Reaction and Separations Development
Typical strained flame solvers (Chemkin, Cantera, FlameMaster, etc.) leverage damped Newton methods to solve for the steady flame. However, as a consequence of the Newton methods they employ, these solvers struggle to converge when using complex chemical kinetic models of 500 species or more. At the same time, researchers and industry are interested in increasingly complex fuel blends which require kinetic models of this scale or larger to capture the relevant chemistry. Ember addresses this need while simultaneously providing additional transient solution functionality. The software begins with the transient form of the 1D reacting flow problem and then leverages Rebalanced Strang splitting to break the differential system into reactive, convective, and diffusive systems that are each easier to solve and parallelizable while maintaining accurate steady state behavior. In particular, there is a significant benefit to decoupling the stiff reactive terms which can then be solved in parallel using a quasi-steady state approach. For demonstration, Ember is applied to the calculation of Extinction Strain Rate (ESR), a fundamental flame parameter shown to be of increasing importance when studying turbulent flame behavior and used to investigate Lewis number effects on ESR using a detailed chemical kinetic model with 500 species for simulation of strained extinction of lean (Le > 1) and rich (Le < 1) propane/air flames. The complicated chemistry introduces some subtle phenomena not seen with single-step models. Ember is further used to examine the behavior of ESR at elevated pressures, identifying key kinetic model parameters for accurate predictions. Ember is open-source and freely available online.
Figure. Methane twin disc flames. = 0.7. Initial temp. 298K, atmospheric pressure. Parallelized over 4 CPUs. Fit to t=AsB where t is time and s is number of species. (a) Computation times for a strain rate of 500 s-1 demonstrating the performance of Ember compared with Chemkin and the benefits of implementing the QSS chemistry integrator and mixture averaged transport approximation. (b) Comparison of Ember and Chemkin ESR computation times for varying kinetic model sizes.