Scale effects on ejector performance: The critical role of boundary layer dynamics

Ejectors are widely employed in various industries. However, conventional design approaches often overlook the impact of scale effects on ejector performance, resulting in limited predictive accuracy and impeding further improvements in thermodynamic efficiency. This study systematically investigates how boundary layer development serves as a key factor in scaling geometrically similar ejectors. The flow characteristics and energy dissipation mechanisms are analyzed through the development of multi-scale thermodynamic models and high-fidelity computational fluid dynamics simulations. The results demonstrate that small-scale ejectors exhibit a relatively thicker boundary layer and higher wall shear stress due to lower Reynolds numbers, resulting in increased frictional losses and reduced isentropic efficiency. Furthermore, under lower Reynolds number conditions, enhanced vortex breakdown and turbulent dissipation contribute to higher entropy generation. In contrast, large-scale ejectors maintain more stable sonic line distributions and superior resistance to adverse pressure gradients, thereby achieving higher critical back pressures and entrainment ratios. An exponential correlation is proposed to correct the entrainment ratio across different scales, significantly improving prediction accuracy. These findings provide novel insights into ejector scale mechanisms and offer a practical framework for optimizing ejector design in advanced energy systems, particularly in applications requiring miniaturization and high thermodynamic perfection.