Methane-air Diffusion Flame Research Articles

CoFlame: A refined and validated numerical algorithm for modeling sooting laminar coflow diffusion flames

Mitigation of soot emissions from combustion devices is a global concern. For example, recent EURO 6 regulations for vehicles have placed stringent limits on soot emissions. In order to allow design engineers to achieve the goal of reduced soot emissions, they must have the tools to so. Due to the complex nature of soot formation, which includes growth and oxidation, detailed numerical models are required to gain fundamental insights into the mechanisms of soot formation. A detailed description of the CoFlame FORTRAN code which models sooting laminar coflow diffusion flames is given. The code solves axial and radial velocity, temperature, species conservation, and soot aggregate and primary particle number density equations. The sectional particle dynamics model includes nucleation, PAH condensation and HACA surface growth, surface oxidation, coagulation, fragmentation, particle diffusion, and thermophoresis. The code utilizes a distributed memory parallelization scheme with strip-domain decomposition. The public release of the CoFlame code, which has been refined in terms of coding structure, to the research community accompanies this paper. CoFlame is validated against experimental data for reattachment length in an axi-symmetric pipe with a sudden expansion, and ethylene–air and methane–air diffusion flames for multiple soot morphological parameters and gas-phase species. Finally, the parallel performance and computational costs of the code is investigated. Program summaryProgram title: CoFlameCatalogue identifier: AFAU_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AFAU_v1_0.htmlProgram obtainable from: CPC Program Library, Queen’s University, Belfast, N. IrelandLicensing provisions: GNU General Public License, version 3No. of lines in distributed program, including test data, etc.: 94964No. of bytes in distributed program, including test data, etc.: 6242986Distribution format: tar.gzProgramming language: Fortran 90, MPI. (Requires an Intel compiler).Computer: Workstations.Operating system: Linux.RAM: From 16 GB to over 1000 GB depending on size of system being simulatedClassification: 22.Nature of problem:Soot formation in laminar diffusion flames with detailed description of thermodynamics, kinetic, and transport dataSolution method:Finite volume method utilizing the pseudo-transient SIMPLE algorithm and locally coupled chemistry solverAdditional comments:The code was specifically developed for modeling soot formation in laminar diffusion flamesRunning time:From hours to a month depending on the complexity of the chemical mechanism and the disparity between the initial guess and the final solution.

Experimental study of bluff body stabilized laminar reactive boundary layers

Experiments have been carried out to analyze the characteristics of laminar cross-flow methane–air diffusion flames in the presence of bluff bodies. The stable operating regime having steady and oscillating flames, and unstable operating regime that includes flame blow-out and extinction, are identified by varying the air and fuel flow rates systematically. Detailed stability maps indicating these regimes have been proposed for bluff bodies having rectangular, isosceles triangular and semicircular shapes. Three types of flames, namely plate stabilized, bluff body stabilized and separated flames, have been captured using a high-definition digital camera. The flame anchoring locations, flame shapes and transitions between different regimes of stabilization have been analyzed thoroughly. Hysteresis effects in the transition between the regimes, when the air flow rate is systematically increased or decreased, are also presented in the stability maps. The interesting features of the semicircular cylinder bluff body have been discussed in comparison to other two bluff bodies. Additionally, the effect of addition of hydrogen in small quantities to methane has also been envisaged. When 10% hydrogen by volume is added to methane, the plate stabilized regime is seen to be widened. Also, the hysteresis effect is seen to reduce with the injection of hydrogen.

Radiation effect on temperature distribution and NO formation in a diffusion flame under reduced gravity conditions

Combustion of hydrocarbon fuel is accompanied with the formation of nitric oxide (NO) amongst other harmful emissions. In this work, a numerical investigation has been made for understanding the effect of radiative heat transfer on temperature distribution and formation of thermal NO in a methane–air diffusion flame under different reduced gravity environments. Conservation equations of overall mass, species concentration, momentum and energy for the reactive flows have been numerically solved with the use of finite difference scheme. In addition to that a semi-empirical soot model and an optically thin radiation model have been incorporated in the simulation. Gravity level is varied by the changed values of acceleration due to gravity. A thermal NO model incorporated accounts for the NO formation process which is decoupled from the hydrocarbon combustion. The relevant conservation equations have been solved as a post combustion reaction process. The flame height drops marginally with the reduction of gravity. Temperature becomes more uniformly distributed at lower gravity. NO formation boosts up with the fall of gravity below normal level when no radiation effect is considered. However, when radiation is considered, NO formation declines marginally with the reduction of gravity levels. Also in this case, concentration values of NO compare substantially lower with those without radiation. The upsurge of NO formation due to decline in gravity; and on the other hand, a shrinkage in concentration values of NO due to radiation effect can be attributed mainly to the rise and fall of temperature respectively in the computational zone.

