Laminar Diffusion Flames Research Articles

Modeling of silica synthesis in a laminar flame by coupling an extended population balance model with computational fluid dynamics

In the present study, we propose a novel extended population balance equation (PBE) model for aggregation and sintering and couple it with computational fluid dynamics (CFD) to investigate synthesis of silica nanoparticles in a laminar diffusion flame. The extended PBE includes finite-rate sintering of primary particles by solving the PBE together with a transport equation for the number concentration of primary particles. In the process simulated, the particles are formed via the oxidation of a vapor precursor, hexamethyldisiloxane (HMDSO), and the aerosol processes include nucleation, condensation, aggregation and sintering. The model is validated with detailed experimental in-situ SAXS data found in the literature and is also compared with a monodisperse and a two-PBE approach. Good agreement is found between the extended one-PBE and two-PBE models, while both of them provide a substantial improvement over the monodisperse one. Furthermore, the coupled CFD-PBE simulation with the extended one-PBE model reduces substantially the computational time as compared with the two-PBE model and requires less than twice the time needed for the monodisperse model. Excellent agreement is found between numerical predictions and experimental data for temperature along the centerline and reasonably good agreement is found between numerical predictions and SAXS data for primary particle diameters. While results for the particle number concentration are in qualitative agreement with the experimental data, the particle formation rate is overpredicted, leading to an overestimation of the number concentration of the primary particles. This is attributed to uncertainties in the experimental data and precursor decomposition kinetics.

Mechanism and effect assessment of sub-atmospheric pressure and co-flow air to suppress the flicker of buoyancy-driven methane laminar diffusion flame

To further reduce the combustion instability behavior in high-altitude areas, a synergistic suppression of buoyancy-driven methane laminar diffusion flame flicker was achieved by sub-atmospheric pressure combined with co-flow air, and the suppression mechanism was revealed through the schlieren technique. As the pressure decreases, the fluid density gradient within the shear layer decreases, slowing down the formation of vortices. When the co-flow air increases, the vortex is accelerated to rise until the vortex forms downstream of the flame and the flame flicker is completely suppressed. The suppression of flame flicker by sub-atmospheric pressure and co-flow air was nonlinear. The flicker frequency, oscillation amplitude, and flame mean height of flames were assessed for a pressure of 0.5–1.0 atm and co-flow air of 4–16 L/min. The sub-atmospheric pressure reduces the flicker frequency, which increases with increasing co-flow air. The reduction rate of oscillation amplitude at sub-atmospheric pressure varied among different co-flow air flow rates. The initial and that almost realize flame stabilization co-flow air flow rates are more effective than the intermediate flow rate in reducing the oscillation amplitude. The flame mean height decreases monotonically with decreasing pressure, but there is a small peak in the downward trend with increasing co-flow air.

Study on effects of NH3 and/or H2 addition on the characteristics of soot formation and gas emissions in a laminar ethylene diffusion flame

The effects of NH3 and/or H2 addition on the soot formation and gas emission in a laminar ethylene diffusion flame were investigated with laser diagnosis methods, thermophoretic sampling method and a Fourier Transform infrared device in this work. The flame doped with N2 was also studied to evaluate dilution effect. The soot, polycyclic aromatic hydrocarbon (PAH), OH evolutions with flame height and the gas emission were obtained. The flame temperature was simulated to identify the thermal effect, and found the temperature rise of H2 enriched flame is slightly larger than that of the flame with NH3 by <50 K. This indicates the temperature does not dominate the soot evolution. NH3 addition can obviously suppress PAH and soot formation by both dilutive and chemical effects, and the rich soot region moves from the two wings to the flame centerline with flame height increases. However, H2 addition chemically promotes soot formation. The soot formation of flames with NH3-H2 blends is between the flames doped with pure NH3 and H2. Adding the blends with low ratio NH3 and high ratio H2 can achieve dramatic soot reduction and moderate NOx emission. This result is significant for guiding future utilization of carbon-free fuel.

