Transient flow characteristics and energy loss investigation in a desalination energy recovery device under rotor system axial sliding conditions: Focusing on the turbine side

The energy recovery integrated machine can effectively reduce the energy consumption of a seawater desalination system by harnessing the residual pressure energy in high-pressure seawater waste liquid. However, the axial force imbalance between the pump side and the turbine side can lead to axial sliding of the rotor system, and then change the axial clearance size. The objective of this paper is to investigate the unsteady flow pattern in the integrated machine with rotor system axial sliding, focusing specifically on the turbine side under coupled operation conditions. The results show that the increased axial clearance size can lead to the turbine performance decreases obviously but that of the pump side is little affected. The results of entropy production analysis show that impeller, draft tube and volute are core regions of energy loss in the turbine. The flow instability in the impeller, draft tube, and chamber is amplified by the increased axial clearance size, resulting in a greater turbulent kinetic energy dissipation. Unsteady flow phenomena such as jet, flow separation and vortex in the front chamber cause great energy loss. The dominant frequency of pressure fluctuation in the impeller is generally the axial frequency f(n). The dominant frequency in the chamber is 6f(n). With the increased axial clearance size, the flow instability in the clearance and the front chamber is enhanced. The increased axial clearance size results in the generation of large-scale leakage vortex in the draft tube outlet region. The dominant frequency near the wall of the draft tube gradually changes from 6f(n) to f(n). As the axial clearance size increases, the flow instability in the draft tube is intensified by the gradual increased strength of clearance leakage vortex outside the draft tube.