DEVELOPMENT OF A RADIAL PISTON EXPANDER FOR VAPOR COMPRESSION CYCLES

In recent years, heat pumps and refrigeration systems are widely used in both residential and industrial applications. The possibility of recovering the large throttling losses by using an expander could give a substantial contribution to the performance improvement. In this thesis, a reciprocating expander developed from a hydraulic motor was numerically and experimentally analyzed. A numerical model was developed to identify the needed small modifications to be made on the expander without change its architecture. Successively, an extensive experimental activity on the modified expander has been carried out to characterize it in detail and evaluate the effective performance. With this aim, a dedicated test rig and a measurement system have been developed. The expander was tested in a R134a heat pump cycle and in a CO2 refrigeration cycles. Despite of the mechanical losses due to the different original application of the machine, the thermodynamic cycles showed very promising results with the adoption of this solution. For this reason, a redesign and manufacturing of the machine was done in order to improve the efficiency in HFC cycles and to decrease the mechanical losses. The new version of the expander was tested in a "hot-gas bypass cycle", which has been designed and manufactured. The aim of this cycle is to obtain high stability and flexibility, and lower size due to the lack of the evaporator. The results showed improvement in both the thermodynamic behavior and mechanical losses. Finally, a 1D thermal conduction model has been developed to study the two-phase expansion with R134a with an improved accuracy.