Laminar Coflow Diffusion Flames Research Articles

Study on Soot and NOx Formation Characteristics in Ammonia/Ethylene Laminar Co-Flow Diffusion Flame.

The formation of soot and NOx in ammonia/ethylene flames with varying ammonia ratios was investigated through experimental and numerical analysis. The spatial distribution of the soot volume fraction and NOx concentrations along the flame central line were measured, and the mechanism of soot and NOx formation during ammonia/ethylene co-combustion was analyzed using CHEMKIN 17.0. The experimental results indicated that the soot volume fraction decreases with an increase in ammonia ratio, with the soot peak concentration occurring in the upper region of the flame. The distribution of NOx is complex. In the initial part of the flame, a higher concentration of NOx is generated, and the lower the ammonia ratio, the higher the concentration of NOx. As the combustion process progresses, the concentration of NOx initially decreases and then subsequently increases rapidly, with higher ammonia ratios leading to higher concentrations of NOx. The addition of ammonia results in a decrease in CH3, C2H2, and C3H3, and an increase in CN concentration. This leads to a transformation of carbon atoms within the combustion system, reducing the available carbon for soot formation and suppressing its generation. A higher ammonia ratio increases the likelihood that NH3 will be oxidized to N2, as well as increasing the probability that any generated NO will undergo reduction to N2 through the action of the free radicals NH2 and NH.

Numerical simulation of soot and NO formation in DME/gasoline laminar co-flow diffusion flames

Dimethyl ether (DME) is regarded as a promising alternative fuel to reduce greenhouse gases and soot emission. However, the application of DME in actual engines is difficult because of its low calorific value and viscosity. DME is commonly utilized as a supplementary component in traditional fuels to enhance their combustion efficiency and emission performance. This study investigated the effect of DME blending on the soot and NO formation in laminar diffusion gasoline flames. The results showed that the peak soot volume fraction (SVF) reduced by 12.3 %, while the peak temperature increased by 19.2 K and the peak concentration of NO increased by 18.2 % when the blending ratio of DME increased from 0 % to 15 %. The primary reason for the reduction in the soot inception rate was the decrease in benzo(ghi)fluoranthene (BGHIF) concentration. The formation of BGHIF was controlled by the C6H5CH3 → A1 → A1-→A1C2H → A2R5 → A2 → A2-→BGHIF pathways. The decrease of initial soot particles led to a reduction in Hydrogen Abstraction Carbon Addition (HACA) surface growth rate. The increase in A4 concentration was due to the increase in the concentration of C4H2 in the reaction A2R5 + C4H2 = A4. Therefore, more benzo[a]pyrene (BAPYR) was formed through A4 + C4H2 = BAPYR, promoted the process of soot condensation. The increase in OH concentration promoted the oxidation of OH, and the decrease in soot inhibited the oxidation of O2. The majority of NO was primarily produced through the NO thermal pathway in the DME/gasoline flame, and N + OH = NO + H was the main reaction to form NO. The increase of O, H and OH radicals caused the increase in NO concentration.

Effects of ammonia addition on the soot nanostructure and oxidation reactivity in n-heptane/toluene diffusion flames

Co-firing NH3 with conventional hydrocarbon fuels is an important approach for reducing CO2 emissions in existing combustion systems. Besides CO2, the blending of NH3 would also notably affect soot formation and its oxidation behaviors. In the present study, we focus on the effects of NH3 on the nanostructure and oxidation characteristics of soot produced in diffusion flames of n-heptane/toluene mixtures. Two configurations of laminar co-flow diffusion flame, including both normal and inverse diffusion flames (NDF and IDFs), were used for investigation. High-resolution transmission electron microscopy (HRTEM), Raman spectroscopy (Raman), and Thermogravimetric analysis (TGA) were employed for soot characterization. The HRTEM and Raman spectra showed that with the increase of NH3 blending ratio, the fringe length (La) and the degree of graphitization decreased while the microcrystal tortuosity (Tf) increased. The results are in consistent with TGA analysis which suggests the promoting effects of NH3 on the soot oxidation reactivity. Difference between NDF and IDF with respect to the soot nanostructure and oxidation activity were discussed. It is our hope that the present results could deepen our understanding on the effects of NH3 on soot nanostructure and oxidation behavior and benefit the design of particulate filters for combustion devices fueled with hydrocarbon/NH3 mixtures.

