Numerical Investigation of Mixing Tabs in Multi-Ring Entraining Diffuser in Air-Air Ejectors

Abstract

This research studied the effect of mixing tabs used in entraining diffusers in subsonic short air-air ejectors. Although numerous studies exist on mixing tabs in nozzles, none was found that placed tabs in an entraining diffuser system used with an ejector. Given that mixing tabs led to improvements at the nozzle, they were analyzed in entraining diffusers for this research. Numerical studies of tab performance were carried out using two-equation κ-ɛ turbulence models with enhanced wall treatment available in ANSYS Fluent on an ejector obtained from a prior thesis [1]. Only triangular tabs were considered on the ejector. Tab number was varied with 4, 8 and 12 tabs. Increasing tab number from 4 to 8 led to increased diffuser pumping while increasing to 12 tabs led to marginal decrease, although higher than the base case. Addition of 4 tabs led to unchanged overall pumping, while 8 and 12 tabs led to reduced overall ejector pumping performance. Tab blockage ratio was also varied at 1%, 2% and 4% blockage per tab. Increasing blockage ratios was associated with increasing diffuser pumping, but worsened pumping at standoff, however, total pumping was unchanged. A location study identified that the placement of tabs just before the diffuser is not as effective as tabs placed at the nozzle exit for similar blockage ratios. Additionally, tabs placed in the primary flow region were associated with higher back pressure penalties than those in the secondary flow region.