Numerical study of the effect of liquid flow parameters on the three-dimensional film thickness distribution outside the horizontal tube

Abstract

In the horizontal-tube falling film evaporators, the overall heat transfer performance is directly affected by the liquid film flow outside the tube. To investigate the flow characteristics of the evaporating falling film outside the tube, a three-dimensional (3-D) two-phase model for falling film flow of water and seawater was developed. The heat transfer and evaporation were considered in this model. The Tanasawa model was applied for simulating the phase transition process. The spatial distribution of the local film thickness (δ) along the circumferential angle (θ) and dimensionless axial distance (L*) was discussed in detail. The effect of spray density (Γ), salinity (S) and spray fluid temperature (T_sf) on the average tangential velocity, local spray density and δ was analyzed within the range of 0.03 ≤ Γ ≤ 0.07 kg/(m·s), 0 ≤ S ≤ 100 g/kg and 323.15 ≤ T_sf ≤ 343.15 K. The results indicates that the impact of Γ, S and T_sf on δ is obviously different in space. The collision of liquid film at L* = 0.5 results in a reflux vortex within the liquid film. When θ is less than 60^°, there is a slight decrease in the distribution of δ along L* before a rapid increase with L* due to the secondary collision between the liquid film spreading along L* and the reflux flow. As S increases from 0 to 100 g/kg, the maximum difference of δ in the axial direction decreases by 0.038 mm on average. It indicates that increasing S makes the liquid film more evenly distributed along the axial direction.