Separation control of turbulent developing boundary layer flow in planar symmetric diffuser

Abstract

This manuscript presents a systematic parametric investigation aimed at identifying the optimal vortex generator (VG) design for a planar symmetric diffuser with opening angles of 12° and 15° and with an area ratio of 4.7, operating under steady, developing turbulent inlet flow conditions. The numerical study correlates the geometric parameters of VG with the performance parameters of the diffuser to meet specific design criteria. Computational fluid dynamics simulations were conducted by solving the Reynolds-averaged Navier–Stokes equations, coupled with the shear stress transport k–ω turbulence model. The local boundary layer thickness around the diffuser inlet, δ/(H/2), ranges from ∼20% to 70%, while the diffuser inlet flow speeds vary from 7.76 to 38.8 m/s. The impact of three-dimensional isotropic scaling of the diffuser geometry on optimal VG design is systematically analyzed and compared with the corresponding unscaled and baseline cases. A careful selection of the geometric parameters for the VG is crucial for preventing boundary layer separation induced by adverse pressure gradients, thereby maximizing diffuser performance and exit flow quality. One important finding is that streamwise mounting position of the VG blade row is independent of location of separation point in the corresponding baseline case, provided that trailing edge of the VG blade row is positioned at a streamwise distance of 0.15H–0.2 H from the diffuser entrance. A symmetrically mounted row of counter-rotating vane-type VGs, with its geometric parameters taking the following optimal values: h/δ ∼ 1.1, g/h = 1, e/h = 8, β = 18°, and xvg/H = 0.15–0.2, is recommended as the best choice for a planar symmetric diffuser exposed to a developing turbulent flow with δ/(H/2) ∼ 0.20%.