scholarly journals Numerical Study of the Structure and NO Emission Characteristics of N2- and CO2-Diluted Tubular Diffusion Flames

Energies ◽  
2019 ◽  
Vol 12 (8) ◽  
pp. 1490
Author(s):  
Harshini Devathi ◽  
Carl A. Hall ◽  
Robert W. Pitz
Keyword(s):  
Diffusion Flames ◽  
Numerical Study ◽  
Reaction Pathway ◽  
Flame Structure ◽  
Reaction Rates ◽  
No Production ◽  
Emission Characteristics ◽  
No Emission ◽  
Nitrogen Radicals ◽  
Lower Temperature

The structure of methane/air tubular diffusion flames with 65 % fuel dilution by either CO2 or N2 is numerically investigated as a function of pressure. As pressure is increased, the reaction zone thickness reduces due to decrease in diffusivities with pressure. The flame with CO2-diluted fuel exhibits much lower nitrogen radicals (N, NH, HCN, NCO) and lower temperature than its N2-diluted counterpart. In addition to flame structure, NO emission characteristics are studied using analysis of reaction rates and quantitative reaction pathway diagrams (QRPDs). Four different routes, namely the thermal route, Fenimore prompt route, N2O route, and NNH route, are examined and it is observed that the Fenimore prompt route is the most dominant for both CO2- and N2-diuted cases at all values of pressure followed by NNH route, thermal route, and N2O route. This is due to low temperatures (below 1900 K) found in these highly diluted, stretched, and curved flames. Further, due to lower availability of N2 and nitrogen bearing radicals for the CO2-diluted cases, the reaction rates are orders of magnitude lower than their N2-diluted counterparts. This results in lower NO production for the CO2-diluted flame cases.

scholarly journals A Numerical Investigation of NOx Formation in Counterflow CH4/H2/Air Diffusion Flames

2006 ◽  
Author(s):  
Hongsheng Guo ◽  
Stuart W. Neill ◽  
Gregory J. Smallwood
Keyword(s):  
Diffusion Flames ◽  
Numerical Study ◽  
Flame Structure ◽  
Pure Hydrogen ◽  
No Emission ◽  
Nox Formation ◽  
No Formation ◽  
Hydrogen Enrichment ◽  
Air Diffusion ◽  
Emission Index

A detailed numerical study was carried out for the effect of hydrogen enrichment on flame structure and NOx formation in counterflow CH4/air diffusion flames. Detailed chemistry and complex thermal and transport properties were employed. The enrichment fraction was changed from 0 (pure CH4) to 1.0 (pure H2). The result indicates that for flames with low to moderate stretch rates, with the increase of the enrichment fraction from 0 to 0.5~0.6, NO emission index keeps almost constant or only slightly increases. When the enrichment fraction is increased from 0.5~0.6 to about 0.9, NO emission index quickly increases, and finally NO formation decreases again when pure hydrogen flame condition is approached. However, for flames with higher stretch rates, with the increase of hydrogen enrichment fraction from 0 to 1.0, the formation of NO first quickly increases, then slightly decreases and finally increases again. Detailed analysis suggests that the variation of the characteristics in NO formation in stretched CH4/air diffusion flames is caused by the change of flame structure and NO formation mechanism, when the enrichment fraction and stretch rate are changed.

scholarly journals Effects of strain rate and CO2 on no formation in CH4/N2/O2 counter-flow diffusion flames

2018 ◽  
Vol 22 (Suppl. 2) ◽  
pp. 769-776
Author(s):  
Fei Ren ◽  
Longkai Xiang ◽  
Huaqiang Chu ◽  
Weiwei Han
Keyword(s):  
Strain Rate ◽  
Diffusion Flames ◽  
Numerical Study ◽  
Flame Temperature ◽  
Co2 Concentration ◽  
No Production ◽  
Counter Flow ◽  
Detailed Chemistry ◽  
No Formation ◽  
Key Factor

