Computational Two Phase Marangoni Flow in a Microgravity Environment

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Abstract

The lack of significant buoyancy effects in zero-gravity conditions poses an issue with fluid transfer in a stagnant liquid. In this thesis, the movement of a bubble or droplet in both stagnant and rotating liquids is analysed and presented numerically using computational fluid dynamics (CFD). The governing continuum conservation equations for two-phase flow are solved using the commercial software package (2011). The Volume of Fluid (VOF) method is used to track the liquid/gas interface in 2D and 3D domains. User-Defined Functions (UDFs) are employed in order to include the effect of surface tension gradient and fluid properties as a function of temperature, with a view to efficiently investigating temperature effects on the properties of the two phases. The flow is driven via Marangoni influence induced by the surface tension gradient, which in turn drives the bubble/droplet from the cold to the hot region. For stationary liquid, the results indicate that the scaled velocity of the bubble decreases with an increase in the Marangoni number, which agrees with the results of previous space experiments. An expression for predicting the scaled velocity of a bubble has been regressed based on the obtained data from the present numerical study for thermal Marangoni numbers up to 10,721. An expression for predicting the scaled velocity of a Fluorinert droplet migrating in oil has also been presented for an MaT range from 24.05 to 2771. The interactions of two droplets in thermocapillary motion have also been studied and compared with the results obtained for the isolated droplet. The results have shown that the leading droplet will not move faster than if it were isolated, as the trailing droplet has no influence on the velocity of the leading droplet. Three-dimensional results show that no bubbles broke in any of the cases observed and agglomeration could occur during thermocapillary migration for bubbles placed side by side. The results of the motion of a singular and multiple bubbles incorporating thermocapillary forces in a rotating liquid in a zero-gravity environment have been presented for the first time. When the Rossby number is 1, the effects of rotation are important. Furthermore, the deflection of the gas bubble motion increases towards the axis of rotation with a decrease in the Rossby number (Ro). Bubble population balance modelling has been investigated in normal gravity using Luo kernels for breakage and agglomeration and two different laminar kernels for zero-gravity conditions. The simulations covered a wide range of scenarios and results are presented as a bell and histogram shapes for number density and particle percentage distribution, respectively.

Layman's Abstract

The present CFD results show that different temperature gradients lead to different droplet migration velocities, and it was proven that bubble migration velocity varies linearly with the temperature gradient for the given conditions. An expression for predicting the scaled velocity of a bubble has been derived based on the data obtained in the present numerical study for Marangoni numbers up to 10,721. An expression for predicting the scaled velocity of a Fluorinert droplet migrating in oil has also been presented for Ma ranging from 24.05 to 2771.The results of the motion of a singular and multiple bubbles incorporating thermocapillary forces in a rotating liquid in a zero-gravity environment have been presented for the first time. When the Rossby number is 1, the effects of rotation are significant. Furthermore, the deflection of the gas bubble motion increases towards the axis of rotation with a decrease in the Rossby number (Ro).Finally, two axisymmetric dimensional Eulerian/Eulerian simulations of two-phase (gas/liquid) transient flow were performed using a multiphase flow algorithm based on the finite-volume method to simulate bubble population balance modelling for both stationary and rotating columns in normal and zero gravity. Quantitative agreements between the experimental data and simulations are obtained for the averaged axial liquid velocity profiles. An important conclusion from these numerical results is that the centrifugal force is pulling the bubbles towards the axis of rotation and shifts them away from the wall. Rotation could accelerate the moving bubbles throughout the column in zero-gravity conditions.

Keyword(s)

Thermocapillary; Rotation; Two bubble interaction; Two Phase; Bubbles/droplets; Surface tension gradient; Marangoni flow

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