Structure Of Laminar Premixed Flames Research Articles

Optimized two-step (OTS) chemistry model for the description of partially premixed combustion

Once properly optimized, single-step chemistry models are able to recover essential characteristics of flames such as burned gases temperature, flame propagation velocity and thickness. Ignition delay values can be reproduced as well. However, an improved description of either the premixed flame inner structure or the ignition process requires the consideration of the multi-step nature of chemical kinetics. Indeed, single-step chemistry models can neither describe properly the internal structure of laminar premixed flames nor the development of ignition processes. These limitations arise because single-step chemistry does not take into account intermediate species, the importance of which is essential. Thus, the present work is aimed at extending the recently-proposed optimized single-step (OSS) framework to multi-step chemistry. To this purpose an unbranched chain reaction with only two consecutive steps relevant to chain initiation and chain termination is considered. The corresponding kinetics model does involve two fictive species, the characteristics of which are optimized together with the two-step chemical scheme parameters, e.g., pre-exponential factors and activation energies. One of this fictive species is relevant to combustion products while the other represents the whole pool of radicals. The chemistry optimization procedure makes use of a well-defined in-house genetic algorithm and the resulting optimized two-step (OTS) model is assessed through comparisons made with data obtained from detailed chemistry computations used as reference. For similar (and even lower) CPU costs, the computations performed with the OTS model put into evidence significant improvements compared to the results obtained with the OSS description.

Local effects in vortex-flame interactions: Implications for turbulent premixed flame scaling and observables

A computational investigation was carried out to assess the effects of highly energetic vortices on the local structure of laminar premixed flames. The study involved a lean atmospheric methane-air flame interacting with vortex sizes ranging from five times to half of the laminar flame thickness. The characteristic velocities of the vortices were chosen to correspond to extreme turbulence intensities. Vortices smaller than the flame thickness were found to have a minimal effect on the flame structure due to the high rate of dissipation. Vortices with sizes greater than or equal to the flame thickness were found to induce flame siphoning and/or product-side local flame annihilation and as a result transport considerable amounts of formaldehyde and thermal energy to the unburned side thereby modifying the state of the reactants. The results of the present study for high activation energy reacting flows raise questions regarding the general validity of: (1) conjectured mechanisms of the so-called flame broadening under intense turbulence conditions; and (2) qualitative arguments regarding sub-flame-thickness vortex effects on the preheat zone in the context of the Borghi–Peters regime diagram, as originally postulated by Damköhler.

Structure of laminar premixed flames of methane near the auto-ignition limit

Auto-ignition and flame propagation are the two different controlling mechanisms for stabilizing the flame in secondary stage combustion in hot vitiated air environment and at elevated pressure. The present work aims at the investigation of the flame stabilization mechanism of flames developing in such an environment. In order to better understand the structure of turbulent flames at inlet temperature well above the auto-ignition temperature, the behavior of laminar flames at those conditions needs to be analyzed. As an alternative to challenging and expensive measurements at high temperature and pressure, the behavior of laminar flames at such conditions can be predicted from theory using mathematical simulation. In the present work, the laminar burning velocities and flame structures of premixed stoichiometric methane/air mixtures for inlet temperatures from 300 to 1450K and absolute pressures from 1 to 8bar have been calculated using a freely propagating laminar, one dimensional, planar flame model. The prediction shows that at inlet temperatures below the auto-ignition temperature, the predicted laminar burning velocity which corresponds to the unburned mixture velocity in order to create a steady laminar flame decreases with increase in pressure. When the inlet temperature of the mixture goes well beyond the auto-ignition temperature of the mixture, however, the unburned mixture velocity increases steeply at higher pressure level, because of a complete transition of the flame structure.

An experimental study of the structure of laminar premixed flames of ethanol/methane/oxygen/argon.

