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Research Papers: Gas Turbines: Controls, Diagnostics, and Instrumentation

Unsteady Entropy Measurements in a High-Speed Radial Compressor

[+] Author and Article Information
M. Mansour, N. Chokani, R. S. Abhari

LSM, Turbomachinery Laboratory, Department of Mechanical and Process Engineering, ETH Zürich, 8092 Zürich, Switzerland

A. I. Kalfas1

LSM, Turbomachinery Laboratory, Department of Mechanical and Process Engineering, ETH Zürich, 8092 Zürich, Switzerland

1

Current address: Department of Mechanical Engineering, Aristotle University of Tessaloniki, GR-54124 Greece.

J. Eng. Gas Turbines Power 130(2), 021603 (Jan 22, 2008) (9 pages) doi:10.1115/1.2799525 History: Received April 27, 2007; Revised August 14, 2007; Published January 22, 2008

The time-dependent relative entropy field at the impeller exit of a centrifugal compressor is measured. This study is part of a broader effort to develop comprehensive measurement techniques that can be applied in the harsh environment of turbomachines. A miniature unsteady entropy probe (diameter of 1.8 mm) is designed and constructed in the present study. The unsteady entropy probe has two components: a one-sensor fast-response aerodynamic probe and a pair of thin-film gauges. The time-dependent total pressure and total temperature are measured from the fast-response aerodynamic probe and pair of thin-film gauges, respectively. The time-dependent relative entropy derived from these two measurements has a bandwidth of 40 kHz and an uncertainty of ±2 J/kg. The measurements show that for operating Condition A, φ=0.059 and ψ=0.478, the impeller exit flowfield is highly three dimensional. Adjacent to the shroud there are high levels of relative entropy and at the midspan there are low and moderate levels. Independent measurements made with a two-sensor aerodynamic probe show that the high velocity of the flow relative to the casing is responsible for the high relative entropy levels at the shroud. On the other hand, at the midspan, a loss free, jet flow region and a channel wake flow of moderate mixing characterize the flowfield. At both the shroud and midspan, there are strong circumferential variations in the relative entropy. These circumferential variations are much reduced when the centrifugal compressor is operated at operating Condition B, φ=0.0365 and ψ=0.54, near the onset of stall. In this condition, the impeller exit flowfield is less highly skewed; however, the time-averaged relative entropy is higher than at the operating Condition A. The relative entropy measurements with the unsteady entropy probe are thus complementary to other measurements, and more clearly document the losses in the centrifugal compressor.

Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

Photograph of the tip of the unsteady entropy probe

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Figure 2

FRAP: (a) miniature silicon piezoresistive pressure sensor chip and (b) schematic of Wheatstone bridge

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Figure 3

Unsteady total temperature probe (before integration with the FRAP)

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Figure 4

Static calibration results for unsteady total temperature probe

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Figure 5

Measurement concept of a FRAP two sensor in a virtual four-sensor mode

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Figure 6

System layout of the compressor facility “Rigi”

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Figure 7

3D rendering of impeller, showing also location of the unsteady entropy and two-sensor probes and measurement area

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Figure 12

Time-resolved contours of (a) axial velocity, (b) radial velocity, and (c) tangential velocity, at operating Point A (ϕ=0.059)

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Figure 17

Circumferential variation of the relative entropy at operating Point B (ϕ=0.0365) for three axial positions: Z=10% (shroud), Z=40% (midspan), and Z=70% (hub)

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Figure 16

Time-resolved contours of (a) total pressure, (b) total temperature, and (c) relative entropy, at operating Point B (ϕ=0.0365)

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Figure 15

Time-resolved contours of (a) axial velocity, (b) radial velocity, and (c) tangential velocity, at operating Point B (ϕ=0.0365)

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Figure 14

Circumferential variation of the relative entropy at operating Point A (ϕ=0.059) for three axial positions: Z=10% (shroud), Z=40% (midspan), and Z=70% (hub)

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Figure 13

Time-resolved contours of (a) total pressure, (b) total temperature, and (c) relative entropy, at operating Point A (ϕ=0.059)

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Figure 11

Circumferentially averaged profiles of total pressure, total temperature, and entropy: (a) operating Point A: ϕ=0.059 and (b) operating Point B: ϕ=0.0365

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Figure 10

Time-resolved distribution of relative flow angle αrel (a) operating Point A: ϕ=0.059 and (b) operating Point B: ϕ=0.0365

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Figure 9

Circumferentially averaged profiles of relative flow angle αrel at operating Points A (ϕ=0.059) and B (ϕ=0.0365)

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Figure 8

Operating line for the compressor. A and B are the two conditions at which time-resolved flow measurements are made with the unsteady entropy probe.

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