The Effect Of Viscosity On The Erosion Of Gradient- Zone In A Laboratory Solar Pond

Abstract

Double diffusive phenomena occur in many natural systems and in a variety of engineering applications, such as solar ponds. In a solar pond, the erosion of gradient zone due to convection and diffusion is the main cause of inefficiency and hence, requires lot of maintenance. In this thesis, control of erosion of the gradient zone in a solar pond situation has been studied through experiments and transient numerical simulations. A laboratory solar pond was setup in a tank of size 19cm X 19cm X 19cm, and was heated from below. A polymer additive, Carboxy Methyl Cellulose (CMC) was used to increase the viscosity of the working fluid. With 0.01% CMC, viscosity enhancement of the working fluid by 25 to 30 times was achieved, without changing any other properties. The transient momentum, energy and species conservation equations along with continuity equation were solved numerically, using the SIMPLER algorithm with 2 mm grid spacing.

The experiments conducted with the addition of CMC, showed delays in the onset of convection and reduced erosion of the gradient zone. The erosion rates obtained from the numerical simulations agreed with the experimental observation. The impact of viscosity on the onset of convection, kinetic energy, convection pattern, frequency of bursts and erosion rate of the gradient zone, were simulated. Numerical simulations revealed that, there is a nonlinear relationship between the viscosity and erosion rate of the gradient zone. Increase in viscosity by a factor of 15 reduced the erosion of gradient zone completely, indicating the dominant role played by Turbulent entrainment.

The present work indicates that the control of erosion of gradient zone in solar pond using the polymer additive shows lot of promise. The use of the polymer additive as a method to increase viscosity will lower the maintenance costs and, increase the reliability and efficiency of solar ponds, with less than 5% increase in the initial cost.