Multiphysics software speeds analysis of next-generation filtration systems
With air pollution on the rise and the EPA (Environmental Protection Agency) heading it off with strict regulations, auto manufacturers look to multiphysics modeling to take a serious bite out of engine emissions. So many of the items that we take for granted (such as air, fuel, and water) stand to benefit from filtration design advances. Fleetguard brought about a series of major advances when they began optimizing the performance of an electrostatic particle separator through COMSOL simulations.
In this cross section of the fiber media, the blue lines represent particles along a streamline whereas aqua-colored points represent captured particle or particles. COMSOL can model the particle tracking of an entire particle perimeter rather than just its center of mass.
A technology leader in the heavy-duty filtration industry, Fleetguard (a wholly owned subsidiary of Cummins Engines) uses COMSOL to analyze separators that remove particulates from airflow streams. In operation, an electrostatic separator first builds up a corona field around a highvoltage electrode. Particles passing through the field pick up a positive charge and then are attracted to a grounding element such as a plate. A higher voltage creates a more powerful corona field and thus a more efficient filter-but when the field strength exceeds a certain level, sparking within the device can arise. Lost efficiency is only one negative consequence of sparking, which can also cause premature failure of the power supply. Clearly, the designers want to find a field strength that has the highest efficiency that will not provoke sparks.
Beyond voltage, two other key variables that affect electrostatic separator performance are electrode size and shape. Further, the current in the corona and sparking depend on the field in a highly nonlinear way and are sensitive to small local variations in field strength. While it is possible to fabricate and test various configurations in the lab, engineers at Fleetguard knew it is far more efficient to examine design options in software.
Finding the right software was a challenge in itself. "We're part of a large corporation and thus have access to many modeling packages," notes research engineer Ismail Bagci, "but our primary concern was examining the electric field, and none of those packages were able to handle the job to our liking. We looked around and decided that COMSOL offered capabilities we needed at an attractive price."
In this symmetric section of a filter canister, the red geometry shows the elextrode, whereas the green isosurfaces show the elextric field. The large isosurfaces at the top and the bottom, where the field protrudes out from the elextrode, reveal areas with greater probabilities of high-voltage.
"Even though we bought the package primarily for its ability to analyze electric fields," he adds, "we purchased all the optional modules. COMSOL´s ability to combine several multiphysics aspects easily has broadened its use into the exploration of other filtration problems we studied primarily through experimentation. We´ve barely tapped the package's enormous potential, and we're still learning what it can do for us."
Apart from looking just at the corona field, the team can simultaneously study flow. Air flow through the device fluctuates between laminar and turbulent as the engine runs in different conditions, but the designers need to know what flow looks like at all times. The group modeled turbulence simultaneously coupled with electrostatic properties and particle tracking. This study is important because if a charged particle touches a boundary wall or another particle, it tends to be held in place. By looking at the combined physics (flow characteristics along streamlines, particle size, electric forces, mass and density), a more accurate simulation results.
Multiphysics capabilities also prove quite useful when exploring particle tracking through porous media. The design goal is a permeable basis in the filter that collects particles effectively under all conditions. "This isn't the kind of capability you find in a standard package," notes research engineer Kevin South, "so we asked one supplier of modeling software if they'd help us create a particle-tracking routine. They said it was possible but would be a `major undertaking.´ We then approached COMSOL, and they wrote our routine in a week." It consists of Matlab code, which provides a tightly coupled computational engine behind COMSOL Multiphysics for such applications.
The group also harnessed COMSOL´s integration with Matlab in other ways. They wrote a geometry-generating program that quickly creates simulated fiber environments based on the supplied physical parameters for fiber media. Importing DXF files from microscopic photographs allowed for more complex mediastructure modeling. Then, with COMSOL's subroutine, the team monitored injected batches of mono-disperse particles for particle surfaces touching any fiber surface in the model domain (walls) as well as contact with other "tagged or halted" particles. This is a unique capability not possible with other specialized codes that can only track a particle's center of mass and not the entire perimeter.
"Our ultimate objective," notes principal engineer Peter Herman, "is modeling the depth loading characteristics of our gradient StrataPoreTM filter media to maximize its capacity and efficiency for liquid-filter applications. Of special interest for particle capture in fibrous liquid filters is "interception" mode, where a particle doesn't inertially deviate from flow streamline but is captured anyway due to the edge of a particle contacting a fiber. Thus COMSOL's ability to track particle perimeter versus simply the center of mass is a prerequisite."
With COMSOL's Structural Mechanics module, the researchers have modeled 2D and 3D pressure vessels and cross-sectional housings to develop an understanding of the unit's burst and fatigue properties. The only stumbling block the engineers encountered was importing 3D models they created in ProEngineer, whose drawing capabilities they prefer for sophisticated configurations. After drawing a filter with ProEngineer they must create an IGES file, read that file into SDRC (now part of the PLM family at EDS), have that package create an IGES file, and then read it into COMSOL Multiphysics. This method was necessary with COMSOL Multiphysics (FEMLAB) ver 2.3, but with the latest release, ver 3.0, COMSOL reports that such importing problems have disappeared.
The designers were also impressed with COMSOL´s speed. The model for a typical electrostatic particle separator has close to 40,000 elements, and a standard electrostatic model solves in less than 10 minutes; a multiphysics model takes from 20 to 30 minutes. With their first few designs, they compared the results from COMSOL from those with another modeling package that took three times as long to run, and the results were identical. They also corresponded very closely to laboratory findings. With this confidence, Fleetguard started examining a variety of electrode designs, evaluating far more than they could with any other package and far, far more than they could if they had to do the work experimentally.