Effect of Obstruction Near Fan Inlet on Fan Heat Sink Performance

Compact electronic enclosures requiring forced air cooling often have airflow obstructions located very close to the fan inlet. The adverse effects of these obstructions are not well understood even using modern CFD techniques. In this study a thorough experimental investigation was carried out on the thermal, airflow and acoustic effects of an obstruction very near to the fan inlet for four fan-heat sink combinations. It was found that as the obstruction was brought closer to fan inlet, the device case temperature increased significantly. This was due to a combined effect of reduction in the fan airflow and an increase in the fan inlet air temperature. The overall degradation in performance was more severe for obstruction heights less than 15mm. A reversal in the fan flow direction was observed for one fan when the obstruction was very close to the fan inlet (less than 7mm). The acoustic noise level (sound power) also went up for 3 of the 4 fan heat sinks which was partly due to an observed natural increase in the fan rotational speed. The thermal performance improved when the fan speed was artificially increased but with higher sound power levels