Flow modelling of scroll compressors and expanders
The use of scroll compressors in refrigeration systems is becoming increasing popular. However the development and use of a scroll machine as an expander to replace a throttle valve has never been reported. This thesis describes the work carried out to develop a two phase scroll expander for use in vapour compression refrigeration systems. By this means the power requirements of the system would be decreased and the refrigeration effect increased. A computer simulation of the geometry of the scroll device was first developed. This was attached to a package of subroutines developed to analyse compressor and expander thermodynamic performance to obtain estimates of both scroll expander and compressor performance. The results of the expander compressor analyses were used to estimate the performance of a scroll expander driving a second scroll compressor in a sealed unit used in a large industrial chiller of 4MW thermal output. By this means the power requirement for the main compressor was reduced by approximately 10% while, due to the expansion in the expander being more nearly isentropic than in the throttle valve, the refrigeration effect was simultaneously increased by nearly 2%. This lead to an overall improvement in coefficient of performance of nearly 13.5%. Further investigation of the system and more specially on the scroll device appears to be justified. A 3D simulation was then performed by the use of a CFD package called CFX4. This gave an excellent understanding on the flow 'inside' the machine. From the flow patterns obtained it is obvious that the scroll device behaviour appears to be excellent as compressor but not so good as an expander. This is mainly due to the huge recirculation during the filling process and due to the significant pressure drop of approximately 2 bar (Max) associated with it.