Laser Insight Into the Turbulent Swirl Flow Behind the Axial Flow Fan

Abstract

Complex experimental study of the turbulent swirl flow behind the axial fan is reported in this paper. Axial fan with nine blades, designed to generate Rankine vortex, was positioned in the circular pipe entrance transparent section with profiled free bell mouth inlet. Two test rigs were built in order to study the turbulent swirl flow generated on the axial fan pressure side in the case of axially unrestricted and restricted swirl flows. One-component laser Doppler anemometry (LDA) and stereo particle image velocimetry (SPIV) were used in the first test rig in the measuring section 3.35D, measured from the test rig inlet. One of the latest measurement techniques, high speed SPIV (HSS PIV), was used for the measurements in the second test rig in the section 2.1D downstream the fan’s trailing edge. Achieved Reynolds numbers in the first test rig are Re = 182600 and 277020, while in the second Re = 186463. Turbulent velocity field non-homogeneity and anisotropy is revealed using the LDA system. Calculated turbulent statistical properties, such as moments of the second and higher orders, reveal complex mechanisms in turbulent swirl flow. It is shown for the used axial fan construction that swirl number has almost constant value for two various duty points generated by changing rotation number. Study of the instant and mean velocity fields obtained using SPIV discovers vortex core dynamics. Obtained percentage of the unique positions of the total velocity minimum are 10% for the first regime, while 11.5% for the second regime in the first test rig. HSS PIV experimental results have also shown the three-dimensionality and non-homogeneity of generated turbulent swirl flow. Experimentally determined and calculated invariant maps revealed three-component isotropic turbulence in the vortex core region.