Small scale Organic Rankine Cycle (ORC) systems have a big potential for waste heat recovery in the market. Due to the smaller volume flows inside these systems, non-conventional expansion technologies such as screw expanders become more interesting. Recent economic studies have shown the important role of screw machines in such cycles. However, in order to get a better understanding of the expansion behaviour in an ORC, appropriate simulation models of screw expanders are necessary. The flow inside an oil-injected twin screw expander is modeled in detail with 3D CFD (Computational Fluid Dynamics) calculations. These simulations are challenging because of the deforming domain and the narrow gaps between the screws or between a screw and the casing. The deforming mesh motion is handled by an in-house code which generates a block-structured grid with the help of the solutions of the Laplace problem. The oil-phase was modeled with an Eulerian multiphase model and the working fluid is treated compressible. The performance of the screw expander is strongly affected by the oil-injection which provides lubrication and a better sealing of the gaps. Therefore, the different types of leakages inside the screw expander are studied and monitored. As the result of the simulations, knowledge about the flow process and the losses inside the oil-injected screw expander is built up.

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