Laser-Induced Plasma Spectrometry With Chemical Seeding and Application to Flow Mixing Analysis in Methane–Air Flames

Spectroscopic measurements of flames are amongst the most important analytical diagnostic techniques that allow one to improve thermal and energy efficiency of industrial furnaces. A chemical seeding laser-induced plasma spectroscopy (CS-LIPS) was successfully developed and applied for mixing analysis of a methane–air diffusion flame. The results obtained showed that sensitivity of this system was much improved using silica rod as the target material in place of the tungsten material used in our previous studies. Profiling of Mg spectral emission and mixing in the flame was made more clearly with the introduction of magnesium aerosols as a tracer into the combustion air flow.

Comparison of structures of laminar methane–oxygen and methane–air diffusion flames from atmospheric to 60 atm

A combined experimental and numerical study was conducted to examine the structure of laminar methane–oxygen diffusion flames in comparison with methane–air flames. Soot measurements made in these flames indicated that the maximum soot yields of methane–air flames are consistently higher than methane–oxygen flames at all pressures. The maximum soot yield of the methane–oxygen flames reaches a peak near 40atm and then starts decreasing as the pressure further increased. The maximum soot yield of the methane–air flames plateaus at about 40atm and does not change much with further increases in pressure. Methane–oxygen flames display a distinct two-zone structure which is visible from atmospheric pressure up to 60atm. The inner zone, similar to hydrocarbon-air diffusion flames, has a yellow/orange colour and is surrounded by an outer blue zone. This outer zone was shown to have a stratified structure with a very steep equivalence ratio gradient. The main reactions in this zone were shown to be the oxidation of hydrogen and carbon monoxide produced within the inner zone. The methane–air diffusion flames had a thin layer of blue outer zone at atmospheric pressure; however, this zone completely disappeared when the pressure was increased above atmospheric. The presence of the two-zone structure in the methane–oxygen flames was attributed to the intensified penetration of oxygen into the core flow. The higher diffusivities, steeper oxygen concentration gradients, and enhanced entrainment increase the transport of oxygen to the flame. As such, there is sufficient oxygen present near the base of the flame to support the diffusion flame in the inner zone of the methane–oxygen flames. The abundance of oxygen near the centerline, even in the lower portion of the flame, also promotes the oxidation of soot.

Evaluation of the sooting properties of real fuels and their commonly used surrogates in a laminar co-flow diffusion flame

In an effort to help determine the fidelity of simple surrogate fuels to represent real fuel chemistry in computational fluid dynamic simulations of engines, quantitative two-dimensional soot volume fraction measurements were made in a laminar coflow methane–air diffusion flame seeded with approximately 2200ppm of real and surrogate fuels. A combined laser extinction and laser-induced incandescence (LII) method was used to measure soot volume fraction. Additionally, soot particles were thermophoretically sampled from the flame and soot morphology data were collected with a transmission electron microscope (TEM). Vaporized liquid fuels were seeded at low concentrations to maintain constant thermodynamic conditions for each experiment. In all, 14 different fuels were investigated, including: three real fuels (gasoline, diesel, and jet fuel), two alkanes, and a variety of simple surrogate fuels. A toluene reference fuel (TRF) (30% aromatics) and gasoline (28% aromatics) were found to have similar soot volume fractions and soot morphology. The addition of toluene to the long-straight chain of n-tetradecane in similar concentrations (30vol.%) as the aromatic concentration of diesel (31.1vol.%) resulted in soot volume fractions that were very similar, although the primary particle size and mass-weighted radius of gyration were both smaller for the surrogate than for the conventional diesel fuel. Finally, the jet-fuel surrogate tested was found to have a lower sooting tendency than the jet-A fuel despite the jet-A fuel having a lower concentration of aromatics than the surrogate. Soot morphology between jet-A and the jet-fuel surrogate were the same within experimental uncertainty. The current work provides an experimental dataset for validation of fuel-surrogate chemistry and soot models.

