Investigations on the Swirl Flow Caused by Axial Fans - A Contribution to the Revision of ISO 5801

E2 - Codes, Standards and Legislation

Actually the international standard ISO 5801 "Industrial fans — Performance testing using standardized airways" is in revision. In the setups with an "outlet common segment", as defined in ISO 5801, the fan pressure of an axial fan without outlet guide vanes can differ considerably to higher or lower values from the value under duty conditions. This difference depends on the details of the swirl flow at the exit of the impeller with its specific blade design (airfoil, blade shape, vortex design, ...).

The estimation of characteristics at standardized setups with an "outlet common segment" can create a benefit of pressure rise for the fan, but cause also an increase of losses at the flow straighteners, which cannot be corrected with the defined "loss allowances" in ISO 5801. Setups with "outlet duct simulation" or "short duct at the outlet" perform correct measurements of the pressure difference.

To get an impression of the flow and to visualize the vortex which causes this discrepancy High-Speed-Stereo-PIV and pressure measurements were carried out in the downstream tube of an axial fan without guide vains. The HSS-PIV is performed in a cross section as well as in a vertical meridian plane perpendicular to that cross section to visualize the behavior of the vortex further downstream. The intention of the paper is to document and to help to understand the complex flow pattern behind an axial fan. Therefore it delivers detailed information of the velocity component distribution, furthermore comparing them at three different operating points.
The results are presented in movies and time averaged contour plots.

Keywords: axial fan, turbulent swirl flow, High-Speed-Stereo-PIV.