Control of Separation in Turbine Boundary Layers

Abstract

The opportunity for the reattachment and control of separated flows occurs in inlets, compressors, transition ducts and turbines. Passive and active control of separated flows has been demonstrated successfully by a number of techniques which employ the introduction of longitudinal or streamwise vortices. The role of these vortices is initially to reenergize the wall boundary layer flow by entraining and redistributing momentum from the primary flow to the wall layer and enhance early transition. A chain of non-linear interactions of these unsteady vortices with large scale unsteady separation vortices and the shed shear layer results in significant alteration of the circulation. The resulting increased circulation allows higher blade loadings, reduced part count, as well as increased performance at low Reynolds numbers. Flow control location has been investigated at chord locations ahead, at, and after separation. Passive dimples with single and multiple rows, varied dimple location and dimple shape have been investigated. Initial investigations of a single row of dimples and their wakes on a high pressure turbine vane ring have been performed at Reynolds numbers down to 13,500 in a full scale matched parameter rig. Properly placed dimples reattach separated flows at all Reynolds numbers investigated. Computations for the dimple geometries with VBI, MISES, and Fluent have been carried out to determine initial separation, compressible implications, reattachment locations, and predicted wake profiles or loss coefficients. Steady and pulsed vortex generator jets with duty cycles down to 1%, have both demonstrated reattachment and reduction of total losses in excess of 40% at Reynolds numbers down to 25,000, without incurring significant additional losses at higher Reynolds numbers. Pulsed vortex generator jets with a duty cycle of 1% have demonstrated blowing coefficients of <10 -4 .