Droplet-Laden Flows in Multi-Stage Compressors an Overview of the Impact of Modeling Depth on Calculated Compressor Performance

There are various mechanisms through which water droplets can be present in compressor flows, e.g. rain ingestion in aeroengines or overspray fogging used in heavy duty gas turbines to boost power output. For the latter, droplet evaporation within the compressor leads to a cooling of the flow as well as to a shift in the fluid properties which is beneficial to the overall process. However, due to their inertia, the majority of droplets is deposited in the first stages of a multistage compressor. While this phenomenon is generally considered in CFD-computations of droplet-laden flows, the subsequent re-entrainment of collected water, the formation of new droplets and the impact on the overall evaporation is mostly neglected because of the additional computational effort required, especially with regard to the modeling of films formed by the deposited water. The work presented here shows an approach which allows to integrate the process of droplet deposition and re-entrainment based on relatively simple correlations and experimental observations from literature. Thus, the two-phase flow in multistage compressors can be modelled and analyzed very efficiently. In this paper, the models and assumptions used are described first, then the results of a study performed based on a generic multistage compressor are presented, whereby the various models are integrated step by step to allow an assessment of their impact on the droplet evaporation throughout the compressor and overall performance. It can be shown that evaporation becomes largely independent of droplet size when deposition on both rotor and stator and subsequent re-entrainment of collected water is considered. In addition, open issues with regard to a future improvement of models and correlations of two-phase flow phenomena are highlighted based on the results of the current investigation.