Experimental studies of heat transfer and flow regimes during flow boiling of water and alumina nanofluids at different heat and mass fluxes

The flow boiling heat transfer in a vertical pipe of inner diameter 7.5 mm was investigated with pure water and Al2O3/water nanofluid as working fluids. The main heater section was made up of borosilicate glass for better visualization of flow regime. For this study, particle concentrations of 0.001%, 0.005% and 0.01% were considered. The influence of mass flux and heat flux, on flow boiling heat transfer was analysed. From the results, it is observed that boiling heat transfer coefficient is increasing with mass flux for both water and nanofluids. Use of nanofluid decreases wall superheat. The average reduction of wall superheat, as compared to water, at mass flux of 905.42 kg/s-m(2) for 0.001%, 0.005% and 0.01% nanofluids is 10.8%, 21.34% and 26.79% respectively. It is also observed that heat transfer coefficient increases with particle concentration due to the changed heater surface characteristics and amendment in bubble formation mechanism. The average enhancement in heat transfer coefficient, as compared to water, for the particle concentrations of 0.001%, 0.005% and 0.01% at a mass flux of 905.42 kg/s-m(2) is found to be 12.11%, 21.75% and 27.97%, respectively. Flow visualization study was also done to differentiate flow patterns of water and nanofluids. Churn flow regime was observed for water at moderate heat fluxes. However, in case of nanofluids, churn flow was not observed. The flow boiling heat transfer coefficient is observed to be high for the nanofluids compared to water. An effort has been made to explain the heat transfer mechanism, based on the existing flow boiling regime under the given conditions.