Numerical Simulation of Air–Steam Mixture Condensation in a Falling Film Condenser

Shell-and-tube falling film condensers are critical in fields like energy, petrochemicals, and waste heat recovery. Their operation predominantly involves the complex mixed condensation of steam and non-condensable gases. This process couples multi-physical phenomena—gas flow, liquid film dynamics, phase change, and non-condensable gas accumulation—making accurate prediction challenging. To better understand the underlying mechanisms, this paper develops a practical CFD simulation scheme. The scheme strongly couples the Species Transport model and the Eulerian-wall-film (EWF) model via User-Defined Functions (UDFs) to simulate condensate generation, inter-tube migration, and interphase transfer. Its reliability is validated through grid independence tests and comparisons with theoretical and experimental data. Using this model, the effects of inlet velocity, temperature difference, and non-condensable gas mass fraction are analyzed. Results indicate that higher inlet velocity significantly enhances heat and mass transfer, with the average wall heat transfer coefficient stabilizing at high velocities. While a larger temperature difference increases total heat transfer, its marginal benefit decreases, accompanied by reduced efficiency. A high non-condensable gas mass fraction severely inhibits condensation. Furthermore, the outer wall heat transfer coefficient of the tube bundle is highly non-uniform, with the liquid film thickness varying by over two orders of magnitude. This study provides theoretical and numerical insights for the optimal design and operation of falling film condensers.