The structures of three laminar premixed stoichiometric flames at low pressure (6.7 kPa): a pure methane flame, a pure ethanol flame and a methane flame doped by 30% of ethanol, have been investigated and compared. The results consist of concentration profiles of methane, ethanol, O2, Ar, CO, CO2, H2O, H2, C2H6, C2H4, C2H2, C3H8, C3H6, p-C3H4, a-C3H4, CH2O, CH3HCO, measured as a function of the height above the burner by probe sampling followed by on-line gas chromatography analyses. Flame temperature profiles have been also obtained using a PtRh (6%)-PtRh (30%) type B thermocouple. The similarities and differences between the three flames were analyzed. The results show that, in these three flames, the concentration of the C2 intermediates is much larger than that of the C3 species. In general, mole fraction of all intermediate species in the pure ethanol flame is the largest, followed by the doped flame, and finally the pure methane flame.

Laminar burning velocity of oxy-methane flames in atmospheric condition

The laminar burning velocity of oxy-CH4 flames in atmospheric conditions (300 K and 1 atm) was studied with a lab-scale Bunsen burner. CH chemiluminescence (CH*) was measured, and Schlieren photography was used to derive the laminar burning velocity. The experimental result was compared with the numerical calculation by using a GRI-Mech. ver. 3.0 in a CHEMKIN package. The global equivalence ratio (φG) of an oxy-CH4 mixture varied from φG = 0.5 to 2.0 in 0.1 steps in a laminar flow region. From the observation of the experimental measurements, the structure of laminar premixed flames was composed of a preheating zone, a reaction zone, and a burned gas region. The adiabatic flame temperature (TAd) was predicted to be 3056 K at φG = 1.1 from the numerical calculation. With the angle method, the laminar burning velocity (SL) of premixed oxy-CH4 flames at a normal temperature (300 K) and pressure (1 atm) was measured as SL = 2.95 m/s from Schlieren photographs and SL = 2.91 m/s from CH* images.

Simulation and Analysis of the Structure of Laminar Premixed Flame

The aim of our work is to contribute to the analysis of the structure of laminar premixed Methane-Air flames using two methods. This allows us to validate the chemical mechanisms, to know the fine structure of the flame front and to get, for a given pressure and temperature of fresh gases, the speed and the mass fractions of all chemical species of the combustion reaction. The first method is based on controlling combustion parameters of laminar premixed flame. The numerical resolution strategy used consist in the discretization of the balance equations completed by the transport properties and the thermodynamic variables expressions, as well as the kinetic mechanisms concepts of chemical reactions and boundary conditions, using the first-order finite difference spatial scheme technique. The final solution is obtained, thereafter, iteratively using a recursive method. The calculations stop when equilibrium is reached. The second method consists in the use of FDS (Fire Dynamics Simulator) in order to simulate the propagation speed of the flame for different equivalence ratio in the cylindrical combustion chamber. This geometry is used by Tahtouh et al. (2009), and Bouvet et al. (2010) in their experimental devices for calculating the flame velocity. This study examines the influence of temperature variation of unburned gases on the structure of the flame front, as well as the effect of equivalence ratio on the flame front speed, combustion products and pollutants formation that allows us to deduce which parameters ensure higher efficiency with less fuel consumption and fewer pollutants.

Asymptotic analysis of a premixed laminar flame governed by a two-step reaction

Once properly optimized, single-step chemistry models are able to recover essential characteristics of flames such as burned gases temperature, flame propagation velocity and thickness. Ignition delay values can be reproduced as well. However, an improved description of either the premixed flame inner structure or the ignition process requires the consideration of the multi-step nature of chemical kinetics. Indeed, single-step chemistry models can neither describe properly the internal structure of laminar premixed flames nor the development of ignition processes. These limitations arise because single-step chemistry does not take into account intermediate species, the importance of which is essential. Thus, the present work is aimed at extending the recently-proposed optimized single-step (OSS) framework to multi-step chemistry. To this purpose an unbranched chain reaction with only two consecutive steps relevant to chain initiation and chain termination is considered. The corresponding kinetics model does involve two fictive species, the characteristics of which are optimized together with the two-step chemical scheme parameters, e.g., pre-exponential factors and activation energies. One of this fictive species is relevant to combustion products while the other represents the whole pool of radicals. The chemistry optimization procedure makes use of a well-defined in-house genetic algorithm and the resulting optimized two-step (OTS) model is assessed through comparisons made with data obtained from detailed chemistry computations used as reference. For similar (and even lower) CPU costs, the computations performed with the OTS model put into evidence significant improvements compared to the results obtained with the OSS description.