Description
The effect of an array of jet injectors on the flow over a nozzle blade of a power turbine is investigated computationally and parametrically by varying the chord-wise array location and the spanwise distribution. A jet configuration that optimally reduces the mass flow rate and has minimum aerodynamic losses at part load operating conditions is determined. Steady-state nozzle flows are simulated with ANSYS CFX-Pre using the k ! turbulence model. Meshes are generated with ANSYS TurboGrid. Injector jet flows are modeled through source points which avoids the modeling of the local jet geometry and hence saves computational cost. Based on the two dimensional blade analysis reported in [Soto, M.S. Thesis, SDSU, 2020], four chordwise jet locations are considered, three on the suction side at thirty-five, forty-four, and fifty-three percent axial chord length and one near the trailing edge on the pressure side at eighty nine percent axial chord. The jet array is placed along the span ranging from the hub to the shroud. The effect of the array configuration is systematically investigated by perturbing the suction side array with harmonic modes of varying frequencies and amplitudes. The wall-normal jet flow on the suction side causes flow separation. Depending on the jet location, the negative streamwise pressure gradient in the power turbine yields a reattachment further downstream. Between the flow separation and reattachment location a separation bubble forms that displaces the outer potential flow and so increases the effective camber of the blade. The separation bubble also reduces the effective throat area in the nozzle, which leads to a reduced mass flow rate. With a more downstream location of the jet array, the separation bubbles are larger, which results in a higher mass flow reduction, but also in increased aerodynamic losses. The pressure side jet acts like a Gurney flap by changing the angle of the trailing edge shear layer, the so-called Kutta condition, which reduces the effective throat area and increases the flow turning. The separation bubble length at the midspan location is larger than it is near the hub or shroud for an unperturbed jet injector array configuration. The passage vortices re-energize the flow at the end-wall and so promote an earlier flow reattachment. A single mode perturbation of the array which yields a more upstream placement of the injector at the midspan reduces the midspan bubble length and enhances uniformity in the spanwise coordinate. Jet arrays with perturbations of higher frequency act like virtual vortex generators. that also re-energize the flow and lead to an earlier reattachment.
