In this study the surge cycle is thoroughly analysed with the main focus on identification of the recirculation zones. Surge is the most dangerous global flow instability occurring in centrifugal compressors. It can be described as low-frequency and high-amplitude fluctuations of the pressure and the mass flow rate. In most severe conditions in can lead to reversed flows, eventually preceding machine/installation failure. Therefore, providing the stable and undisturbed working conditions of such machines is essential, not only from the economic point of view, but also in terms of operating crew safety.

Presented research is a part of a project aiming at design and construction of more efficient anti-surge systems. The overall aim is to eventually introduce the anti-surge systems that are made to fit the particular machine. Understanding the local flows during and prior to surge are one of key issues that need to be understood to achieve this target.

The full surge cycle modelled with an application of unsteady RANS methods is presented. The significant push was put to realistic boundary conditions definition, narrowed to establishing pressure setting at both inlet and outlet positions located far from the impeller. The outlet condition is located behind the valve to emulate its dynamics. This approach does allow to analyse both steady and unsteady regimes of the flow in a centrifugal compressor. Previous studies have shown that the mass flow boundary condition, or boundary conditions located too close to the impeller, “stiffen” the flow dynamics compared to the experimental case. The investigated geometry is a virtual representation of a test rig consisting of an impeller and casing of a large industrial compressor VRK-3 located in the Lodz University of Technology. The computational model includes: inlet channel, impeller, vaneless diffuser, volute, outlet pipe, valve and the region downstream the valve. Surge conditions are achieved by realization of static valve closure.

The presented analysis is mainly focused on recirculation zones appearing during the surge cycle. The regions of inflow and backflow at different stages of the surge period are investigated. The analysis includes the size and position of these regions throughout the surge cycle. Study shows that even in the backflow period there are regions with downstream flow. Both structures coexist throughout the cycle and change their size. This gives a room for better understanding of both: the surge phenomenon and possible ways of avoiding or controlling it. Being able to predict a location of recirculation zones can be an important step towards custom, machine-dedicated anti-surge systems design.


Recirculation Zones During the Centrifugal Compressor Surge Cycle: CFD Simulation

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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