A Numerical Study on Falling Film Evaporation with Wall Heat Flux and Pulsating Airflow

In the current work, pulsating airflow and an external heat source are proposed to improve the evaporation performance of falling films. A volume of fluid (VOF) multiphase flow model is established based on the commercial software ANSYS Fluent 2023 R1 (ANSYS, Inc., Canonsburg, PA, USA) to simulate the falling film evaporation process. Numerical simulations are conducted under combined working conditions with variable average inlet velocity (u0), relative pulsating amplitude (A), and wall heat flux (qw). The spatial and temporal distributions of physical fields are visualized via numerical contours and characteristic curves, and the heat and mass transfer enhancement mechanism is revealed from three aspects, namely, the promotion of driving potential difference by wall heat flux, the increase in heat and mass transfer gradients induced by pulsating airflow, and the synergistic effect of the above two factors. The results indicate that the evaporation ratio (Ψ) increases monotonically with the rise of u0, with a maximum growth rate of 83.8%. By contrast, under the condition of fixed A0 = 1 m/s while varying u0, the evaporation ratio exhibits a convex variation with the relative amplitude A = A0/u0, and the global optimal value is achieved at A = 1/6, corresponding to u0 = 6 m/s and A0 = 1 m/s. Comparative analysis demonstrates that wall heat flux exerts a more significant influence on evaporation performance than pulsating airflow. Specifically, the evaporation ratio at qw = 10,000 W/m2 is 3~4 times higher than that under the adiabatic wall condition. The reliability of the numerical model is first confirmed by comparing the predictions with published experimental data for vertical falling film evaporation. Based on this validated model, the quantified parametric effects and optimal operating conditions provide practical design references for falling film evaporators in seawater desalination and related thermal separation applications.