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Research Assistant, Purdue University

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Various numerical methods. looking into prediction and importance of unsteady aerodynamic effects.

A centrifugal compressor consists of two primary parts: a rotating component (the impeller) and a stationary component (the diffuser). The impeller rotates at a high speed to impart energy into the airflow, and the diffuser acts to slow that airflow down and turn that kinetic energy into an increase in the pressure. The diffuser used in this study was a “vaned diffuser,” which means that it contains many slender metal vanes that act to obtain this pressure increase efficiently. These devices are similar to pumps (which work with liquids), but work with gases! Computer simulations are used to predict the performance of these devices, however some difficulties arise because of the fact that the impeller is rotating and the diffuser is static. Typically, a steady simulation is run in which the flow leaving the impeller is averaged around the perimeter, using what is called a “mixing-plane,” before it is "passed" to the diffuser. A full unsteady run takes very short snapshots in time of the impeller rotation and fully models how the flow will move in both space and time as the impeller rotates. These simulations allow for the unsteady interaction between the impeller and diffuser; however are much more "expensive" computationally, taking roughly 50 times longer to run and requiring more computer memory.

Studies looking at the drawbacks of the mixing-plane method have generally showed that, in most cases, the prediction of the overall performance of the compressor is minimally affected by using the simplified simulation. Because of this and the much faster nature, steady simulations have become the primary tool of designers.

For this study, steady and unsteady simulations of a centrifugal compressor for use in a jet engine were conducted. The results were compared and the prediction of the overall performance parameters (efficiency and pressure increase) were largely unchanged. However, a closer look at the internal aerodynamics, specifically in the diffuser, showed some significant differences. These differences arose from the fact that the unsteady flow leaving the impeller oscillated in direction over a range of approximately 20 degrees, which changed the aerodynamic behavior of the diffuser vanes. The big conclusion was that although the different simulations did not yield different overall performance predictions, the unsteady simulation yields information, which could be useful to a designer in obtaining a more efficient machine.

13 ITEMS PINNED

Optimum design of diffuser in a small high-speed centrifugal fan using CFD & DOE

Abstract: Small fans with powerful performance are being developed recently, reflecting the trends of the times in which electrical home appliances are becoming smaller and smaller. In order to develop high-performance, high-efficiency fans, an analysis of the effects of design parameters and an optimum design process are essential. This study was conducted to analyze the effects of design parameters of the diffuser in a small, high-speed centrifugal fan, and to derive an optimum model based on the results. Six design parameters (independent variables) were considered for this study: the number of Guide vanes (GVs), the meridional plane length of the GV(rear), the crosssectional area of the Leading edge (LE) in the GV(rear), the beta angle of the Trailing edge (TE) in the GV(rear), the maximum thickness of the airfoil in the GV(rear), and the maximum thickness position of the airfoil in the GV(rear). In addition, the dependent variables were fan performance (vacuum and fan efficiency), and the results were converted to dimensionless values. For screening design, the 26-1 fractional factorial design method was used. To check the existence of the curvature effect, the center point was added. For optimum design, the central composite design method of the Response surface methodology (RSM) was used for two design variables. P-value and T-value were used to determine whether each compounded factor was appropriate for the analysis object of the design of experiments. The results of the screening design were expressed by Pareto chart and main effects plot, and the results of the optimum design by surface plot, overlaid contour plot, and Response optimization. The reliability of the Computational fluid dynamics (CFD) was verified through a comparison between the experiment results and CFD results of the optimum model. As a result of the screening design, the design parameter that had the greatest influence on fan performance was the beta angle of the TE in the GV(rear), followed by the number of the GV(rear) and the maximum thickness of airfoil in the GV(rear). It was judged that the vacuum increase was determined by the beta angle of the TE in the GV(rear), and that the main cause of the vacuum decrease was the increase of pressure loss due to the decreasing cross-sectional area between the GVs and the generation of a vortex at the hub of TE in the GV(rear).

