Effects of tip-gap width on the flow field in an axial fan

Abstract

The turbulent low Mach number flow through an axial fan at a Reynolds number of 9.36 × 105 is investigated by large-eddy simulation (LES). Computations are performed for a fixed flow rate coefficient Φ=0.165 and two tip-gap widths, i.e., s/Do=0.01 and s/Do=0.005. A finite-volume flow solver in an unstructured hierarchical Cartesian setup for the compressible Navier–Stokes equations is used. To account for sharp edges, a fully conservative cut-cell approach is applied. A periodic boundary condition is used in the azimuthal direction such that only one out of five blades is resolved. The focus of this numerical analysis is on the impact of the tip-gap size on the overall flow field. Special attention is paid to the vortical structures around the tip-gap region and the turbulent transition on the suction side of the blade. It is shown that the reduction of the tip-gap size totally changes the flow field. The blade-wake interaction, which was evident for s/Do=0.01, vanishes for s/Do=0.005. Furthermore, depending on the tip-gap width different mechanisms are responsible for the turbulent transition on the suction side of the blade, i.e., for s/Do=0.01 a cyclic transition is observed which is triggered by the blade-wake interaction, where for s/Do=0.005 a separation bubble leads to a permanent transition. A reduction of the tip-gap width decreases the amplitude of the tip-gap vortex wandering and as such the region of influence of the turbulent wake and increases the frequencies of the dominant modes. Since this is confirmed by the experimental results of the sound power level, it also shows the dominant role of the tip-gap vortex in the noise emission.