Abstract
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.
Highlights
Current efforts on limiting environmental impact of human activities result in an increased attention towards renewable energy sources and energy efficiency
For low-enthalpy energy recovery, Organic Rankine Cycles (ORCs) are commonly used since they provide better performances when compared with other available technologies, such as Stirling engines, thermo-electric generators, and inverted Brayton cycles [5]
The research has focused on the development of mathematical models that help to predict the scroll expander performances under different operating conditions
Summary
Current efforts on limiting environmental impact of human activities result in an increased attention towards renewable energy sources and energy efficiency. Ma et al [10] developed a scroll expander model that takes into account the friction between stator and rotor during their relative motion With such a model, it is possible to determine the expander rotating speed and mass flow rate for specific load conditions. Zhang et al [12] have studied the effects of specific operating conditions (inlet pressure and temperature) and geometric parameters (clearance and vanes height to pitch ratio) on the expander performances. The study found that, for a given speed, the volumetric efficiency is the same, whereas scroll expander mass flow rate and power output increase with the inlet pressure. This work is organized as follows: first the scroll expander geometry is described, along with two interesting modifications of the starting segments, the mathematical and numerical models are given, the results are discussed and, the conclusions are summarized
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