Analyzing an Air Filter with a RANS Turbulence Mannequin

Joseph Carew | Comsol

HVAC techniques do greater than present the sleek, chilled air that flows when the temperature outdoors rises. Inside these techniques, air strikes via filters to make sure excessive air high quality. With clear air at stake, modeling and simulation can be utilized to achieve an in-depth understanding of the physics behind the conduct of air because it strikes via a filter…

Modeling an Air Filter

The filters inside HVAC techniques depend on a cloth (usually fiberglass or cotton folds) able to straining the air and catching particulates like mud, pollen, and micro organism. These supplies impression the circulation of the air, catching the undesirable particulates whereas concurrently permitting the filtered air to circulation via. Modeling these gadgets and the turbulent circulation they induce permits for figuring out the effectiveness of various supplies when they’re used for filters, serving to designers to slender down the fabric choices earlier than investing in real-life, experimental variations.

On this weblog put up, we’ll have a look at a standard air filter geometry (proven beneath) as our instance.

Modeling this air filter begins with the CFD Module, an add-on product to the COMSOL Multiphysics® software program, which permits customers to create Reynolds-averaged Navier–Stokes (RANS) turbulence fashions in open and porous domains. On this instance, the air filter is modeled as a extremely porous area with 90% of the fabric occupied by cylindrical pores with a diameter of .1 mm. The help of the air filter is represented by a body with no-slip partitions. For this instance, we employed the Turbulent Stream, k-ω interface due to its accuracy for fashions with many partitions, together with no-slip partitions. (An in-depth have a look at the mannequin setup might be discovered within the mannequin documentation, which might be accessed by way of the button on the finish of this weblog put up.)

Evaluating the Outcomes

Fixing the mannequin permits for visualizing the change in turbulence, velocity, and strain as air strikes towards, via, and previous the filter. The computation begins with the air shifting towards the filter (purple within the picture beneath). When the air passes via the filter, the interstitial velocity will increase (though the porous-averaged velocity stays fixed), leading to a rise in turbulence kinetic vitality. Moreover, there’s an abrupt strain drop because of the enhance in velocity and the elevated friction and strain losses, which stem from the excessive variety of wall surfaces. As for the conduct of the air because it strikes away from the filter, the body of the filter prevents the air from shifting freely, as an alternative inflicting downstream wakes of air.

The visualization of the air shifting via the filter can be utilized to conclude whether or not or not the filter will take away contaminants from the air. To substantiate this conclusion, the answer might be evaluated with totally different slice plots. One of many slice plots for this instance signifies that the speed of the air is most impacted by the porous air filter and the body and that it homogenizes via the wake area. A slice plot measuring the turbulence kinetic vitality exhibits that the turbulence kinetic vitality peaks noticeably throughout the filter and attains typical values on the no-slip partitions.

Typically, the mannequin factors to a strain drop and a dramatic enhance in turbulence throughout the filter, creating perturbations in velocity perpendicular to the principle route of the circulation thus additionally rising the chance of the particles to collide with the pore partitions and keep there. In different phrases, the rise in turbulence supplies the blending required for filtering out the undesirable particulates, which in any other case would circulation via the pores undisturbed.