Thermomechanical effects in supercritical binary fluids

Abstract

Thermomechanical effects, i.e., Piston effect (PE), Soret effect (SE), and Dufour effect (DE), occur in supercritical binary fluids when subjected to boundary thermal perturbation due to the diverging compressibility, vanishing thermal diffusivity and mass diffusion. We numerically study those effects in a 1-D slab with a size of 10mm by solving a complete set of governing equations, which are derived considering the supercritical hydrodynamics, mass transfer and energy conservation simultaneously. The characteristics of these thermomechanical effects in the supercritical binary fluid and liquid binary fluid on different timescales (acoustic and diffusion timescales) are clarified, respectively. Because of the existence of the strong PE in the supercritical binary fluid, the fluid bulk is heated up uniformly, and the SE appears on both sides simultaneously. The direction of mass diffusion is determined by the relative magnitude between concentration gradient and temperature gradient, i.e., gradient ratio γ, and there is a balance gradient ratio γb in each specific binary fluid under a certain condition. The DE is verified by comparing the results of binary fluid and the corresponding pseudo-pure fluid with the same thermo-physical properties. The DE is considerable in supercritical binary fluid, but negligibly small in liquid ethanol/water binary fluid because of the weak SE. These thermomechanical effects in different binary fluids (including supercritical, liquid and gaseous ones) mainly differ in the relative magnitudes and the traveling speed of thermoacoustic wave.