Modeling fan-induced flow distortion in a push-type telecom shelf

Slot airflow measurements in a push-type telecom shelf revealed significant non-uniformity produced by the strongly swirling and highly directional annular exhaust flow from the three high hub-tip ratio, mixed-flow fans. Similar flows were measured with plenum heights of 30 mm and 58 mm above the fans. A shelf CFD model was created, complemented by testing and modeling of a single fan in a test rig. The fan geometry was modeled exactly, but without the blades and motor struts, and tangential/axial body forces were added to the appropriate momentum equations in the fan volume to reproduce the test rig swirl and pressure rise respectively. Exhaust velocities were also measured, one-half tip diameter from the fan discharge, providing the radius of peak exhaust velocity. This radius was then matched in the test rig CFD model by imposing a radial body force in the fan volume. The resulting calibrated body forces were then applied to the shelf model, improving agreement with experiment compared to the same fan model without the calibrated radial force. To facilitate such calibration, either by electronic cooling software vendors or users, fan manufacturers should provide measured exhaust swirl (most important) and radial spreading (less important) data as well as the usual pressure rise curves.