Experimental Investigation of a Centrifugal Compressor Stage With Focus on the Flow in the Pipe Diffuser Supported by Particle Image Velocimetry (PIV) Measurements

Abstract

Centrifugal compressor stages with pipe diffusers are considered to perform with high efficiency. Yet very little information on these kinds of diffusers is available to this day. Therefore, experimental investigations have been performed on a centrifugal compressor stage with a pipe diffuser. An extensive measurement series using various steady, unsteady and laser optical measurement techniques has been performed to detect the highly three dimensional diffuser flow and create a benchmark for further development steps. Whereas this paper presents the test rig and the results gathered under nominal conditions, in a follow-up paper the results of the parameter variation studies covering bleed variation, impeller tip clearance and impeller-diffuser misalignment are compared to the nominal baseline and evaluated with respect to the compressor stage performance. For the investigations performed under nominal conditions it was found that the diffuser flow separates on the pressure side in the first half of the pipe. In the last 30% of the pipe hardly any deceleration of the flow takes place. From this, special attention is drawn to the parameter variations regarding a first proposal for a diffuser design change, which consists in shortening the diffuser. Ongoing investigations cover this design proposal. Along with the results of the ongoing investigations, the numerical investigations accompanying the experimental work will also be presented in follow-up papers.

Investigation of two pipe diffuser configurations for a compact centrifugal compressor

“…Similarly, when the reverse flow caused by the impeller passes through the ridge of the pipe diffuser near the shroud, a vortex is formed at the shroud/suction side region. This is different from the results shown in the previous studies, [1][2][3] where just two vortices were found induced by scallop leading edge of pipe diffuser. As the fluid convects along the pipe passage, these three vortices are enhanced until the throat at Plane III.…”

“…This configuration is normally employed by General Electric. [1][2][3][4][5]10 As shown in Figure 2, at the throat of the pipe diffuser, the cross section is circular, and then the cross section becomes oblong with semi-circular ends attached by straight lines, like an oval along a straight centerline. The long axis of the cross section gradually increases, whereas the short axis and semicircular radius are equal to the diffuser throat diameter along a straight centerline.…”

“…Many researchers have focused on investigating both the particularly little understood flow mechanisms between the impeller exit and pipe diffuser throat and the diffusing passages behind the throat. Zachau and Buescher, 1 Zachau and Niehuis, 2 and Kunte et al 3,4 performed flow investigations in diffusers using a nonintrusive measurement technique and numerical simulations. A comparison of predicted and measured flow fields was conducted in detail and strong agreement was found.…”

“…Furthermore, altering the flow from the radial to axial direction in a pipe diffuser is a difficult challenge for pipe diffuser design. In previous publications, two main configurations, which will be discussed in more detail later, have been presented: The first is a radial pipe diffuser connected to a dump followed by a deswirler, which is mainly used by General Electric, [1][2][3][4][5]10 and the second is a fishtail diffuser, which is employed by Pratt & Whitney Canada. [11][12][13][14] Despite the extensive studies on pipe diffusers, few comprehensive results have been published addressing the complex flow characteristics in fishtail diffusers.…”

Parametric Studies of Pipe Diffuser on Performance of a Highly Loaded Centrifugal Compressor

“…In recent years, most investigations on pipe diffuser are focused on flow phenomena in diffuser passages [2][3][4][5][6] investigations were earned out by use of nonintrusive measure ment technique such as PIV (particle image velocimetry) or LDV (laser Doppler velocimetry) meanwhile numerical simulations were done with computational fluid dynamics (CFD) solvers. Pre dicted and measured flow fields were compared at several critical planes.…”