A Quasi-dimensional Numerical Investigation of the Scroll Expander of an Organic Rankine Cycle Unit

Scroll compressors are often used in air conditioning and refrigeration systems thanks to their high efficiency, low noise and vibrations, light weight and high reliability. Scrolls are also widely used as positive displacement expanders in small-scale power generation systems, such as Organic Rankine Cycles (ORCs). In recent years, the research has focused on the development of mathematical models that help to predict the scroll expander performances under different operating conditions. In this work, a quasi-dimensional model of a scroll expander of an Organic Rankine Cycle unit is presented. Such an expander consists of two identical circle involute spirals, with appropriate starting segments. Specifically, the model is able to design the scroll expander geometry with either a “circular cutter” or a “perfect mesh profile” (PMP) approach, which are two of the most common scroll geometry starting segments. As regards the thermo-fluid dynamic aspects, specific sub-models have been used to account for radial and axial leakage, wall heat transfer, intake and exhaust of the working fluid. The model has been validated with available experimental data in order to assess its accuracy and, at the same time, to calibrate the implemented sub-models. The influence of some geometrical parameters on the expander performances has been assessed. In particular, a study of the influence of the wrap geometry is presented. The results show that the circular cutter approach returns better performances at the expenses of a higher mass consumption. Nevertheless, the circular cutter modification returns a higher specific work. Lastly, the influence of one of the PMP parameters on performances has been assessed. The results show a fairly strong dependence of both mechanical power and specific work, suggesting that the global optimization of all geometric parameters of the scroll expander may radically improve its performances.