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Background Simulation of practical-scale combustion devices is an immense undertaking. The problem is inherently multi-scale both in time and space, the fuel is often turbulent, and the combustion process may involve hundreds of species and thousands of chemical reactions. Traditional direct numerical simulation (DNS) approaches based on explicit numerical methods for the compressible flow equations on uniform grids require very fine spatial grids to resolve the local flame structure. In addition, they require small time steps to resolve the acoustic and chemical time scales inherent in the model. DNS is normally reserved in combustion applications for small idealized problems geared at the fundamental nature of turbulence/chemistry interactions. For engineering design applications on the other hand computational models have been developed to approximate underresolved physics. But these models are incomplete, do not have general applicability, and certainly provide no means of exploring fundamental fluid/chemistry interactions.

The research approach taken by CCSE has explicitly targeted both the temporal and spatial multiscale aspects of combustion modeling. First, a low Mach number formulation is used instead of the traditional compressible equations, thereby eliminating the acoustic time step restriction while fully maintaining the compressibility effects due to heat release. Second, adaptive mesh refinement (AMR) is used to focus computational resources in regions of interest without wasting resources in regions requiring less resolution. Third, robust integration methods are employed to allow reasonable solution behavior with a minimum of computational resolution. The combination of AMR and a robust low Mach number implementation for reacting flows has reduced the computational requirements of simulating laboratory-scale low-speed methane combustion by a factor of 10,000 relative to traditional approaches (compressible equations solved on a uniform grid).

With these advanced methods, we can simulate time-dependent, laboratory-scale, turbulent premixed combustion experiments in three dimensions, while including detailed chemical mechanisms to describe the combustion process and the differential diffusion of the various chemical species.

Laboratory-Scale Turbulent Premixed Flames

Without invoking phenomenological or heuristic models for subgrid-scale behavior, CCSE's low Mach number model incorporates the detailed chemistry and transport of up to 20 species in this premixed methane flame. The modelled domain includes the entire relevant flow field (tens of centimeters from the nozzle outflow). Additionally, since little is known about the details of the high-speed cold flow within the nozzle itself, we simulate that flow as well using a geometry-capable adaptive model for compressible gas. Results from the auxiliary compressible calculation are coupled into the low Mach simulation through numerical boundary conditions at the inlet plane.

Burke-Schuman Flames (non-premixed, laminar diffusion flames)

We have applied our adaptive low Mach number combustion code to the study of axisymmetric laminar non-premixed diffusion flames. We have looked at steady and time-dependent scenarios for purposes ranging from software validation excercises to detailed pollutant formation analysis. Working in two dimensions, we were able to include a diverse set of combustion chemistry descriptions corresponding to the level of detail necessary for each study. The validation excercises, for example required a reasonable model for ignition chemistry, and were therefore based on a 26-species mechanism. Studies geared at understanding gravitational effects on the thermal field from a buouyant flame required only a two-step scheme. And detailed nitrogen pollutant analysis was based on mechanisms that included up to 65 species and 486 reactions.

It should be noted that the steady calculations we've performed are not exactly done while operating at our full 'algorithmic strength', in terms of efficiently getting to a solution. Our low Mach simulation algorithm is based on a time-dependent model of the flame and fluid physics, so we need to integrate from a simulated start-up condition, all the way through to a steady flame. That being said, the refined steady solutions with the largest of mechanisms provided a great deal of spatial information about these Burke-Schumann type flames, including the precise creation and transport mechanisms of nitrogen-based flame intermediates.

Freely Propagating Premixed Hydrogen Flames

Freely-propagating lean premixed hydrogen flames spontaneously develop into the well-known cellular burning structures, where the fuel consumption and heat release are highly variable along the flame surface. The instabilities at play in this system saturate quickly and result in robust, slowly evolving cellular burning features which prevent the establishment of a true steady solution. Here, we explore the structure of three premixed hydrogen-air flames: stoichiometric and lean flat steady flames (one-dimensional), and a time-dependent lean freely-propagating (two-dimensional) case. More information can be found here......

Freely Propagating Hydrogen-Methane Flames

Recent interest in alternative fuels such as hydrogen or syngas, obtained from coal gasification, has sparked the development of burners that can operate over a broad range of fuels. We have run simulations of a range of mixed (hydrogen and methane) flames, focusing primarily on the structure of the heat release and reaction paths of carbon chemistry for the mixed fuels, which provides insight on understanding flame dynamics. More information can be found here......

Turbulence-Flame Interactions in Lean Premixed Hydrogen Flames

Turbulent flames have been reported to be quenched when turbulence levels become sufficiently high. However, in a recent study by Aspden et al. (2008) in supernova flames, global extinction was not observed, even at a Karlovitz number of 230. In this study, we consider lean premixed hydrogen flames at a range of Karlovitz numbers from 10 to 1560. The figure on the left shows density, burning rate and temperature for the four Karlovitz numbers at an equivalence ratio of 0.40. More information can be found here......