为探究入口压强小于1个标准大气压时对离心压缩机性能及流动结构的影响, 该文以Krain离心式叶轮为研究对象, 采用Spalart-Allmaras(S-A)方程湍流模型的有限体积法求解Reynolds时均Navier-Stokes(Reynolds time-averaged Navier-Stokes, RANS)方程组, 得到入口体积流量恒定时入口压强400~101 325 Pa条件下离心叶轮的性能及内部流场结构。结果表明： 随着入口压强减小, Krain叶轮总压比和总等熵效率先缓慢减小再急剧减小; 相比于在101 325 Pa压强下, 当压强为400 Pa时, Krain叶轮总压比和总等熵效率减小幅度超过10%。入口压强减小使叶轮及无叶扩压器内气体比熵增增加, 其中由叶轮出口处反流、 无叶扩压器轮盘侧边界层导致的比熵增显著增加。入口压强在1 333~101 325 Pa变化时, 叶轮及扩压器内流场特征差异性较小, 但轮盘侧边界层厚度随压强减小而增加, 当入口压强减小至400 Pa时, 叶轮出口处反流速度增大、 区域扩大, 扩压器内轮盘侧边界层厚度显著增加, 使流动损失增加。故在负压压缩机初始设计阶段中采用基于正压的一维模型预测性能时, 应考虑对性能进行负修正, 且修正程度随入口压强减小而增加。
[Objective] A centrifugal compressor is a key equipment in the gas diffusion isotope separation cascade at pressures below 5 000 Pa, and its performance affects the economics of the separation cascade. Currently, only a few studies have focused on the performance and internal flow characteristics of centrifugal compressors under negative pressure conditions (inlet pressure ≤101 325 Pa), especially when the inlet pressure is as low as 400 Pa. Additionally, in our experiments with the designed centrifugal compressor, an increase in inlet pressure improved the pressure ratio for the same inlet volume flow rate. Therefore, investigating the impact of the inlet pressure on the performance of centrifugal compressors is necessary. [Methods] Taking the Krain centrifugal impeller as the research object, the finite volume method is used to solve the three-dimensional Reynolds time-averaged Navier-Stokes (RANS) equations, and the ideal gas state and Spalart-Allmaras turbulence model equations are substituted into the RANS equations. A single flow channel calculation model, including the inducer, impeller, and vaneless diffuser, is meshed by 1.88 million hexahedral structural grids. The thickness of the first layer of the grid near the walls is 6 μm, and the y+ value in most calculation domains is set to be less 10, satisfying the grid independence verification. The boundary conditions are as follows: total pressure, total temperature, and velocity along the axis direction are given at the inlet, while the mass flow rate is provided at the outlet; all walls are no-slip adiabatic boundaries. The relative deviation between the calculated values of the verification case and experimental values is approximately 3%, indicating the accuracy and reliability of the calculation method. [Results] The numerical results were summarized as follows. (1) As the inlet pressure decreased, the total pressure ratio and total isentropic efficiency of the compressor first decreased slowly and then decreased rapidly. When the pressure was 400 Pa, performance was reduced by 10% compared with that at 101 325 Pa, and the range of stable operating conditions decreased. (2) The specific entropy increase in the compressor components gradually increased with decreasing inlet pressure, implying that the specific flow loss was improved. Among them, the specific entropy increase in the vaneless diffuser increased faster than the two other components. (3) The distribution of the specific entropy increase revealed that the decrease in the inlet pressure reinforced backflow and secondary flow losses at the impeller outlet, as well as boundary layer lossed on the hub and shroud side of the diffuser. (4) The flow characteristics showed that when the inlet pressure decreased to 400 Pa, the backflow velocity at the impeller outlet increased, and the backflow region expanded. In the vaneless diffuser, as the inlet pressure decreased, the gradient of the radial velocity on the hub side and that of tangential velocity on the hub and shroud sides decreased, indicating a thicker boundary layer. [Conclusions] The decrease in the inlet pressure increases the thickness of the boundary layer on the hub side of the vaneless diffuser and enhances backflow and secondary flow at the impeller outlet, increasing the boundary layer, backflow, and secondary flow losses and subsequently decreasing compressor performance. Therefore, during the initial design stage of negative-pressure centrifugal compressors, performance predicted using a one-dimensional performance model based on positive pressure is high and needs to be corrected.
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