Flow boiling critical heat flux of DI-water and nanofluids inside smooth and nanoporous round microchannels

Abstract

This work concerns an experimental evaluation of the flow boiling critical heat flux of DI-water and nanofluids inside a 1.1 mm ID channel. Results were obtained for DI-water and solutions of Al2O3 (40–80 nm)/DI-water, Al2O3 (10 nm)/DI-water and SiO2 (80 nm)/DI-water for volumetric concentrations of 0.1% and 0.01%. Additional experiments were performed for pure DI-water on surfaces previously covered with nanoparticles through the flow boiling process. Experiments were performed for mass velocities ranging from 133 to 494 kg/m2s, saturation temperature of 130 °C and vapor qualities at the CHF location ranging from 0.65 to 0.84. The CHF results for DI-water in the tube before covering its surface with nanoparticles were compared with well reputed CHF prediction methods from literature and a reasonable agreement was found. Despite of the large augmentation of the wettability/wickability of the surfaces covered by a layer of nanoparticles, the differences between the CHF values for DI-water for the surfaces with and without the nanoporous covering were within the uncertainty of the CHF measurements. This behavior suggests a negligible effect of the nanoporous layer and, consequently, of wettability/wickability on the saturated CHF in microchannels for high vapor qualities and the range of conditions evaluated in present study. Based on an analysis of the CHF behaviors observed in this study, the saturated CHF mechanism was associated to periodical lumps of liquid that promotes intermittently the surface dryout and its re-wetting. The CHF is established when the time between two consecutives liquid lumps is low enough, so that the surface reaches a temperature for which its re-wetting is not possible.