Numerical Simulation Of Flow Field Characteristics Of Supercritical Carbon Dioxide Centrifugal Compressor

Supercritical carbon dioxide（SCO₂）Brayton cycle system has attracted much attention for its small equipment volume,simple cycle arrangement and high cycle efficiency.Centrifugal compressor is one of the key components of SCO₂ circulation system,which plays a decisive role in the efficiency and stable operation of the system.Compared with the air compressor,the physical property parameters of CO₂ change dramatically near the critical point,leading to the condensing phenomenon of the internal flow of the compressor.The internal flow field characteristics of SCO₂centrifugal compressor are quite different from that of the traditional air compressor.Taking the SCO₂ centrifugal compressor of Sandia Laboratory as the research object,this paper analyzed the 3D flow phenomenon in impeller machinery in detail through the full 3D numerical numercial simulation.In view of the fact that CO₂ is prone to phase transformation near the critical point,in order to accurately describe the physical properties of CO₂ two-phase region,the equilibrium phase transformation model and CO₂ real properties table are used to simulate.Secondly,the performance,flow field structure,blade load,condensation and other characteristics of the compressor are deeply studied.Finally,the influence of speed,inlet conditions,blade number and other factors on the performance,flow characteristics and condensation characteristics of SCO₂ centrifugal compressor and its mechanism were investigated.The results show that:（1）The low-temperature and low-pressure region is mainly located near the leading edge of the main blades and the splitter blades at the design speed.Its formation reason mainly includes two aspects,one is due to the local acceleration of gas;Second,there is a certain angle of attack when CO₂ flows into the impeller,resulting in large area of flow separation and loss near the tip leading edge of the main blades and the splitter blades,resulting in the decrease of local temperature and pressure values.With the increase of flow rate,the flow angle of attack changes from positive to negative,and the flow separation area,reverse flow area and condensation area at the leading edge of the main blades transfer from suction surface to pressure surface.（2）Under the design speed,internal loss of the SCO₂centrifugal compressor impeller includes flow separation loss of the main blades tip and the splitter blades tip on the leading edge caused by flow angle-of-attack,flow separation loss and mixing loss resulting from the pressure difference of suction and pressure surface,and mixing loss caused by the mainstream and blade boundary layer in the radial secondary flow mixing.（3）Under constant flow coefficient conditions,different rotational speeds and different inlet conditions,the low-temperature and low-pressure regions have the same location and formation reasons.The impeller efficiency and pressure ratio can be improved by reducing the inlet temperature and increasing the inlet pressure.However,the lower the inlet total temperature is,the greater the possibility of phase transformation in the impeller is,the higher the degree of condensation is,and the range of stable working conditions of the compressor is reduced.When selecting compressor parameters,it is necessary to make a reasonable choice among efficiency,pressure ratio and stable operating range.（4）The increase in the number of blades improves the reverseflow phenomenon at the splitter blades and the flow separation phenomenon at the trailing edge of the blade at the cost of loss compressibility energy,thus reducing the loss.The results of this paper have certain reference value for the design and research of SCO₂ centrifugal compressor.