Dual influence of temperature and gas composition of selected helium-based binary gas mixtures on the thermal convection enhancement in Rayleigh-Bénard enclosures

This paper addresses the potential augmentation of natural convection heat transfer in Rayleigh-Bénard enclosures when filled with a certain type of binary gas mixture. To form the binary gas mixtures, helium (He) is the primary gas and the secondary gases are nitrogen (N2), oxygen (O2), carbon dioxide (CO2) and methane (CH4). Each of the thermo-physical properties participating in the binary gas mixtures viscosity ηm, thermal conductivity λm, density ρm, and heat capacity at constant pressure Cp,m depends on the molar gas composition, temperature and pressure. Results are presented in terms of the maximum allied heat transfer coefficient hm,max/B at the optimal mole gas composition wopt, in the w-domain [0, 1] for the entire range of laminar and turbulent conditions. In the conduction regime, He provides the best heat transfer regardless of temperature. In the convection regime at 300 K a He-CO2 mixture usually provides the maximum heat transfer, whereas at 1000 K pure methane CH4 is the optimum. In addition, a detailed thermo-fluidic structure of the thermal convection patterns in the Rayleigh-Bénard enclosure was analyzed by performing 2-D numerical simulations.