All-diode-pumped quasi-continuous burst-mode laser for extended high-speed planar imaging

An all-diode-pumped, multistage Nd:YAG amplifier is investigated as a means of extending the duration of high-power, burst-mode laser pulse sequences to an unprecedented 30 ms or more. The laser generates 120 mJ per pulse at 1064.3 nm with a repetition rate of 10 kHz, which is sufficient for a wide range of planar laser diagnostics based on fluorescence, Raman scattering, and Rayleigh scattering, among others. The utility of the technique is evaluated for image sequences of formaldehyde fluorescence in a lifted methane-air diffusion flame. The advantages and limitations of diode pumping are discussed, along with long-pulse diode-bar performance characteristics to guide future designs.

Numerical investigation of NOx reduction in a sudden-expansion combustor with inclined turbulent air jet

Axisymmetric sudden-expansion geometry of a co-flowing methane-air diffusion flame is considered to investigate the effect of air inlet conditions on NOx formation, flow field and temperature distribution using the k-ɛ turbulence and β-PDF combustion model. The predicted results are in acceptable agreement with the published experimental and numerical data. The obtained results show that increasing air turbulence intensity results in considerable decrease in NO formation. Increasing the inlet angle of the air causes the NO formation to decrease due to raising vorticity strength. As a new index, the mass-averaged integral of vorticity magnitude is introduced to investigate the effect of altering inlet angle of the air on the flow field.

The Effect of Air Preheating on a Sudden-Expansion Turbulent Diffusion Air-fuel Flame

An axisymmetric sudden-expansion geometry of a co-flowing methane–air diffusion flame is considered to investigate the effect of air preheating on pollutant formation using \({k-\varepsilon}\) turbulence and β-PDF combustion models. The governing equations are solved by iterative numerical approach using Finite Volume Method and a second-order upwind scheme. NO and CO2 concentration and peak combustor temperature as well as combustor efficiency are studied in this paper. The obtained results show that air preheating increases NO formation and maximum temperature in the combustor. Air preheating improves the combustor efficiency and save fuel as well.

Quasi-continuous burst-mode laser for high-speed planar imaging

The pulse-burst duration of a compact burst-mode Nd:YAG laser is extended by one order of magnitude compared to previous flashlamp-pumped designs by incorporating a fiber oscillator and diode-pumped solid-state amplifiers. The laser has a linewidth of <2 GHz at 1064.3 nm with 150 mJ per individual pulse at 10 kHz. The performance of the system is evaluated by using the third-harmonic output at 354.8 nm for high-speed planar laser-induced fluorescence of formaldehyde in a lifted methane-air diffusion flame. A total of 100 and 200 sequential images of unsteady fluid-flame interactions are acquired at repetition rates of 10 kHz and 20 kHz, respectively.

Observation of Superaggregates from a Reversed Gravity Low-Sooting Flame

Copyright 2012 American Association for Aerosol Research

Effects of gravity and pressure on laminar coflow methane–air diffusion flames at pressures from 1 to 60 atmospheres

The effects of pressure and gravity on the sooting characteristics and flame structure of coflow methane–air laminar diffusion flames between 1 and 60 atm were studied numerically. Computations were performed by solving the unmodified and fully-coupled equations governing reactive, compressible flows which include complex chemistry, detailed radiation heat transfer and soot formation/oxidation. Soot formation/oxidation was modeled using an acetylene-based, semi-empirical model which was verified with previously published experimental data to correctly capture many of the observed trends at normal-gravity. Calculations for each pressure considered were performed under both normal- and zero-gravity conditions to help separate and identify the effects of pressure and buoyancy on soot formation. Based on the numerical predictions, pressure and gravity were observed to significantly influence the sooting behavior and structure of the flames through their effects on buoyancy and temperature. Zero-gravity flames generally have lower temperatures, broader soot-containing zones, and higher soot volume fractions than normal-gravity flames at the same pressure. Buoyancy forces caused the normal-gravity flames to narrow with increasing pressure while the increased soot concentrations and radiation at high pressures caused the zero-gravity flames to lengthen. Low-pressure flames at both gravity levels exhibited a similar power-law dependence of the maximum carbon conversion on pressure that weakened as pressure was increased. In the zero-gravity flames, increasing pressure beyond 20 atm caused the maximum carbon conversion factor to decrease.

