Experimental Investigation of Heat Transfer Characteristics of Copper Nanofluid Under Subcooled Flow Boiling Conditions

Flow boiling heat transfer is a critical process in various industrial applications due to its efficient heat transfer characteristics. The present study investigates the influence of Cu nanoparticles on the flow boiling characteristics in a vertical pipe of 8 mm internal diameter. Water is considered as the base fluid, and Cu nanoparticles with diameters ranging from 40 to 50 nm are dispersed at various concentrations of 0.001%, 0.005% and 0.01%. Experiments were conducted under atmospheric pressure with consistent inlet subcooling, by varying heat fluxes from 50 to 200 kW/m(2) and mass fluxes from 380 to 955 kg/m(2)-s. Flow visualisation helps in understanding the flow boiling regimes and the associated mechanism of heat transfer. The results revealed a positive correlation between mass flux and the flow boiling heat transfer coefficient for both water and nanofluids. Compared to water, nanofluids exhibited a decrease in wall superheat required for boiling initiation. At a mass flux of 954.21 kg/m(2)-s, the average reductions in wall superheat for 0.001%, 0.005% and 0.01% Cu/water nanofluids were 11.41%, 17.51% and 20.67%, respectively. Notably, nanofluids demonstrated significantly higher boiling heat transfer coefficients in comparison with water, with the enhancement proportional to the volume concentration of the nanofluid. The enhanced heater surface characteristics and altered bubble formation mechanism brought by the presence of Cu nanoparticles were identified as primary factors contributing to enhanced heat transfer. Furthermore, the flow boiling behaviour of nanofluids differs from water in terms of nanoparticle-induced nucleation site creation, faster flow regime transitions, and bypassing the churn flow stage during the boiling process. These findings suggest that Cu/water nanofluid can be treated as a promising fluid for enhancing heat transfer effectively in vertical pipes that can be used in compact heat exchangers and electronic cooling systems.