A 1D model for the unsteady gas dynamics of ejectors

Ejectors are passive mixing devices that might operate in time-varying conditions, e.g., at start-up or during transients. Nevertheless, modeling efforts in literature have mostly been focused on steady conditions. In this work, we propose a novel 1D unsteady model for supersonic single-phase ejectors based on a pipeline analogy with two inlets and one outlet. Conservation of mass, momentum and energy is internally enforced in each domain through 1D gas dynamic equations, and across the domains by a junction model. The characteristics-based formulation of the junction admits a solution in the critical and the sub-critical regime, including backflow, as long as the primary nozzle is choked. Furthermore, the model is not restricted to sonic or supersonic ejectors. To the authors’ knowledge, this formulation is the first to enable 1D ejector simulations with time-varying boundary conditions across operating regimes. The model was calibrated and validated on experimental data in steady state and benchmarked against a 2D URANS simulation featuring a sudden increase in back pressure, triggering traveling pressure waves and a burst of backflow. The results show that the model can correctly predict the ejector performance and the stream-wise evolution of relevant integral quantities in both steady and transient conditions.