<p dir="ltr">Investigating clearance flows and blockage generation in axial compressors represents a longstanding area of research for enhancing aerodynamic performance and operational stability in turbomachinery. With advancements in computational fluid dynamics (CFD), opportunities to explore these phenomena have expanded, allowing a deeper understanding of the turbomachine's inherently complex and highly unsteady flow fields. This work delves into these topics, focusing on the Purdue 3-Stage (P3S) compressor, an engine-representative, multistage, high-speed compressor.</p><p dir="ltr">The primary objective of this research is to compare the performance and stability characteristics of two distinct stator configurations: a shrouded baseline configuration and a cantilevered stator configuration. This comparison reveals the impacts of clearance flows and blockage generation on compressor operation. Through a series of experimental investigations, this study aims to identify the differences in performance and stability traits between these configurations and the flow structures responsible.</p><p dir="ltr">Experimental characterization has a central role in this study, involving the analysis of leakage flow structures, corner separations, wake structures, and resulting endwall blockage generation. This research seeks to provide detailed insights into the flow phenomena within the compressor by utilizing detailed measurement techniques, such as circumferential interrogation of the flow field using 7-element Kiel-head rakes. Pressure deficits associated with leakage flows, corner separations, and wakes are quantified to assess their impact on compressor performance.</p><p dir="ltr">In conjunction with experimental investigations, this work outlines the development and validation of the supporting high-fidelity CFD models. These models, employing scale-adaptive turbulence model simulations, aim to simulate the flow field within the compressor with accuracy and reliability. Validation of these models against experimental data ensures their fidelity in capturing the complex flow phenomena observed experimentally. Furthermore, a detailed exploration of convergence aspects, including iterative convergence, grid convergence, and periodic-unsteady signals, lays the foundations for building confidence in the model predictions.</p><p dir="ltr">The computational models complement experimental findings, allowing for a comprehensive flow field analysis focusing on endwall flow structures. Visualization of vortex core and three-dimensional blockage regions provides valuable insights into the flow physics governing compressor performance. Moreover, the comparative nature of computational simulations facilitates systematic exploration of geometric changes and their effects on compressor operation. This study leverages complementary methodologies of experimental measurements and high-fidelity computational models to advance the understanding of clearance flows and blockage generation in axial compressors.</p><p dir="ltr">The experimental analysis concludes that the cantilevered configuration achieves better performance and stability than the shrouded stator configuration. However, this conclusion is not apparent when the machine is considered holistically. The cantilevered stages show significant performance improvements, with increases in total pressure ratio of up to 1% and an increase in isentropic efficiency of as much as 2%. However, the common Stage 3 shrouded Stator 3 shows a corresponding deficit of as much as 2% loss in efficiency relative to the fully shrouded stator configuration baseline. These contrasting benefits in the cantilevered stator compressor show that Stage 3 seems to cancel the overall benefits gained by the cantilevered stator. Similar studies have been done on low-speed multistage compressors, but this shows the value of the study in a high-speed research compressor with appreciable stagewise temperature and density increase.</p>

Unsteady three-dimensional flow phenomena have major effects on the aerodynamic performance of, and heat transfer to, gas-turbine blading. Investigation of the mechanisms associated with these phenomena requires an experimental facility that is capable of simulating a gas turbine, but at lower levels of temperature and pressure to allow conventional measurement techniques. This thesis reports on the design, development and commissioning of a new experimental facility that models these unsteady three-dimensional flow phenomena. The new facility, which consists of a 62%-size, high-pressure gas-turbine stage mounted in a transient wind tunnel, simulates the turbine design point of a full-stage turbine. The thesis describes the aerodynamic and mechanical design of the new facility, a rigorous stress analysis of the facility’s rotating system and the three-stage commissioning of the facility. The thesis concludes with an assessment of the turbine stage performance.

