Analysis of Laminar-to-Turbulent Threshold with Water-γ Al 2 O 3 and Ethylene glycol-γ Al 2 O 3 Nanofluids in Free Convection

In the present work, a theoretical model based on the integral formalism approach for both laminar and turbulent external free convection is extended to Newtonian nanofluids. By using an empirical model for computing nanofluid viscosity, a close attention is focused first on the way the heat transfer is modified by increasing the volume fraction of nanoparticles and then to the transitional threshold between laminar and turbulent regimes. Using this viscosity model, a systematic degradation in the heat transfer is observed using nanofluids while compared to the basefluid. Nevertheless, some interesting major conclusions are drawn and show that the heat transfer depends strongly on the flow regime: results have shown that the Ethylene glycol-γAl2O3 mixture gives a better heat transfer enhancement than the water-γAl2O3 mixture in the laminar regime while the reversed trend is found for a turbulent flow. Moreover, for such Newtonian nanofluids, increasing the volume fraction of metallic oxide particles delays the occurrence of the flow transition to turbulence. Key-Words: Heat transfer, Water-γAl2O3, Ethylene glycol-γAl2O3, Free convection, Laminar-to-turbulent transition, Transition threshold.