Numerical Simulation of Surge in a Multi-Stage Compressor

Abstract

This thesis will discuss the approach taken to develop a numerical model for the calculation of surge in a multi-stage compressor. The focus of the work is to obtain a suitable numerical model that can perform at least one full surge cycle without the need to change the flow or boundary conditions during the cycle. As this is a preliminary investigation no experimental data is used for validation and the results obtained are analysed from their characteristics as opposed to reference data. The work is performed through use of the computational fluid dynamics (CFD) software TRACE, a code developed specifically for use in turbomachine analysis by the German Aerospace Centre (DLR). A CFD model is created of the compressor Rig 250, a 4.5 stage axial flow compressor owned and operated by the DLR Institute of Propulsion Technology at their site in Cologne Germany. It is an axial flow compressor containing an IGV and four rotor stator pairs(stages). The model will be simulated using multiple nodes on the DLR computational cluster, with each node containing 24 processors, therefore allowing for a significant decrease in the time required to perform the simulations. Initial analysis is performed on the compressor to assess the suitability of the model in both conventional and reversed flow conditions. Speedlines are also developed to assess the compressor behaviour close to surge, allowing for values such as the back pressure at which surge occurs to be identified. Following on from the preliminary findings, some additional geometry is added to the compressor to assist in the simulation of surge. These consists of a bypass, plenum duct and variable geometry nozzle. The set-up of the final model is achieved through an iterative process where a range of the simulation set-up parameters are altered and tested to assess if they allow the model to work. The main parameters altered are the boundary conditions within the compressor, particularly for the unsteady calculations where simplified boundary conditions are used. Once the iterative process is finished the model is run and the results throughout the system are analysed. The model in its current state is capable of performing close to two surge cycles prior to a breakdown in the flow solution. During the surge cycles there are times where the mean flow direction in the entirety of the compressor is reversed. The flow through the compressor is analysed between each blade row as well as at the entrance and exit to the core compressor (Rig 250 without the additional geometry) and results obtained at the system exit are also checked to ensure the model is performing as expected. Overall the work presented within this thesis provides an insight into the initial work required for a numerical surge model. A number of items of future work are highlighted into areas that are deemed important with the progression of the work and in areas that would benefit the sector.