Spatially Resolved Laser-Induced Breakdown Spectroscopy in Methane—Air Diffusion Flames

A new setup for spatially resolved laser-induced breakdown spectroscopy (SR-LIBS) is used for the first time to analyze methane-air diffusion flames. Using this configuration, background continuum emission is reduced, signal-to-background noise ratio is increased up to eight times, and spatial resolution is enhanced. The local equivalence ratio is also quantitatively estimated and the width of the secondary combustion region at a specified height above the burner is determined for two different methane flow rates. Furthermore, the threshold energy for spark formation is measured for regions inside and outside the flame. The results show that threshold energy is larger at the secondary combustion region, near the border of the flame, than inside the flame.

Leading-Edge Flame Fluctuations in Lifted Turbulent Flames

Studies are presented that examine the fluctuations in liftoff height of lifted methane flames in the presence of air coflow. At a certain jet exit velocity, a flame will lift from the burner exit and stabilize at some downstream position. The partially-premixed flame front of the lifted flame oscillates in the axial direction, with the fluctuations becoming greater in flames stabilized further downstream. These fluctuations are also observed in flames where blowout is imminent. This work investigates the role of fuel velocity and air co-flow on flame fluctuations in both stable and unstable regimes. The results of video imaging of a lifted methane-air diffusion flame are presented and discussed. Images are used to ascertain the changes in the reaction zone that influence these fluctuations and relate the movement to blowout.

Methane Diffusion Flame Dynamics at Elevated Pressures

The chamber pressure and fuel flow rate effects on the flickering behavior of methane-air diffusion flames was studied over the pressure range of 1 to 10 bar. Photomultipliers and high-speed imaging techniques have been used to study the frequency and magnitude of the flame oscillation and the change in global flame shape. The instability behavior of the flame was observed to be sensitive to both the fuel flow rate and pressure. Particularly, it has been observed that the flame responds to the change of pressure more when the pressure is relatively low. High-speed imaging has shown that the periodical break-up of the methane flame at higher flow rates is almost symmetric. However, the methane flames at lower flow rates oscillate in a more waving manner due to the alternating lateral nature of the outer vortices. The average flame luminosity was observed to increase with pressure up to 6 bar and then starts to decrease with the further increase of pressure. The flame oscillation magnitude (L f ) and oscillation wavelength (λ) were obtained from the high-speed imaging database. It has been observed that the trends of these parameters correlate well with the standard deviation (σ) of mean pixel intensity (MPI), measured from the flame high-speed images. The increase in fuel flow rate was observed to increase the magnitude of oscillation. The dominant flickering frequency of a methane diffusion flame varies with the chamber pressure as a function of P n (f = 15.7P 0.17).

Soot formation and temperature structure in small methane–oxygen diffusion flames at subcritical and supercritical pressures

An experimental study was conducted to examine the characteristics of laminar methane–oxygen diffusion flames up to 100 atmospheres. The influence of pressure on soot formation and on the structure of the temperature field was investigated over the pressure range of 10–90 atmospheres in a high-pressure combustion chamber using a non-intrusive, line-of-sight spectral soot emission diagnostic technique. Two distinct zones characterized the appearance of a methane and pure oxygen diffusion flame: an inner luminous zone similar to the methane–air diffusion flames, and an outer diffusion flame zone which is mostly blue. The flame height, marked by the visible soot radiation emission, was reduced by over 50% over the pressure range of 10–100 atmospheres. Between 10 and 40 atmospheres, the soot levels increased with increasing pressure; however, above 40 atmospheres the soot concentrations decreased with increasing pressure.