<p dir="ltr">This research presents a culmination of work into uncovering the underlying fluid dynamic behaviors of supercritical CO2 as it relates to high pressure turbine design using a combined fundamental and practical numerical and experimental analysis. The fundamental analysis of the thermo-fluid dynamic properties of supercritical CO2 boundary layers and separation is analyzed against the air counterparts for non-dimensional quantities of interest – pressure ratio, Mach number, Reynolds number – and combinations of these quantities. The coupling of density derivatives with pressure and temperature are investigated within the operating conditions of the first stage turbine of a supercritical CO2 oxyfuel power cycle. Armed with the information garnered from this analysis, a 3D optimization is run using computational fluid dynamics to investigated nearly 3000 unique blade shapes, focusing on increasing the isothermal corrected efficiency and decreasing the heat load to the blade. Three different families of blade shapes are identified from the analysis and their aerodynamic qualities discussed. A single advanced blade design is chosen for in depth analysis and experimental testing against the baseline blade from which the optimization was started. Mechanical design for the experimental campaign in the Big Rig for Aerothermal Stationary Turbine Analysis (BRASTA) is presented for a novel sector-based off-axis design and the results of the aerothermal measurements discussed. In tandem with blade design and analysis, the Tip Gap Experimental Research Article for Large Scale Injection Layouts (Tiger Lily), a canonical model for the large-scale investigation of tip flows in high Reynolds number flows, is developed and the mechanical and aerodynamic design discussed. Aerothermal analysis for different tip coolant injection configurations is performed using Improved Delayed Detached Eddy Simulation (IDDES) computational fluid dynamics analysis to resolve turbulent structures resulting from coolant injection and over tip flow interaction. Experimental investigation of Tiger Lily is presented, validating the structures and features seen in the numerical analysis. The conclusion of these investigations results in the increased understanding of the underlying fluid dynamic behaviors of supercritical CO2 in high pressure turbines.</p>

<p>The objective of this research was to develop stability enhancing design features for aeroengine centrifugal compressors. The motivation for this research is based on climate change and fuel-efficiency concerns, which call for improvements in achievable pressure ratios and surge margins. Specifically, this research aimed to develop diffuser recirculation channels and provide more insight into their design space. These channels are passive casing treatments in the diffuser and have been successfully demonstrated to improve stage surge margin. Diffuser recirculation channels are secondary flow paths that connect an opening near the diffuser inlet to one further down in the passage. Flow is recirculated by relieving the static pressure differential between the two openings. The basic design concept of these features is to add blockage upstream of the diffuser inlet, reducing the amount of diffusion in the vaneless space. In addition, channel geometries can be optimized to specifically target adverse flow properties, such as high incidence on the diffuser vane leading edge.</p> <p><br></p> <p>This design development was purely computational and served as the first approach to implementation of these features in a future generation of the Centrifugal Stage for Aerodynamic Research (CSTAR) at the Purdue Compressor Research Lab. Design development consisted of a computational design study, which quantified the effects of changing diffuser recirculation channel geometries on stage stability and performance metrics. Moreover, the CFD model for this future configuration of CSTAR was created and served as the baseline comparison for design iterations. The design study was comprised of controlled variation of channel geometry parameters and iterative solving of those cases in unsteady full stage single passage CFD models. Further design optimization studies were completed on specific down-selected recirculation channel geometry configurations. In total, 16 unsteady CFD cases with varied geometry configurations and 43 steady models were solved. Once a final optimized design was confirmed, a pressure characteristic at 100 % corrected design speed was generated. Compared to the baseline speed line, the implementation of diffuser recirculation channels resulted in a more gradual numerical surge and apparent numerical surge margin enhancement. Furthermore, the variation in incidence at the diffuser vane leading edge near the shroud was significantly reduced with diffuser recirculation. For the baseline compressor, incidence grew by about 70 degrees from the design aerodynamic loading to numerical surge at that location. However, flow stabilization due to diffuser 16 recirculation resulted in a change of approximately 2 degrees through that range. In conclusion, a first approach design recommendation for diffuser recirculation channels is CSTAR was generated through computational studies. Using this recommendation, diffusers with this recirculation channel design can be manufactured and tested for experimental concept validation.  </p>