The effect of pressure on the critical heat flux in water-based nanofluids containing Al2O3 and Fe3O4 nanoparticles

Water-based nanofluids are colloidal dispersions of nanoparticles in pure water and have significantly higher critical heat flux (CHF) values compared to pure water at atmospheric pressure. This paper describes the effects of pressure on the CHF in a pool of water-based nanofluids of magnetite (Fe3O4) and alumina (Al2O3) nanoparticles using Ni–Cr wire. To evaluate the effect of pressure on the CHF enhancement of water-based nanofluids, pool boiling CHF experiments using pure water were first conducted at 101 kPa (atmospheric pressure) to 1100 kPa of absolute pressure. The CHF values for pure water increased with increasing system pressure, as expected. The results for pure water were compared with the CHF correlations and were found to be well matched. Based on the results for the pure water case, pool boiling CHF experiments using magnetite water nanofluid (MWNF) and alumina water nanofluid (AWNF) were performed at higher pressures within the same range as those used for the pure water case. It was found that the CHF using water-based nanofluids also can be enhanced at high pressures in a pool. Based on the obtained results, several methods (i.e., analyses of the bubble dynamics during nucleate boiling, of the contact angle with a droplet of pure water, and of the weight of deposited nanoparticles on the heating surface after CHF) were used to investigate the mechanism of CHF occurrence and the effect of pressure on the CHF in water-based nanofluids. The bubble frequency in the nanofluids was approximately 2 times higher than that for pure water. Due to the higher frequency of departing bubbles, the opportunity to rewet the hot spots on the heating surface was increased. The contact angles of the nanofluid were lower than that for the pure water cases at all ranges of pressure tested. The weight of the deposited nanoparticles on the heating surface decreased as the system pressure was increased. We discuss the effect of pressure on the surface characteristics in detail based on these analyses.