Nucleate boiling heat transfer in nanofluids with carbon nanotubes up to critical heat fluxes

Abstract

In this study, pool boiling heat transfer coefficients (HTCs) and critical heat fluxes (CHF) are measured on a smooth square flat copper heater in a pool of pure water with and without carbon nanotubes (CNTs) dispersed at 60°C. Tested aqueous nanofluids are prepared using multi-walled CNTs whose volume concentrations are 0.0001, 0.001, 0.01, and 0.05%. For the dispersion of CNTs, DISPERBYK 184 is used in distilled water. Pool boiling HTCs are taken from 10 kW/m2 to critical heat flux for all tested fluids. Test results show that the pool boiling HTCs of the nanofluids are lower than those of pure water in entire nucleate boiling regime. On the other hand, critical heat flux is enhanced greatly showing up to 150% increase at the CNT concentration of 0.001% as compared to that of pure water. This is related to the change in surface characteristics by the deposition of CNTs. This deposition makes a thin CNT layer on the surface and the active nucleation sites of the surface are decreased due to this layer. The thin CNT layer acts as the thermal resistance and also decreases the bubble generation rate resulting in a decrease in pool boiling HTCs. The same layer, however, decreases the contact angle on the test surface and extends the nucleate boiling regime to very high heat flux range and reduces the formation of large vapor canopy at near CHF. Thus, a significant increase in CHF results.