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Research Papers: Gas Turbines: Microturbines and Small Turbomachinery

The Efficiency of Turbocharger Compressors With Diabatic Flows

[+] Author and Article Information
Michael V. Casey, Thomas M. Fesich

Institute of Thermal Turbomachinery and Machinery Laboratory (ITSM), Universität Stuttgart, Pfaffenwaldring 6, D-70569 Stuttgart, Germany

J. Eng. Gas Turbines Power 132(7), 072302 (Apr 21, 2010) (9 pages) doi:10.1115/1.4000300 History: Received June 17, 2009; Revised June 18, 2009; Published April 21, 2010; Online April 21, 2010

In most compressors the flow is adiabatic, but in low-speed turbochargers, the compression process has both heat transfer and work input. This paper examines different compressor efficiency definitions for such diabatic flows. Fundamental flaws in the use of the isentropic efficiency for this purpose are identified, whereas the polytropic efficiency can be used with or without heat transfer without ambiguities. The advantage of the polytropic approach for a practical application is demonstrated by analyzing the heat transfer in a turbocharger compressor. A simple model of the heat transfer allows a correction for this effect on the polytropic efficiency at low-speed to be derived. Compressor characteristics that have been corrected for this surprisingly large effect maintain a much higher efficiency down to low-speeds.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

Compressor efficiency definitions in Eq. 1. 1-2i: isothermal (x=2i), 1-2s: isentropic (x=2s), and 1-2: polytropic (x=2).

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Figure 2

Diabatic process and isentropic efficiency

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Figure 3

Simplified h-s model for nonadiabatic compression with heat added before and after adiabatic compression

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Figure 4

(a) p-v diagram of processes in Table 1. (b) h-s diagram of processes in Table 1.

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Figure 5

Diabatic polytropic efficiency displayed in terms of entropy differences in an h-s diagram, as given in Eq. 31

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Figure 6

Apparent isentropic efficiency of a turbocharger compressor stage with different turbine inlet temperatures and rotational speeds

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Figure 7

Work coefficient versus outlet flow coefficient characteristic (derived from a measured performance map of a turbocharger compressor in Ref. 17)

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Figure 8

Comparison of test data from a turbocharger test rig operating with a turbine inlet temperature of 600°C, for two turbochargers with the constant heat transfer model of Eq. 46(kq=0.01). The different symbols represent measurements at different speeds.

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Figure 9

Correction for the heat transfer effect on the apparent efficiency for a typical turbocharger compressor

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