On the combined effects of compositional inhomogeneity and ammonia addition to turbulent flames of ethylene

This paper is part of a broader program aimed at investigating the effects of co-firing clean fuels such as ammonia or hydrogen with hydrocarbons. The focus is on soot formation as well as flame stability in turbulent mixed-mode combustion, which is highly relevant in practical combustors. Ammonia substitution for nitrogen results in reduced flame stability, and this is correlated to differences in flame speed and extinction strain rate. While it is known that the addition of ammonia suppresses soot, visual inspection of compositionally inhomogeneous flames of ethylene-ammonia indicates a reduction in ammonia's ability to suppress soot formation. Measurements of soot volume fraction and laser-induced fluorescence in selected UV and visible bands are made along the centreline in selected flames to test this hypothesis. Experimental results are then compared to simulations in laminar diffusion flames, stratified counterflow flames, and partially premixed flames. All results confirm the soot-inhibiting ability of ammonia. Increasing inhomogeneity, leading to higher centreline mixture fractions, enhances soot formation, and the level of enhancement is greater for flames with ammonia than without. Moreover, it is found that partial premixing is ultimately responsible for determining the amount of soot formed as opposed to stratification of fuel mixtures near the pilot.

Chemical, radiative, and dilutive effects of CO2 addition on soot formation in jet A-1 kerosene co-flow diffusion flame

With the development of aviation industry, it is urgently to investigate the soot formation properties of aviation kerosene to better control the soot emissions. The dilutive, chemical and radiative effects of CO2 on the soot inception, condensation and hydrogen abstraction acetylene addtion (HACA) growth processes in laminar co-flow Jet-A1 kerosene diffusion flames were numerically investigated by employing detailed chemical mechanisms and soot sectional models. The results showed that the addition of CO2 dramatically decreased the maximum temperature (by 92 K) and soot volume fraction (by 41.0%). The dilutive effect of CO2 contributed the most to the decrease of temperature and soot volume fraction. It also was the main factor in the decrease of soot inception, condensation and HACA growth processes. The chemical effect of CO2 had little impact on the decomposition of fuels into light hydrocarbons, but obviously limited the growth of light hydrocarbons to A1. The radiative effect of CO2 decreased the maximum temperature and soot volume fraction by 13 K and 5.2% (from 1.92-1.82 ppm). It had little impact on the soot inception, condensation and HACA growth rates.

Experimental study of the effect of CO2 on temperature and soot volume fraction in C2H4/air co-flow laminar diffusion flame.

The threat of global warming caused by greenhouse gases such as CO2 to the environment is one of the most intractable challenges. The capture and utilization of CO2 are essential to reduce its emission and achieve the goal of being carbon neutral, in which CO2-diluted combustion is an efficient carbon capture technology. In this research, the effects of CO2 addition in the fuel side (CO2-F), oxidizer side (CO2-O) and both sides (CO2-F/O) on temperature and soot formation in C2H4/air laminar co-flow diffusion flames were researched. The flame images were measured by a complementary metal-oxide-semiconductor (CMOS) imaging equipment. The two-dimensional distributions of temperature and soot volume fraction in C2H4/air laminar co-flow diffusion flames were measured employing the inverse Abel transform. The results demonstrated that the effect of amount variation of CO2-F on the decrease of flame temperature was enhanced by the CO2-O. The reduction in peak flame temperature was 4 K in the CO2-F cases, while the reduction in peak flame temperature was 83 K in the CO2-F/O cases. The soot formation was suppressed significantly by the effects of CO2-F/O. Compared with the CO2-F cases, the reductions in peak soot volume fraction were 22.5% and 23.5% in the CO2-F/O cases. The suppression effect of amount variation of the CO2-F on soot formation became more significant with the increase of flame height. The reductions in peak soot volume fractions were 0.3%, 3.07% and 6.38% at the flame heights of 20 mm, 30 mm and 40 mm in the CO2-F cases, and the corresponding reductions were 4.92%, 5.2% and 16% in the CO2-F/O cases, respectively.

