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Unsteady Entropy Measurements in a High-Speed Radial Compressor

[+] Author Affiliations
M. Mansour, N. Chokani, A. I. Kalfas, R. S. Abhari

Swiss Federal Institute of Technology, Zürich, Switzerland

Paper No. GT2007-27450, pp. 625-637; 13 pages
doi:10.1115/GT2007-27450
From:
  • ASME Turbo Expo 2007: Power for Land, Sea, and Air
  • Volume 1: Turbo Expo 2007
  • Montreal, Canada, May 14–17, 2007
  • Conference Sponsors: International Gas Turbine Institute
  • ISBN: 0-7918-4790-X | eISBN: 0-7918-3796-3
  • Copyright © 2007 by ASME

abstract

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 1.8mm) 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 40kHz and uncertainty of ±2J/kg. The measurements show that at the near design operating condition, the impeller exit flowfield is highly three-dimensional. Adjacent to the shroud there are high levels of relative entropy and at the mid-span there are low & 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 otherhand, at the mid-span, a loss free, jet flow region and a channel wake flow of moderate mixing characterize the flowfield. At both the shroud and mid-span, there are strong circumferential variations in the relative entropy. These circumferential variations are much reduced when the centrifugal compressor is operated 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 near design operating condition. 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 © 2007 by ASME

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