Research Papers

The Matching of a Vaned Diffuser With a Radial Compressor Impeller and Its Effect on the Stage Performance

[+] Author and Article Information
Michael Casey

PCA Engineers Limited,
Lincoln LN5 7SB, UK
Institute of Thermal Turbomachinery (ITSM),
University of Stuttgart,
Stuttgart D-70569, Germany
e-mail: Mick.Casey@pcaeng.co.uk

Daniel Rusch

Compressor Development (Dept. ZTE),
ABB Turbo Systems Ltd.,
Bruggerstrasse 71a,
Baden CH-5401, Switzerland
e-mail: Daniel.Rusch@ch.abb.com

Contributed by the International Gas Turbine Institute (IGTI) Division of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received July 10, 2014; final manuscript received July 15, 2014; published online August 26, 2014. Editor: Ronald Bunker.

J. Turbomach 136(12), 121004 (Aug 26, 2014) (11 pages) Paper No: TURBO-14-1130; doi: 10.1115/1.4028218 History: Received July 10, 2014; Revised July 15, 2014

The matching of a vaned diffuser with a centrifugal impeller is examined with a one-dimensional (1D) analysis combined with extensive experimental data. A matching equation is derived to define the required throat area of the diffuser relative to the throat area of the impeller at different design speeds and validated by comparison with a wide range of compressor designs. The matching equation is then used to give design guidelines for the throat area of vaned diffusers operating with impellers at different tip-speed Mach numbers. An analysis of test data for a range of high pressure ratio turbocharger compressor stages is presented in which different matching between the diffuser and the impeller has been experimentally examined. The test data includes different impellers with different diffuser throat areas over a wide range of speeds. It is shown that the changes in performance with speed and diffuser throat area can be explained on the basis of the tip-speed Mach number which causes both the diffuser and impeller to choke at the same mass flow. Based on this understanding, a radial compressor map prediction method is extended to include this parameter, so that more accurate maps for matched and mismatched vaned diffusers can be predicted.

Copyright © 2014 by ASME
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Fig. 1

Schematic of the effect of the diffuser matching on the performance map of a turbocharger compressor

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Fig. 2

Variation of diffuser to impeller throat area ratio for a variation in tip-speed Mach number and different mean impeller inlet diameters

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Fig. 3

The theoretical diffuser to impeller area ratio, A*d /A*i, predicted by Eq. (4) compared to the design area ratios

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Fig. 4

Design guideline for the variation of diffuser to impeller throat area ratio over a range of flow coefficients and tip-speed Mach numbers, after Rusch and Casey [10]

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Fig. 5

Normalized polytropic efficiency versus normalized flow coefficient for a range of tip-speed Mach numbers for test data (symbols) and map prediction method (lines)

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Fig. 6

Normalized efficiency ηp/ηp0 for a range of speeds, including test data and predictions

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Fig. 7

Normalized flow coefficient ϕp/ϕp0 for a range of tip speeds, including predictions and test data

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Fig. 8

Different trends of the impeller work coefficient toward choke from test data for the same impeller at different tip speeds with a small and a large diffuser throat

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Fig. 9

Map predicted by the map prediction method (lines) compared to test data (triangles) at different speeds for a case with a small diffuser and a nominal design Mach number of 1.6. The nominal design point and reference conditions are shown as a white circle.

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Fig. 10

Map predicted by the map prediction method (lines) compared to test data (triangles) at different speeds for the stage in Fig. 9 equipped with a large diffuser giving a nominal design Mach number of 1.3. Reference conditions are the same as shown in Fig. 9, and the nominal design point is shown as a white circle.

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Fig. 11

Predictions of work coefficient against flow coefficient for the stage in Fig. 9 with a small diffuser area ratio where the impeller only chokes at the highest tip-speed Mach number. The nominal design point and reference conditions are shown as a white circle.

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Fig. 12

Predictions of work coefficient against flow coefficient for the stage in Fig. 10 with a larger diffuser area ratio. The impeller chokes on speeds lines above the nominal design tip-speed Mach number. Reference conditions are the same as shown in Fig. 11, and the design point is shown as a white circle.

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Fig. 13

Summary of matching behavior on the performance map and the work coefficient at speeds above and below the nominal design Mach number, Mu2d




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