The importance of accurately modelling soot and radiation coupling in laminar and laboratory-scale turbulent diffusion flames

Ultrafine and highly light absorbing soot particles have been known to be harmful to human health and a major climate forcer. Understanding the mechanisms of soot formation and developing reliable and robust soot models are essential to design efficient and clean combustion systems. Thermal radiation is often the dominant heat transfer mode in both turbulent and laminar flames and affects soot formation through its influence on temperature. Significant progress has been made in modelling soot formation, thermal radiation transfer, and turbulence radiation interaction in hydrocarbon flames over the last three decades. However, the optically thin approximation has still been used to simplify thermal radiation transfer modelling and turbulence radiation interaction has often been neglected in soot formation modelling without proper justification. In this paper, soot models developed for laminar and laboratory scale turbulent jet diffusion flames, non-gray radiative heat transfer models, and turbulence radiation interaction models are reviewed. The aim of this review is to highlight the importance of thermal radiation heat transfer in both laminar and turbulent diffusion flames and turbulence radiation interaction in turbulent jet diffusion flames. Use of the optically thin approximation is often not justified even in laminar diffusion flames. This is particularly true in diffusion flames at elevated pressures or microgravity. Although the prediction of soot in both laminar and turbulent flames is primarily influenced by soot kinetic model, it is also significantly affected by aerosol dynamic model, thermal radiation heat transfer model, and turbulence radiation interaction model. It is important to accurately model soot aerosol dynamics and non-gray thermal radiation transfer in laminar diffusion flames and additionally to adequately model turbulence radiation interaction in turbulent flames for the purpose of validating reaction mechanisms and soot kinetic models.

Experimental study on soot suppression efficiency of acetylene diffusion flames in a Rijke tube

• The soot suppression effect of acetylene diffusion flames in a Rijke-type combustor is reported for the first time. • Extremely high soot suppression efficiencies are only achieved at the tube’s naturally resonant frequencies (169 Hz and 346 Hz). • As the flow state turns into turbulence, soot is nearly completely suppressed. • With the generation of harmonics, different flame shapes are observed in adjacent periods. • Temperature downstream of the flame is increased from 1773 K to 2250 K. This work reports soot suppression effect of acetylene diffusion flames in a Rijke-type combustor for the first time. The soot generation and suppression from a laminar and turbulent diffusion flame in the Rijke tube by introducing an external acoustic forcing signal are investigated experimentally. A loudspeaker is applied at the bottom of the Rijke tube to induce resonance and control the frequency in the tube. The flame is fluctuated violently at the tube’s naturally resonant frequencies (169 Hz and 346 Hz), and extremely high soot suppression efficiencies are only achieved at these frequencies. Besides, the soot suppression efficiency gradually increases and the flame height gradually decreases with the increase of the acoustic pressure. Different flame shapes are observed in adjacent periods, which is the result of harmonics in the tube. The Reynolds number is calculated based on the velocity of the flame fluctuation. Results show that in laminar flow, the soot suppression efficiency is low. Differently, in turbulent flow, soot suppression efficiency (up to about 100 %) is extremely high. At turbulent condition, the mixing of fuel and air is enhanced, resulting in more soot to be oxidized. The flame temperature fluctuates periodically and the peak flame temperature shows a 96 degrees lag behind the peak acoustic pressure. Under the acoustic resonance, the temperature downstream of the flame gradually increases from 1773 K (without acoustics) to 2250 K.

Quantitative analyses of NO formations from different reaction pathways in methane-air combustion at various pressures and temperatures

Numerical calculations of the laminar counterflow diffusion flames of methane-air are conducted to quantitatively study NO formations from different reaction pathways (mechanisms) at various pressures and/or initial air temperatures. A modified method, which deactivates the initiating reactions in a specific reaction pathway only for NO production while maintaining interactions with other pathways, is developed to calculate NO formation in each mechanism. Results indicate that in the thermal, prompt, N2O, and NNH reaction mechanisms, the NO production rate largely increases as the pressure or the initial air temperature increases, and in particular, as these two parameters are increased together as in a practical combustion system. In the thermal pathway, as the pressure is increased to 15 atm, NO is produced almost entirely on the fuel-lean side. In the prompt pathway, NO production generally occurs in regions with the progress variable ranging from 0.8 to 0.95; the maximum NO production rate moves from the fuel-rich towards the stoichiometric region as the pressure is increased. NO formation from the N2O or NNH mechanism occurs mainly at high pressures, but the corresponding production rate is an order of magnitude smaller than that from the thermal or prompt mechanism, respectively. In all the reaction mechanisms, NO consumption occurs through the reburn mechanism on the fuel-rich side, and the consumption rate significantly increases as the pressure and initial air temperature increase. Results herein provide a quantitative understanding of the effects of pressure and temperature on NO formations from different reaction mechanisms.