Effect of ammonia addition on nanostructure of soot in laminar coflow diffusion flames of ethylene diluted with nitrogen

The effect of ammonia addition on the nanostructure of soot particles was studied experimentally for ethylene diffusion flames. To compensate the thermal effect and nitrogen-containing species production when ammonia was added, the total mole fraction of ammonia and nitrogen was fixed in the fuel stream. Soot particle size, fringe length, and fringe tortuosity were measured through transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). A complementary X-ray photoelectron spectroscopy (XPS) analysis provided information about the chemical bonding of soot. A significant delay in soot growth was observed and the particle size increased with the addition of ammonia. While there was no obvious correlation between the fractal dimension of soot and ammonia mole fraction (XNH3) or sampling location. With the addition of ammonia, the mean fringe length increased reasonably linearly with XNH3 and the fringe tortuosity increased up to XNH3 ≈ 0.17 and then decreased with XNH3, which suggested that ammonia addition led to higher graphitization and lower oxidative activity. The soot from ammonia diluted flame exhibited lower reactivity, implying the delay of soot surface growth. With the addition of ammonia, the value of sp2/sp3 (indication of the graphitization degree of soot particles) did not change much for XNH3 from 0 to 0.17 then increased significantly, which indicated the degree of graphitization of soot particles significantly increased with ammonia addition. The intensity of the N1s peak (indication of the N-containing species in soot) increased with the addition of ammonia. This study confirmed that the addition of NH3 promotes the graphitization of soot.

Detailed modeling of the radiative feedback in the large eddy simulation of a lab-scale heptane pool fire

This article proposes a detailed modeling of gas/soot radiation for large eddy simulation (LES) of sooting pool fires with a focus on the radiative heat feedback through the fuel vapor dome. The numerical model combines a non-adiabatic steady laminar flamelet (SLF) model, a two-equation acetylene/benzene-based soot production model, the Rank-Correlated Full-Spectrum K (RCFSK) and a presumed filtered density function (FDF) approach to model the subgrid-scale (SGS) interactions between turbulence, chemistry, soot production and radiation. This LES model is applied to simulate 15 cm heptane pool fire. The reliability of the RCSFK and the acetylene/benzene soot model for heptane pool fire scenarios is investigated through a priori analysis. First, decoupled radiative heat transfer calculations along lines of sight, representative of heptane pool fires and covering a wide range of optical thicknesses, demonstrate that the RCFSK computes the absorption of the fuel vapor dome with a similar accuracy as the narrow-band correlated-k (NBCK) model. Second, simulations of laminar coflow diffusion flame are used to extend the acetylene/benzene-based soot production model, previously validated for C1-C3 hydrocarbons, to n-heptane flames. LES of the 15 cm heptane pool fire reproduces with fidelity the flame structure in terms of mean and fluctuating temperatures and mean soot volume fraction (SVF). Simulations with and without considering heptane vapor radiation evidences its effects on the radiative outputs and the heat feedback. The radiative contribution of heptane vapor noticeably enhances the radiative loss to the surroundings and the peak of axial radiative flux. On the hand, the heat feedback to the pool surface is predicted within the experimental uncertainty when this contribution is taken into account with the radiative component being reduced by more than 10 %.

Study of soot microscopic characteristics in hydrogen/methane/ethylene co-flow diffusion flame

