A passive method for enhancing the performance of a symmetric diffuser in turbulent flow regime

The object of the present work is testing the capabilities of a passive method for improving the efficiency of a plane symmetric diffuser. The passive control method consists in introducing small localized flow recirculations. These localized flow separations are obtained modifying the geometry of the diffuser diverging walls using couples of appropriately contoured cavities. The turbulent flow regime is considered. The Reynolds number, based on the diffuser width at the inlet section and the inlet velocity on the axis in a certain section is Re = 20000. The diffuser area ratio in 4.7 and the divergence angle is 5°. The inlet flow is fully-developed turbulent. A preliminary mesh sensitivity analysis for two turbulence models (SST k-ω and RSM) is performed for the flow inside the reference diffuser configuration (without cavities). Based on this analysis the SST k-ω model is chosen for an optimization procedure in order to identify the cavity geometries that maximize the pressure recovery in the diffuser and minimize the main boundary separation extent. Three different configuration have been considered: one, two and three couples of subsequent couples of general contoured cavities. In the optimized diffuser configuration with one couple of contoured cavities there are not small localized flow recirculations, while in the other two optimized diffuser configurations with two and three couples of contoured cavities small localized flow recirculations are present. The passive control increases the performance of the diffuser in all three configurations (the maximum pressure recovery is reached with the optimized diffuser configuration with three couples of cavities), leading to an increase of the mean pressure recovery coefficient by reducing the extent of the main flow separation region. In particular, the success of the passive control is due both to a virtual geometry modification of the flow inside the diffuser and to a favourable effect of the cavities in reducing the momentum losses near the wall. A classical shape optimization with Bézier curve is also carried out in order to mazimixe the pressure recovery. The resulting optimized diffuser shape with Bézier curve does not show small local flow recirculations but produces a diverging wall shape similar to the one optimized with contoured cavities. Since the optimized diffuser with Bézier curve provides a pressure recovery very similar but not equal to the one of the optimized diffuser with three couples of cavities it seems that the "optimum" configuration can be given by the union of a high degrees of freedom Bézier curve and the presence of small localized flow recirculation placed at precise locations