The reduction of nitrogen oxides in the high temperature flame is the key factor affecting the oxygen-enriched combustion performance. A numerical study using an OPPDIF code with detailed chemistry mechanism GRI 3.0 was carried out to focus on the effect of strain rate (25-130 s?1) and CO2 addition (0-0.59) on the oxidizer side on NO emission in CH4 / N2 / O2 counter-flow diffusion flame. The mole fraction profiles of flame structures, NO, NO2 and some selected radicals (H, O, OH) and the sensitivity of the dominant reactions contributing to NO formation in the counter-flow diffusion flames of CH4\/ N2 /O2 and CH4 / N2 / O2 / CO2 were obtained. The results indicated that the flame temperature and the amount of NO were reduced while the sensitivity of reactions to the prompt NO formation was gradually increased with the increasing strain rate. Furthermore, it is shown that with the increasing CO2 concentration in oxidizer, CO2 was directly involved in the reaction of NO consumption. The flame temperature and NO production were decreased dramatically and the mechanism of NO production was transformed from the thermal to prompt route.

scholarly journals A Numerical Study on the Effect of CO Addition on Flame Temperature and NO Formation in Counterflow CH4/Air Diffusion Flames

2008 ◽  
Vol 130 (5) ◽  
Author(s):  
Hongsheng Guo ◽  
W. Stuart Neill
Keyword(s):  
Carbon Monoxide ◽  
Strain Rate ◽  
Diffusion Flames ◽  
Numerical Study ◽  
Formation Rate ◽  
Flame Temperature ◽  
No Emission ◽  
No Formation ◽  
Air Diffusion ◽  
Emission Index

A numerical study was carried out to understand the effect of CO enrichment on flame temperature and NO formation in counterflow CH4/air diffusion flames. The results indicate that when CO is added to the fuel, both flame temperature and NO formation rate are changed due to the variations in adiabatic flame temperature, fuel Lewis number, and chemical reaction. At a low strain rate, the addition of carbon monoxide causes a monotonic decrease in flame temperature and peak NO concentration. However, NO emission index first slightly increases, and then decreases. At a moderate strain rate, the addition of CO has negligible effect on flame temperature and leads to a slight increase in both peak NO concentration and NO emission index, until the fraction of carbon monoxide reaches about 0.7. Then, with a further increase in the fraction of added carbon monoxide, all three quantities quickly decrease. At a high strain rate, the addition of carbon monoxide causes increase in flame temperature and NO formation rate, until a critical carbon monoxide fraction is reached. After the critical fraction, the further addition of carbon monoxide leads to decrease in both flame temperature and NO formation rate.

scholarly journals A computational study of soot formation and flame structure of coflow laminar methane/air diffusion flames under microgravity and normal gravity

2021 ◽  
Author(s):  
Armin Veshkini ◽  
Seth B. Dworkin
Keyword(s):  
Normal Gravity ◽  
Volume Fraction ◽  
Diffusion Flames ◽  
Soot Formation ◽  
Numerical Study ◽  
Computational Study ◽  
Flame Structure ◽  
Chemical Kinetic ◽  
Surface Growth ◽  
Rate Of Increase

A numerical study is conducted of methane-air coflow diffusion flames at microgravity (μg) and normal gravity (lg), and comparisons are made with experimental data in the literature. The model employed uses a detailed gas phase chemical kinetic mechanism that includes PAH formation and growth, and is coupled to a sectional soot particle dynamics model. The model is able to accurately predict the trends observed experimentally with reduction of gravity without any tuning of the model for different flames. The microgravity sooting flames were found to have lower temperatures and higher volume fraction than their normal gravity counterparts. In the absence of gravity, the flame radii increase due to elimination of buoyance forces and reduction of flow velocity, which is consistent with experimental observations. Soot formation along the wings is seen to be surface growth dominated, while PAH condensation plays a more major role on centerline soot formation. Surface growth and PAH growth increase in microgravity primarily due to increases in the residence time inside the flame. The rate of increase of surface growth is more significant compared to PAH growth, which causes soot distribution to shift from the centerline of the flame to the wings in microgravity. Keywords: laminar diffusion flame,methane-air,microgravity, soot formation, numerical modelling

