University of Birjand Water Harvesting Research 2476-6976 9 1 2026 03 01 Studying Unsteady Heat and Mass Transfer Condition in a Tubular Solar Still Used for Water Harvesting Implications 18 27 3919 10.22077/jwhr.2026.11055.1202 EN Mahdi Hedayatizadeh Associate Professor, Faculty of Agriculture, University of Birjand, Birjand, Iran & ​Member of the Unconventional Water Resources Research Group, University of Birjand, Birjand, Iran. Journal Article 2025 09 19  <span class="fontstyle0">This study presents a comprehensive unsteady-state mathematical model for analyzing heat and mass transfer processes in a tubular solar still (TSS), aimed at enhancing the accuracy of temperature predictions and freshwater productivity assessments. The model incorporates time-dependent energy balance equations for key components—including the saline water, trough, humid air, and transparent cover—while accounting for convective, evaporative, radiative, and condensative heat transfer mechanisms. Assumptions such as uniform water temperature, negligible vapor leakage, saturated vapor near the water surface, and minimal solar absorption by humid air are employed to simplify the analysis. Mass transfer coefficients are derived from established correlations, and natural convection is modeled using Rayleigh and Grashof numbers for curved surfaces. Simulations under constant solar radiation (750 W/m²) and ambient temperature (28°C) reveal that the trough exhibits the highest temperature (up to 60°C), followed by saline water, humid air, and the cover (lowest at ~40°C), with all components stabilizing after approximately 2 hours. Cumulative distillate yield reaches approximately 0.5 kg after 8 hours of operation. Parametric analyses demonstrate that a 2.5-fold increase in solar radiation intensity results in a 3.4-fold rise in productivity, while enlarging the diameter from 0.1 m to 0.4 m yields a 4.5-fold enhancement, though at increased cost. The model provides valuable insights for optimizing TSS designs in water-scarce regions, highlighting the potential for sustainable, low-cost desalination.</span>  https://jwhr.birjand.ac.ir/article_3919_eca77dba0b73f82e53cfb7d4660bacc4.pdf