Numerical simulation of combined screw compressor-expander machines for use in high pressure refrigeration systems

Recent interest in natural refrigerants has created a new impetus for studies of CO2 as a working fluid in vapour compression systems for refrigeration and air conditioning. Two major drawbacks to its use are the very high pressure differences required across the compressor and the large efficiency losses associated with the throttling process. One of potential advantages of screw machines over other types of positive displacement machine is their ability to perform both the compression and expansion functions simultaneously, using only one pair of rotors. A further feature is the use of the rotors which seal on both contacting surfaces so that the same profile may be used both for the expander and the compressor sections. By using the same profile for both, the compressor and expander rotors can be machined or ground in a single cutting operation and then separated by machining a parting slot in them on completion of the lobe formation. Computational Fluid Dynamics and Structure Analysis are used in this paper for investigation of fluid and solid interaction in such machines.