Follow the chemistry to better CFD combustion simulations

  • 04-Jan-2017 12:32 EST
Combustion In Cylinder.jpg

ANSYS Forte accurately simulates IC engine combustion by precisely accounting for real chemical kinetics.

For internal-combustion (IC) engine design, companies need to stay ahead of not only the competition but also of new emissions mandates.

Computational fluid dynamics (CFD) can help engine designers create higher performance, lower emissions IC engines without costly physical prototyping. CFD lets engine designers visualize and test fuel and ignition behaviors within a virtual combustion chamber, providing a faster way to design cleaner, more efficient engines by simulating ignition and fuel dynamics.

But combustion CFD can only be of value if the results predict real-life behaviors. To predict actual performance and pollutant emissions, simulations need to accurately account for the chemical kinetics of the combustion process. Simulations that rely on drastically simplified fuel chemistries and ever-finer meshing technologies fall far short of this goal. And when they fall short, designers must fall back on expensive, time-consuming physical testing for answers.

Combustion CFD challenges

Combustion CFD is complex and computation-intensive, especially for applications such as soot formation and engine knock. Times can easily stretch into days for traditional CFD multi-stage simulations with thousands of variables. Incremental changes to engine geometries and fuel models can stretch the total time to weeks or months before an optimized design is realized.

To speed design time, many CFD solutions simplify combustion chemistry, trusting that severe mesh refinements can make up in detail what they lack in precise chemistry. These simplified fuel models rely on weakly validated, third-party mechanisms from disparate and incompatible sources. Using models from multiple sources makes it very difficult to blend or customize fuels in simulations because species and reactions may be duplicated—perhaps in a contradictory way—in different sources.

Better models are needed now because motor fuels have become more complex. Fuels vary by seasonal formulation (U.S. summer gasoline contains less butane than winter formulations), by region and by application (U.S. diesel has different properties than European diesel). Alternative fuels such as ethanol and biodiesel now supplement petroleum-derived fuels.

To understand the effects of these diverse fuel types, chemically correct fuel models are required. Unfortunately, fuel model algorithms in conventional CFD packages are complex and compute time can multiply exponentially when they are combined to represent multicomponent fuels.

Simulating soot formation and engine knock

New particulate matter (PM) regulations present particular challenges for engine designers. Soot phenomena are notoriously difficult to simulate and too complex to run in conventional CFD software, due to the physics and chemical reactions leading up to soot formation. As a result, optimization of soot in conventional combustion-engine design typically requires years of building and testing prototypes.

Knocking occurs when the highly compressed fuel and air mixture in the combustion chamber auto-ignites, either before or after the spark that is meant to trigger ignition. Accurately modeling the location and structure of the flame front as it expands into the combustion chamber is extremely important for predicting knock. But simulating auto-ignition is very difficult with conventional CFD approaches that rely on mesh refinement and simplified chemistry.

Since the scale of the flame front thickness is significantly smaller than computational mesh—even with severe grid refinement—CFD simulations that rely on mesh to resolve the flame location will require an inordinately large number of tiny cells to resolve the flame topology sufficiently. Simulations with large numbers of small cells can easily get bogged down by the tiny time steps needed to maintain simulation stability, and require an impractical amount of computation time.

What’s an engine designer to do?

Follow the chemistry

Chemistry is crucial. It’s at the heart of combustion, and for internal-combustion CFD to accurately predict real-world engine behavior, it must precisely account for real chemical kinetics.

ANSYS Forte, for example, changes the well-established scaling equation: instead of scaling with the cube of the number of species, the simulation time scales linearly. This allows an engine designer to include as many reactions as he or she requires for accurate simulations, without incurring a compute time penalty. Even larger, more accurate fuel models achieve compute times comparable to those with severely reduced, less accurate models. As a result, designers can quickly and accurately predict emissions that translate reliably to actual engine designs—with far less trial-and-error hardware prototyping.

Fuel components derived from the industry-validated Model Fuel Library enable ANSYS Forte to simulate combustion for a large variety of new or existing fuel blends and foresee what emissions will occur for a wide range of operating conditions.

By using accurate fuel models based on precise chemistry, engine designers greatly increase the predictive quality of combustion simulations, to more quickly and effectively meet strict regulatory guidelines and create advanced clean engine and fuel technologies.

Bill Kulp, the lead product marketing manager for Fluids at ANSYS, wrote this article for Truck & Off-Highway Engineering. He has more than 25 years of experience marketing complex software products across the globe.

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