Abstract
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.
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.
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.
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.