Hydrogen (H2) has been regarded as a highly competitive alternative fuel that does not produce CO2 and soot during combustion. However, the effect of hydrogen addition on soot formation and emission is questionable. The purpose of this paper is to study the influence of hydrogen/methane on the morphology of soot and the evolution of nanoparticle structure in laminar flame of ethylene. In this study, the soot morphology and nanostructure in H2/CH4/C2H4 laminar co-flow diffusion flames were investigated with thermophoretic sampling and high-resolution transmission electron microscopy (HRTEM). The results showed that the average primary particle diameters (Dp) of pure ethylene flame gradually decreased along the flame height direction, showing the competition between soot surface growth and soot oxidation. When the hydrogen is added into the fuel flow of ethylene, the Dp decrease at most sample points. Methane affects the inhibitory effect of hydrogen doping on soot, and with the increase of methane mixing ratio, the change trend of Dp of each hydrogen mixture with flame height is becoming more and more consistent. The nanostructure analysis showed that hydrogen doping made soot particles shorter, more tortuous and denser fringes. There is a synergistic effect to promote the growth of primary soot particles at low methane ratio, but it disappears after mixing 20 % hydrogen. The fringe length and tortuosity of soot particles gradually increase with the methane ratio.

Maturity characterization of soot in laminar coflow diffusion flames of methane/anisole under different oxygen indices

Anisole fuel has gained increased attention, since it is considered a biofuel, and its addition to gasoline increases the octane number, improving the engine performance. This study focuses on the comprehensive characterization of soot maturity and sooting propensity of a well-controlled laminar coflow diffusion flame fueled with anisole and using methane as the carrier gas. Four laminar non-premixed flames were established below the smoke point. The oxidizer composition ranged from a molar fraction of oxygen (oxygen index, OI) of 21% to 35% (doped with pure oxygen), while keeping constant the total volumetric flow rate of oxidizer stream. Multi-wavelength line-of-sight attenuation experiments were performed from visible to infrared wavelengths, and the measurements of the extinction coefficient and flame temperature were used to retrieve two dimensional fields of soot temperature, volume fraction, radiative intensity and degree of soot maturity. In general, the sooting propensity and radiative behavior of the anisole flames follows a similar trend than typical hydrocarbon fuels when increasing the OI. However, the spatial distribution of soot volume fraction is enhanced, particularly near the flame centerline. In contrast, the soot production is promoted by the OI, near the flame wings. The same effect can be observed for the soot temperature. Indeed, temperature increases, promoting formation and oxidation processes, specially at the locations near the maximum soot volume fraction. In general, the maturity of soot particles is affected as the OI is changed. Also, it is observed that the degree of maturity is enhanced by flame temperature. In order to support these local observations, a statistical analysis was carried out. Finally, the results obtained yield a curated database for validating detailed soot production models of oxygenated fuels, and to gain further insights on the evolution of soot maturity and propensity of these particular fuels under different oxidizer conditions.

Investigation of the impact of nitrogen and water vapor dilution in oxidizer stream on flame structure, soot production and extinction of a laminar ethylene-fueled co-flow diffusion flame

This paper discusses the effect of nitrogen (N2) and water vapor (H2O) dilution in oxidizer stream on the flame structure, and soot production and extinction of laminar ethylene (C2H4)-fueled co-flow diffusion flame (CFDF). The total oxidizer stream flow rate is kept constant while varying the flow rates of air, N2, and H2O to achieve the desired oxygen concentration called oxygen index (OI). As OI reduces, the flame starts to lift off from the burner surface and shows upward and downward oscillations before extinction through blow-off. The OI at which the flame blows off is called the limited oxygen index (LOI), and it depends on burner configuration, fuel, and oxidizer flow conditions. LOI value for C2H4-fueled laminar CFDF were found to be 12.4% and 14% when the diluent was N2 and H2O, respectively. The experiment and simulation results are presented from 21% O2 concentration in oxidizer to very low oxygen concentration in oxidizer close to extinction. Through quantitative measurements of soot volume fraction (SVF), the result shows a notable reduction in the peak concentrations of SVF as the OI decreases. This decrease is more pronounced with H2O than N2 dilution. One-dimensional (1D) opposed flow diffusion flame simulations are performed to understand various effects of the addition of diluents like H2O and N2. It is observed that in addition to inert, thermal and diffusion effect of H2O in reduction of flame temperature and temperature-dependent production rate of key species, H2O also affects flame chemistry through OH + H2 = H + H2O, 2OH = O + H2O. As a result there is a further drop in the concentration of active radicals (like H, O) which leads to quicker soot inhibition and flame extinction.

