Real gas expansion with dynamic mesh in common positive displacement machine
Nicola Casari, Alessio Suman, Mirko Morini, Michele Pinelli
Session: Session 3B: CFD
Session starts: Wednesday 13 September, 16:30
Presentation starts: 16:50
Room: Building 27 - Lecture room 02
Nicola Casari (Università degli Studi di Ferrara)
Alessio Suman (Università degli Studi di Ferrara)
Mirko Morini (Università degli Studi di Parma)
Michele Pinelli (Università degli Studi di Ferrara)
Fluids that are elaborated by the machinery involved in ORC undergo several transformations among which the expansion in
positive displacement machines. The gas 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 overall efficiency both of
the expander and the whole cycle. In this work the fluid dynamics of a real gas during the expansion through the gap during the
operation is numerically investigated. The eects of the gap formation and its evolution on the processed fluid is studied thanks to a
dynamic mesh approach. Different geometries have been considered, each one related to different positive displacement machines
used as an expander in ORC applications. In particular, the variable gap between the fixed and mobile spirals of a scroll expander
and the clearance between the star wheel and the screw of a single screw expander are 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 represents 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 the representation of the actual behavior involved in the positive displacement
machine operation. This analyses show the local fluid dynamic phenomena due to the variable clearances. Both air and R134a,
fluid widespread in the ORC cycles, are used in this work. Different operating conditions are considered and effects like separation
and shock wave are highlighted.
This analysis allows the comprehension of how local phenomena could aect the overall machine operation and efficiency. In
addition, information about flow coefficient related to dierent time instant could be useful to setup the lumped parameter models.
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.