Investigation of geometry-induced heat and mass transfer mechanisms in square, pentagonal, and hexagonal pyramid solar stills

Pyramid solar stills have gained attention for their larger condensation area and improved solar exposure, yet the influence of geometry on internal heat and mass transfer mechanisms remains insufficiently understood. Most existing studies focus mainly on productivity, leaving the underlying transport physics unexplored. In this work, a three-dimensional transient Computational Fluid Dynamics (CFD) model is used to compare square, pentagonal, and hexagonal pyramid geometries under identical operating conditions and to examine how they shape natural convection, vapor transport, and phase change behavior. The simulations reveal that geometry fundamentally alters circulation strength, vapor residence time, and condensation distribution. The square pyramid forms a single weak loop with strong stagnation zones, whereas the pentagonal geometry reduces stagnation by ∼ 50%, and the hexagonal design strengthens overall circulation by ∼ 29%. Condensation uniformity also improves markedly, with the global CUI increasing from 0.42 (square) to 0.81 (hexagonal), while vapor residence time decreases from 14.8 s in the square pyramid to 7.6 s in a hexagonal shape. A key finding is the decoupling between evaporation and condensation performance, demonstrating that similar basin temperatures do not guarantee effective condensation unless vapor is uniformly redistributed. Two transport-based performance indicators—the circulation strength factor and the condensation uniformity index—are introduced to enable systematic geometric comparison. By translating classical cavity convection principles into the context of solar desalination, this study demonstrates the decoupling of evaporation and condensation performance. The work introduces practical, transport-based indicators, the circulation strength factor and the condensation uniformity index, to provide a systematic framework for engineers to evaluate geometry-induced transport efficiency beyond standard empirical yield measurements.