Dinamica da dupla difusao: Aplicacao a lagos solares

This work presents a stability analysis and a model of Solar Ponds. This type of solar energy device prevents convection in a body of water by creating a vertical negative salt gradient, that counteracts the thermal gradient. The performance of the Pond depends on the stability of the gradient zone where both thermal and molecular diffusion are present. Due to the fact that thermal diffusion coefficient is greater then the saline diffusion coefficient "double diffusion" instabilities may occur, leading to the onset of a convective process. The first part of this work establishes the limits for the maintenance of a diffusive heat transfer process in the gradient zone considering the effect of solar radiation absorption, the variation of the diffusion coefficients and both effects coupled. The analysis is made considering the Boussinesq approximation of the Navier Stokes system of equations, solved using a semi-analytical process. In the second part of the work a 2D semi-analytical model is made in order to analyse the evolution of temperature, salinity and velocity fields and the transition from diffusion to convection, being possible to validate the stability criteria previously obtained. It is possible to conclude that the onset of convection in a Solar Pond is function of pond's geometry, of the imposed initial saline profile and of pond's transparency. The transition from diffusive to convective flow occurs through oscillatory motion depending on pond's characteristicsAvailable from Fundacao para a Ciencia e a Tecnologia, Servico de Informacao e Documentacao, Av. D. Carlos I, 126, 1249-074 Lisboa, Portugal / FCT - Fundação para o Ciência e a TecnologiaSIGLEPTPortuga