Research On Expansion Theory Of CO₂ Transcritical Cycle And Focus On Rotary Expander-Compressor

CO₂ is under serious consideration worldwide as a replacement for conventionalrefrigerants because the severe environmental problems. The objective of this dissertationis to develop a CO₂ transcritical cycle that has comparable or better performance whencompared to a cycle with conventional refrigerants. Therefore this research will try topush the CO₂ transcritical cycle into practice application that may be benefit to thesustainable development in the refrigeration and air-conditioning field.Based on the existing theoretical and experimental research on CO₂ transcriticalcycle of other talent researchers worldwide, this dissertation takes the performanceimprovement of CO₂ transcritical cycle as the main target and focuses on the developmentof rotary expander-compressor in theory analysis and experimentation.The typical characteristics of CO₂ transcritical cycle may be somewhat higheroperating pressure and lower performance in baseline system when compared with asystem using conventional refrigerants. The safety problem and the effects of theoptimism high pressure to its performance are analyzed toward its high operating pressure.In principle, a large number of modifications to the basic CO₂ transcritical cyle arepossible. Those potential improvements of components and advance cycles withadditional components such as IHX, ejector, economizer and expander etc. are analyzedtheoretically. The technology modification can be approximately divided into two classes:reducing the pressure drop power loss and reducing the irreversible loss of heat rejectionprocess.It is investigated on the characteristics of the supercritical CO₂ expansion power inthe process of decompression and flashing process theoretically adopting thermodynamics.Gas-liquid phase change hysteresis in the meta-stable state is investigated and theaffecting factors are proposed. The irreversible thermodynamic model about the bubblenucleation in the CO₂ superheated liquid in the expansion process of expander is founded.The critical radius and the critical potential barrier of CO₂ nucleation are calculated. TheCO₂ energy fluctuation of meta-stable CO₂ liquid is calculated with the statisticthermodynamic method. Therefore the theories about adopting disturbance factors such asforeign substance and ultrasonic to accelerate the nucleation and the bubble growth areproposed.Employing an expander is paramount important to improve the performance of theCO2 transcritical cycle. Integrating an expander and a compressor together will be good toutilize the recovered power directly since the transmission loss is expected to be small.The design parameters of the swing piston type rotary expander-compressor adopting thenewly designed inlet control method are calculated according to the heat pump systemcapacity. The lubrication of the prototype is analyzed with tribology. Those four kind ofsynthetic lubricants: POE, PAG, PAO and AN/AB are compared and valued on their maincharacteristics such as miscibility, fluidity, solubility, viscosity-temperature character,long-term stability and lubricity.The model on the swing piston type rotary expander-compressor is founded tosimulate the operating process e.g. suction, compression and discharge for compressorand inlet, expansion and discharge for expander. The main thermodynamic parameterssuch as pressure, temperature and density of the refrigerant in the unit and othercharacteristics such as internal leakage loss, friction loss and etc. are numerical calculated.The expander-compressor potential can be well estimated and optimization geometricparameters can be gotten through the comprehensive analysis on the simulation results.The CO2 transcritical cycle water-water heat pump test rig was founded to test theperformance of the expander-compressor unit. The experiments indicated that theexpander had obvious improvement effects on the heat pump performance, COP of thesystem can be increased about 20%⁴0% which are favorable results. However, theinternal leak rates were still too high and both an optimized design and improvedmachining accuracy are required to fabricate and operate a CO2 expander at acceptableperformance levels. The disturbance action of minor non-condensation gas NC-1 totrigger the phase change in expander is tested. The result shows the disturbance isfavorable to improve the efficiency of expander. But further research in experiment andtheory is still needed to investigate the mechanism of disturbance to expansion process.