Jacqueline H. Chen

Unsteady strain rate and curvature effects in turbulent premixed methane-air flames

Turbulent premixed stoichiometric methane-air flames modeled with reduced kinetics have been studied using the direct numerical simulation (DNS) approach. The simulations include a four-step reduced mechanism for the oxidation of methane and the molecular transport is modeled with Lewis numbers for individual species. The effects of strain rate and curvature on the intermediate radical concentrations and heat release rate are evaluated. The topology of the flame surface is interpreted in terms of its propagation and statistics. The correlation of radical species with strain rate and curvature is found to be strongly dependent upon their individual mass diffusion rates; hence, a global Lewis number representation of all of the species may be inadequate in predicting the heat release rate and evolution of the flame surface. It is found that highly diffusive and fast reactive species, H and H2, are well correlated with curvature, while less diffusive species, CO, with a slow oxidation rate is more susceptible to unsteady strain rate effects. The global response of the flame is presented in terms of volumetric heat release and fuel consumption rates. The contributions of flame surface wrinkling and flame structure are identified.

Direct numerical simulation of autoignition in non- homogeneous hydrogen-air mixtures

The autoignition of spatially non-homogeneous hydrogen-air mixtures in 2-D random turbulence and mixture fraction fields is studied using the Direct Numerical Simulation (DNS) approach coupled with detailed kinetics. The coupling between chemistry and the unsteady scalar dissipation rate field is investigated over a wide range of different autoignition scenarios. The simulations show that autoignition is initiated at discrete spatially localized sites, referred to as kernels, by radical build-up in high-temperature, fuel-lean mixtures, and at relatively low dissipation rates. Detailed analysis of the dominant chemistry and the relative roles of reaction and diffusion is implemented by tracking the evolution of four representative kernels that characterize the range of ignition behaviors observed in the simulation. This evolution yields different autoignition delay scenarios as well as extinction at the different sites based on the local dissipation rates and their temporal histories. Where significant autoignition delay and extinction are observed, a shift in the relative roles of dominant reactions that contribute to radical production and consumption during this induction phase is observed. This shift is particularly characterized by an increased role of termination reactions during the intermediate stages of the induction period, which results in extinction in approximately two thirds of the ignition kernels in the computational domain. The fate of the different kernels is associated with: (1) the dissipation of heat that contributes to a slowdown in chemical reactions and a shift in the balance between chain-branching and chain-termination reactions; (2) the dissipation of mass that keeps the radical pool growth in check, and that is promoted by slower reaction rates; and (3) counter to the effects of dissipation of heat and intermediate species, the preferential diffusion of H2 relative to both heat and its diluent, N2, that promotes ignition. Ultimately, the balance between radical production and dissipation determines the success or failure of a given kernel to ignite. A new criterion for unsteady ignition is presented based on the instantaneous balance between radical production and dissipation. A Damkohler number, so defined, must remain above a critical value of unity at all times during the induction period if the kernel is to eventually ignite. Inherent in a multi-step kinetic description of ignition phenomena is the disparate time scales associated with different elementary reactions that, coupled with the characteristic scales of heat and mass dissipation, may yield different dominant chemistries at different stages of the induction process for a given kernel. To capture the strong history effects associated with radical build-up, new ignition progress variables based on key radical species are investigated.

Analysis of the contribution of curvature to premixed flame propagation

In this analysis, the authors attempt to identify the contributions of curvature to the displacement speed by using the governing transport equation for the deficient reactant. They show that the displacement speed is a balance of three components: reaction, normal diffusion, and curvature. The contribution of the three components is then evaluated using results from direct numerical simulations (DNS) of an unsteady stoichiometric methane-air flame embedded in a 2-D vortical flow field.

Structure and Propagation of Methanol–Air Triple Flames

Abstract The structure and propagation for a methanol (CH3OH)–air triple flame are studied using direct numerical simulations (DNS). The methanol (CH3OH)–air triple flame is found to burn with an asymmetric shape due to the different chemical and transport processes characterizing the mixture. The excess fuel, CH3OH, on the rich premixed flame branch is replaced by more stable fuels CO and H2 which burn at the diffusion flame. On the lean premixed flame side, a higher concentration of O2 leaks through to the diffusion flame. The general structure of the triple point features the contribution of both differential diffusion of radicals and heat. A mixture fraction–temperature phase plane description of the triple flame structure is proposed to highlight some interesting features in partially premixed combustion. The effects of differential diffusion at the triple point add to the contribution of hydrodynamic effects in the propagation of the triple flame. Differential diffusion effects are measured using two methods: a direct computation using diffusion velocities and an indirect computation based on the difference between the normalized mixture fractions of C and H. The mixture fraction approach does not clearly identify the effects of differential diffusion, in particular at the curved triple point, because of ambiguities in the contribution of carbon and hydrogen atoms’ carrying species.

In Situ Visualization for Large-Scale Combustion Simulations

As scientific supercomputing moves toward petascale and exascale levels, in situ visualization stands out as a scalable way for scientists to view the data their simulations generate. This full picture is crucial particularly for capturing and understanding highly intermittent transient phenomena, such as ignition and extinction events in turbulent combustion.