Initial interpretation of Laser-induced Incandescence (LII) signals from flame-generated TiO2 particles: Towards a quantitative characterization of the flame synthesis processes

Among various optical diagnostics for the characterization of particle formation in flames, laser-induced incandescence (LII), developed for soot particles, is attracting attention for the study of flame synthesis of metal-oxides. Among them, TiO2 nanoparticles are widely used for pigments and photocatalysts. Recent works have shown the feasibility of LII for flame-synthesized TiO2, but extensive research is still needed to quantitatively characterize TiO2 production in flames with LII measurements. In this work, the first attempt towards the characterization of TiO2 synthesis in flames is provided as a normalized volume fraction. To achieve this, TiO2 nanoparticles are generated in a laminar coflow diffusion flame of argon-diluted hydrogen and air with pre-vaporized titanium isopropoxide (TTIP). A 355 nm laser is used to irradiate the flame-generated particles. Spectral, temporal, and spatial measurements are performed at various flame heights. First, laser-induced emission (LIE) at prompt is investigated for different laser fluences to identify the operating conditions that ensure the LII-like nature of the measured signals. The LIE at high fluence presents sharp features that contain information on the atomic composition of the particles and of the vaporized species when compared to reference spectra of carbon black and high-purity TiO2 particles. Then, the LII signal at lower fluence is used to obtain an estimation of the spatial evolution of the normalized volume fraction and of the LII signal decay time. These results are finally used to discuss the major aerosol processes along the flame centerline.

Effects of Radiation Model on Soot Modeling in Laminar Coflow Diffusion Flames at Elevated Pressure

ABSTRACT Detailed numerical simulations of C2H4/Air coflow laminar sooting flames at atmospheric and elevated pressures are performed with fully coupled flow, gas-phase reactions, soot dynamics, and nongray radiative heat transfer. The soot dynamics is modeled using a hybrid method of moments combined with a polycyclic aromatic hydrocarbon (PAH) mechanism. Nongray radiative heat transfer by CO2, H2O, and soot is calculated using different methods to study the effect of radiation model on the predictions. The discrete ordinates radiation model (DOM) and P1 radiation model are compared. Three radiative property models with different accuracy and efficiency are included: a) full-spectrum correlated- distribution (FSCK) model, b) weighted sum of gray gas model with original parameters (WSGG-Smith), and c) WSGG model with optimized parameters for variable mole ratios and elevated pressure (WSGG-SK). The optically thin approximation (OTA) model is also compared to a simple baseline case. The results show that the temperature and soot volume fraction predicted by the DOM combined with the WSGG-SK model are consistent with the FSCK model, with errors less than 2%. When the pressure increases to 8 bar, errors of P1 and OTA models increase significantly, the maximum temperatures predicted by the P1 and OTA models have errors of 36 K and 63 K, and the relative errors of the peak soot volume fraction are 18% and 24%, respectively.

Investigation on the Effect of Charge Injection from Non-Thermal Plasma on Soot Formation in Laminar Coflow Diffusion Flame

ABSTRACT A novel, modified coflow burner was developed to study the effect of charge injection from a non-thermal plasma into three helium-diluted laminar coflow diffusion ethylene flames. The frequency of the high voltage (HV) signal was varied to control the ion concentration (charge) injected into the flames. Optical emission spectroscopy was used to characterize the non-thermal plasma while a bias plate methodology was used to gauge the relative amount of charge generated. For different HV signal frequencies, the laser-induced fluorescence of OH, chemiluminescence of CH*, and laser-induced incandescence of soot in flames were measured. The OH and CH* measurements showed that the flames retained the classic flame shape with charge injection. Significant soot reduction was observed at low HV signal frequencies, corresponding to an increase in charge injection. Notably, at low HV signal frequency, soot reduction in highly concentrated (60%) ethylene flame is three times lower than the less concentrated (32%) ethylene flame. This can be attributed to the decrease in the injected charge to soot precursor concentration ratio when the concentration of ethylene in the flame is increased. These results demonstrate that the current system is a promising candidate for studying the charge effect from non-thermal plasma on soot formation in laminar coflow diffusion flames.

