Study of real-gas effects on aerodynamic performance of SCO2 centrifugal compressors
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摘要:
利用相似理论和数值模拟对压气机中真实气体气动与工质热物性耦合影响性能的问题进行解耦，对比研究了不同进口工况下超临界二氧化碳离心压气机特性和内部流场，探讨了近临界点真实气体物性变化和冷凝对压气机焓增、效率、堵塞边界的影响。结果表明：真实气体效应的影响可分为蕴含于气动性能参数热力学定义中的热力学影响和蕴含于压气机内部流场中的气动影响，其中气动影响占主导；研究气动影响时可借助相似性能参数排除热力学层面的干扰，当真实气体效应增强时，工质压缩性的减弱将导致叶轮扭速降低从而使相似焓增减小，叶轮负荷的减轻会减小负荷相关气动损失从而使相似效率提升，两相流的存在以及喉道冷凝诱发的提前堵塞会导致堵塞边界的前移。
Abstract:By utilizing similitude theory and numerical simulation, the coupling effects of real-gas aerodynamics and thermophysical properties on the performance of compressors were decoupled. The characteristics and internal flow fields of a supercritical carbon dioxide centrifugal compressor were investigated under different inlet conditions, and the real-gas effects on the enthalpy increase, efficiency, and choke margin of the compressor were explored. The results showed that real-gas effects on the compressor performance can be divided into two categories: thermodynamic effects inherent in the thermodynamic definitions of the performance parameters and aerodynamic effects within the compressor's internal flow field. Among them, aerodynamic effects played a dominant role. By leveraging similitude parameters, the thermodynamic effects can be eliminated. When the real-gas effects were intensified, the reduction in fluid compressibility decreased the impeller’s Euler work, leading to a reduction in enthalpy increase. The alleviation of the blade loading reduced load-related aerodynamic losses, thereby improving the efficiency. The presence of two-phase flow and the onset of choking at the throat led to the reduction of choke margin.
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图 4 不同温度分辨率下的比定压热容曲线
Figure 4. Curves for constant-pressure specific heat at different temperature resolutions
图 6 均相平衡态相变数值模拟得到的喷管内部湿度分布
Figure 6. Wetness distribution inside the nozzle obtained by numerical simulation with homogeneous equilibrium model
图 7 Sandia压气机数值与试验特性曲线对比
Figure 7. Comparison of numerical and experimental characteristic curves for the Sandia compressor
图 10 沿近临界点等熵线膨胀过程的密流变化
Figure 10. Mass flux density distribution in an expansion process along an isentrope
图 11 沿等熵线的声速变化与喷管马赫数分布
Figure 11. Variation of sonic speed along an isentrope and the Ma distribution in the nozzle
图 12 不同工况的堵塞状态下喉道、相变位置、Ma=1位置的空间分布
Figure 12. Spatial distribution of throat, phase-transition position and Ma=1 position under different inlet conditions
图 13 真实气体效应影响压气机气动性能的分析思路
Figure 13. Analytical strategies for real-gas effects on the aerodynamic performance of compressors
图 14 进口工况对CO2压气机气动性能的热力学影响
Figure 14. Thermodynamic effects of inlet conditions on aerodynamic performance of CO2 compressors
图 15 近临界点与远临界点压缩过程的等熵压缩功对比
Figure 15. Comparison of isentropic compression work for compression processes beginning from near and far critical points
图 16 ΠU =1.33时压气机的特性曲线对比
Figure 16. Comparison of the characteristic curves at the speed line ΠU =1.33
图 17 ΠU=1.33时压气机的相似性能曲线对比
