Effect of Protrusions on the Falling Film Flow and Heat Transfer of Oily Wastewater Outside an Elliptical Tube

This study addresses the optimized design of falling-film heat exchanger tubes, aiming to enhance heat transfer efficiency and reduce thermal losses, thereby offering potential pathways for efficient green energy utilization. Ten tube models were established and analyzed using computational fluid dynamics (CFD) under constant heat flux conditions. The study investigated the effects of the position, number, and ellipticity (e) of external protrusions on the flow characteristics and heat transfer performance of oily wastewater. The simulation revealed that different protrusion configurations significantly influence hydrodynamic behavior and heat transfer mechanisms. It was found that introducing flow disturbances at an early developmental stage enhances the overall heat transfer performance of the external fluid. Specifically, for a tube with e = 0.5, the heat transfer coefficients (HTC) initially increases and then decreases with increasing Reynolds numbers (Re). This behavior is attributed to the reduction in flow stability caused by the protrusions at higher Re values, which promotes vortex shedding and leads to more complex flow patterns, thereby impairing heat transfer efficiency. Furthermore, as the number of protrusions increases, the overall HTC of the enhanced elliptical tube also follows a trend of an initial increase and then decrease. These results suggest the existence of an optimal protrusion density that enhances turbulence without incurring excessive resistance that would degrade thermal performance.