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 project has advanced SwRI's engineering service capabilities related to rotating stall in the diffusers of high-performance centrifugal compressors. Centrifugal compressors impart angular momentum to the gas flowing radially through a bladed wheel (the impeller). The gas decelerates after exiting from the impeller into the diffuser, thereby converting kinetic energy into pressure. Diffuser rotating stall tends to occur in high-head, low-flow compressors. When the flow and radial velocity are low, the adverse pressure gradient can cause recirculation in the diffuser. As a result, the diffuser flow develops three-dimensional unstable vertical structures with pressure modulation in time and space, potentially leading to rotor vibration and piping pulsations. This poorly understood phenomenon represents one of several limiting factors that must be considered in applying these compressors for oil and gas production and processing.

Approach - The project combines three technical tracks to enhance knowledge of diffuser stall and its severity, including computational fluid dynamics (CFD), compressor system testing, and analysis of available data from literature and other studies. The CFD uses time-transient, three-dimensional solution of the Navier Stokes equations to model flow in the entire diffuser, subject to highly tangential flows at inlet. The testing will take place in a facility designed to allow the two-way interaction between a centrifugal compressor and pulsations in the attached piping. In addition, a substantial existing database showing stall pulsation over a range of conditions has been subjected to regression analysis resulting in quantitative relationships.

Accomplishment - All three tracks have met their objectives. The CFD analysis, which has successfully predicted stall in the diffuser, has been correlated against literature data defining the conditions under which stall occurs (onset conditions). The CFD can now provide a means to extrapolate onset conditions to the high pressures typical of advanced applications, but beyond the bounds of available data. Secondly, the data analysis effort has successfully developed relationships between compressor stage pressure rise, and the most severe dynamic stall pressure to be expected, if and when stall occurs. It has also provided a quantitative basis for predicting the vibrations of a rotor under the influence of stall. Thus, the project has already provided the minimum knowledge needed to help operating companies determine the likelihood and worst-case consequences of stall. Thirdly, the test facility is now operational. The compressor and piping are installed, together with instrumentation for vibration and dynamic pressure. The compressor exhibits diffuser rotating stall at expected conditions, and the phenomenon is now being mapped in more detail.

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