Experimental Investigation And Performance Optimization For CO₂ Transcritical Refrigeration Cycle

The application of natural refrigerant CO₂ is of great significance to reduce the greenhouse effect and ozone depletion. CO₂ transcritical refrigeration cycle is presently an important aspect of natural refrigerant alternatives research. Based on the natural refrigerant of CO₂ for the study, the CO₂ transcritical refrigeration cycle system is experimental studied and theoretical system optimization is made. Some fundamental data for the development and application of the CO₂ transcritical system are obtained.The natural refrigerant of CO₂ has great advantages in environment protection and physicochemical properties over the traditional synthesized refrigerant HCFCs and HFCs through a comprehensive evaluation. Theoretical analysis on CO₂ transcritical single stage compression refrigeration cycle is made. Results indicate that the evaporation temperature, gas cooler exit temperature and heat rejection pressure have important influence on the system performance. In this dissertation, an experimental study is made in order to testify the high-pressure-side regulation character and the influence of external heat source. The experimental results show that there is an optimum high pressure to achieve the maximum COP, the evaporation temperature and gas cooler exit temperature have great impact on the COP. Increase of gas cooler exit temperature led to a rapid increase in the optimal high pressure, but effect of evaporation temperature is not obvious; cooling water temperature, chilled water flow rate and temperature have a great impact on system performance, while the cooling water flow rate has no obvious effect.CO₂ transcritical two-stage cycle has better performance than the single stage one, two-stage compression is an effective method to improve the system performance. In this dissertation, system optimization of CO₂ transcritical two-stage cycle was employed mainly by theoretical analysis. Two different throttle types of two-stage cycle were compared, results show that the double-throttling cycle has a higher COP than the single-throttling cycle, and its optimum high pressure is less. The performance of CO₂ transcritical two-stage compression/ejection refrigeration cycle is analyzed. Results indicate that the COP of two-stage compression/injection cycle is better than the two-stage cycle obviously. A vortex tube expansion CO₂ transcritical two-stage refrigeration cycle is established and thermodynamic analysis results indicate that the COP improvement is 2.4%¹6.8% at given conditions compared to the two-stage cycle.