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This paper describes the measurement of the Soret coefficient of a hydrocarbon binary liquid mixture at different orientations to the gravity vector. The working fluid in the experiment was composed of 1,2,3,4‐tetrahydronaphtalene (THN) and dodecane (C12) at mass fractions of 0.50/0.50 and a temperature of 298.15 K. The experimental technique used in this study was an optical digital interferometry method using a Mach‐Zehnder Interferometer. A fast Fourier transform method was used for image processing. The experimental approach was supplemented with a numerical solution using the finite volume method. Experimental and numerical results both confirmed that cell inclination during the thermodiffusion process had a tremendous effect on the measurement of the Soret coefficient.

In a multi-component liquid mixture, the process of disassociation of the components induced by thermal gradient is called thermal diffusion or Soret effect. This effect plays a crucial role in separation of the components in hydrocarbon mixtures of oil. Accordingly, the main goal of this study is to experimentally investigate the Soret effect in binary and ternary hydrocarbon mixtures. Optical interferometry technique with Mach-Zehnder scheme was used to conduct the experiments. The interferometry techniques are not intrusive and the separation of the components in the mixture is not affected by the measurement instrument. A Soret cell is defined as a cubic cavity where the sample mixture is placed in it and, the separation of the components takes place in the cell by heating it from the above. Soret cells are used in convectionless experiments and natural convections are undesirable. The Soret cell used in space experiments was re-designed and optimized for ground-based experiments to avoid the natural convections. Computational studies were made on the both cells to obtain the temperature and velocity fields. Then a set of thermal diffusion experiments conducted in order to compare the performance of the cells. The results shows that the induced convective motions in the second cell are significantly weaker than those in the previous cell which is desirable. In the next step, the effect of the inclination of the cell on the thermal diffusion was studied. First numerical analysis was made to find the velocity and temperature fields in different inclinations and then a set of experiments was performed and the concentration distribution of the components in a binary mixture in different inclinations of the cell was found. Finally, ground based experiments were performed to study the thermal diffusion in five ternary hydrocarbon mixtures. Optical interferometry with Mach-Zehnder scheme using two laser sources with different wavelengths was used. The Soret information of one of the mixtures is available in the literature and this mixture was studied here to validate the present experimental setup. The temperature and concentration of the mixtures were measured successfully in the Soret cell and a table of the measured Soret coefficient were provided.

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