What Determines Accuracy in a Combustion Simulation

Today’s combustion system design objectives focus on efficiency, pollutant emissions, and fuel flexibility. Combustion simulation can be a valuable aid to designers in meeting their goals but only if the results of their modeling can give true insight into their engine’s behavior. Obtaining accurate results from combustion simulation requires the capture of both the physical and chemical characteristics that can change radically over a complete engine duty cycle. In an internal combustion engine, for example, spray breakup and evaporation, turbulence, ignition delay and flame propagation are all factors that must be modeled accurately to yield meaningful results. Inaccuracies can sometimes be overcome by a substantial amount of tuning or calibration of the simulation models, but the cost of calibration is usually significant and the resulting calibrations are typically not transferable over the course of a design’s evolution. 

Thanks to massively parallel computers, engine geometries can now be represented with amazing detail using computational meshes in CFD that approach 100 million cells. Advanced turbulence modeling in CFD provides better simulation of turbulence-chemistry interactions, albeit with severely reduced chemistry. But because the chemistry solver technology included with most CFD packages is slow relative to the flow calculations, it is common for engineers to use single-component, severely reduced fuel models (or mechanisms) in their combustion simulations. These severely reduced fuel models lack the detail that is required to accurately predict key engine performance factors such as ignition delay, flame propagation, NOx, CO and PM (soot) emissions. Through the work of the Model Fuels Consortium (MFC), it was clearly shown that the use of severely reduced fuel models can compromise the accuracy of important parameters in a simulation. In the figure below, the results of a simulation using a 34 species n-heptane fuel model that is commonly used to model diesel combustion are shown. This fuel model was extensively tuned to provide reasonable accuracy in standard diesel engine designs. However, it fails to accurately predict trends for important combustion and emissions performance when compared to actual engine measurements for high EGR cases. The more accurate 174 species mechanism is able to accurately capture ignition and heat release for the high EGR cases.

Severely reduced chemistry fails to predict ignition and heat release for high EGR cases.  More accurate chemistry is able to predict combustion for high EGR cases

 

The impacts of using oversimplified chemistry can be significant. Inaccurate results require significant calibration or tuning such that the overall Time-to-Solution of a combustion simulation is long and the results are not transferrable to new or modified designs. Through the work of the MFC, the combustion simulation community now has access to the most widely validated library of fuel model components available. When coupled with Reaction Design’s software suite, these models have been shown to greatly increase the accuracy of results across a wide range of operating conditions and fuel types without negatively impacting Time-to-Solution.