Research On The Flow Characteristics Of Supercritical Carbon Dioxide Centrifugal Compressor

With the shortage of energy and the proposal of new energy,more and more attention has been paid to the high performance dynamic cycle.The Brayton cycle with supercritical carbon dioxide?S-CO₂?as a working medium can obtain high energy output with a high efficiency of up to 50%.The compressor is the key to the S-CO₂ Brayton cycle,and its flow characteristics are directly related to the operation of the whole system.In this paper,the CFD numerical simulation method is used to study the centrifugal compressor with S-CO₂ as the working medium.Its thermodynamic process,performance and flow field characteristics,and the generation and influence factors of phase transformation in S-CO₂ impeller are analyzed.The following are the main contents and related conclusions:1.The special properties of S-CO₂ are analyzed and the method of establishing S-CO₂ property variable model are given,successful simulated the S-CO₂ centrifugal compressor media by using the commercial CFD software.According to the results,obtained the numerical simulation analysis of S-CO₂ compression process of thermal power conversion process.And the paper presents a thermodynamic formula suitable for S-CO₂ is different from the common law,pointed out the gas constant and the compression factor Z of S-CO₂ is smaller,larger specific heat capacity at constant pressure,which can lead to S-CO₂ more easily compressed and compressed to the same compression ratio caused by the temperature increase of the consumption of the compression process is small compared to the ideal gas.It embodies the superiority of S-CO₂ medium centrifuge compressor compared to the ideal gas medium compressor.This is also the core and the key advantage of the S-CO₂ Brayton cycle.2.Through analyzing and comparing the flow pressure ratio characteristics and the flow efficiency characteristics of centrifugal compressor impeller and the specific state parameters of similar working points,we get the difference between the performance and characteristics of S-CO₂medium centrifugal compressor and ideal gas medium centrifugal compressor.Under similar working conditions,the pressure ratio of the impeller can be 2.7 times that of the ideal gas when the S-CO₂ is a working fluid.The efficiency of the S-CO₂ medium impeller can reach 0.888,which is higher than the efficiency of the ideal gas medium impeller 0.799.3.Detailed description for the details of the flow field characteristics of S-CO₂ and ideal gas medium centrifugal compressor impeller,the difference between S-CO₂ and ideal gas flow characteristics of centrifugal compressor impeller are analyzed,as well as the special flow field of centrifugal compressor impeller that caused by S-CO₂ medium.4.Through the analysis of the flow field in the S-CO₂ medium impeller,the position and region of the phase transition in the S-CO₂ medium impeller and the causes are summarized.The phase transition is concentrated in the suction surface at the front of the main blade,and the range of phase change increases with the height of the blade height.The reason of phase transformation can be summarized as the particularity of S-CO₂ physical property,leading to the formation of complex vortex structure at the leading edge of main blade,which causes the working temperature drop to below the critical temperature.5.The effects of impeller flow,impeller speed,inlet total temperature and inlet flow angle on phase transformation are discussed.With the increase of flow rate and speed,the range of phase transformation increased.With the increase of inlet total temperature and inlet flow angle,the range of phase transformation decreased.It is found that when the given inlet total temperature is 309K or the inlet air angle is given 40 degrees,the phase transition area decreases obviously.