Mitchell D. Smoke

Bio: Mitchell D. Smoke is an academic researcher from Yale University. The author has contributed to research in topics: Flame speed & Diffusion flame. The author has an hindex of 1, co-authored 1 publications receiving 231 citations.

Large-eddy simulation of a turbulent piloted methane/air diffusion flame (Sandia flame D)

Abstract: The Lagrangian Flamelet Model is formulated as a combustion model for large-eddy simulations of turbulent jet diffusion flames. The model is applied in a large-eddy simulation of a piloted partially premixed methane/air diffusion flame (Sandia flame D). The results of the simulation are compared to experimental data of the mean and RMS of the axial velocity and the mixture fraction and the unconditional and conditional averages of temperature and various species mass fractions, including CO and NO. All quantities are in good agreement with the experiments. The results indicate in accordance with experimental findings that regions of high strain appear in layer like structures, which are directed inwards and tend to align with the reaction zone, where the turbulence is fully developed. The analysis of the conditional temperature and mass fractions reveals a strong influence of the partial premixing of the fuel.

Recent advances in understanding of flammability characteristics of hydrogen

Abstract: The current increasing interest in hydrogen utilization and increasing understanding of hydrogen combustion motivate this review of flammability characteristics of hydrogen. The intent is to present a thorough and self-contained tutorial that covers the existing fundamental knowledge in a uniform and concise manner. The presentation begins with an up-dated exposition of the elementary chemical mechanism of hydrogen oxidation, including the latest chemical-kinetic results, with evaluated selections of reaction-rate parameters. Understanding of the mechanism is emphasized through presentation of systematically reduced overall steps and their associated rates. Useful simplifications of the chemistry are thereby exposed and appraised, identifying applicable quasi-steady-state approximations. The status of our knowledge of the fundamental transport properties for hydrogen combustion is then summarized, with indication of the relevance of thermal diffusion for hydrogen. Hydrogen–oxygen autoignition processes are next analyzed, including the important differences found under conditions above and below the crossover temperature at which the rates of the branching and recombination steps are equal, with an explanation of the classical explosion diagram that exhibits three explosion limits. Time-dependent and counter-flow mixing layers are addressed in the context of ignition processes. Knowledge of hydrogen deflagrations is reviewed, including their flame structures, burning velocities, and flammability limits, with special emphasis on peculiarities and simplification that occur in the vicinity of the lean limit. Deflagration instabilities and effects of strain and curvature on deflagrations are described, resulting under appropriate circumstances in flame balls, the structures, characteristics, and importance of which are analyzed. The structures and stabilization mechanisms of hydrogen diffusion flames are reviewed, pointing out the current state of knowledge and current uncertainties in their extinction conditions. Hydrogen detonations also are considered, with explanations given of their detonation velocities, structures, and instabilities, including cellular detonations and emphasizing the importance of future studies of vibrational relaxation effects in these detonations. Finally, some comments and observations on the applications and future prospects for hydrogen usage are offered from viewpoints of safety and energy production.

Direct numerical simulation of hydrogen-enriched lean premixed methane–air flames

Abstract: The effect of hydrogen blending on lean premixed methane–air flames is studied with the direct numerical simulation (DNS) approach coupled with a reduced chemical mechanism. Two flames are compared with respect to stability and pollutant formation characteristics—one a pure methane flame close to the lean limit, and one enriched with hydrogen. The stability of the flame is quantified in terms of the turbulent flame speed. A higher speed is observed for the hydrogen-enriched flame consistent with extended blow-off stability limits found in measurements. The greater flame speed is the result of a combination of higher laminar flame speed, enhanced area generation, and greater burning rate per unit area. Preferential diffusion of hydrogen coupled with shorter flame time scales accounts for the enhanced flame surface area. In particular, the enriched flame is less diffusive-thermally stable and more resistant to quenching than the pure methane flame, resulting in a greater flame area generation. The burning rate per unit area correlates strongly with curvature as a result of preferential diffusion effects focusing fuel at positive cusps. Lower CO emissions per unit fuel consumption are observed for the enriched flame, consistent with experimental data. CO production is greatest in regions which undergo significant downstream interaction. In these regions, the enriched flame exhibits faster oxidation rates as a result of higher levels of OH concentration. NO emissions are increased for the enriched flame as a result of locally higher temperature and radical concentrations found in cusp regions.

State-of-the-art in premixed combustion modeling using flamelet generated manifolds

Abstract: Flamelet based chemical reduction techniques are very promising methods for efficient and accurate modeling of premixed flames. Over the years the Flamelet Generated Manifold (FGM) technique has been developed by the Combustion Technology Group of Eindhoven University of Technology. Current state-of-the-art of FGM for the modeling of premixed and partially-premixed flames is reviewed. The fundamental basis of FGM consists of a generalized description of the flame front in a (possibly moving) flame-adapted coordinate system. The basic nature of the generalized flamelet model is that effects of strong stretch in turbulent flames are taken into account by resolving the detailed structure of flame stretch and curvature inside the flame front. The generalized flamelet model, which forms the basis on which FGM is built, is derived in Part I. To be able to validate numerical results of flames obtained with full chemistry and obtained from FGM, it is important that the generalized flamelet model is analyzed further. This is done by investigating the impact of strong stretch, curvature and preferential diffusion effects on the flame dynamics as described by the local mass burning rate. This so-called strong stretch theory is derived and analyzed in Part I, as well as multiple simplifications of it, to compare the strong stretch theory with existing stretch theories. The results compare well with numerical results for flames with thin reaction layers, but described by multiple-species transport and chemistry. This opens the way to use the generalized flamelet model as a firm basis for applying FGM in strongly stretched laminar and turbulent flames in Part II. The complete FGM model is derived first and the use of FGM in practice is reviewed. The FGM model is then validated by studying effects of flame stretch, heat loss, and changes in elements, as well as NO formation. The application to direct numerical simulations of turbulent flames is subsequently studied and validated using the strong stretch theory. It is shown that the generalized flamelet model still holds even in case of strong stretch and curvature effects, at least as long as the reaction layer is dominated by reaction and diffusion phenomena and not perturbed too much by stretch related perturbations. The FGM model then still performs very well with a low number of control variables. Turbulent flames with strong preferential diffusion effects can also be modeled efficiently with an FGM model using a single additional control variable for the changes in element mass fractions and enthalpy. Finally FGM is applied to the modeling of turbulent flames using LES and RANS flow solvers. For these cases, the flame front structure is not resolved anymore and unresolved terms need to be modeled. A common approach to include unresolved turbulent fluctuations is the presumed probability density function (PDF) approach. The validity of this FGM-PDF approach is discussed for a few test cases with increasing level of complexity.

Transient plasma ignition of quiescent and flowing air/fuel mixtures

Abstract: Transient plasmas that exist during the formative phase of a pulse-ignited atmospheric pressure discharge were studied for application to ignition of quiescent and flowing fuel-air mixtures. Quiescent methane-air mixture ignition was studied as a function of equivalence ratio, and flowing ethane-air mixture was studied in a pulse detonation engine (PDE). The transient plasma was primarily comprised of streamers, which exist during approximately 50 ns prior to the formation of an equilibrated electron energy distribution. Results of significant reduction in delay to ignition and ignition pressure rise time were obtained with energy costs roughly comparable to traditional spark ignition methods (100-800 mJ). Reduction in delay to ignition by factors of typically 3 in quiescent mixes to >4 in a flowing PDE (0.35 kg/s), and other enhancements in performance were obtained. These results, along with a discussion of a pseudospark-based pulse generator that was developed for these applications, will be presented.