Stability of thermal convection in basin-type solar stills

Abstract

The onset of thermal convection in a basin-type solar still is studied using linear stability theory. The system is idealized as a horizontal layer of an absorbing, scattering and emitting liquid with a lower insulated rigid boundary and an upper convective free boundary subjected to uniform solar radiant heat. Radiative transfer in the liquid is modelled by the differential approximation. At low optical thickness values, the lower boundary emissivity is the dominant factor controlling the fluid instability with the mirror boundary cases being more stable than the corresponding cases for a black boundary. Generally, lowering the following parameters, Planck number, scattering albedo, weighted non-grayness and increasing the heat losses from the top surface, all delay the onset of convection.