Analysis of the heat and mass transfer processes in solar stills – The validation of a model

The outcome of the earlier systematic research work on the theoretical modeling of the complex transport phenomena occurring in solar stills was the development of the fundamental Dunkle’s model, already known almost four decades ago. Although it has been based on several simplified assumptions, this model has extensively been employed over the years as a convenient and sufficiently accurate predictive tool for solar stills working under ordinary operating conditions. However, it has occasionally been reported that it fails under unusual operating conditions, mainly corresponding to higher average temperatures, usually leading to higher distillate yields. The aim of the present investigation was to relax the initially established simplified assumptions of the fundamental Dunkle’s model and to evaluate the comparative accuracy of both, the refined and the earlier fundamental models against an extensive body of previously reported measurements from the literature, both field and laboratory. The comparative presentation of results indicates that although both models are impressively correct for ordinary low temperature operating conditions where the humid air thermophysical properties are close to those of dry air, the saturation vapor pressure at the brine and condensing plate temperatures are negligible compared to barometric pressure and the familiar Jakob’s dimensionless Nusselt–Rayleigh correlation for natural convection heat transfer appears to be valid, they both fail at higher operational temperatures. It appears that as far as Dunkle’s simplified model is concerned, this occurs not only owing to the first two counteracting effects but also to the effect of the dimensionless convective heat transfer correlation affecting also the accuracy of the refined model, which fails to predict precisely the natural convection conditions at higher Rayleigh numbers representing conditions of strong turbulence in the solar still cavity. Assuming a constant asymptotic value of the exponent n = 1/3 which persists over a broad region of high Rayleigh numbers relevant to solar still operation, an improved value of the proportionality constant C around the value of 0.05 was estimated for the accurate prediction of measurements, at least as far as the available data from the literature is concerned.