Superior Convective Heat Transport for Laminar Boundary Layer Flow Over a Flat Plate Using Binary Gas Mixtures With Light Helium and Selected Heavier Gases

Abstract

This paper addresses the laminar boundary layer flow of certain binary gas mixtures along a heated flat plate. To form the binary gas mixtures, light helium (He) is the primary gas and the heavier secondary gases are nitrogen (N2), oxygen (O2), xenon (Xe), carbon dioxide (CO2), methane (CH4), tetrafluoromethane (CF4), and sulfur hexafluoride (SF6). The central objective of this paper is to investigate the potential of this group of binary gas mixtures for heat transfer intensification. From fluid physics, two thermophysical properties, i.e., viscosity η and density ρ, influence the fluid flow, whereas four thermophysical properties, i.e., viscosity η, thermal conductivity λ, density ρ, and heat capacity at constant pressure Cp, affect the forced convective heat transfer. The heat transfer augmentation from the flat plate is pursued by stimulating the forced convection mode as a whole. In this regard, it became necessary to construct a specific correlation equation to handle binary gas mixtures owing Prandtl number Pr∊(0.1,1). Whenever there is heat transfer invigoration in forced flow, drag force accretion seems to be inevitable. A standard formula for estimating the drag force Fd exerted on the flat plate is available from the fluid dynamics literature. The descriptive equations for the heat transfer rate Qmix and drag force Fd,mix associated with the seven binary gas mixtures are channeled through the four thermophysical properties, i.e., density ρmix, viscosity ηmix, thermal conductivity λmix, and heat capacity at constant pressure Cp,mix, which depend on the molar gas composition w. Two case studies suffice to elucidate the modified convective heat and momentum transport that the binary gas mixtures bring forward. At a film temperature Tf=300 K and 1 atm, the He+SF6 mixture delivers the absolute maximum for the relative heat transfer Qmix,abs max/B=16.71 at an optimal molar gas composition wopt=0.96. When compared with the light primary He gas with a relative heat transfer rate QHe/B=12.04, the He+SF6 mixture generates a significant heat transfer enhancement of 39%. At a film temperature Tf=600 K and the same 1 atm, the relative heat transfer QHe/B for the light primary gas He comes down to 10.77. In reference to this, the He+SF6 mixture furnishes an absolute maximum heat transfer Qmix,abs max/B=18.11 at an optimal molar gas composition wopt=0.96, yielding a remarkable heat transfer enhancement of 68%. In the global context, the usage of exotic binary gas mixtures with light helium and selected heavier gases may be envisioned for special tasks in industries that demand high heat transfer rates.