The Design of Aspirated Stators for Compressor Bleed

Abstract The operation of gas turbines requires air to be extracted from the primary compressor flow path for a variety of purposes, including turbine cooling and off-design stage matching. This “bleed air” is typically extracted through the compressor casing using either: a continuous circumferential slot; or discrete holes, each with a diameter on the order of a blade pitch. In this paper, the benefits of extracting air through an array of holes on the stator blade suction surface, each with a diameter on the order of one percent of blade chord, are identified. Such blades are referred to as aspirated compressor blades. Using a combined experimental and computational approach, two findings are presented. First, the operating range of the aspirated compressor blade is widened through the control of corner separations. The aspiration reduces the streamline curvature of the surface limiting streamlines, inhibiting corner separation formation. This control is sensitive to the removal of air from the blade surface close to the endwall where the separation is expected to occur, and that robustness is increased by employing more aspiration holes in this region compared to mid-span. This approach is shown to increase the operating range of a stator by 1.8° in experimental measurements, and increase adiabatic efficiency of a three-stage representative industrial gas turbine compressor by 3 percentage points in simulations. The second finding is that the loss of the integrated compressor passage and bleed system can be reduced by extracting air via a combined circumferential bleed slot and aspirated compressor blade approach. This strategy combines the beneficial control of blade separations at high loading achieved by aspiration, with the low extraction loss of the circumferential bleed slot. Both findings are demonstrated in experiments and simulations of a low-speed research compressor and in simulations of a high-speed compressor design representative of a modern industrial gas turbine.