Computational Studies of Stable and Unstable Growth of Vapor bubbles in Superheated Liquid

Author

Sai Manohar Ramachandran, Southern Illinois University CarbondaleFollow

Date of Award

5-1-2017

Degree Name

Doctor of Philosophy

Department

Engineering Science

First Advisor

Esmaeeli, Asghar

Abstract

Two thermo-fluid problems of physical and practical significance are studied; (i) growth of a vapor bubble in a superheated liquid and (ii) thermocapillary-driven fluid flow of two superimposed fluids. In the first, the dynamics of stable and unstable growth of a vapor bubble in a superheated liquid is investigated using Direct Numerical Simulations. A one-field formulation is used wherein one set of equations for conservation of mass, momentum, and energy are solved using the front tracking/finite difference method. The governing equations are solved on a fixed grid and the phase boundary is tracked explicitly by a moving front. The numerical method is validated by comparison with benchmark analytical solutions. Unstable growth leads to wrinkling at the surface of the bubbles and evaporative mass fluxes that are several order of magnitude larger than those of stable boiling. Stable/unstable growth of vapor bubble is studied, by changing the governing nondimensional parameters and the material property ratio. The effect of the ratio of the bubble radii, and the distance between the center of two bubbles on the growth rate, and bubble surface area are studied. The growth of the vapor bubble is considerably influenced by density ratio, conductivity ratio and Jacob number. The hydrodynamic and thermal interactions of two vapor bubbles significantly reduces the bubble radius and bubble surface area. In the second problem, the thermocapillary-driven convection of two-superimposed fluids occupying the space between the walls of a microchannel, heated from below with a sinusoidal nonuniform temperature and imposing a uniform temperature on the top wall, in zero gravity is studied. The problem is studied to explore the effect of the thickness ratio, conductivity ratio, and the viscosity ratio on fluid flow and interface deformation. This is achieved, by solving the equation of conservation of mass, momentum, and the energy balance analytically using the domain perturbation analysis and numerically using a front tracking/finite difference method, in the limit of creeping flow regime, and negligible convection of heat. The relative thickness of the fluid, and the ratio of the material properties, have direct effect on the flow strength. The sense of the interface deformation is studied, by varying the thickness ratio, and the viscosity ratio. The flow strength decreased, with an increase in the conductivity ratio ekc = ka/kb, and the viscosity ratio. The relative thickness, and the viscosity ratio determine the sense of deformation. It is seen that the deformation decreases with increase in the conductivity ratio. It is also seen that as the viscosity ratio increases for different thickness ratio, at lower eμ the deformation is towards the bottom wall, and further increase in viscosity ratio achieves a zero deformation and then reverses the sense of deformation, towards the top wall increasing the deformation.