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Understanding The Adverse Effects Of Axial Swirl And Limited Plenum Space Behind An Airmover Within A High-Density Electronic Enclosures And Mitigation Constructs. Disclosure Number: IPCOM000236790D
Publication Date: 2014-May-15
Document File: 4 page(s) / 249K

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Described are methods of understanding the adverse effects of axial swirl and limited plenum space behind an airmover within high-density electronic enclosures and mitigation constructs.

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Page 01 of 4

Understanding The Adverse Effects Of Axial Swirl And Limited Plenum Space Behind An Airmover Within A High -

-Density Electronic Enclosures And Mitigation Constructs

Density Electronic Enclosures And Mitigation Constructs .

In order for a fan to achieve its rated performance, i.e., the stated performance from the manufacturer used to select the fan, the airflow at the inlet must be fully developed, symmetrical, and free from swirl. Figure 1 illustrates the concept of fully-developed channel flow where, at a 100% of the effective duct length, the profile of airflow velocity becomes uniform and consistent.

Figure 1: Fully developed channel flow.

    The space behind the fan exhaust must be designed such that the asymmetrical flow profile from the fan is allowed to diffuse. The effect on fan performance when these conditions are not met can disrupt the airflow distribution and volumetric flow rate to an electronic enclosure's components downstream of the fan. At normal slow fan speeds in Figure 2, the effective duct length may be relatively compact, and airflow fills in behind the hub prior to flowing past the array of components with little impact to cooling until the fan reaches such speeds that the airflow behind the hub becomes nonexistent or possibly negative (reverse flow).

Figure 2: Airflow velocity profiles of same fan at fast and slow fan speeds.


Page 02 of 4

    Thus, if components are placed within the system that encroaches on the distance needed for fully-developed airflow, there will be difference in cooling for an array of components. The larger the diameter of the axial fan, the larger the footprint of the hub containing the motor and thus leads to larger variations in the airflow velocity profile. Airflow directly behind the hub of the axial fan can be effectively zero, while the highest velocity can be found around the outer circumference of the fan. The larger the hub, more components may be effected by the airflow only being delivered near the top or the bottom of a standing array of components.

    In addition, the effects of swirl also become difficult to predict and control as fan speeds and fan placement are varied. At normal speeds, there may be no noticeable effects of fan swirl; however, when the system increases the fan speeds to provide additional cooling, the negative impact of fan swirl can also be multiplied. Air comes off the rotating airfoils at an angle to the face of the fan and causes airflow to behave in unexpected manners. This complex, tangential flow is called swirl and can also impact the ability to provide the array of components with equal airflow rates (see Figure 3 below).

Figure 3: Swirl

    The common solutions to uneven airflow created by the conditions previously discussed can fall into a large category: fan attachments or airflow straighteners in the foam of either egg crates (see Figure 4 below) or radial fins are often used to eliminate or reduce swirl exiting an axial fan. Most designs must re...