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Session: Session 4A: Volumetric Expanders (1)
Session starts: Tuesday 10 September, 14:00
Presentation starts: 14:20
Room: Olympia

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Saverio Randi ()
Nicola Casari ()
Ettore Fadiga ()
Alessio Suman ()
Michele Pinelli ()
Eckhard Groll ()
Davide Ziviani ()
Bryce Shaffer ()

Abstract:
Computational Fluid Dynamics (CFD) studies of volumetric machines, such as scroll ones, are becoming more and more widespread due to the interest in these type of devices by Organic Rankine Cycle (ORC) systems manufacturers and because of the increasing computational power, which makes these approaches more affordable than in past years and capable to provide useful information regarding transient phenomena. It is known that the peculiarity of scroll devices is the orbiting motion of one or both spirals. In the first case, to prevent rotation of the moving spiral during machine operation, an Oldham coupling is used. In those applications where this mechanism is placed between the moving and fixed spirals, there could be instants during motion in which one of the inlet or outlet ports, depending if the device is running in compression or expansion mode, is partially overlapped by the Oldham ring. This fact can cause issues such a non-uniform flow profile at the outlet of the port or flow pulsations, leading to vibrations of the machine. In this paper, a CFD analysis of such a scroll expander is showed. The geometry of the machine, provided by the manufacturer, is used to set-up an unsteady, 3D simulation in CONVERGE CFD software. An orthogonal grid is generated and then the scroll surface is mapped on this mesh: in this way, the complex motion of the machine can be simulated, and the gaps can be discretized thanks to an adaptive mesh refinement (AMR) technique, which can guarantee shorter computational time if compared to the Overset method, already presented by the authors in a previous work. Flow field is also solved in both radial and axial gaps, while the motion of both the moving spiral and the Oldham ring is imposed by means of a user-defined function. Working fluid R245fa has been modelled thanks to Redlich-Kwong equation of state. In this way, it has been possible to characterize the operation of the expander for different working conditions, such as rotational speeds and refrigerant inlet temperature and pressures.