Structural Improvement and Experimental Study on CO2 Double Rolling Piston Expander
|Keywords||Natural refrigerant CO2 Double rolling piston expander Leakage loss Isentropic efficiency|
The negative environmental effects of refrigerant have attracted more and more attention. The natural refrigerant CO2 should be commended and promoted because of its superior environmental protection. Expander is the fundamental measure to improve the efficiency of CO2 tanscritical cycle. Aimed at the preliminary designed CO2 double rolling piston expander, the theoretical analysis and experimental research were to identify the reasons for its low efficiency and the corresponding improvement programs were raised. At last the effect of improved was proved through experiments.Firstly the structure and movement of double rolling piston expander are introduced in general. After check and analysis of the structural size of the original expander, some key parameters of design have been recommended. We analyze the impacting factor of expansion ratio, find the middle-channel volume affect expansion ratio severely, and raise the diameter and length designed reasonably.The theories of friction and wear are introduced firstly. The model of adhesive wear most in CO2 expander is established and the key factors are analyzed. In the gas-liquid two-phase state, the oil film would be destroyed easily. We suggest that fluid lubrication fit for expander and lubricant be provided by hydrodynamic-hydrostatic pressure. The appropriate materials are selected based on different leak points of CO2 expander.The external leakage and internal leakage of the double rolling piston expander are analyzed in the paper. The external leakage can be eliminated by integration of intermediate plate and sealing ring in the end of cylinder. The end of rolling piston coated with a layer of copper can reduce the leakage gap, but also prevent adhesive wear. The piston end can also use O-ring to enhance seal, but need wearable composite rubber. A single lubricant-supplied system is design for expander, which can reduce friction and wear, while the oil film can make leak channel narrowed, and reduce the leakage loss of the expander.Make use of CO2 water-water heat pump system to complete the rolling piston expander experiments including solving leakage problems and efficiency test, and get the initial operating characteristics of the expander, the isentropic efficiency up to 31.79%. In any experimental condition, the measured expansion ratio is far less than the design level, which is considered as the main reason for low efficiency, also the focus improving direction of the next step.