Pub.: 12 Mar '16, Pinned: 16 Aug '17

Large Eddy Simulation for Surge Inception and Active Surge Control in a High Speed Centrifugal Compressor with Vaned Diffuser

Abstract: This paper presents a numerical study for active surge control and the performance improvement for an aero engine centrifugal compressor NASA CC3, using self-recirculating bleed slots "SRBS" with different bleeding positions. The investigation has been carried out using unsteady three dimensional numerical simulation based on Large eddy simulation. Three bleeding slots positions have been studied and compared with the compressor with no surge control. The purpose of the recirculating bleed slots is to remove some of the reversed flow during the surge inception from the impeller inducer, which would influence the stable operating-rang and compressor pressure ratio. The impact of three bleeding slots positions on the internal flow and performance of the compressor was highlighted through a detailed analysis of the impeller flow field. The surge event stages are well detected inside the impeller and diffuser. The results revealed that an effective flow bleeding system can increase the surge limit when compared to the classical compressor without bleeding system. The comparison study between the studied cases presented in this paper showed that the surge limit increased by 8% for bleeding slots position closer to the main blade leading edge, while lower increase in the surge limit for slots nearby the splittered blade leading edge.

Pub.: 28 Jul '16, Pinned: 16 Aug '17

Experimental investigation of surge and stall in a turbocharger centrifugal compressor with a vaned diffuser

Abstract: The stable flow range of a compressor is dominantly limited by flow instability, known as surge and stall. In this paper, surge and stall in a turbocharger centrifugal compressor with a vaned diffuser are investigated by experiments. The transitional process from stable to unstable conditions is quite different at different rotating speeds. With decreasing mass flow rate, the compressor successively experiences stable conditions and deep surge at low speed, while the compressor experiences stable conditions, mild surge and deep surge at high speed. During experiments, deep surge occurs at all tested rotating speeds with different characteristics and behavior, and a deep surge cycle includes three periods, named recovery period, oscillation period and breakdown period. The dynamic experimental signal shows that pressure behavior in the oscillation period differ at different rotating speeds: high frequency fluctuation occurs at low speed, while mild surge appears at high speed. Mild surge occurs independently or appears within a deep surge cycle with a frequency close to the Helmholtz resonant frequency of the compression system. The behavior of mild surge at the vaned diffuser inlet is different along the circumferential direction, which is a subversion of the conventional view that surge is an axisymmetric phenomenon. It is highly possible that the non-axisymmetric behavior of mild surge is induced by the volute. Because the volute affects the flow field structure significantly, a new volute design method considering the circumferential non-uniformity of the flow field may be established and developed to improve the compressor stability.

Pub.: 29 Nov '16, Pinned: 16 Aug '17

Effect of a volute on the unsteady flow in the vane diffuser of a centrifugal compressor

Abstract: A volute is the only circumferential asymmetric component in a centrifugal compressor, and thus, it should account for the circumferential asymmetry of the flow in a vane diffuser. This study performs a transient numerical analysis to investigate the effect of a volute on the flow in the vane diffuser of a centrifugal compressor under three operating conditions (near-stall, middle, and high mass flow). We compare numerical and experimental performance of the compressor, including polytropic efficiency, total pressure ratio, and unsteady pressure on a diffuser vane. The numerical scheme is proven valid owing to the fact that the numerical and experimental results considerably agree well with each other. Under middle and high mass flow conditions, the time-averaged static pressure recovery and the total pressure loss coefficients for all the diffuser passages indicate that the performance of the passages near and upstream of the volute tongue is affected negatively by the volute, whereas that of the passages downstream of the volute tongue is less affected. Under near-stall condition, the performance of all the passages is disturbed, and the diffuser passage marked as DP 3 demonstrates the worst performance. Investigation on the time-averaged aerodynamic forces, loading, and pressure on the vanes yields results that are consistent with those of the investigation on the performance of the passages. The harmonics with 0.5fb and fb , which are included in the unsteady loading and pressure on the pressure and suction sides of the vanes, are dominant, where fb is the impeller main and splitter blades passing frequency. Their amplitude values increase as mass flow deviates from the middle mass flow condition. Under middle and high mass flow conditions, the harmonic with 0.5fb is affected more negatively because of the larger amplitude on the vanes near and upstream of the volute tongue than those downstream, whereas the harmonic with fb is less affected by the volute. Under the near-stall condition, the transient vorticity fields along with the harmonics of 0.5fb and fb are investigated to evaluate the performance of the diffuser passages. DP 3, which is located at approximately 90° downstream of the volute tongue, suffers the strongest flow deterioration and is inferred to stall first. Further researches for designing more matching diffuser/volute combination will be performed by referring this study.

Pub.: 01 Dec '16, Pinned: 16 Aug '17