• EFFICIENCY OF PLANE LAMINAR DIFFUSER FLOWS

The object of the present work is the improvement of the efficiency of symmetrical plane diffusers by means of localized separations along their diverging walls. The laminar flow regime is considered. The Reynolds number, based on the diffuser half-width at the inlet section and the inlet velocity on the axis, is Re = 500. Three different diffuser configurations are considered, which are characterized by the same area ratio (AR=2), and by different expansion angles, viz. alpha=2°, alpha=3.5° and alpha=5°. The variation of the expansion angle produces different flow patterns. In particular, in the diffuser with alpha=2° the flow is attached along the diffuser diverging walls, while the diffusers with alpha=3.5° and alpha=5° are characterized by asymmetric zones of separated flow of different extension, which anyway both reattach before the end of the diffuser. The localized separations are obtained, first, by introducing one or more subsequent couples of contoured cavities in the diverging walls. Cavity-shape optimizations are carried out to identify the cavity geometries that maximize the pressure recovery in the diffuser and minimize, whether present, the main boundary layer separation extent. The introduction of the optimal cavities leads to increase in pressure recovery for all the considered diffuser geometry, even in the configuration that, without the introduction of the flow control, does not present flow separation. This result is due to a favourable modification of the velocity and vorticity fields in the near-wall region. In particular, the success of the control is due both to a virtual geometry modification of the diffuser and to a favourable effect of the cavities in reducing the momentum losses near the wall. The capabilities of classical shape optimization are also investigated. The shape of the diverging walls is optimized by using Bézier curves with different degrees of freedom to maximize the pressure recovery, as previously for the cavities. If the optimization is carried out with an adequate number of degrees of freedom, the optimal configuration is caracterized by one or more localized recirculation regions along the diffuser optimized diverging walls, i.e. by a flow configuration very similar to that in the diffuser with the optimized cavities. The efficiency gains obtained by shape optimization are similar to those given by the cavities.