System Performance of Highly Coupled Fan and High-Resistance Medium

This article presents an experimental study on the performance of a system consisting of a fan and a high-resistance medium (or an HRM). The HRM used in this study is a simplified model for cooling fins, heat exchangers, memory dual in-line memory module (DIMMs), air filters, etc. The study focuses on comparing the pressure difference across a closely coupled fan and HRM arranged in the draw-through configuration (HRM placed in front of a fan) and the blow-through configuration (HRM placed behind a fan) as a function of the gap distance between the fan and the HRM, along with the efficiency of moving air through these systems. System efficiency is defined to quantify system performance; it is a measure of the actual power input to push the air through the system relative to the ideal power required to achieve the same task. For the specific fan and HRM configurations studied here, the results show that system efficiency is a strong function of gap when the gap is approximately 15% of fan diameter or less for the draw-through configuration, and 10% of fan diameter or less for the blow-through configuration. Furthermore, it has been determined that the blow-through configuration is significantly less efficient than the draw-through configuration.

I. Introduction

Computing power density is continually increasing and reaching the point where electronic components are packed tightly into server boxes. Consequently, the available flow area for air cooling is reduced, resulting in higher flow resistance as the airflow speed is higher. At the same time, airflow rate requirements also go up to accommodate higher cooling demands. Hence, fans are required to generate more pressure rise and higher flow rates, and they must operate efficiently and quietly in the presence of surrounding electronic components. Because of the server’s size restrictions, enlarging the fan size is not an option. Therefore, a common method to increase fan pressure rise and flow rate is to increase fan speed. Unfortunately, energy usage and noise depend strongly on fan speed, with energy usage scaling with RPM to the power of three and noise scaling with RPM to the power of six. Finally, flow interaction between closely coupled fan and surrounding components can result in a significant change in fan and system performance.