An investigation of the effect of nanoparticle composition and dimension on the heat transfer coefficient during flow boiling of aqueous nanofluids in small diameter channels (1.1Â mm)

The present paper concerns an experimental investigation of the effect of adding nanoparticles to DI-water on the heat transfer coefficient during flow boiling in a 1.1 mm ID tube. Results were obtained for DI-water and solutions of Al2O3 (20-30 nm)/DI-water, Al2O3 (40-80 nm)/DI-water, SiO2 (80 nm)/DI-water, SiO2 (15 nm)/DI-water and Cu (25 nm)/DI-water for nanoparticles volumetric concentrations of 0.001, 0.01 and 0.1%. Additional results were obtained for pure DI-water before and after performing tests with nanofluids in order to evaluate the effect of nanoparticles deposition due to the boiling process on the heat transfer coefficient. Experiments were carried out for heat fluxes up to 350 kW/m2, mass velocities ranging from 200 to 600 kg/m2 s, vapor qualities up to 0.5 and saturation temperatures at the test section outlet of 102 °C. An analysis of the surface roughness and a characterization of its texture using an optical profiler were also performed before and after the experiments with nanofluids. According to most of the results, the heat transfer coefficient for flow boiling of nanofluids is lower than for DI-water in a tube without nanoparticles deposition. The data analyses revealed that the nanoparticles dimension affects the heat transfer behavior. This fact is associate to the nanoparticles deposition on the surface and its effect on the density of active nucleation sites. This hypothesis is corroborated by the fact that the deposition characteristics depends on the nanoparticles size.