Soot reduction in strongly forced lifted CH4–air laminar flames

Previous work has shown that for sufficiently high periodic forcing amplitudes, laminar diffusion flames can burn in an effectively partially premixed mode. Experimental observations show that the luminosity and sooting properties of the forced flames are significantly modified by the presence of strong forcing. In this work, simulations were performed to examine the effects of strong forcing on soot production in lifted methane (CH4)–air laminar diffusion flames. The gas phase combustion reactions were modelled using detailed gas-phase chemistry and complex thermo-physical properties. A two equation soot chemistry model for soot nucleation, surface growth and oxidation was used. The radiation heat transfer was modelled using the S-6 discrete ordinates method. First, an unforced flickering methane–air diffusion flame was modelled and then the flame was forced by varying the amplitude and frequency of the fuel axial velocity in the nozzle. Cases where the peak velocity in the fuel stream reached six times the mean velocity are examined. The internal nozzle flow was also simulated since the near-nozzle region was of particular interest owing to the strong mixing processes occurring there and the subsequent effect on the flame properties. The lifted forced flames showed drastically reduced soot concentrations as compared with the non-forced attached flame. The changes in the flow field and local soot chemistry were examined to explain the changes in soot production in forced lifted flames.

Flowfield-flame structure interactions in an oscillating swirl flame

A swirling methane-air diffusion flame at atmospheric pressure is stabilized in a gas turbine model combustor with good optical access. The investigated flame with a thermal power of 10 kW and an overall equivalence ratio of 0.75 exhibits pronounced thermoacoustic oscillations at a frequency of 295 Hz. The main goal of the presented work is a detailed experimental characterization of the flame behavior in order to better understand the flame stabilization mechanism and the feedback loop of thermoacoustic instability. OH* chemiluminescence imaging is applied for determining the flame shape and estimating the heat release rate. Laser Raman scattering is used for simultaneous detection of the major species concentrations, mixture fractions, and temperature. The velocity fields are measured by particle image velocimetry (PIV) or stereo PIV, simultaneously with OH planar laser-induced fluorescence. The dynamic pressure in the combustion chamber is determined by microphone probes. The flow-field exhibits a conically shaped inflow of fresh gases and inner and outer recirculation zones. The instantaneous flame structures are dominated by turbulent fluctuations; however, phase-correlated measurements reveal phase-dependent changes in all measured quantities. The paper presents examples of measured results, characterizes the main features of the flame behavior, explains the feedback loop of the oscillation, and discusses the flame stabilization mechanism.

Behavior of Moderately Oscillating Sooting Methane-Air Diffusion Flames

The properties of oscillating sooting methane air diffusion flames have been investigated by different methods in order to examine instationary effects in these flames. The pulsation has been induced by modulation of the methane gas flow with an amplitude of 30% of the mean gas flow. The focus of the investigations is on the flame oscillated at 10 Hz, which is close to the frequency of self-induced flickering. Additionally, further measurements at varying frequencies have been performed to determine the transition towards steady-state behavior. Different measurement techniques allowed the determination of soot volume fractions, particle number densities, mean particle radii, particle temperatures, and OH*-chemiluminescence. The oscillating flame shows strong instationary effects and increased soot concentrations compared to the steady-state flame of equivalent mean fuel flow. Accompanying calculations are based on a kinematic analysis of diffusion flames. The model can sufficiently well reproduce the flame height and the contour of the flame. Furthermore, the model describes the asymmetric course of the OH*-emission signal. A simple numerical approach is deduced that explains qualitatively the strong variations of the soot volume fraction in an oscillating flame.

Flow characterization of diffusion flame oscillations using particle image velocimetry

Particle image velocimetry (PIV) was used to measure velocity fields inside and around oscillating methane-air diffusion flames with a slot fuel orifice. PIV provided velocity and directional information of the flow field comprised of both the flame and air. From this, information on flow paths of entrained air into the flame were obtained and visualized. These show that at low fuel flow rates for which the oscillations were strongest, the responsible mechanism for the oscillating flow appeared to be the repetitive occurrence of flame quenching. PIV findings indicated that quenching appears to be associated primarily with air entrainment. Velocity was found to be considerably larger in regions where quenching occurred. The shedding of vortices in the shear layer occurs immediately outside the boundary of the flame envelope and was speculated to be the primary driving force for air entrainment.