Effects of moving endwall on the unsteadiness of tip leakage flow in compressor cascades

Unsteady flow phenomena are common in turbomachinery and have serious impacts on its stable operation. This paper reports a self-induced unsteady phenomenon inside a centrifugal impeller due to strong secondary flow that has not been previously reported. A high-loading centrifugal impeller is selected as the research object, an accurate three-dimensional model is established, and exhaustive numerical simulations are performed based on unsteady compressible Navier–Stokes equations. The unsteady performance of the compressor is measured in detail during the experiment, and the experimental results are compared with the numerical simulation results to verify the reliability of the numerical simulation method. The results of the unsteady pressure signal show the existence of unsteady phenomenon when the operating point is far away from the choke boundary. The power spectrum results of the unsteady pressure distribution at the impeller blade tip are analyzed, and two different modes of this unsteady phenomenon are confirmed. Higher-order frequency characteristics are not observed near the stall boundary. The detailed analysis of the flow field inside the impeller clarifies that the instability of the interface formed between the secondary flow and the main flow induces this unsteady phenomenon. This unsteady pressure disturbance propagates inside the blade passage through pressure waves, which contributes to the instability of the interface and the existence of this self-induced unsteady phenomenon.

Numerical investigation of the self-induced unsteadiness phenomenon in the impeller of a high-loading centrifugal compressor

Li,

Cheng,

et al. 2023

The unsteadiness of tip leakage vortex breakdown and its role in rotating instability

Yang,

Wu,

Chen

et al. 2023

The unsteadiness due to tip leakage vortex (TLV) breakdown was studied using a special experimental test campaign in parallel with numerical simulations. The back flow vortex (BFV), an isolated vortex caused by TLV spiral-type breakdown, was found to play a key role in rotating instability (RI). High-speed pressure transducers were used to measure the unsteady pressure field at the casing end wall of the blade in an isolated subsonic compressor rotor, which identified a low-frequency fluctuation at the near stall condition. A single-passage unsteady Reynolds-averaged Navier–Stokes simulation was used to study the evolution of unsteady flow structures, validated by the experimental measurements. Two distinct kinds of periodically unsteady flow were revealed by the simulations. A high-frequency fluctuation corresponding to 1.0 blade pass frequency (BPF) was caused by the spiral-type breakdown of the TLV. The other low-frequency fluctuation corresponding to 0.5BPF was caused by the feedback interaction between the BFV and the blade loading. The BFV was generated by the TLV breakdown, which was separated from the twisted vortex core of the TLV, and it moved downstream along the pressure side of the adjacent blade. A larger sized BFV reduced the local loading of the adjacent blade. The TLV was weakened as a consequence of the reduced loading, resulting in a smaller sized BFV. The blade tip loading was relatively less affected by the small sized BFV rather than the larger sized BFV. Therefore, the blade loading recovered and the size of the BFV increased, repeating the cycle. This feedback mechanism produced a pressure fluctuation with a frequency equal to 0.5BPF, which was closely related to RI.

Influence of lightweight material on tip flow of a transonic centrifugal impeller based on bidirectional fluid-structure coupling

Li,

Zhang

2023

The energy consumption and stability of centrifugal impellers can be effectively improved by using a lightweight material. Tip flow is the main factor affecting energy consumption and operational stability of centrifugal impellers. In this study, the mechanism underlying the influence of material weight on the flow in the tip region of a centrifugal impeller was explored. First, a numerical model of a bidirectional fluid-structure coupling was established and validated. Then, a comparative analysis was conducted on the vibration deformation of 17-4PH, titanium alloy, aluminum alloy, and epoxy carbon UD (CFP) impellers under extreme stall conditions. Finally, the changing trends of shock wave structure, leakage flow, and secondary flow in the tip region of these four kinds of impellers were compared and analyzed. The results show that tip clearance decreases gradually with decreasing impeller material density. By comparing with a stainless-steel impeller, the tip clearance of a CFP impeller decreased by 53% at most, and the total displacement decreased by nearly 100% (except in the case of resonance). The shock wave of the CFP impeller can be characterized by fast detachment, fast dissipation, and minimal countercurrent. The leakage flow of the CFP impeller was uniform, the leakage vortex moved forward slowly, the volume of the vortex was small, and the flow velocity on the blade surface was also small. With decreasing impeller density, the influence of the secondary flow on the main flow gradually weakened. These results lay a theoretical foundation for optimizing the structural and aerodynamic design of centrifugal impellers.