A new process for the determination of the Soret coefficient of a binary mixture under microgravity

Abstract

In the presence of a gravity field or under microgravity, pure thermo-diffusion leads to very weak species separation in binary mixtures. To increase the species separation in the presence of gravity, many authors use thermo-gravitational diffusion in vertical columns (TGC). For a given binary mixture, the species separation between the top and the bottom of these columns depends on the temperature difference, ΔT, imposed between the two vertical walls facing each other, and the thickness, H, between these two walls (annular or parallelepipedic column). These studies show that, for a fixed temperature difference, the species separation is optimal for a thickness, H opt , much smaller than one millimetre. The species separation decreases sharply when the thickness H decreases with respect to this optimum value. It decreases progressively as H increases with respect to H opt. In addition, for mixtures with a negative thermo-diffusion coefficient, the heaviest component migrates towards the upper part of the column and the lightest one towards the lower part. The loss of stability of the configuration thus obtained leads to a brutal homogeneity of the binary solution. The objective of this study in microgravity was to increase the optimum of species separation. For this purpose, the binary fluid motion was provided by uniform velocities imposed on the two walls of the cavity facing each other. This forced flow led to species separation between the two motionless walls of the cavity. In this case, the fluid motion generated in the cavity was not dependent on the imposed temperature difference, ΔT contrarily to the case of thermogravitational column. Under these conditions and for a given column of thickness H, there are three independent control parameters: ΔT and the two velocities of the walls facing each other. Using the parallel flow approximation for a cell of large aspect ratio, the velocity, temperature and mass fraction fields within the cavity were determined analytically. Thus the parameters leading to optimal species separation were calculated. The analytical results were corroborated by direct numerical simulations. The present paper thus proposes a new process for the determination of the Soret coefficient, the thermodiffusion coefficient and mass-diffusion coefficient.

On species separation in n-component mixture under microgravity

“…The present work is an extension of the study performed by Mojtabi [9] with various mixtures, from binary to n-component mixtures. An application with a ternary mixture was presented.…”

“…Recently, Mojtabi [9] showed that it is possible to significantly increase the importance of species separation for binary mixtures, in microgravity, using forced convection obtained by a uniform translational displacement of the isothermal walls maintained at T h and T c respectively, with ∆T = T h − T c . In this new configuration, the species separation is several orders of magnitude greater than the one obtained in thermo-gravitational columns (TGC).…”

Analytical, numerical and scale analysis study of thermal diffusion effect on free convection boundary layer flows in a slender porous layer

“…It is to note here that, except some experimental tests devoted to analyse some specific behaviors [4,5], most of these studies have addressed the measurement of the thermal diffusion coefficients of various multicomponent mixtures [6−9]. To this end, novel methods based on Thermogravitational [6], electrochemical impedance spectroscopy [7], thermal lens spectrometry [8] and microgravity [9] process have been presented and adopted in these studies. Recent experimental techniques used to simulate the thermal diffusion effect in binary and ternary liquid mixtures have been described and interpreted in the review conducted by Köhler et al [10].…”

The Influence of Bounding Plates on Species Separation in a Vertical Thermogravitational Column

“…The results were compared with the experimental values obtained on the Earth. Mojtabi [19] presented a new process for the determination of the Soret coefficient of a binary mixture under microgravity.…”