding nanoparticles to a fluid on the critical heat flux during convective boiling in micro-scale channels and surface rewetting under film boiling conditions. In the proposed study, boiling curves will be obtained for convective boiling of nanofluids inside micro-scale channels for different experimental conditions including different nanoparticle material and size and covering a broad range of mass velocities, heated lengths, heat fluxes and degrees of subcooling at the test section inlet. The CHF experiments will be run by increasing progressively the heat flux up to its critical value. For each heat flux step, it will be determined the flow pattern, the pressure drop and the heat transfer coefficient. To evaluate the effects on the surface rewetting promoted by nanofluids, a stainless steel cylinder having a diameter of 10 mm and a length of 30 mm at a temperature of 800°C will be submerged in a nanofluid pool. The internal temperature of the cylinder will be monitored by thermocouples placed in its interior and the mean heat transfer coefficient on its external surface estimated through inverse methods with the heat flux based on the cooling cylinder cooling rate, using a code already developed. From these results, boiling curves based on the instantaneous decreasing heat flux will be built and hydrodynamic of the convective cooling process of the cylinder registered by a high speed camera (100.000 frames/s). For this purpose, illumination techniques will be developed. The nanofluid will be prepared from deionized water and nanoparticles, by ultrasonication without adding any stabilizing products. Experiments will be performed for the following nanoparticles: (i) gama alumina, mean diameters of 25 nm and 60 nm; (ii) silicon oxide, mean diameters of 15 nm and 80 nm and (iii) carbon nanotubes, characteristic dimension of 15 nm. The deposition of nanoparticles will be analyzed by electronic microscopy examining the test sections before and after the experiments. Based on these results, the critical heat flux and surface rewetting mechanisms for nanofluids will be analyzed and correlations to predict these parameters will be developed.
News published in Agência FAPESP Newsletter about the scholarship: