Case Studies in Thermal Engineering (Apr 2021)
Hydrodynamic behavior of liquid falling film over horizontal tubes: Effect of hydrophilic circular surface on liquid film thickness and heat transfer
Abstract
Falling film configurations have widely been used in various types of refrigeration systems, and wettability has played an important role in characterizing the heat and mass transfer due to varying hydrodynamic behavior. The present paper focuses on a three-dimensional computational fluid dynamics (CFD) simulation to investigate the effect of hydrophilic circular surface on hydrodynamic behavior of liquid falling film over the heated horizontal circular tubes. First, the thickness of the liquid falling film and the liquid distribution around the circumferential and axial directions were analyzed at a feed water temperature of 20 °C for three film Reynolds numbers of 12, 24, and 36 (50 ≤ Q̇feed ≤ 150 ml·min-1) and three heat fluxes of 5, 10, and 15 kW m-2. The influences of the contact angle on the liquid film hydrodynamics and heat transfer were then analyzed. The results showed that at a low feed film Reynolds number, even for super-hydrophilic nature (e.g., contact angles of 0° or 10°), the liquid film failed to cover the first-row tube surface. Increasing the feed film Reynolds number enhanced the liquid film distribution over the tubes and the dry spot decreased, which enhanced the heat transfer performance. At relatively higher contact angles of 30° and 60° (but still hydrophilic nature), the wetted area decreased and dry spots covered most of the second- and third-row tubes, which significantly influenced the thermal performance. Validation of the CFD model based on the volume-of-fluid approach was achieved by comparison with the experimental data, and the experimentally measured values of local heat transfer coefficient were employed for the comparison to support the validity of the present study.