Effects of mechanically-induced oscillations on soot formation and flame temperature in laminar diffusion flames of ethylene

An experimental study was performed to measure the effects of flame oscillations on soot concentration and flame temperature in co-flow laminar diffusion flames of ethylene. One novel aspect of this study is that flame oscillations were mechanically induced by moving the burner assembly in a reciprocating motion with an industrial computer-controlled linear stage. Motor speed was used to control the oscillation frequency. The peak-to-peak oscillation amplitude was defined as stroke, which was then used to calculate the Strouhal number. Oscillation frequencies of 5‒20 Hz and peak-to-peak amplitudes of 0.5‒3 mm were studied. At 1 mm peak-to-peak oscillation amplitude, flame oscillations switched from tip flickering type to tip-clipping type oscillations above 10 Hz. Time-averaged soot concentration increased from its value in the steady-state flame when the flame was perturbed by small oscillation frequencies and amplitudes but then decreased as the oscillation intensity was increased. The peak time-averaged soot volume fraction was 9.5 ppm in the steady-state flame and 4.5 ppm at a peak-to-peak oscillation amplitude and an oscillation frequency of 1 mm and 20 Hz, respectively. Similarly, the peak soot volume fraction was 5.8 ppm at a peak-to-peak oscillation amplitude and an oscillation frequency of 3 mm and 10 Hz, respectively. The time-averaged flame temperatures were in the range of 1500‒2000 K. The low-temperature zone was observed at the flame core in the steady-state flame and for low-intensity oscillations, and at the flame tip for high-intensity oscillations. The literature has discrepancies on how flame oscillations affect the total soot loading in ethylene flames. Our results show that the total soot loading increased by about 10% from steady-state to low-intensity oscillations, then slightly decreased as either oscillation frequency or amplitude was increased. The effects of increasing the oscillation frequency or amplitude were similar and well captured by the revised Strouhal number.

2D 2-Colour time-Resolved laser induced incandescence sizing of ultra-fine soot particles in a methane diffusion flame

Planar, two-dimensional, two-colour, time-resolved laser-induced incandescence (2D-2C-TiRe-LII) is employed to investigate the formation of soot in a standard methane laminar diffusion flame for the first time. The work builds on previous developments in 2D LII by time-resolving the signal decay by shifting the gating time of ICCD cameras. The two-colour configuration allows application of the technique to extract the initial temperature of soot particles immediately after the laser pulse, instead of relying on estimates of the soot absorption coefficient, thus allowing higher accuracy in the determination of very small particle sizes produced in methane flames, using a minimum error approach. The technique is combined with a previous work on Continuous Wave Laser Cavity Extinction (CW-LCE) to allow absolute measurements of sub-ppm low soot volume fractions with very small particle sizes.

Effect of maturity on soot volume fraction measurements using the AC-LII technique in a laminar coflow ethylene diffusion flame

