Exit Flow Behavior of Axial Fan Flows With/Without Impingement

The exit flow patterns of an axial flow fan widely used in electronics cooling are experimentally characterized both in free exit and in the presence of a flat impingement plate. The axial fan is rotated with 12.0 V input from a dc power supply, leading to a nominal Reynolds number of $Re=9.0×103$ based on fan diameter. One shear layer each is found to form between the exit flow from the axial fan and the surrounding fluid at rest, and between the exit flow and the flow along the fan axis. In addition to creating a highest wall pressure region (the primary stagnation region), the presence of the flat plate induces a flow recirculation zone (the secondary stagnation region) at the plate center. When the fan exit-to-plate spacing normalized by fan diameter $(H/D)$ equals to about 0.6, the wall pressure is minimized in the secondary stagnation region due to the maximized “recirculation” as a result of intensified flow interaction. Within the range considered $(0.2≤H/D≤2.0)$ and with the case of $H/D∼0.6$ serving as a reference, the flow interaction tends to be suppressed by the proximity of the plate at $H/D=0.2$ and weakened due to the momentum dissipation at $H/D∼2.0$⁠.