A visualization study of vacuum enhancement on vapor flow and yield in tubular solar still

Abstract

Operating solar still under vacuum is known to significantly boost the freshwater yield, and the bonus yield could be mainly attributed to the enhanced vapor flow. Smoke visualization technique can characterize the vapor flow field within a distillation chamber, and help to reveal structural/operational shortcomings and achieve an optimal design. To quantitatively explain the mechanism of vacuum enhancement on vapor flow within a tubular solar still (TSS), a visualization experiment using laser sheet and smoke tracer was conducted to capture the flow pattern and diffusion process of vapor. The accelerations of the vapor flow with decreasing pop and elevating Tw were captured. At Tw = 60 °C, average velocity of vapor flow (vavg) under 40 kPa was 66.6 mm/s, increased by 153% compared with that under 95 kPa. An empirical correlation was then presented with an accuracy of −12% ∼ 22%. Based on the correlation, a novel model for predicting the yield of solar still was proposed, and validated through independent data, showing an average deviation of 5.11%. Furthermore, three flow regimes (laminar, transition, turbulent) were classified with different Grashof number, where laminar flow was observed at Gr < 3.0 × 104 and turbulence was occurred when Gr exceeded 5.6 × 104. This work visualized the acceleration of vapor transportation under vacuum, and supplied guidance for further optimal design in solar stills.