Figure 17. Comparison of the similitude performance at the speed line ΠU=1.33
图 18 设计点进口速度三角形形状对比
Figure 18. Comparison of the inlet velocity triangles at design point
图 19 设计点进口相对欧拉数的展向分布对比
Figure 19. Comparison of the inlet spanwise distribution of the relative Eu at design point
图 20 径向速度增大导致扭速减小
Figure 20. Decrease of twisted velocity due to increase in radial velocity
图 22 3个算例中湍流耗散熵产对比
Figure 22. Comparison of turbulent dissipative entropy production in three cases
图 23 3个算例中湍流耗散熵产的各项分量对比
Figure 23. Comparison of turbulent dissipative entropy production components in three cases
图 24 离心叶轮内叶根、叶中、叶尖负荷分布对比
Figure 24. Comparison of blade loading at root, mid and tip in impeller
图 25 离心叶轮内不同流向位置的二次流强度对比
Figure 25. Comparison of the secondary flow intensity at different streamwise locations in impeller
图 26 无叶扩压器内不同流向位置的流动不均匀度对比
Figure 26. Comparison of flow non-uniformity at different streamwise locations in vaneless diffuser
图 27 堵塞工况下进口速度三角形对比
Figure 27. Comparison of inlet velocity triangles at choking condition
图 28 堵塞工况下90%叶高截面的湿度分布对比
Figure 28. Comparison of wetness contour on 90% span at choking condition
图 29 堵塞工况下90%叶高截面的相对马赫数分布对比
Figure 29. Comparison of the relative Mach number contour on 90% span at choking condition
图 30 3个算例中压气机流量增大时进口速度三角形的变化
Figure 30. Changes in inlet velocity triangles with increasing mass flow in three cases
表 1 SCO2压气机设计工况与几何尺寸
Table 1. Operation conditions and dimensions of the studied SCO2 compressor
类型 参数 数值 设计工况 进口总温T01/K 305 进口总压p01/kPa 7000 转速N/（r/min） 50000 流量$ \dot m $/（kg/s） 20.5 总压比πtt 3.25 几何尺寸 进口轮毂半径/mm 10 进口轮缘半径/mm 25.87 进口轮缘叶片角/（°） −65 出口半径/mm 50.57 出口叶高/mm 4 出口后弯角/（°） −50 叶顶间隙/mm 0.2 主叶片数 9 分流叶片数 9 
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表 2 叶轮机通用相似参数
Table 2. Similitude parameters for gas turbomachinery
相似参数 表达式 流量相似参数$ {\varPi _{\dot m}} $ $ \dfrac{{\dot m\sqrt {{R_{\text{g}}}{T_{01}}} }}{{{p_{01}}}}\sqrt {\dfrac{{{Z_{01}}}}{{{n_{\text{s}}}}}{{\left( {\dfrac{{{n_{\text{s}}} + 1}}{2}} \right)}^{\tfrac{{{n_{\text{s}}} + 1}}{{{n_{\text{s}}} - 1}}}}} $ 转速相似参数$ {\varPi _U} $ $ \dfrac{{{u_2}}}{{\sqrt {{n_{\text{s}}}{Z_{01}}{R_{\text{g}}}{T_{01}}} }} $ 黏性相似参数$ {\varPi _\nu } $ $ \dfrac{{\rho {u_2}{r_2}}}{\mu } $ 焓增相似参数$ {\varPi _{\Delta H}} $ $ \dfrac{{\Delta H}}{{{n_{\text{s}}}{Z_{01}}{R_{\text{g}}}{T_{01}}}} $ 效率相似参数$ {\varPi _\eta } $ $ {\eta _{\text{p}}} $
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表 3 3种工质的相似运行工况
Table 3. Similar operating conditions for the three fluids
算例 工质 T01/K p01/kPa Z01 ns N/（r/min） Re/108 理想气体 理想气体CO2 305 7000 1 1.285 68 300 2.98 真实气体 真实气体CO2 305 7000 0.5 1.384 50 000 3.24 近临界点真实气体 真实气体CO2 （近临界） 305 7500 0.33 1.477 42 222 3.25
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表 4 真实气体效应在热力学层面和气动层面对等熵效率影响的对比
Table 4. Comparison of real-gas effects on isentropic efficiency at thermodynamic and aerodynamic levels
% 算例 效率 效率提升 （相对于理想气体） 理论ηis（相似转换） 实际ηis（CFD） 热力学层面 气动层面 合计 理想气体 89.5 89.5 真实气体 89.8 90.5 +0.3 +0.7 +1.0 近临界点真实气体 90.1 91.4 +0.6 +1.3 +1.9
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