Variable Physical Properties in Natural Convective Gas Microflow

Abstract

Anisothermal flow prevails in a heated microchannel. It is desirable to understand the influence of temperature-dependent physical properties on the flow and heat transfer characteristics for natural convective gas microflow. In this study, formulas for the shear viscosity, thermal conductivity, constant-pressure specific heat, density, and molecular mean free path are proposed in power-law form and validated through experimental data. Natural convective gas flow with variable physical properties in a long open-ended vertical parallel-plate microchannel with asymmetric wall temperature distributions is further investigated. The full Navier–Stokes equations and energy equation combined with the first-order slip∕jump boundary conditions are employed. Analysis process shows that the compressibility and viscous dissipation terms in balance equations are negligible. Numerical solutions are presented for air at the standard reference state with complete accommodation. It is found that the effect of variable properties should be considered for hotter-wall temperatures greater than 306.88K. The effect is to advance the velocity slip and temperature jump as well as the velocity symmetry and temperature nonlinearity. Moreover, it tends to reduce the mass flow rate and the local heat transfer rate excluding on the cooler-wall surface where the temperature-jump effect prevails over the temperature-nonlinearity effect. Increasing the cooler-wall temperature magnifies the effect on flow behavior but minifies that on thermal behavior.

Second-order slip flow and heat transfer in a microchannel

“…where U is the density, P is the shear viscosity, p c is the constant-pressure specific heat, k is the thermal conductivity, T is the temperature, and p is the pressure defect, related to the pressure p by It should be noted that assuming a small temperature difference between the heated wall and the reservoir supports the constant-property assumption [14] and that considering the low-speed flow of a low-Prandtl-number fluid supports the neglect of viscous dissipation in the energy equation [4,14].…”

Natural convection slip flow in a vertical microchannel heated at uniform heat flux

“…For all cases, the boundary layer theory-based predictions were in very good accord with the results obtained from numerical predictions. More recently, the effect of variable thermophysical properties, in natural convection, in vertical parallel-plate microchannels, at uniform wall temperatures, was studied in [31]. The parallel walls were heated symmetrically and the physical properties depended on the temperature in power-law form.…”

Rarefied Natural Gas Transport

“…It should be noted that assuming a small temperature difference between the wall and the ambient gas supports the constant-property assumption (Weng & Chen 2008b) and that considering the low-speed flow of a low-Prandtl-number fluid supports the neglect of viscous dissipation in the energy equation (Chen & Weng 2005;Weng & Chen 2008b). We can think of the fully developed region as the flow section situated far from the entrance such that y u is negligible.…”

“…It becomes necessary to have formulas for temperature-dependent physical properties of methane. The property formulas adopted in this chapter was originally proposed by Weng & Chen (2008b). According to the nature of gases, the power-law expressions for the shear viscosity, the thermal conductivity, the specific heat, and the density can be written, respectively, as ,…”