Abstract
Low-temperature thermal desalination (LTTD) is widely used to recover freshwater from seawater using low-grade thermal energy, typically operating between about 5 and 30 °C. In such systems, removing the water droplets entrained in the vapour is essential for maintaining vapour quality and overall performance. This study examines how surface roughness influences the droplet-separation efficiency of hook-free demisters operating under LTTD conditions, across inlet velocities of 2–10 m/s and droplet diameters of 1–100 µm. A computational fluid dynamics (CFD) framework coupling the continuous vapour phase with a Discrete Phase Model (DPM) is used to resolve the interaction between the vapour flow and the entrained droplets, with a two-dimensional formulation adopted to represent the predominantly planar flow within the demister channel. Surface roughness is assessed through its effect on separation efficiency, pressure drop, Euler number, and turbulent kinetic energy. The results show that roughness intensifies near-wall turbulence and droplet–wall interactions, raising the separation efficiency by 6–12% for droplets in the 5–50 µm range and reaching a maximum of 97.4% at 10 m/s. This gain is accompanied by a pressure-drop increase of up to 99% at the higher velocities, which produces a nonlinear rise in the Euler number. Because the absolute pressure losses remain small (below 25 Pa), however, the corresponding fan-power penalty is minor; the Separation Performance Index (efficiency per unit pressure drop) nonetheless falls as velocity increases, indicating that a low-to-moderate inlet velocity (≈4–6 m/s) gives the most favourable balance between separation efficiency and energy use. Relative to other demister geometries, the rough-walled hook-free demister captures both fine (20 µm) droplets effectively while keeping the absolute pressure loss low. The study further clarifies the droplet–wall interactions and the adverse pressure gradients that develop under different surface-roughness conditions.