In response to strict EPA air pollution regulations, savvy auto manufacturers are relying on multiphysics modeling methodologies to help reduce engine emissions. Fleetguard (a wholly owned subsidiary of Cummins Engines) is one such organization that uses Comsol software (Burlington, MA) to analyze separators that remove particulates from airflow streams. During operation, an electrostatic separator first builds up a corona field around a high voltage 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. As a result, Fleetguard designers wanted to find a field strength that has the highest efficiency that will not provoke sparks.

Beyond voltage, electrode size and shape are key variables that affect electrostatic separator performance. In addition, 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. Fleetguard selected Comsol software to help them examine electric fields, as well as its breadth of multiphysics capabilities to explore other filtration issues.

In this cross-section of the fiber media, the blue lines represent particles along a streamline. The aqua-colored points represent captured particle(s). Comsol can model the particle tracking of an entire particle rather than just its center of mass.

Using the software, the engineering team can focus on the corona field while simultaneously studying flow. Air flow through the device fluctuates between laminar and turbulent as the engine operates under different conditions. However, the engineers wanted to know what the flow looks like at all times. As a result, 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 studying the combined physics including flow characteristics along streamlines, particle size, electric forces, mass, and density, a more accurate simulation results.

Coupling technologies to achieve goals
Multiphysics capabilities also prove useful when exploring particle tracing through porous media. The design goal is a permeable basis in the filter that collects particles effectively under all conditions. “This is not the kind of capability you find in a standard package,” notes Fleetguard research engineer Kevin Smith. “So we asked one supplier of modeling software if they would help us create a particle-tracking routine. They said it was possible but would be a major undertaking. However, Comsol wrote our routine in one week.” The resulting particle-tracking routine uses Matlab code that provides a tightly coupled computational engine behind Comsol Multiphysics for such applications.

The group also harnessed Comsol’s integration with Matlab in other ways. For instance, 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 media structure 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,” says Fleetguard principal engineer Peter Herman, “is modeling the depth loading characteristics of our gradient StrataPore™ 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 does not inertially deviate from flow stream line 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 a mass is a prerequisite.”

In this symmetric section of a filter canister, the red geometry shows the electrode, whereas the green iso-surfaces show the electric field. The large iso-surfaces at the top and the bottom, where the field protrudes out from the electrode, reveal areas with greater probabilities of  high-voltage.

Exploring additional scenarios
Using Comsol’s structural mechanics module, the researchers have modeled 2D and 3D pressure vessels and cross-sectional housings to develop an understanding of the units’ burst and fatigue properties.

In addition, model solving speed was an important factor when Fleetguard considered software packages. The model for a typical electrostatic particle separator has an estimated 40,000 elements. A standard electrostatic model solves in less than 10 minutes. A multiphysics model takes between 20 to 30 minutes. With their first few designs, Fleetguard engineers compared the results from Comsol from those with other modeling packages 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 deigns, evaluating far more than they could with any other software product and far more than they if they had to do the work experimentally.

Comsol
www.comsol.com

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