Integrated Flow Control Devices for the Design of Enhanced Low Pressure Turbines

Flow separation limits the efficiency of low-pressure turbines (LPTs) in aircraft engines. Experiments with vortex generator jets (VGJs), conducted in AFRL's low-speed cascade at Wright-Patterson AFB, have demonstrated dramatic reductions in separation losses. The critical science that will enable this design innovation to reach its potential is a comprehensive understanding of the effect of VGJs on a separating boundary layer. Experiments are underway at BYU to better understand the basic physics of the separation control phenomenon and establish the quantitative links between the underlying flow physics and LPT performance. During the three years of this study, the following tasks were accomplished. Two-component velocity measurements of VGJ evolution were made in a linear cascade for various flow conditions. Data clearly show the presence of streamwise vortices which provide the necessary boundary layer mixing to inhibit separation. A design criterion for the use of steady VGJs is proposed. The effects of elevated freestream turbulence (from 0.3% to 10%) are also addressed in detail. Finally, 3-component PIV measurements of the flowfield with steady and unsteady VGJs highlight major differences between the two control methods. Understanding gained from these experiments was used to guide the design of a new, high-performance LPT blade at AFRL. The Air Force design codes used to generate the new airfoil included provisions for flow control using vortex generator jets. Experiments with the new profile confirmed the design goal of a 17% increase in blade loading compared to industry standard designs. Phase-locked three-component PIV measurements of the suction surface flowfield with unsteady VGJs document the unsteady response of the separation bubble. Time-resolved flow measurements were taken with pulsed VGJs for 2 blade configurations: the Pack B baseline and a new, high-performance design from AFRL (LIM). At low Reynolds number, the LIM has a reattaching separation while the Pack B does not. Measurements indicate that this feature significantly influences the unsteady interaction of the pulsed jet with the separation bubble. Specifically, the non-reattaching bubble has a longer phase lag to recover its fully separated state after being perturbed by the jet. Experiments are underway to assess the impact of upstream wakes on the proposed control methodology. Full Report Objectives: Flow separation limits the efficiency of modem low-pressure turbines in aircraft gas turbine engines. Experiments with vortex generator jets, conducted in AFRL's cascade at Wright-Patterson AFB, demonstrated reductions in separation losses at low Reynolds numbers [1]. This was followed by demonstrations of VGJ separation