Counterflow Methane-air Flames Research Articles

A computational study of the effects of DC electric fields on non-premixed counterflow methane-air flames

Two-dimensional axisymmetric simulations for counterflow non-premixed methane-air flames were undertaken as an attempt to reproduce the experimentally observed electro-hydrodynamic effect, also known as the ionic wind effect, on flames. Incompressible fluid dynamic solver was implemented with a skeletal chemical kinetic mechanism and transport property evaluations. The simulation successfully reproduced the key characteristics of the flames subjected to DC bias voltages at different intensity and polarity. Most notably, the simulation predicted the flame positions and showed good qualitative agreement with experimental data for the current–voltage curve. The flame response to the electric field with positive and negative polarity exhibited qualitatively different characteristics. In the negative polarity of the configuration considered, a non-monotonic variation of the current with the voltage was observed, along with the existence of an unstable regime at an intermediate voltage level. With positive polarity, a typical monotonic current–voltage curve was obtained. This behavior was attributed to the asymmetry in the distribution of the positive and negative ions resulting from ionization processes. The present study demonstrated that the mathematical and computational models for the ion chemistry, transport, and fluid dynamics were able to describe the key processes responsible for the flame-electric field interaction.

Effect of multistage combustion on NO x emissions in methane–air flames

Coflow and counterflow methane–air flames are simulated over a complete partially premixed regime in order to characterize the effects of dominant combustion modes (i.e., single-, two-, and three-stage combustion) on NO x emissions. Simulations employ a comprehensive numerical model that uses detailed descriptions of transport and chemistry (GRI-2.11 mechanism) and includes radiation effects. It is demonstrated that a complete partially premixed regime, which extends from premixed flames to triple flames and then to double flames, can be simulated by suitably varying the equivalence ratios in the fuel-rich and fuel-lean streams, while maintaining the global equivalence ratio fixed. Both counterflow and coflow simulations show that NO x emissions decrease significantly from the premixed to the triple flame regime, and then increase from the triple to the double flame regime. Therefore, triple flames not only extend the rich and the lean flammability limits, but also exhibit superior NO x characteristics compared to the corresponding premixed flames and double flames, with thermal, prompt, NNH-intermediate, and N 2O-intermediate routes being the important contributors (in descending order) to NO x formation. Coflow and counterflow flames exhibit qualitatively similar NO x characteristics in the entire partially premixed regime and an optimum level of partial premixing that yields the lowest NO x emission. The quantitative differences in NO x emissions between the two configurations can be attributed to geometry-dependent effects. In particular, compared to counterflow flames, the coflow flames have significantly larger flame volume and therefore lower peak temperature and NO x emission index in the triple flame regime.

Concentrations of Nitric Oxide in Laminar Counterflow Methane/Air Diffusion Flames

Results of nearly 20 different measurements of profiles of concentrations of oxides of nitrogen in steady counterflow methane-air flames are studied and compared with predictions of four different chemical-kinetic mechanisms. The conditions include air-side strain rates between about 10 and 200 s - 1 , pressures between 1 and 15 atm, air temperatures between about 300 and 560 K, partial premixing of air on the fuel side down to an equivalence ratio of 1.5, and dilution of the fuel with nitrogen down to a fuel mole fraction of 0.25, all for a fuel temperature of about 300 K. Although most of the experimental results were taken from the literature, two new measurements were made, employing a newly devised NO x -scrubber technique to correct chemiluminescent NO x -analyzer results for interference from other species, principally hydrocarbons on the fuel side containing two or more carbon atoms. The results show that differences between measured and predicted NO x concentrations tend to be smaller on the fuel-lean side of the flame than they are on the fuel-rich side, where typical discrepancies reach values on the order of 50%. Whereas the Milan mechanism, the San Diego mechanism, and the Gas Research Institute (GRI) mechanism version 2.11 usually produce reasonable agreement with the experimental data, the newer GRI mechanism version 3.0 tends to overpredict NO x under most of the conditions investigated. Because the prompt mechanism is the principal source of NO x in these flames, this might be associated in some way with the tendency of the GRI mechanisms to overpredict acetylene concentrations by about a factor of two, an overprediction that is demonstrated to occur by comparisons with additional experimental data on concentration profiles of intermediate hydrocarbons.

Effect of sodium bicarbonate particle size on the extinction condition of non-premixed counterflow flames

The effect of chemically active sodium bicarbonate (NaHCO 3) particles and inert silica particles on suppression of laminar, non-premixed counterflow methane-air flames is investigated. In experiments, the NaHCO 3 particles are separated into size ranges of <10, 10-20, 20-30, and 40-60 μm and are introduced with the air stream using two types of particle seeders. To distinguish physical effects from chemical effects of particles on flame extinction condition, inert silica particles of similar size ranges are also used as control experiments. Counterflow flame extinction results suggest that NaHCO 3 particles below 10 μm are equally effective as the 10-20 μm particles, while 20-30 μm particles are only half as effective. Unlike previously reported non-monotonic variation in effectiveness with particle size, the present results do not indicate a reversal in effectiveness, for the particle size range investigated. The observed experimental trends on NaHCO 3 particle size are analyzed using a numerical model that includes interactions between the particles and the gas-phase flame. In the present numerical model, a global particle decomposition step is assumed and the need for a comprehensive particle decomposition model is identified.