Sooting propensity and maturity of gasoline/anisole blends in a laminar coflow diffusion flame

Liquid oxygenated biofuels such as anisole have been used as additives, since they increase the octane number and improve spark-ignition engines’ performance. In this study, the sooting propensity of a gasoline surrogate, anisole, and gasoline/anisole mixture is quantified by measuring bidimensional fields of soot volume fraction, temperature and radiative heat flux in a coflow laminar flame using well-established non-intrusive optical diagnostics. The maturity of soot particles is also measured using innovative multiwavelength line-of-sight attenuation and emission measurements. Experimental measurements shows that increasing anisole content in fuel mixture results in a greatly enhanced soot volume fraction, while overall temperature is slightly decreased. In addition, maps of soot maturity index are reported based on the spectral dependence of soot optical properties with the amount of immature soot. Mature soot is found near the outer edge of the flame for the three flames, and mature soot content was found to decrease with increased anisole addition to the fuel mixture. In addition, high temperature zones were found to correlate with the presence of more graphitic compounds within the soot particles for the investigated fuels. The comprehensive dataset reported in this study provides rich information for a better understanding of soot formation processes, and for the development and validation of soot models in coflow laminar diffusion flames using complex fuels of interest for practical applications.

Experimental evaluation of dimensionless extinction coefficient of soot in co-flow laminar diffusion flames at elevated pressures

Dimensionless extinction coefficient (Ke) is one of the most important optical parameters of soot since it affects the accuracy of laser extinction measurement of soot volume fraction. In literature, Ke values have been measured for soot from laminar and turbulent flames with different fuels at atmospheric pressure. However, relevant studies about the measurement of Ke values in flames at elevated pressures are still lacking. In this study, a simultaneous gravity sampling and laser extinction system (GSLE) capable of operating in a combustion chamber was proposed to evaluate the value of Ke in laminar diffusion flames at elevated pressures. Methane and two straight-chain alkanes (n-heptane and n-dodecane) were selected as fuels for comparative studies at 0.1–0.8 MPa and 0.1–0.5 MPa, respectively. The morphology properties of soot samples were analyzed quantitatively by transmission electron microscopy (TEM). The results show the Ke values of in-flame soot for methane, n-heptane, and n-dodecane range from 6.1 to 9.7, 6.5 to 10.7, and 7.4 to 10.9 at the corresponding experimental pressures, respectively. The Ke values of in-flame soot for all three fuels increase with pressure. The above results conclude that using a constant Ke value measured at atmospheric pressure may result in larger errors in the measurement of soot volume fraction in flames at elevated pressure. The quantitative analysis of TEM images shows that the significant variation of soot morphology parameters with pressure is one of the main factors for the variation of Ke value with pressure.

Investigation of soot suppression by ammonia addition to laminar ethylene flames at varying pressure

Blending ammonia with hydrocarbon fuels is a promising pathway to expedite its use in different industries, as it might offer a good compromise between energy output and minimizing the emissions of both NOx and carbonaceous combustion products, including soot. This work describes the effects of pressure on soot formation in ethylene/ammonia laminar co-flow diffusion flames. A high-pressure chamber was used to study the flames at pressures between 1 to 7 atm, and ammonia addition mole fractions between 0 and 50% (38% in mass fraction). Soot volume fraction and soot temperature measurements using a Spectral Soot Emissions technique show that adding a 50% mole fraction of ammonia at atmospheric pressure reduces soot concentration below the detection limit of the technique and between 70% and 80% for all the remaining studied pressures; that is, the ammonia soot suppression characteristics are demonstrated when increasing pressure. Maximum soot yield and total volumetrically integrated soot volume fraction results are investigated to reveal the contrasting effects of pressure and ammonia addition on the flame structure and the soot formation mechanisms. The results showcase the potential of NH3 in high-pressure applications when mixed with hydrocarbon fuels to reduce carbon emissions.

