Evaluation of airflow prediction methods in compact electronic enclosures

In forced convection cooled electronic enclosure design, one of the most important parameters is enclosure airflow. Enclosure airflow mainly depends upon enclosure pressure drop and fan characteristics. Fan curves are often used in conjunction with system pressure drop (impedance) characteristics to determine airflow. CFD tool accuracy depends mainly on accurate modeling of system pressure loss (grilles, filters, etc.) and fan curve data accuracy. The fan curves, which show fan air delivery capacity at various pressure drops, are usually generated with no obstructions close to the fan. However, electronic systems contain densely packaged components, including airflow obstructions such as inlet and outlet grilles in close proximity to the fan, so methods using fan curves can often be inaccurate for airflow prediction. Inaccuracies can also occur by using grille pressure loss data from handbooks. The objective of this study is to understand the accuracy of airflow prediction methods use pressure loss and fan curve data compared to experimental results obtained in a wind tunnel. The system used is representative of typical electronic systems, which include major components such as fans, inlet and outlet grilles and an array of stacked PCBs. Further components such as capacitors, inductors, transformers and heat sinks are also included to increase the total pressure drop. Base configuration variations are made by changing grille open area, fan size and using fans in series and parallel configurations. It is found that differences of up to 20% can occur depending on the method used to calculate flow.