Laminar Premixed Methane-air Flames Research Articles

Inhibition of Premixed Methane–Air Flames by Fluoroethanes and Fluoropropanes

This paper presents experimental and modeling results for laminar premixed methane–air flames inhibited by the fluoroethanes C2F6, C2HF5, and C2H2F4, and experimental results for the fluoropropanes C3F8 and C3HF7. The modeling results are in good agreement with the measurements with respect to reproducing flame speeds. For the fluoroethanes, calculated flame structures are used to determine the reaction pathways for inhibitor decomposition and the mechanisms of inhibition, as well as to explain the enhanced soot formation observed for the inhibitors C2HF5, C2H2F4, and C3HF7. The agents reduce the burning velocity of rich and stoichiometric flames primarily by raising the effective equivalence ratio and lowering the adiabatic flame temperature. For lean flames, the inhibition is primarily kinetic, since inhibitor reactions help to maintain the final temperature. The peak radical concentrations are reduced beyond that due to the temperature effect through reactions of fluorinated species with radicals.

Laminar premixed flame extinction limits. I. Combined effects of stretch and upstream heat loss in the twin-flame unburnt-to-unburnt opposed flow configuration

Numerical methods have been used to examine the combined effects of stretch and localized upstream heat loss on the properties of a stoichiometric laminar premixed methane-air flame. A symmetric unburnt-to-unburnt opposed flow configuration was investigated, in which fresh combustible premixture was discharged towards the stagnation plane from identical coaxial plug flow nozzles maintained at constant temperature, and separated by a fixed distance. A continuation method was used in tandem with Newton solutions for individual flame structures, in order to study the flame behaviour as the stretch rate (or nozzle inlet velocity) was varied. High- and low-stretch extinction limits were observed as positions of vertical tangency on the steady-state solution curves of flame property versus stretch rate. The complete series of solutions between these positions formed a set of isolae with both stable and unstable branches. The chemical behaviour at the limits and along both branches is discussed, with particular emphasis on the behaviour near the low-temperature low-stretch-rate limit. The importance of changes not only in the size of the radical pool, but also in the ratio of hydrocarbon (CH 3 ) to non-hydrocarbon (H, O and OH) species therein, is demonstrated both near the limits and along the unstable solution branch. Attention is drawn to the significance of the conclusions in the context of flame extinguishment. The presentation and discussion of the flame behaviour is preceded by a full description of the computational approach.

Modeling of coal pyrolysis kinetics

We report on modifications to the widely used astrophysical code, FLASH (Fryxell, Olson et al., 2000) that enable accurate simulations of chemically reacting flows with heat addition. The enhancements to FLASH include the implementation of extensive hydrogen–air and methane–air chemistry through multiple, detailed mechanisms (Smooke, 1991; Katta and Roquemore, 1995; Mueller, Kim et al., 1999; Billet, 2005), accomplished by building on the existing infrastructure of nuclear reaction network solvers. The chemical reaction network is represented as a system of coupled ODEs, that are then solved either through the Kaps–Rentrop (Rosenbrok) method (Kaps and Rentrop, 1979) or the Bader–Deuflhard method (Bader and Deuflhard, 1983), supplemented by a sparse matrix package for solving linear systems of equations. Furthermore, an existing gamma-law equation of state solver was modified to describe multiple species, each with temperature-dependent properties necessary for realistic simulations of combustion applications. The calculation of temperature-dependent transport properties of constituent species is accomplished through a comprehensive expansion of the materials database. We take advantage of the capability in FLASH to handle the diffusion of heat, mass, and momentum either through an update of the fluxes of each quantity across cell faces, or by directly solving a diffusion equation for each, and the relative merits of each approach for reacting flows are discussed. The capabilities of the modified tool are extensive and in some instances unique, and are documented in detail, along with numerical properties. We also present results from validation of the above capabilities through comparison with analytical solutions, and published numerical and experimental data of chemically reacting flows. Our validation cases include comparison of temporal evolution of species and temperature in a well stirred reactor, comparison of adiabatic flame temperature data, advection of reacting and non-reacting 1D fronts, 2D laminar premixed methane–air flame in a Bunsen burner configuration, shock-driven combustion of an initially circular hydrogen bubble, and a reacting Richtmyer–Meshkov instability not studied previously.

No x in parametrically varied methane flames

Fifteen laminar premixed methane-air flames are computed in the pressure range 1–10 atm. and the equivalence ratio range 0.6–1.0. The kinetic scheme includes thermal and prompt NO x , C 2 chemistry and pressure fall-off data for the kinetic rates. Sensitivity analysis shows the influence of pressure and the relative contributions of the different NO x mechanisms. NO x is insensitive to pressure in very lean flames. Thermal NO x dominates in stoichiometric flames, while prompt NO x and N 2 O mechanisms have an influence in the lean flames. The prompt NO x reactions are more influential in NO 2 than in NO production. NO 2 peaks early due to reactions between HO 2 and NO diffusing forward into the cool part of the flame, and then decays to 1% or less of NO. Reactions which control flame speed are very influential on NO. Based on mass fractions, CH scales as CH 4 ·OH·H in the 10 atm. flame, suggesting a simplified scheme for prompt NO x .