This study describes the deterioration of a small axial fan's supply flow rate in high-density packaging electronic equipment. Forced convection driven by cooling fans is the general method for removing heat from electronic equipment. A cooling fan flow rate can be predicted by its P–Q curve, which shows a relationship between a pressure rise at a fan (ΔP) and a supply flow rate (Q). An operating point of a fan, defined as operating pressure and a flow rate, is a point of intersection of the P–Q curves and flow resistance curves in electronic equipment. An accurate prediction of supply flow rate by fans strongly affects the reliability of the cooling design.Ordinarily, P–Q curves are given by fan manufacturers as simple catalog values. Catalog values are usually measured by a chamber method in accordance with measurement standards such as AMCA　Standard 210, JIS B 8330, or JBMS-72-2003. However, such catalog values are obtained under ideal conditions that differ significantly from actual operating conditions. Especially in high-density packaging electronic equipment, the fan performance is affected by the mounting components around the fans, and the accurate prediction of the supply flow rate becomes difficult. A quantitative understanding of how fan performance is affected by an actual operating environment is therefore required.Our previous research has conducted the relationship between the fan performance deterioration and the obstructions mounted near the fan. In this paper, we tried to do flow visualization around a small axial cooling fan's impellers when the obstruction was mounted in front of the fan through CFD analysis. By comparing the deterioration of the fan performance with the flow pattern, the fan performance deterioration mechanisms by the obstructions were clarified. As the test fan, a 40 mm-scale small axial fan was used. The Large-Eddy Simulation (LES) with the moving element was conducted. As the obstruction, a simple square plate (90 mm x 90 mm) was mounted in front of the test fan. A relationship between the supply flow rate by the fan and the flow pattern around the impellers was investigated while changing the distance between the test fan and the obstruction. The comparison with the experiment confirmed the reliability of predicting the supply flow rate by the proposed CFD analysis.Through the research, the following information can be obtained. The proposed CFD analysis can predict the deterioration of the P-Q curve by the obstruction. In the rotating stall region and the high flow rate operating points, the fan's flow is stable regardless of the obstruction's presence or absence. At the lower flow rate conditions, reverse-flow vortices generate from the fan to the upstream side, and the formation of a complex unsteady flow is reproduced. As the flow rate decreased, the attack angle of the flow to the impellers increases concerning the flow, and the separation point of the flow becomes closer to the leading edge of the impeller. When the obstruction is mounted, the flow's axial velocity into the fan is decreased, and the radial velocity is increased. The change of the flow pattern causes the larger attack angle. Besides, the fan's restricted flow direction makes it easier to interfere with the reverse flow caused by the pressure increase in the wake side. Due to the restriction, the smaller the distance between the obstruction and the fan causes more significant deterioration of the fan itself's supply flow rate.

CFD - Based Investigation of Effects of Obstruction in Front of Small Axial Cooling Fan and Deterioration of Supply Flow Rate

Paper Proceedings: https://asmedigitalcollection.asme.org/InterPACK/InterPACK2021/volume/85505

ASME 2021 International Electronic Packaging Technical Conference and Exhibition (InterPACK2021)

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