The auto-compensating laser-induced incandescence (AC-LII) technique has been frequently used to measure soot volume fraction (fv) in flames. AC-LII relies on an assumption about the spectral variation of soot absorption function E(m,λ) at the two detection wavelengths for soot temperature determination. It has been a common practice to assume a constant E(m,λ) based on the believe that LII measures mature soot only. Since soot maturity in flames is a continuous phenomenon from young to mature, it is unclear how soot maturity affects the AC-LII measured fv. In this study, a correction to the AC-LII measured fv with the conventional hypothesis of mature soot is proposed based on the soot maturity coefficient β and absorption function E(m,λ) in a laminar coflow ethylene diffusion flame. The application of this correction is first demonstrated for synthetic time-resolved LII signals based on the spatial distribution of soot properties in the ethylene diffusion flame predicted by the CoFlame code. A significant decrease of the simulated incandescence signals SLII and soot temperature Teff is observed when the maturity coefficient is increased from 0 for fully mature soot to 1.2 for highly immature soot. Then the proposed correction is applied to AC-LII measured fv in the same flame with the help of spatial distributions of β and ratio of spectrally-resolved E(m,λ) by multiwavelength line-of-sight attenuation and emission (MW-LOSA/EMI) measurements. The uncorrected AC-LII measured fv is in general significantly lower than that measured by MW-LOSA/EMI, which inherently considers soot maturity. The corrected AC-LII fv results are in good agreement, within the experimental error, with those measured by MW-LOSA/EMI. Therefore, the proposed methodology can quantify the effect of soot maturity on AC-LII measured fv and largely eliminate the difference in the measurements of fv by AC-LII and MW-LOSA/EMI.

Effects of blending methane, propane, and propylene on soot evolution in ethylene diffusion flames based on optical diagnostics

Soot evolution of the ethylene-based laminar coflow diffusion flame doping with methane, propane, and propylene with the blending ratio ranging from 0 to 50% has been investigated experimentally. These fuels were selected for their different fuel structure. Spatially resolved distribution maps of the flame temperature, soot volume fraction, the fluorescence signals of polycyclic aromatic hydrocarbons (PAHs), and primary soot particle diameter and number density were determined utilizing laser-induced incandescence (LII), laser-induced fluorescence (LIF), and ratio pyrometry (RP). It was found that the flame height surged and the flame temperature dropped dramatically when the propylene blending ratio was 20% while they were almost constant with the increasing addition of methane or propane. The soot/PAHs loading decreased monotonically with the addition of methane and the opposite was true in the cases of blending propylene. The synergistic effect of mixing propane into ethylene was found on both the soot/PAHs loading. The same trends were observed in the percentage of carbon conversion to soot and the primary soot particle diameter. It can be seen from the soot evolution analysis that methane addition suppressed the soot nucleation and surface growth simultaneously. The enhanced soot surface growth for the flames doping with propane at a low addition level and for the whole flame sets doping with propylene were attributed to the earlier inception time and the longer mass growth time. The preferential diffusion and weakened oxidation of soot particles because of the decreased flame temperature perhaps accounted for the transition from a non-smoking flame to a smoking flame when the propylene blending ratio was greater than 40%.

Electric-field effects on methane coflow flames aboard the international space station (ISS): ACME E-FIELD flames

This work describes the preparatory and initial measurements of diffusion flames under the influence of an electric field aboard the International Space Station (ISS), as part of the Advanced Combustion via Microgravity Experiments (ACME) project. Intended as the foundation publication for the experimental effort, the work comprehensively includes the space experiment methods, the capabilities of the ACME insert, experiment procedures, data, and limitations and constraints. The measurements presented include images and ion currents of small diffusion flames of methane (in air), subjected to a ramping electric field in microgravity. While there have been prior microgravity studies of Electric Field Effects on Laminar Diffusion Flames (E-FIELD Flames) using a drop tower, the current measurements represent the first mapping of the effects of an ion-driven wind on flame behavior under an electric field in the absence of the confounding influences of buoyancy. The paper describes the challenges of remote measurement and manipulation of flames on the ISS and presents preliminary results from the first set of coflow flames. The on-orbit tests began in March 2018 using the modular ACME hardware within the ISS’ Combustion Integrated Rack (CIR) and were completed in the same year November. The results show that the flame is most compact at saturation while the measured voltage-current characteristic (VCC) curve demonstrates parabolic behavior after saturation which differs from observations in 1 g on Earth.