Numerical Study on Formation Mechanism and Reduction Methods of NOx in Methane/Oxygen-Enriched Air Diffusion Flame

2012 ◽  
Vol 602-604 ◽  
pp. 1317-1324
Author(s):  
Yao Xun Feng ◽  
Xiao Feng Zheng ◽  
Ming Sheng Jia
Keyword(s):  
Production Rate ◽  
Diffusion Flame ◽  
Numerical Study ◽  
Nox Reduction ◽  
Flame Structure ◽  
Inert Gases ◽  
Maximum Temperature ◽  
No Production ◽  
Detailed Chemical Kinetics ◽  
Reduction Methods

In this study, a methane/oxygen-enriched air counterflow diffusion flame was analyzed numerically using detailed chemical kinetics, on the condition that the oxygen mass fraction in the oxidizer stream varied from 21% to 99%. The obtained results show that as the oxygen concentration in air increases, the maximum temperature increases; the region of combustion reaction is gradually divided into two parts, and the total NO production rate and especially the thermal NO production rate increase greatly. With consideration of the possibility of gas recirculation to minimize NOX in the industrial combustor, the usefulness of NOX reduction in combustion was analyzed numerically when the methane stream was diluted with the inert gases N2 or CO2. The obtained results show that the flame structure and dominant mechanism of NO formation change greatly with the concentration of diluents in fuel; the emission index of NO decreases gradually when the concentration of diluent CO2 increases.

Effect of Strain Rate and Pressure on the Flame Structure and Emission Characteristics of Syngas Flames

2007 ◽  
Author(s):  
Sibendu Som ◽  
Anita I. Rami´rez ◽  
Jonathan Hagerdorn ◽  
Alexei Saveliev ◽  
Suresh K. Aggarwal
Keyword(s):  
Strain Rate ◽  
Reaction Zone ◽  
Laminar Flame ◽  
Flame Structure ◽  
Flame Temperature ◽  
Chemical Kinetic ◽  
No Production ◽  
Emission Characteristics ◽  
Strain Rates ◽  
Fundamental Understanding

Synthesis gas or “Syngas” is being recognized as a viable energy source worldwide, particularly for stationary power generation due to its wide flexibility in fuel sources. There are gaps in the fundamental understanding of syngas combustion and emissions characteristics, especially at elevated pressures, high strain rates and in more practical conditions. This paper presents a numerical and experimental investigation to gain fundamental understanding of combustion and emission characteristics of syngas with varying composition, pressure and strain rate. Two representative syngas fuel mixtures, 50% H2 / 50% CO and 5% H2 / 95% CO (% vol.), are chosen, three detailed chemical kinetic models are used namely, GRI 3.0, Davis et al. and Li et al. mechanisms. Davis et al. mechanism agrees best with the experimental data hence is used to simulate the partially premixed flame structures at all pressures. Results indicate that for the pressure range investigated, a typical double flame structure was observed characterized by a rich premixed reaction zone (RPZ) on the fuel side and a nonpremixed reaction zone (NPZ) at the oxidizer side nozzle with the stabilizing due to the H2 chemistry rather than the CO chemistry. Sensitivity analysis to mass burning rates for unstretched laminar flame shows that flames are more sensitive to H2 chemistry. For both representative mixtures an increase in pressure leads to a significant increase in NO due to increase in flame temperature. The emission index for these flames is also found to follow a similar behavior with pressure. Although flame temperatures were higher for flame A, total NO is lower for these flames due to increases in reburn characteristics. Thermal route dominates NO production while, prompt route is negligible. Experimental analysis on the stability of nonpremixed syngas/air flames showed that the flames were very stable for the range of strain rates investigated. At low strain rates it required 0.5% H2 to establish a stable flame.