Effect of preheating air temperature on sooting tendency in laminar co-flow diffusion flame of n-heptane

This work investigates the effect of preheating air temperature on the sooting tendency in the laminar co-flow diffusion flames of n-heptane. A modified co-flow burner was employed to produce steady n-heptane laminar diffusion flames with the preheating temperatures of co-flow air varying from 344 K to 588 K. The planar laser-induced incandescence (LII) calibrated by the line-of-sight attenuation (LOSA) was performed to quantitatively measure the soot volume fraction and the soot yield in flames and an ICCD camera was used to capture the flame luminosity images. The results reveal that the visible flame height is decreased with the preheating co-flow air temperature, and it is hardly affected by the air velocity with a constant air temperature. As the co-flow air temperature increases, the soot inception and oxidation and the peak soot concentration initiate at the lower positions in flames, and the soot loading in flames has a remarkable increasing trend by assessing the evolution of the soot volume fraction and the soot yield. However, the enhancement rate of total soot loading in flames with the co-flow air temperature below the initial fuel temperature is lower than that for the co-flow air temperature above the initial fuel temperature. In addition, the temperature sensitivity of soot loading is assessed by the maximum soot volume fraction, which is in accordance with those of the alkane in previous research. According to the response of the maximum soot yield to the co-flow air temperature and adiabatic flame temperature, the temperature sensitivity of sooting tendency in n-heptane flames is further confirmed.

Effects of intramolecular oxygen and partial premixing on soot formation in ethane/ethanol flames

Detailed soot concentration and temperature fields were measured in coflow laminar diffusion flames of neat ethane and ethane/ethanol mixtures, and partially premixed flames of ethane/ethanol mixtures. Two-dimensional diffuse-light line-of-sight attenuation and spectral soot emission techniques were employed for soot volume fraction and temperature measurements, respectively. The choice of flow rates prioritized tractability and comparability between flames. To this end, the total carbon mass flow rate and the C/H ratio were kept constant for all ethane/ethanol mixture flames, i.e., oxygen was introduced into the fuel stream either as intramolecular oxygen in ethanol or both as intermolecular and molecular oxygen. Soot concentration was observed to decrease monotonically with an increasing ethanol content in ethane/ethanol mixtures without synergistic effects. Soot concentration was also measured in neat ethane flames with flow rates matching those of ethane in ethane/ethanol mixtures. A comparison of these flames with ethane/ethanol flames revealed a striking soot suppression effect by ethanol addition. Although the carbon flow rate was less in neat ethane flames than the total carbon flow rate of their corresponding ethane/ethanol flames, the peak soot volume fractions were significantly higher in neat ethane flames. This points to a chemical soot suppression effect beyond that can be explained by the lower sooting propensity of ethanol fuel. These results were discussed in light of the synergistic effect studies in the literature. Lastly, measurements from partially-premixed ethane/ethanol mixtures are reported. The increased oxygen content significantly increased the peak soot volume fraction in these flames. These results support that intermolecular oxygen in ethanol and molecular oxygen in the fuel tube behave drastically differently in an ethane base flame. Ethanol addition does not exhibit synergistic effects on soot formation, whereas molecular oxygen addition increases maximum soot concentration, indicating the activation of unique soot production pathways.

Effect of ammonia on the soot surface characteristics in ammonia/ethylene co-flow diffusion flames

As a carbon free and hydrogen rich fuel, ammonia is attracting more and more attention. The co-combustion of ammonia and hydrocarbon fuel can improve the combustion characteristics of ammonia and reduce CO2 emissions, and the influence of co-combustion of ammonia and hydrocarbon fuel on reducing soot emission characteristics has also aroused people's interest. In this study, the laminar co-flow ethylene diffusion flame with different ammonia addition ratios (0 %, 10 %, 20 %, and 30 %) was studied and the soot surface functional groups and graphitization structure characteristics were explored by Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) respectively. The results showed that ammonia accelerated the soot graphitization process, improved the degree of graphitization, and increased the content of oxygen containing functional groups on the surface of soot particles. The XPS N1s analysis showed that, a certain amount of nitrogen-containing functional groups can be detected in ammonia/ethylene co-flow diffusion flames. The N-Ox content increased with the increase of ammonia ratio when the NHAB was>0.7. The N-5 content decreased linearly along the flame height for different the NH3 ratios, and the transforms between N-5, N-6 and N-Q were observed with the soot formation, growth and oxidation.

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.

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.

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.