The formation of SiO2 from hexamethyldisiloxane combustion in counterflow methane-air flames

Silica formation from hexamethyldisiloxane (HMDS) oxidation was studied by means of a CH 4 −N 2 /air opposed diffusion flame technique to which vaporized HMDS was added to the fuel flow. The CH 4 −N 2 /air flame changed color from a pale-blue flame to whitish pink color when small amounts of HMDS were introduced in the flame. Increasing concentrations (1.3 mol %) made the flame more luminous, and a second thin-flame zone, orange in color, appeared on the fuel side. Emission spectroscopy revealed the existence of Si−H and Si−O species. SiO 2 particles were observed only in the postcombustion gases, and the analysis of solid materials suggested the formation of fused silica particles, which were initially about 10 nm in size, forming outside the flame zone in these experiments. The overall mechanism in addition to that for methane oxidation is suggested as follows: C 6 H 18 Si 2 O+OH=2C 3 H 9 SiO+H C 6 H 18 Si 2 O+O 2 =2C 3 H 9 SiO+O C 6 H 18 Si 2 O+HO 2 =2C 3 H 9 SiO+OH C 3 H 9 SiO+M=3CH 3 +SiO+M SiO+HO 2 =HSiO+O 2 SiO+OH=SiO 2 +H SiO+O+M=SiO 2 +M SiO+O 2 =SiO 2 +O together with the formation of SiOH and SiO(OH) species. The agreement between the model predictions using Sandia code OPPDIF and the experimental data was found to be satisfactory. The model appears to be a useful tool in elaboration of chemistry of formation of SiO 2 in flames used to synthesize pure silica in this way.

Extinction of non-premixed methane-and propane-air counterflow flames inhibited with CF 4, CF 3H and CF 3Br

The extinction condition was studied for non-premixed counterflow flames of methane and propane impinging against an air stream with added amounts of CF 4 , CF 3 H, or CF 3 Br. Strain rates near extinction using laser Doppler velocimetry were determined for counterflow methane-air flames, with CF 4 and CF 3 H added to the air stream. The value obtained for the case of methane impinging against an air stream with no agent added, 400±25 s −1 , agrees with the experimental value reported in the literature. For both methane-and propane-air flames, increasing amounts of CF 4 , CF 3 H, or CF 3 Br in the air stream decrease the extinction oxidizer stream mass flow rate (or strain rate) monotonically until, at a given amount of agent, the extinction strain rate rapidly drops. If the molar percentage of agent in the air stream exceeds 2.5% for CF 3 Br and 11% for CF 4 or CF 3 H, non-premixed counterflow methane-air flames cannot be maintained. Counterflow propane-air flames cannot be maintained if agent concentrations in the air stream are greater than 2.7% for CF 3 Br, 11% for CF 3 H, and >14% for CF 4 . For a given amount of agent added to the air stream that does not exceed the limiting agent concentration, the extinction strain rate is comparable for CF 3 H or CF 4 addition in methane-air flames. CF 3 H is more effective than CF 4 in propane-air flames, similar to the reported extinction behavior in heptane-air cup burner studies. CF 3 Br is significantly more effective than CF 4 or CF 3 H in extinguishing methane-or propane-air flames. The net chemical effect of CF 3 H in the inhibition of non-premixed counterflow flames is more pronounced for propane-air flames than for methane-air flames. Explanations for this behavior are discussed briefly.

A comparison between experimental measurements and numerical calculations of the structure of premixed rotating counterflow methane-air flames

In this paper we investigate computationally the structure and extinction of rotating counterflow premixed methane-air flames. By seeking a similarity solution of the governing two-dimensional conservation equations, we reduce the problem to the solution of a two-point boundary value problem along the axial spatial coordinate. We apply arclength continuation methods to investigate the equivalence ratio at extinction versus the rotation rate for four flames with ejection velocities equivalent to the experimental conditions considered by Chen et al. The results of our study indicate that by counterrotating the gases from the burners the lean extinction limit can be lowered. This is due to the lowering of the value of the local strain rate in the neighborhood of the stagnation plane.

Comparison between experimental measurements and numerical calculations of the structure of counterflow, diluted, methane-air, premixed flames

In this paper we investigate experimentally and numerically the structure and extinction of counterflow methane-air flames in the fresh reactant-hot product configuration. The flame is formed in the neighborhood of the stagnation point produced by the counterflow of fresh mixture and hot combustion products. Temperature and species profiles are measured using radiatively corrected thermocouple measurements together with a quartz microproble and a gas chromatograph. The governing conservation equations are solved numerically by employing adaptive continuation techniques. The model includes detailed transport and complex kinetics.