Fluids processed by the machinery involved in ORC cycles undergo several transformations among which the expansion in positive displacement machines. The fluid path inside this component is very complicated and gaps play a crucial role. Due to the importance of this technical detail, gap design and optimization is a decisive step in achieving an high efficiency both of the expander and the whole cycle. In this work the fluid dynamics of several fluids commonly used in ORC cycles is investigated. Particularly, their behaviour during the expansion through the gap in operation is numerically investigated. The effects of the gap formation and its evolution on the processed fluid is studied thanks to a dynamic mesh approach. A typical application has been considered in this work: the variable gap between the fixed and mobile spirals of a scroll expander is analysed. The relative motion and in turn, the variation of the gaps during the machine operation, implies the use of particular numerical strategies able to well represent these localized geometrical features. On the top of that, the modelling of the processed fluids as a real gas determines an extra effort in the way of representing the actual behavior involved in the positive displacement machine operation. This analysis shows the local fluid dynamic phenomena due to the variable clearances. R134a and its replacements R152a and R1234ze(E), fluids widespread in the ORC cycles, are used in this work. The fluids are investigated under the same conditions and effects like separation and shock wave are highlighted. This analysis allows the comprehension of how local phenomena could affect the overall machine operation and efficiency. Gaps are the responsible of the volumetric efficiency of the machine and, coupled with (i) time-variable geometry modification, (ii) relative velocities and (iii) fluid characteristics characterize the global ORC system performance
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