Numerical model development for critical region flow and thermodynamic transitions of CO2 fluids in microchannel

Abstract

Dramatic changes of thermophysical properties in the vicinity of critical point brings significant accelerating effect on the thermodynamic transitions of fluid. This study performs a confined geometric design with CO2 fluids passing through the critical zone in microchannel. Such a problem formulation combines the compressible Navier-Stokes equations with different property formulations to obtain critical anomaly effects at complex critical transport coefficients. Three equations of state (van der Waals (vdW), Redlich-Kwong (R-K) and Peng-Robinson (P-R) state equations) and their corresponding thermodynamic relations are derived in solving the governing equations of flow and heat transfer equations. Basic results are found to be consistent with theoretical asymptotic analysis, with linearly curves of pressure and a sudden shift of density (425 kg/m3–502 kg/m3) and velocity (0.82 m/s to 0.97 m/s) profiles when the microchannel flow across the critical region. The cascade effect between weakened radial thermal conductivity (λ ∼ (ρ – 1)-1/2, cp ∼ (ρ - 1)−2) and enhanced fluid expansion (βp ∼ (ρ - 1)−2) in the critical region is also shown to cause a sharp negative transition in temperature. In addition, the simulations demonstrate a qualitative comparison for three state equations, and the distinctions in quantitative terms (negative temperature value are −0.1 (vdW), −0.17 (R-K), and − 0.23(P-R)) are found to originate from stronger divergence characteristics (the specific heat cp and thermal expansion coefficients βp) in P-R gases than vdW and R-K gases.