Influence of Thermal Deformation on the Characteristic Diagram of a Screw Expander in the Automotive Application of Exhaust Heat Recovery
The present paper responds to the operating behavior of an exemplarily selected screw expander for exhaust heat recovery from combustion engines in the lower power range. The application in vehicles for the purpose of heat recovery, which is characterized by the small machine dimensions due to relatively low system mass flows, is a still unexplored area for the screw-type machine. The screw expander is to be used in an organic Rankine cycle (ORC). Compared to other expander designs in general screw expanders are characterized by high efficiency together with relatively low geometrical dimensions. Within the presented investigation mainly the performance characteristic of a screw expander, geometrically designed for a particular operating point, is determined as a function of the system parameters – inlet pressure and temperature – as well as the rotational speed of the expander. Here, based on an iterative coupling of thermodynamic and thermal simulations the influence of the thermal deformation on the machine performance in particular is analyzed. The results of the thermodynamic simulation, mainly based on so-called chamber models, represent the thermodynamic and fluid dynamic performance of the screw machine by means of mass and energy conservation. The information obtained in this way about the temperature distributions and the heat fluxes provides a basis for the subsequent thermal simulation using a finite element method (FEM) calculation. The resulting thermally deformed machine, whose performance-related clearance heights are now changed, is used for the next iteration step within the thermodynamic simulation.