Controlling Diffuser Stall Effects in Centrifugal Compressor Systems, 18-9148

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Principal Investigators
Anthony J. Smalley
Ralph E. Harris
Robert J. McKee
Larry E. Blodgett
Christopher J. Freitas

Inclusive Dates:07/01/99 - 04/01/02

Background - This research project has advanced SwRI's engineering service capabilities related to rotating stall in diffusers of high-performance centrifugal compressors. Diffusers in centrifugal compressors convert kinetic energy in the high-velocity flow exiting a compressor impeller into pressure. However, as flow rate into a diffuser decreases and pressure rise increases, the flow angle entering the diffuser becomes smaller, resulting in a separated flow structure known as rotating stall. Diffuser stall involves three-dimensional unstable flow structures with pressure modulations in space and time, which lead to piping pulsation and rotor vibration. This poorly understood phenomenon represents a significant limitation, the consequences of which have not been predictable, but which must be considered in applying centrifugal compressors for oil and gas production and processing.

Approach - This effort combines three technical tasks to enhance knowledge of diffuser stall and its severity. These tasks include computational fluid dynamics (CFD), compressor testing, and analysis of published data. The CFD uses a time-dependent, three-dimensional solution of the Navier Stokes equations to model flow in the entire diffuser. Compressor testing in the SwRI turbo compressor has recorded compressor performance, and dynamic pulsation and vibration data before and during stall. Analytically, an extensive database of stalled-induced vibration and pulsation levels over a range of operating conditions has been subjected to regression analysis resulting in quantitative relationships.

Accomplishments - The CFD analysis has successfully predicted stall occurrence in a diffuser, that agrees with both published and SwRI compressor test data, and defines the conditions at which stall begins to develop. A pressure contour plot showing stall cells in a diffuser is shown in the illustration below. CFD can now provide a means to extrapolate stall onset conditions to high pressures typical of advanced applications, but beyond the bounds of the test facility or available data. Testing in the SwRI centrifugal compressor has documented the performance, flows, and diffuser inlet angles at which stall occurs and also recorded stall-related pulsation frequencies, amplitudes, and vibration levels that are well-controlled in the case of the SwRI compressor.

The data analysis effort has successfully developed relationships between compressor stage pressure rise, and the most severe dynamic stall pressures expected, when stall occurs. It also provides a quantitative basis for predicting vibration levels of a rotor under the influence of stall. Thus, the project has confirmed the means to predict when stall will occur and provided the minimum knowledge needed to help operating companies determine the likely and worst-case consequences of stall.

Pressure contours in a centrifugal compressor diffuser during stall show the rotating cells that produce pulsation and vibration in the compressor.

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