Computation of Nitric Oxide and Soot Emissions From Turbulent Diffusion Flames

1985 ◽  
Vol 107 (1) ◽  
pp. 48-53 ◽  
Author(s):  
T. Ahmad ◽  
S. L. Plee ◽  
J. P. Myers
Keyword(s):  
Diffusion Flames ◽  
Soot Oxidation ◽  
Air Entrainment ◽  
Reaction Rates ◽  
Gas Jet ◽  
Maximum Temperature ◽  
Injection Velocity ◽  
Soot Emission ◽  
No Emission ◽  
Jet Flame

An existing steady-state, locally homogeneous flow model of turbulent spray combustion was modified to predict NO emission from a spray flame and soot emission from a gas-jet flame. The effect of turbulent fluctuations on the reaction rates was accounted for. The predicted NO emission from an n-pentane spray with a changing injection velocity could be correlated with the convective time scale of the flow. Calculation of soot emission from a burning turbulent gas jet indicated that the centerline soot concentration reaches a peak upstream of the maximum temperature location and then decreases due to soot oxidation and dilution by air entrainment.

Numerical study of the effects of radiative heat losses on diffusion flames

2006 ◽  
Vol 220 (8) ◽  
pp. 1189-1199
Author(s):  
M D Gaustad ◽  
T Shamim
Keyword(s):  
Radiation Effects ◽  
Diffusion Flames ◽  
Numerical Study ◽  
Flame Structure ◽  
Combustion Products ◽  
Chemical Mechanism ◽  
Strain Rates ◽  
Detailed Chemistry ◽  
Extinction Limit ◽  
Radiative Losses

The effects of thermal radiation are numerically investigated for a methane-air counterflow diffusion flame, using ‘detailed’ chemistry. The radiative losses from combustion products (CO2 and H2O) were considered by using a thin gas approximation. The results show a significant effect of radiative losses causing extinction at low strain rates. On the basis of the radiative losses from gaseous combustion products, an extinction limit was found to be 0.7 s−1. The presence of soot will move this limit to higher strain rates. The radiation effects are relatively less at moderate and high strain rates, where they may cause a reduction in the peak temperatures by ∼ 10 per cent. In addition to decreasing peak temperatures and combustion products, the radiative losses also reduce the flame width. The results show the importance of including detailed chemical mechanism in correctly predicting the extinction limit and the influence of radiative losses on flame structure.

scholarly journals Numerical study of flame/vortex interactions in 2-D Trapped Vortex Combustor

2014 ◽  
Vol 18 (4) ◽  
pp. 1373-1387 ◽  
Author(s):  
Prasad Mishra ◽  
Renganathan Sudharshan ◽  
Kumar Ezhil
Keyword(s):  
Numerical Study ◽  
Flame Structure ◽  
Reaction Rates ◽  
Operating Conditions ◽  
Navier Stokes ◽  
Base Case ◽  
Primary Vortex ◽  
Vortex Interactions ◽  
Wide Range ◽  
Trapped Vortex

The interactions between flame and vortex in a 2-D Trapped Vortex Combustor are investigated by simulating the Reynolds Averaged Navier Stokes (RANS) equations, for the following five cases namely (i) non-reacting (base) case, (ii) post-vortex ignition without premixing, (iii) post-vortex ignition with premixing, (iv) pre-vortex ignition without premixing and (v) pre-vortex ignition with premixing. For the post-vortex ignition without premixing case, the reactants are mixed well in the cavity resulting in a stable ?C? shaped flame along the vortex edge. Further, there is insignificant change in the vorticity due to chemical reactions. In contrast, for the pre-vortex ignition case (no premixing); the flame gets stabilized at the interface of two counter rotating vortices resulting in reduced reaction rates. There is a noticeable change in the location and size of the primary vortex as compared to case (ii). When the mainstream air is premixed with fuel, there is a further reduction in the reaction rates and thus structure of cavity flame gets altered significantly for case (v). Pilot flame established for cases (ii) and (iii) are well shielded from main flow and hence the flame structure and reaction rates do not change appreciably. Hence, it is expected that cases (ii) and (iii) can perform well over a wide range of operating conditions.

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