PIV-Based Analysis of Internal Flow Evolution and Coherent Structures in a Semi-Open Axial Flow Fan

The internal flow of a semi-open axial flow fan is highly three-dimensional and unsteady due to the absence of a confined passage. The evolution of complex vortical structures, such as the tip leakage vortex (TLV) and corner separation vortex (CSV), remains poorly understood. This study used high-resolution particle image velocimetry (PIV) to conduct multi-region, multi-view measurements of the flow field in a semi-open fan for an outdoor air conditioning unit. The generation, development, and breakdown of the TLV were analyzed, revealing transient nonuniform flow and wake evolution. Dynamic mode decomposition (DMD) was applied to extract dominant frequencies and spatial modes. The results show that the TLV has a dominant frequency of 98.5 Hz (2.19 times the rotational frequency), accounting for 88.5% of the total energy, and exhibits periodic shedding and asymmetric breakdown. The CSV dominates at 16.44 Hz, slightly above blade rotation, and interacts with the TLV. In the wake region, the dominant frequency is 248.45 Hz, arising from the nonlinear superposition of TLV harmonics, the CSV frequency, and the blade passing frequency. This study provides an experimental basis and a low-dimensional coherent structure model for internal flow diagnostics and the structural optimization of semi-open axial flow fans.