Comparison study of fluid thermal boundary-bulk behaviors in the close-to-critical region under different property trends

Abstract

The high-efficiency energy system based on the utilization of supercritical CO2 has been widely developed in recent years to meet the growing demands of clean energy and the elimination of CO2 emissions. Within this field, the design of the corresponding equipment requires an understanding of the thermodynamic behavior of supercritical fluid affected by the singular thermophysical properties. The analysis of this study is based on the asymptotic expansion of hydrodynamic equations and the thermophysical properties characterized by the exponential trends. The asymptotic model finds the wave and diffusion modes valid within the bulk region in second-order and within the boundary layer region (BLR) in first-order, respectively. Main findings include: (1) The wave mode is found in the bulk region with the wave celerity Γ. When the critical point is approached, Γ decreases from (γ0Mac0/κT0)0.5 = 1 to [(γ0–1)Mac0/κT0]0.5, which; (2) The diffusion mode found in BLR is characterized by non-dimensional diffusion coefficient ζ. This coefficient has the same behavior as thermal diffusivity, which is enlarged by thermal conductivity but weakened by isobaric specific heat. (3) Mass transport from BLR to bulk leads to the generation of the thermal wave, which can be measured by mass transport coefficient Mb=ζβp. Mb also characterizes the magnitude of the thermal wave as u2=Mb(Tb1)z|z=0 (that is, 0.17 mm/s when ϕ=10−4). The behavior of Mb is similar to one of ζ. However, the effect of cp is limited; (4) Two different modes of viscosity are identified. The increase in viscosity leads to the change of viscous stress mode from the second order to the first order, which also leads to a decrease in mass transport.