CFD simulation and second law analysis of weir-type cascade solar stills with different number and dimensions of steps

Computational fluid dynamics (CFD) simulation of fluid flow in weir-type cascade solar stills with different geometries has been presented in this paper. The results of CFD simulation were also used to estimate local entropy generation inside the weir-type solar stills. The results showed that maximum water production was achieved with 7, 8 and 10 steps when the step height was 2 cm. In addition, considering number of steps and various steps heights, the case with six steps of 3.5 cm height was found to be the best geometry for the investigated solar still. Moreover, the results of the simulation show that the number of recirculating zones and their regularity are the main parameters affecting the productivity of a weir-type cascade solar still. Contours of isotherms, vapor mass fraction, streamlines and local entropy generation have been presented in order to demonstrate their contribution to the behavior of the investigated solar still. Furthermore, the results of second law analysis showed that the most entropy generation occurred near the glass and the water surface, as well as at the tip of the stairs. This entropy generation was mainly due to the heat and mass transfer phenomena and the fluid friction.