Impact of liquid distributor design variables on film dynamics and heat and mass transfer on a horizontal flat tube falling film evaporator

The liquid distributor plays a crucial role in controlling film formation and coupled heat and mass transfer in falling film evaporators. However, its effects on falling film behavior and heat and mass transfer over horizontal flat tubes remain poorly understood. A transient VOF model was therefore employed to investigate the effects of spray height, hole diameter, and hole spacing on film dynamics and the spatial distributions of heat and mass transfer. Based on the results, distributor-dependent correlations for heat and mass transfer were developed. The results show that increasing spray height, hole diameter, and hole spacing generally decreases film thickness and enhances mass transfer. Wave troughs, which occur predominantly over the middle and lower sections of the flat tube, are characterized by thinner liquid films and stronger mass transfer. Increasing hole spacing from 6 to 14 mm reduces the average film thickness by 28.4% and increases the average mass transfer rate by 65.6%. The local heat transfer coefficient decreases along the circumferential direction. Increasing spray height or hole diameter enhances heat transfer, whereas increasing hole spacing weakens it. Increasing spray height from 4 to 16 mm increases the average heat transfer coefficient by 58.9%. Sensitivity analysis identifies hole spacing as the dominant geometric variable. The proposed correlations predict 95.7% of the Nusselt data and 93.7% of the mass transfer rate data within ±15%. These findings provide a mechanistic basis for balancing film uniformity, heat transfer, and evaporation performance in the design of liquid distributors for horizontal flat tube systems.