Investigation On The Influence Of Supercritical CO₂ Centrifugal Compressor Inlet Condition And Correlation Between Leakage Flow And Stall

Supercritical carbon dioxide(CO2)Brayton cycle has many advantages,such as high efficiency,compact equipment,no pollution of working medium and so on.It has promising applications on nuclear energy,solar energy,waste heat utilization and other fields.However,the sharp change in properties of CO2 near the critical point has an important influence on the system parameter selection,control,component design and test.Exploring the influence of the special properties of supercritical CO2 on the performance of system and compressor is of positive significance to the design of efficient and stable supercritical CO2 compressor,the efficiency improvement of supercritical CO2 Brayton cycle and the commercial application of supercritical CO2 Brayton cycle.In this paper,the influence of inlet conditions of supercritical CO2 centrifugal compressor on the system and compressor,as well as the compressor stall are studied.Firstly,the supercritical CO2 Brayton simple and recompression cycle with components are analyzed,in view of the problem that the existing system research seldom consider the component characteristics.The variation of efficiency and investment cost of supercritical CO2 cycle under different constraint conditions are compared.It is found that for simple cycle,the optimal outlet pressure tends to decrease with the increase of constraint strength but always over 30MPa.When the outlet pressure exceeds 25MPa,the achievable efficiency of the cycle is about 0.3-0.9 percentage points lower than the theoretical maximum efficiency due to the constraints of feasibility conditions.After considering the feasibility condition of components,the corresponding investment cost increases by 0.02-0.05k$/kW,and the outlet pressure of compressor corresponding to the lowest investment cost decreases at certain degree,between 22.5-30MPa;for the recompression cycle,the theoretical maximum efficiency is always higher than the realizable efficiency after considering the feasibility condition,the maximum deviation is close to two percentage points,and the investment cost is about 0.01-0.5k$/kW higher than that of simple cycle.Secondly,based on the dimensional analysis method,the dimensionless number affecting the compressor characteristics is derived.Under the premise of ensuring the same dimensionless parameters in operation,the influence of the inlet isentropic exponent on the compressor aerodynamic characteristics is numerically and theoretically studied.It is found that the increase of isentropic exponent will decrease the work coefficient and the Mach number at the impeller outlet,but increase the dimensionless axial thrust,the Mach number at the suction surface and the leading edge blade loading.Based on the conservation law and isentropic relationship,the aerodynamic analysis of one-dimensional and quasi 3-D turbomachinery also confirmed these effects.Moreover,the Reynolds number effect can not be ignored in the aerodynamic design and physical test of three-dimensional supercritical CO2 centrifugal compressor.Thirdly,the reason for the divergence of the solver caused by the properties of supercritical CO2 is analyzed to improve the numerical stability of the solver.Under this basis,the full passages unsteady numerical simulation of supercritical CO2 compressor is carried out.The results of unsteady simulation under different inlet conditions are in good agreement with the experimental results.According to the simulation results of near stall point,it is found that the stall of IET supercritical CO2 centrifugal compressor is caused by the spillage of leakage flow from the leading edge of some blade passages,which shows strong non-axisymmetry characteristic.The correlation between fluctuation number and leakage flow is further analyzed.The circumferential propagation speed of the rotational instability of the MWe supercritical CO2 centrifugal compressor is obtained by using the Fast Fourier transform(FFT)based on the full passage unsteady resultsFinally,on the basis of determining the correlation between the leakage flow and stall inception and the corresponding inducing mechanism,the self circulation casing treatment is used to effectively control the leakage flow intensity near the stall point,restrain the interaction between leakage flow and adjacent blades,broaden the stall margin and increase the blockage margin of compressor.The numerical results shows that a technical approach is provided for realizing high efficiency and high stability supercritical CO2 Brayton cycle in the future.