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Research Papers

Loss Mechanisms and Flow Control for Improved Efficiency of a Centrifugal Compressor at High Inlet Prewhirl

[+] Author and Article Information
Xinqian Zheng

State Key Laboratory of Automotive Safety
and Energy,
Tsinghua University,
Beijing 100084, China
e-mail: zhengxq@tsinghua.edu.cn

Qiangqiang Huang, Anxiong Liu

State Key Laboratory of Automotive Safety
and Energy,
Tsinghua University,
Beijing 100084, China

1Corresponding author.

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received November 27, 2015; final manuscript received February 27, 2016; published online May 3, 2016. Editor: Kenneth C. Hall.

J. Turbomach 138(10), 101011 (May 03, 2016) (11 pages) Paper No: TURBO-15-1279; doi: 10.1115/1.4033216 History: Received November 27, 2015; Revised February 27, 2016

Variable inlet prewhirl is an effective way to suppress compressor flow instability. Compressors usually employ a high degree of positive inlet prewhirl to shift the surge line in the performance map to a lower mass flow region. However, the efficiency of a compressor at high inlet prewhirl is far lower than that at zero or low prewhirl. This paper investigates the performances of a centrifugal compressor with different prewhirls, discusses the mechanisms which are responsible for the production of extra loss induced by high inlet prewhirl and develops flow control methods to improve efficiency at high inlet prewhirl. The approach combines steady three-dimensional Reynolds average Navier–Stokes (RANS) simulations with theoretical analysis and modeling. In order to make the study universal to various applications with inlet prewhirl, the inlet prewhirl was imposed by modifying the velocity direction of inlet boundary condition. Simulation results show that the peak efficiency at high inlet prewhirl is reduced by over 7.6% points compared with that at zero prewhirl. The extra loss occurs upstream and downstream of the impeller. Severe flow separation, which reduces efficiency by 2.3% points, was found near the inlet hub. High inlet prewhirl works like a centrifuge gathering low-kinetic-energy fluid to hub, which induces the separation. A dimensionless parameter C was defined to measure the centrifugal trend of gas and indicate the flow separation near the inlet hub. As for the extra loss which is produced downstream of the impeller, the flow mismatch of impeller and diffuser at high prewhirl causes a violent backflow near the diffuser vanes' leading edges. An analytical model was built to predict diffuser choking mass flow. It proves that the diffuser has already operated unstably at high prewhirl. Based on these two loss mechanisms, the hub curve and the diffuser stager angle were modified and adjusted, respectively, for higher efficiency at high prewhirl. The efficiency improvement benefited from the modification of the hub is 1.1% points, and that benefited from the combined optimization is 2.4% points. During optimizing, constant distribution of inlet prewhirl was found to be another factor for inducing reverse flow at the leading edge of the impeller blade root, which turned out being blamed on the misalignment of the swirl angle and the blade angle.

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Figures

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Fig. 1

Comparisons between Krain's experiments and the simulation results by the numerical methods of this paper: (a) relative Mach number in front of the impeller (“station-1” in Ref. [17]) and (b) streamwise component of relative velocity near the trailing edge (“station 10”in Ref. [17])

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Fig. 2

Model and mesh: (a) compressor model and meridional view and (b) mesh for simulations

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Fig. 3

Performance and velocity diagram: (a) adiabatic efficiency of the compressor with different degrees of prewhirl at maximum speed and (b) velocity diagram at impeller inlet

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Fig. 10

Meridian view of part of the flow passage

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Fig. 11

Efficiency performances of compressor with modified hub curve at 0 deg and 60 deg prewhirl

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Fig. 12

Comparisons of total pressure, swirl angle, and the parameter C between prototype and the modified design: (a) pitch-averaged total pressure distributions along the inlet hub, (b) pitch-averaged swirl angle distributions at station L, and (c) pitch-averaged parameter C distributions at the inlet

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Fig. 9

Predicted choking flow with respect to prewhirl at maximum speed

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Fig. 6

Flow separation in the extended inlet passage: (a) vectors of absolute velocity projection and (b) parameter distributions along radius at the inlet

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Fig. 5

Effects of separation on pressure loss and inlet distortion: (a) total pressure distribution at three paths and (b) distribution of incidence along spanwise at station L (points with arrows are local extremes)

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Fig. 4

Distribution of standard deviation of entropy and absolute velocity projection: (a) computing set including 0 deg, 20 deg, and 40 deg prewhirl and vectors of zero prewhirl, and (b) computing set including all four kinds of prewhirl and vectors of 60 deg prewhirl

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Fig. 8

Components performance at prewhirl of 0 deg and 60 deg

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Fig. 7

Projection of absolute velocity lines on 10% span surface (near hub) of diffuser at peak efficiency point of 60 deg prewhirl

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Fig. 13

Relative velocity projection in meridian view, separation vortex in front of leading edge and velocity diagrams

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Fig. 14

Close diffuser by 5 deg to reduce its throat area

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Fig. 15

Performance of whole compressor and components with modification and adjustment

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Fig. 16

Meridian view of compressor with modification and adjustment and three-dimensional view of velocity lines projection in adjusted diffuser

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