Session: Flow Visualization and Regular Poster Session

Paper Number: 170504

170504 - Numerical Flow Visualization of Tip Leakage Vortices in Axial Fans Using Les 

Abstract: 

In axial fans, the presence of a tip clearance or gap between the impeller blade tips and the outer casing is unavoidable due to manufacturing tolerances and thermal expansion during operation. While a minimal clearance is desirable to reduce aerodynamic losses, even small gaps lead to complex three-dimensional flow phenomena that can significantly degrade the fan performance. One of the most critical features of the blade tip flow is the formation of vortex structures caused by the pressure difference between the suction side and the pressure side of the impeller blades. This pressure difference induces a secondary flow through the tip gap, resulting in the generation of tip leakage vortices. These vortices interact with the main flow and lead to increased turbulence, unsteady flow behavior, and energy dissipation.

This study focuses on the numerical visualization and analysis of vortex structures arising from the tip clearance flow in axial fans. To capture the detailed unsteady flow features, a high-fidelity, transient Large Eddy Simulation (LES) was performed using the commercial solver ANSYS CFX. The computational model represents a realistic axial fan geometry, and the mesh resolution was chosen to adequately resolve the tip region and capture the small-scale flow structures. Post-processing and vortex visualization were performed in ANSYS CFD-Post using well-established vortex identification criteria such as Q-criterion.

The investigation covers multiple operating conditions across the fan performance curve, including part-load and overload conditions. These different flow regimes significantly affect the size, strength, and interaction of the tip vortices. The visualizations reveal how the tip leakage vortices evolve and interact with the main flow and how their behavior changes depending on the operating point. In part-load conditions, for instance, the vortex structures exhibit increased unsteadiness and greater penetration into the passage flow, while in overload conditions, the interaction with other secondary flow phenomena becomes more dominant.

The results provide valuable insight into the loss mechanisms associated with tip clearance flow and underline the importance of minimizing tip gaps in design. Furthermore, the high-resolution flow visualizations offer a deeper understanding of the complex vortex dynamics in the rotor tip region, which is essential for improving fan efficiency and guiding future design optimizations.

Presenting Author: Manuel Fritsche Coburg University of Applied Sciences

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