Hydrodynamic behavior of liquid falling film over horizontal tubes: Effect of hydrophilic circular surface on liquid film thickness and heat transfer

摘要

Falling flm confgurations 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 ?uid dynamics (CFD) simulation to investigate the effect of hydrophilic circular surface on hydrodynamic behavior of liquid falling flm over the heated horizontal circular tubes. First, the thickness of the liquid falling flm and the liquid distribution around the circumferential and axial directions were analyzed at a feed water temperature of 20°C for three flm Reynolds numbers of 12, 24, and 36 (50 ≤ Qfeed ≤ 150 ml?min-1) and three heat ?uxes of 5, 10, and 15 kW m-2. The in?uences of the contact angle on the liquid flm hydrodynamics and heat transfer were then analyzed. The results showed that at a low feed flm Reynolds number, even for super-hydrophilic nature (e.g., contact angles of 0° or 10°), the liquid flm failed to cover the frst-row tube surface. Increasing the feed flm Reynolds number enhanced the liquid flm 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 signifcantly in?uenced the thermal performance. Validation of the CFD model based on the volume-of-?uid approach was achieved by comparison with the experimental data, and the experimentally measured values of local heat transfer coeffcient were employed for the comparison to support the validity of the present study.