Development On Supercritical CO₂ Compressor And Performance Analysis On Compression Process

Centrifugal compressor provides energy for the working medium through the high-speed rotating work of impeller,which has been widely used in industrial design process for its high efficiency,high pressure ratio and large operation range.With the increasing demand of energy and environmental protection,the supercritical carbon dioxide(SCO2)Brayton cycle has become one of the most advanced and challenging research directions.The working fluid near the critical point has the advantages of large specific heat capacity,high density and strong thermal stability,which can be adopted in the thermal cycle to save equipment space.Compared with the compression process at ordinary temperature and pressure,SCO2 compressor usually works near the critical point,the physical properties of the working medium change dramatically,and condensation may occur during the compression process,which makes the SCO2 compressor design face more unpredictable challenges.Therefore,it is of great significance to research the design process and flow structure in the stage of SCO2 centrifugal compressor in detail.Based on the knowledge of hydrodynamics,thermodynamics and aerodynamics,this paper firstly optimize and improve the thermal design process of centrifugal compressor,to make it applicable to the real gas.Combined with the software,the visualized program of compressor thermal design is developed,which improves the accuracy and usability of centrifugal compressor design.Secondly,the compression characteristics of several common chemical gases are analyzed,and the influence from the physical properties of working medium on the centrifugal compressor design is summarized,and the optimization direction of different gas compressor design is obtained.Based on the summary of the design characteristics,the design of SCO2 centrifugal compressor is finished in view of its special physical properties,and the numerical simulation method is used to make research on it.During the process of numerical simulation,different gas models are applied to describe the physical properties of working fluids,and comparisons appear the advantages of the real gas property(RGP)look-up table based on the physical properties of NIST database in SCO2 numerical research.Then RGP table is carefully inserted in ANSYS CFX software to obtain the reasonable numerical simulation results.To further improve the operation performance of SCO2 compressor,numerical method is used to calculate compressor performance under different compression starting point.The results show that the compressor performance will decrease since inlet pressure is close to the critical pressure,not like that the closer the starting point is,the higher the compressor performance is.For the industrial SCO2 compressor design,the recommended total inlet pressure is about 7.8MPa-8.6MPa,and the total inlet temperature is about 310-323KFinally,the passage flow loss factor has been modified to get more accurate theoretical performance prediction method for SCO2 centrifugal compressor.Combined with the predicted performance map and the numerical results,we find that,the passage flow loss covers the most part among all the compressor flow losses.The value of the passage flow loss factor applied in the theoretical performance prediction method ranges between 0.008～0.025,which is higher than that for air and some other traditional chemical gases.Considering the importance of compression starting point on the compression process,the performance evaluation by improved loss models at different compression starting points is compared with that by the numerical results,it was found that,closer to the critical point will make the passage flow loss increase a lot,which may even exceed its rational range and the produced loss is beyond expectation.Concluding from the above analysis results,it can be seen that for different chemical media,the characteristics of the physical properties of the working medium can directly influence flow structures and flow losses in the centrifugal compressor.The analysis results of this paper provide references for the industrial design and application of different media compressors.