On the sudden reversal of soot formation by oxygen addition in DME flames

Dimethyl ether (DME) is a non-toxic and renewable fuel known for its soot emissions reduction tendencies. In laminar co-flow DME diffusion flames, adding oxygen to the fuel stream increases the sooting tendency until a critical point is reached, at which point the trend suddenly reverses. This work unravels the mechanisms behind this reversal process, and characterizes their contribution to controlling soot production. A series of experimental measurements using diffuse-light line-of-sight attenuation and two-colour pyrometry were performed to measure soot volume fraction and soot temperature considering a fixed mass flow rate of DME and variable addition of oxygen. Soot volume fraction increases from 0.095 ppm in the pure DME flame to 0.32 ppm when the added oxygen concentration reaches 33%. When the oxygen concentration is slightly increased to 35%, soot volume fraction is reduced by 60%. To explain the reasons behind the reversal, a series of numerical simulations were performed, which successfully demonstrated the same trend. Results show that the chemical effects of adding oxygen to the fuel stream are exceedingly more important than the thermal and dilution effects. It was found that the reversal occurred when nearly all DME disassociated before exiting the fuel tube, indicating a sudden transition from a partially premixed DME flame, to one which primarily burns C1 fuel fragments. An analysis of soot formation and oxidation rates showed that near the reversal, soot inception is the least affected process; furthermore, soot precursor availability is not significantly affected in magnitude, rather they appear further upstream. It is concluded that the favourable conditions for rapid DME decomposition into soot precursors enhances soot inception while depleting the necessary species for further soot mass growth, dramatically reducing soot concentration.

Assessment of physical soot inception model in normal and inverse laminar diffusion flames

Despite the extensive studies, accurate and reliable modeling of the soot inception process, especially at high pressure conditions, amenable to multi-dimensional flame simulations remains a challenge. In this study, the physical inception model was comprehensively evaluated in the fully-resolved simulations of laminar normal diffusion flame (NDF) and inverse diffusion flame (IDF) at elevated pressures. The effects of inception models on polycyclic aromatic hydrocarbons (PAHs) and soot predictions were quantitatively analyzed, including the selection of soot precursors and collision efficiency models. The results show that the quantitative PAH predicted by different collision efficiency models can differ by an order of magnitude. Compared to the constant efficiency, the temperature-dependent collision efficiency was found to improve the quantitative PAH predictions and the prediction of the spatial soot distribution in NDF, with an increased level of soot on the flame centerline. The inclusion of small-sized PAH species (such as A2, A2R5, and A3) as soot precursors was also found to improve the quantitative prediction of soot volume fraction. The physical inception model performs well in NDF using the optimal parameters. Moreover, simultaneous measurements of PAH and soot were performed in IDF configuration for the evaluation of the physical inception model. Contrary to NDF, PAHs and soot are formed on the outer side of the flame and cannot be oxidized in IDF. The experiment observed that the PAHs concentration increased in the post-flame region, while the soot concentration remained unchanged. However, the opposite trend was obtained in simulations, that is, the PAHs concentration decreased while the soot concentration increased, because the physical inception model predicts the inception behavior in the post-flame area, resulting in persistent transformation of PAHs into soot particles. To improve the predictions in IDF, the radical effects in the inception process need to be considered in the model.

Mechanistic comparison of N2 and H2 dilution effects on soot formation processes in laminar ethylene diffusion flames

Mechanistic comparison of N₂ and H₂ dilution effects on soot formation processes in laminar ethylene diffusion flames

In-situ laser diagnostic and numerical investigations of soot formation characteristics in ethylene and acetylene counterflow diffusion flames blended with dimethyl carbonate and methyl formate