STATISTICS OF FLAME DISPLACEMENT SPEEDS FROM COMPUTATIONS OF 2-D UNSTEADY METHANE-AIR FLAMES

Results of two-dimensional numerical computations of turbulent methane flames using detailed and reduced chemistry are analyzed in the context of a new theory for premixed turbulent combustion for high turbulence intensity. This theory defines the thin reaction zones regime, where the Kolmogorov scale is smaller than the preheat zone thickness, but larger than the reaction zone thickness. The two numerical computations considered in this paper fall clearly within this regime. A lean and a stoichiometric flame are considered. The former is characterized by a large ratio of the turbulence intensity to the laminar burning velocity and the latter by a smaller value of that ratio.

FastBit: interactively searching massive data

As scientific instruments and computer simulations produce more and more data, the task of locating the essential information to gain insight becomes increasingly difficult. FastBit is an efficient software tool to address this challenge. In this article, we present a summary of the key underlying technologies, namely bitmap compression, encoding, and binning. Together these techniques enable FastBit to answer structured (SQL) queries orders of magnitude faster than popular database systems. To illustrate how FastBit is used in applications, we present three examples involving a high-energy physics experiment, a combustion simulation, and an accelerator simulation. In each case, FastBit significantly reduces the response time and enables interactive exploration on terabytes of data.

Correlation of flame speed with stretch in turbulent premixed methane/air flames

In the flamelet approach of turbulent premixed combustion, the flames are modeled as a wrinkled surface whose propagation speed, termed the {open_quotes}displacement speed,{close_quotes} is prescribed in terms of the local flow field and flame geometry. Theoretical studies suggest a linear relation between the flame speed and stretch for small values of stretch, S{sub L}/S{sub L}{sup 0} = 1 - MaKa, where S{sub L}{sup 0} is the laminar flame speed, Ka = {kappa}{delta}{sub F}/S{sub L}{sup 0} is the nondimensional stretch or the Karlovitz number, and Ma = L/{delta}{sub F} is the Markstein number. The nominal flame thickness, {delta}{sub F}, is determined as the ratio of the mass diffusivity of the unburnt mixture to the laminar flame speed. Thus, the turbulent flame model relies on an accurate estimate of the Markstein number in specific flame configurations. Experimental measurement of flame speed and stretch in turbulent flames, however, is extremely difficult. As a result, measurement of flame speeds under strained flow fields has been made in simpler geometries, in which the effect of flame curvature is often omitted. In this study we present results of direct numerical simulations of unsteady turbulent flames with detailed methane/air chemistry, thereby providing an alternative method of obtaining flame structure and propagation statistics. The objective is to determine the correlation between the displacement speed and stretch over a broad range of Karlovitz numbers. The observed response of the displacement speed is then interpreted in terms of local tangential strain rate and curvature effects. 13 refs., 3 figs.

Preferential diffusion effects on the burning rate of interacting turbulent premixed hydrogen-air flames

Abstract The upstream interaction of twin premixed hydrogen-air flames in 2-D turbulence is studied using direct numerical simulations with detailed chemistry. The primary objective is to determine the effect of flame stretch on the overall burning rate during various stages of the interaction. Preferential diffusion effects are accounted for by varying the equivalence ratio from symmetric rich-rich to lean-lean interactions. The results show that the local flame front response to turbulence is consistent with previous understanding of laminar premixed flames, in that rich premixed flames become intensified in regions of negative strain or curvature, while the opposite response is found for lean premixed flames. The overall burning rate history with respect to the surface density variation is found to depend on the mixture condition; the consumption rate enhancement advances (follows) the surface enhancement for the rich-rich (lean-lean) case. For the lean-lean case, a self-turbulization mechanism results in a large positive skewness in the area-weighted mean tangential strain statistics. Because of the statistical dominance of positive stretch on the flame surface, the lean-lean case results in a significantly larger burning enhancement (over a twofold increase) in addition to the surface density production. For the case of rich-rich interaction, the abundance in hydrogen species results in an instantaneous overshoot of the radical pool in the post-flame region, resulting in an additional “burst” in the reactant consumption rate history, suggesting its potential impact on the pollutant formation process.

Combining in-situ and in-transit processing to enable extreme-scale scientific analysis

With the onset of extreme-scale computing, I/O constraints make it increasingly difficult for scientists to save a sufficient amount of raw simulation data to persistent storage. One potential solution is to change the data analysis pipeline from a post-process centric to a concurrent approach based on either in-situ or in-transit processing. In this context computations are considered in-situ if they utilize the primary compute resources, while in-transit processing refers to offloading computations to a set of secondary resources using asynchronous data transfers. In this paper we explore the design and implementation of three common analysis techniques typically performed on large-scale scientific simulations: topological analysis, descriptive statistics, and visualization. We summarize algorithmic developments, describe a resource scheduling system to coordinate the execution of various analysis workflows, and discuss our implementation using the DataSpaces and ADIOS frameworks that support efficient data movement between in-situ and in-transit computations. We demonstrate the efficiency of our lightweight, flexible framework by deploying it on the Jaguar XK6 to analyze data generated by S3D, a massively parallel turbulent combustion code. Our framework allows scientists dealing with the data deluge at extreme scale to perform analyses at increased temporal resolutions, mitigate I/O costs, and significantly improve the time to insight.