Caitlin Smythe

Experimental and Numerical Investigation of Tip Clearance and Bleed Effects in a Centrifugal Compressor Stage With Pipe Diffuser

The subject of this paper is the experimental and numerical investigation of a state-of-the-art high pressure centrifugal compressor stage with pipe diffuser for a jet engine application. This study shows the impact of impeller tip clearance- and bleed-variation on the centrifugal stage. The purpose of this paper is threefold. In the first place, it investigates the effects on the stage performance. Secondly, it seeks to explain local flow-phenomena, especially in the diffuser. Finally, it shows that steady CFD simulations are capable of predicting these phenomena. Experimental data were gathered using conventional pitot and three-hole-probes as well as particle-image-velocimetry. Numerical simulations with the CFD solver TRACE were conducted to get fundamental insight into the flow. Thus, this study contributes greatly towards understanding the principle of the flow phenomena in the pipe diffuser of a centrifugal compressor.

Numerical and Experimental Comparison of a Tandem and Single Vane Deswirler Used in an Aero Engine Centrifugal Compressor

The present work is part of the research project at the Institute of Jet Propulsion and Turbomachinery at the RWTH Aachen University in collaboration with GE Aviation. The subject is the numerical and experimental analysis of two blading strategies used in the diffusion system of an aero engine centrifugal compressor. The transonic centrifugal compressor investigated contains a close-coupled impeller and passage diffuser, followed by a deswirler system. The deswirler redirects the flow towards the combustion chamber, while decreasing swirl and recovering pressure. It is characterized by a high aerodynamic loading, due to a moderate inlet Mach number of 0.35, in combination with a required flow redirection of 70 deg in circumferential and 135 deg in meridional direction. For this purpose, two different blading strategies are investigated, both retaining the same meridional flow path and integral chord length. The first design is a tandem configuration with 30 vanes in the first row and 60 vanes in the second row. In principal, this approach benefits from the small wetted surface, the short and thereby stable boundary layers as well as the positive blade interaction due to the close alignment. The second design contains one row of 75 vanes. The higher solidity is needed to compensate for the longer boundary layers. The two deswirlers investigated are compared to a less compact baseline deswirler with simple prismatic vanes. Experimental and numerical data shows that both new configurations have very similar stage efficiency. The single row design shows a higher static pressure recovery, resulting in a +0.2%-points total-to-static isentropic efficiency increase compared to the tandem design. Detailed flow analysis in the deswirler system shows different characteristics in terms of losses, loss mechanisms and pressure build-up. Due to the required high turning, both designs suffer from flow separation. Nevertheless, the single row design shows its robustness under the impact of 3D flow, whereas the tandem suffers from end wall induced losses. The results show that the classical mechanisms making a tandem favorable for high flow turning in 2D flow are counteracted by 3D flow mechanisms caused by the spanwise pressure gradient. The low aspect ratio even increases the effect of 3D end wall mechanisms. These results, combined with a higher manufacturing effort, show that a tandem configuration is not necessarily the superior design for highly 3D flow conditions.

Detailed Performance Analysis of a Centrifugal Compressor Stage With Pipe Diffuser and Immerged Tandem Deswirler

Commonly, the pipe diffuser and deswirler are aerodynamically decoupled from each other to ensure better mixing of the inhomogeneous pipe flow before entering the deswirler and to prevent upstream effects of the deswirler bend and blading on the pipe diffuser. Close-coupling of these two components, however, can lead to a significantly lower radial extent of the stage, resulting in a decrease of stage weight and engine frontal area. The geometry investigated includes a centrifugal compressor stage with a compact diffusion system, including a pipe diffuser and tandem deswirler whose first blade row is immerged into the pipe diffuser. Stage and component performance of this compact configuration is compared to two different stage configurations with decoupled deswirlers. Performance maps and 1-D pressure build-up data show that the compact stage is able to reach efficiency and pressure build-up of the compared stages which contain the same impeller and same diffuser inlet geometry. The performance of one of the compared stages with significantly higher radial stage extent is even exceeded. However, a loss in surge margin of about 2% has been detected which seems to be the major problem in reducing stage size by close-coupling the deswirl system to the pipe diffuser, since this can have a major effect on the diffuser inlet pressure build-up.Copyright

Numerical Investigation of the Unsteady Interaction Within a Close-Coupled Centrifugal Compressor Used in an Aero Engine

The present work forms part of a research project of the Institute of Jet Propulsion and Turbomachinery at the RWTH Aachen University in collaboration with GE Aviation. The subject is the detailed numerical analysis of the unsteady flow field focusing on the interaction between the impeller and the passage diffuser of a close-coupled transsonic centrifugal compressor used in an aero engine.The centrifugal compressor investigated is characterized by a close-coupled impeller and passage diffuser with a radial gap of only 3.6%. The close coupling tends to provide a high aerodynamic efficiency but simultaneously cause a high unsteady interaction between the impeller and the diffuser. These unsteady effects can have a significant impact on the performance of both components [1,2] and present a challenge to state-of-the-art numerical methods. With increasing compressor efficiency, the more important it is to have a understanding of the detailed unsteady flow physics. Experimental data was obtained from a state-of-the art centrifugal compressor test rig located at the Institute of Jet Propulsion [3]. Steady and unsteady pressure measurements within the impeller and diffuser are used to gain detailed information on the temporal, time-averaged and spectral pressure distributions within the stage to validate the CFD.The work presented shows the unsteady phenomena caused by the interaction as well as the location and propagation of these phenomena within the centrifugal stage. Within the impeller, the exducer is in first order excited by the BPF of the diffuser, whereas in the diffuser both the BPF, as well as the PPF, are present up until the end of the pipe-diffuser. Significant effects on the integral component performance could only be identified for the impeller. Special focus is paid to evaluate the diffuser upstream pressure field, since this is the major source of unsteadiness within the impeller. The performance of the rotor decreases due to the unsteady interaction. This effect is traced back to the unsteady tip-clearance flow, in which the time-averaged mass transport decreases, whereas the specific entropy production increases in a nonlinear way. Within the diffuser, local effects counteracting with respect to the integral performance are found. In front of the throat, there is less decay in total pressure, as a result of tangentially expanding pressure waves. Within the passage an decrease in flow uniformity in the unsteady flow is identified as the reason for the lower diffusion up until the throat and higher losses within the downstream diffuser passage.Copyright