Oxygenated fuels have emerged as promising energy carriers for storing renewable energy given their compatibility with current energy systems and their potential of reducing pollutant emissions when burning in blends with traditional fossil fuels. In particular, dimethyl carbonate (DMC) and methyl formate (MeFo) have raised increasing interest for their use as gasoline substitutes in recent years. However, the effect of fuel blending on soot formation remains unclear, which prevents the utilization of the full potential of these fuels. In this work, soot formation of ethylene-based and acetylene-based fuel blended with DMC and MeFo in laminar counterflow diffusion flames was investigated by laser-induced incandescence (LII) in combination with laser-induced breakdown spectroscopy (LIBS) and spontaneous Raman scattering (RS). When substituting the base fuels with DMC or MeFo, the flame temperature profiles remain unchanged, allowing to exclusively focus on the chemical kinetic effect of fuel blending on soot formation. The soot formation in the ethylene-based flames can be enhanced by blending a small amount of DMC, while the soot volume fraction begins to decline at high substitution rates of DMC. Blending DMC with acetylene shows no soot enhancement effect. In comparison, the blending of ethylene with MeFo leads to a direct reduction of soot production. This soot formation behavior was further examined using a reaction pathway analysis based on chemical kinetic modeling. The propargyl recombination pathway plays a more important role than the HACA pathway in the formation of benzene and naphthalene, especially for ethylene-based flames. Different from MeFo, DMC can decompose to generate CH3 radicals and subsequently enhance propargyl formation. Therefore, blending DMC means a reduction of C2H2 and an increase of CH3, which results in the soot formation characteristics of DMC blends. Our kinetic analysis also implies the need for improving the decomposition kinetics of C2H4 to C2H2.

Determining spatially-resolved thermal radiation from non-intrusive measurements of soot properties

A procedure is presented that uses estimations of soot volume fraction and temperature fields to (i) calculate the spatially-resolved radiative intensity emitted from soot, and (ii) model the incident radiative power released from the flame by soot particles on a surface. The procedure is validated using both experimental and simulated data obtained from a canonical laminar diffusion flame about 5 cm high. First, estimations of soot volume fraction and temperature derived from non-intrusive experimental measurements are used to calculate the reference soot radiative intensity and to simulate the signals captured by radiometers located at different positions relative to the flame. Then, simulated radiometer signals are processed using different viewfactor methodologies to retrieve the values of integrated soot flame radiation, which are then compared to the reference intensity obtained from soot volume fraction and temperature by solving the radiative transfer equation. Results show that point-source methods accurately estimate the reference intensity (within 2% of error) when using simulated signals from radiometers positioned 7 and 10 cm from the flame axis and up to three times the flame height. Moreover, the double cylinder model accurately estimates flame radiation from measurements performed above three times the flame height. • Fields of radiative power determined from soot volume fraction and temperature. • Spatially-resolved radiative power used to simulate and compare radiometer signals. • Assessment of radiant fraction established from different viewfactor methodologies. • Simulated and experimental signals deviations increase radiative information.

The effects of CO2/CH4 ratio on soot formation for autothermal reforming of methane at elevated pressure

Autothermal reforming of methane (ATR) technique is a promising technology for H2 production. Soot formed in the combustion region of ATR may deactivate the downstream catalyst used to enhance the reforming reactions. In this work, the effects of CO2/CH4 ratio on soot production and main gas-phase components involved in reforming reactions were investigated by experiment and numerical simulations in laminar inverse coflow diffusion flames, at conditions close to ATR. The CO2/CH4 ratio ranges from 0.27 to 4.90, and the pressure ranges from 1 atm to 10 atm. The oxidant consists 70% O2 and 30% N2 to mimic the oxy-combustion in the ATR process. The soot, polycyclic aromatic hydrocarbon (PAHs), OH* distribution, and flame structure were measured using planar laser-induced incandesce, planar laser-induced fluorescence, OH* luminescence, and flame luminescence, respectively. High fidelity simulations with a reduced gas-phase kinetic mechanism and soot aerosol models were also undertaken to supplement the findings. The results show that soot formation is suppressed by a higher CO2/CH4 ratio, while the level of soot reduction is attenuated as the pressure increases. The peak soot volume fraction increases with the flame height, and then levels off. PAHs concentration monotonically increases with flame height. The spatial distributions of PAH and soot along flame height were not well captured by numerical simulations, and the used physical based soot inception model is expected to account for the discrepancy. The peak soot volume fraction correlates with CO2/CH4 ratio by an exponentially decaying function, with the exponent decreasing with increasing pressure. A similar trend of PAHs concentration along with CO2/CH4 ratio was also observed. Non-sooting flame is achievable at elevated pressure if the CO2/CH4 ratio is higher than 4.0, at the expense of lowering the syngas yield in the combustion region. The simulations also predicted that the flame temperature and concentration of H2 and CO decrease with CO2/CH4 ratio.