The purpose of the current work is to study the transient behavior of axial flow turbomachines in transonic environments. The first part is focused on the aerodynamic behavior of a coupled boundary layer ingesting inlet – distortion tolerant fan for use in next generation aircraft. The second part is focused on stall detection in transonic compressors. The reason these two are covered is that the fan and compressor of this future system would be mechanically linked. The fan behaves dynamically in the presence of a distortion and exhibits non-uniform flow downstream of the fan. An engine core, comprised of compressor stages, would have to operate in that distorted flow possibly inducing stall. Therefore, understanding the behavior of the fan and developing advanced stall detection methods are important for the successful implementation of these future systems.
Part I – Aerodynamic Response of a Distortion Tolerant Fan Coupled to a Boundary Layer Ingesting Inlet
Future aircraft designs are aimed at three main targets for more sustainable flight practices decreasing noise, emissions, and fuel burn. Boundary layer ingestion (BLI), as a part of aircraft’s propulsion, is an innovative means of achieving improvements to all three targets. The BLI design incorporates engines integrated into the body of an aircraft. The engine ingests low momentum boundary layer flow that develops over the aircraft’s surface when in operation. The advantages of such a system are reductions to weight, drag, noise, and increased propulsive efficiency over standard aircraft in service today. Experimental tests representing a blended wing body propulsor utilizing BLI were performed in NASA’s 8ft by 6ft wind tunnel. These tests were aimed at obtaining the physically realizable benefits achievable for a boundary layer ingesting propulsor. The current work represents an effort to compare a simulated coupled boundary layer ingesting inlet and distortion tolerant fan with the experimental measurements. This work also seeks to characterize the aerodynamic behavior of a BLI system in order to provide insights for future designs. It is found that the ingested boundary layer creates a distortion, which effects the incidence angle on the blades. The non-uniform incidence angle, and the flow characteristics it induces around a blade, depends on radial location. For the higher spanwise sections, >50% span, incidence angle is dominated by axial velocity. The incidence angle for lower spanwise sections is most effected by the swirl velocity induced by the distortion. The non-uniform incidence angle, induced by the distortion, results in non-uniform work on the flow and thereby the total pressure rise across the fan. However, at lower mass flow rates, the changes in incidence angle do not necessarily correlate proportionally to changes in the total pressure rise. The changes to incidence angle induce significant behavioral changes in the flow field of a blade passage. This has an effect on blade loading and shock losses, inducing the observed non-linear changes to total pressure rise. The variable total pressure rise is associated directly with non-uniform work by the fan. The non-uniform work also affects the profile of the ingested distortion making the downstream profile less uniform. However, the work reduces the magnitude of the pressure deficit and creates a region of higher than average pressure and velocity. This region would be beneficial if ingested by an engine core as it would produce a stabilizing effect. The blade tip region appears to be the critical factor in stall for the stability of the fan. The tip flow and dynamic stalling behavior is similar to a rotating stall cell, but its inception is fixed spatially to the counter-rotating region of the distortion. Blade tip stall is incited by the breakdown of the tip clearance vortex and strengthened by the tip clearance flow, inducing a blockage of significant size in the passages. As a blade moves out of the distortion region, the blockage decreases in size and is eventually expelled out of the blade passage. The circumferential position that the stall cell forms, and is eventually fully expelled from a blade passage, is a function of the operating condition.
Part II – Rotating Stall Inception in Transonic Axial Compressor Stage using Statistical Anomaly Analysis
Tip clearance flow in transonic rotors is known to have a significant effect on compressor performance and stability. When operating near stall, instabilities can grow and form into passage blockages that propagate around the rotor, referred to as rotating stall. Usually a small number of rotor passages are experiencing stall at any given time during rotating stall. However, the rotating stall can evolve into full stall depending on the operating condition. Determining the inception point at which rotating stall begins is difficult as its formation occurs over multiple revolutions. The purpose of this study is to employ a statistical anomaly analysis method, as a stall precursor detector, and investigate the flow physics underlying stall inception in a high speed transonic compressor. Allowing the natural evolution of stall is crucial, therefore, a full annulus simulation of a transonic axial compressor stage (NASA Stage 35) is performed. Due to the size of the data set, ~25 TB, a statistical analysis was chosen to rapidly analyze the entire spatial and temporal simulation domain. The statistical method utilized entropy in a Grubb’s test on the domain’s grid to reveal regions, times, and trends that are statistically anomalous that need to be investigated. Through use of the anomaly detecting method, rotating stall could be tracked back in time, 18 revolution, at an operating condition very close to the predicted stalling point. It is discovered that oscillatory behavior in the tip clearance vortex between pairs of rotor passages eventually lead to rotating stall. The oscillatory behavior is the result of a spiral-type vortex breakdown of the tip clearance vortex. The spiral-type breakdown in induced after interacting with the passage shock that dominated the pre-stall regime of NASA’s experimental compressor, stage 35. The anomaly analysis also uncovered that a rotating disturbance region, moving faster than the rotor speed, interacted and amplified the breakdown of the tip clearance vortex. The spiral-type vortex breakdown caused a flow blockage that grew in size in both the radial and circumferential directions. Eventually, the disturbance region/vortex breakdown induced rotating stall through decreasing mass flow rate, and thereby increasing blade loading. This work shows the efficacy of a statistical Grubbs’ test using entropy is a method that can detect the earliest signs of rotating stall.