LAB 4 - FAN system Fan Configuration and Airflow Impedance Lab Manual ME114 Fall 2008

LAB 4 - FAN system Fan Configuration and Airflow Impedance Lab Manual ME114 Fall 2008

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Mechanical and Aerospace Engineering Fan Configuration, Airflow Impedance, and Heat Sink Design ME 114 Lab 4 Fall 2008 Introduction As the sizes of electronic enclosures decrease and power levels increase, it is important to understand the effects that this can impose on thermal management. A poorly designed enclosure may increase the restriction of airflow to areas where cooling is most needed. Fan selection and placement can also improve cooling effectiveness. This lab will demonstrate the following design parameters and their effects in electronics cooling: System airflow impedance Series vs. parallel fan configurations System Airflow Impedance As the word implies, impedance is an obstruction to movement, in this case an obstruction to the flow of air. As the flow slows down due to ‘impedances’, a pressure gradient is formed upstream of the obstacle. This static pressure drop represents the impedance of the system and works in opposition to the flow. The summation of individual impedances contributes to the system’s overall static pressure drop, which is represented by an impedance curve (Figure 1). Figure 1. Impedance Curves. Increasing or decreasing the impedance dictates how much air can flow within a system. The impedance curve represents the amount of static pressure a flow must overcome to achieve the desired flow rate. An impedance curve is generated empirically by measuring the static pressure drop through the system using a wind tunnel. For this lab, we will change the airflow impedance by adjusting the size of our airflow inlet. Besides adjusting the inlet and outlet areas, what other factors would cause an increase in static pressure in a computer chassis? 1
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Series vs. Parallel Fan Configurations To compensate for the impedance of the system, either a single fan or a combination of fans either in series or parallel is used. Figure 2 illustrates a series fan configuration and its relation to the impedance of a system. The effect of this configuration doubles the amount of system impedance the fans can overcome at a given flow rate. For high impedance systems, a series fan configuration would be optimal. Figure 2. Comparison of Series and Parallel Fan Configurations in Relation to the System Impedance Curve. Also depicted in Figure 2, are the parallel configuration and its relation to the impedance of a system. Fans in this configuration double the rate of flow for a given impedance. For low impedance systems, this configuration is optimal. When selecting a fan, it is important to ensure the maximum airflow is achieved to maximize heat transfer. The operation point of the fan for a given chassis is indicated by the intersection of the fan curve and the system impedance curve. The system and fan curves used in this lab for both series and parallel configurations can be seen in Figure 3 and Figure 4, respectively. Just based on these figures, which configuration will have the least amount of airflow? Overall, which configuration do you think will be most effective in cooling the computer chips? 2
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LAB 4 - FAN system Fan Configuration and Airflow Impedance Lab Manual ME114 Fall 2008

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