Peak Soot Concentration Research Articles

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.

On the relative contributions of soot to radiative heat transfer at different oxygen indices in ethylene – O2/CO2 laminar diffusion flames

Soot concentration measurements in ethylene-O2/CO2 diffusion flames (fuel Re 120) encompassing oxidizer oxygen mole fractions (oxygen index, OI) of 35% to 90% were employed to validate a methodology for modeling soot production. A skeletal 10-step reaction mechanism and modeling the soot nucleation parameter (Cα) in the Moss-Brookes soot model as a function of OI provided adequate agreement with measurements. Cross-validation of the OI based Cα model was ascertained through ethylene-O2/CO2 diffusion flame simulations in a different experimental configuration (fuel Re 43, 35%-50% O2) by ensuring that peak soot concentrations were in good agreement with previously published results. A mesh resolution of 0.0625 mm within the flame was deemed necessary to obtain grid independent results. Despite increases in peak soot volume fractions and flame temperatures with OI, the radiant fraction (χR) (in-flame) decreases due to decreasing flame lengths. Although in-flame radiation is dominated by gases, the spectrally-continuous soot radiation plays an important role on χR (when it is assessed across the entire domain) due to the attenuation of flame radiation by the cold, radiatively-participating, co-flow. Consequently, in-flame radiation show a strong sensitivity to gas-radiative property models whereas χR when assessed across the entire domain is impacted by the choice of soot radiative property model.

Influence of nozzle diameter on soot evolution in acoustically forced laminar non-premixed flames

The current study investigated the soot evolution in a series of acoustically forced laminar flames affected by the variations in the nozzle diameter, in-flame residence time and buoyancy. Measurements were performed for three laminar co-flowing non-premixed jet flames in which sinusoidal pressure fluctuations were imposed to the fuel stream. Non-linear excitation regime Two-Line Atomic Fluorescence (NTLAF), Laser-Induced Incandescence (LII), Time-Resolved Laser-Induced Incandescence (TiRe-LII) and Planar Laser-Induced Fluorescence of hydroxyl radicals (OH-PLIF) were performed simultaneously to acquire the phase-resolved measurements of temperature, soot concentration, primary particle size and the location of reaction zones. Additionally, Particle Imaging Velocimetry (PIV) was employed independently to characterise the velocity field. The peak soot concentrations in all the forced flames are double those measured in their steady counterparts, consistent with previous measurements, whereas the maximum particle size for the forced flames is only 10% larger than that for the steady flames and is independent of the nozzle diameter. Nevertheless, a systematic variation of the fuel tube diameter shows that the toroidal vortex scale affects the flame structure which leads to an impact on the spatial distribution and the total volume of soot. In addition, residence time analysis shows that the enhancement of the largest particle size, as well as the peak soot volume fraction, scales with the in-flame residence time.

Laser-diagnostic mapping of temperature and soot statistics in a 2-m diameter turbulent pool fire

We present spatial profiles of temperature and soot-volume-fraction statistics from a sooting, 2-m base diameter turbulent pool fire, burning a 10%-toluene/90%-methanol fuel mixture. Dual-pump coherent anti-Stokes Raman scattering and laser-induced incandescence are utilized for simultaneous point measurements of temperature and soot. The research fuel-blend used here results in a lower soot loading than real transportation fuels, but allows us to apply high-fidelity laser diagnostics for spatially resolved measurements in a fully turbulent, buoyant fire of meter-scale base size. Profiles of mean and rms fluctuations are radially resolved across the fire plume, both within the hydrocarbon-rich vapor-dome region near fuel pool, and higher within the actively burning region of the fire. The spatial evolution of the soot and temperature probability density functions is discussed. Soot fluctuations display significant intermittency across the full extent of the fire plume for the research fuel blend used. Simultaneous, spatially overlapped temperature/soot measurements permit us to obtain estimates of joint statistics that are presented as spatially resolved conditional averages across the fire plume, and in terms of a joint pdf obtained by including measurements from multiple spatial locations. Within the actively burning region of the fire, soot is observed to occupy a limited temperature range between ∼1000 and 2000 K, with peak soot concentration occurring at 1600–1700 K across the full radial extent of the fire plume, despite marked changes in the local temperature pdf across the same spatial extent. A wider range of soot temperatures is observed in the fuel vapor-dome region low in the pool fire, with detectable cold soot persisting into conditionally averaged statistics. The results yield insight into soot temperature across a wide spatial extent of a fully turbulent pool fire of meaningful size, which are valuable for development of soot radiative-emission models and for validation of fire fluid-dynamics codes.

Soot Emission Analysis in Combustion of Biogas Diesel Dual Fuel Engine

Soot emission in bio-gas diesel dual fuel engine has been analyzed by numerical simulation with 2-stape soot formation model of Magnussen. The result shows that soot formation mainly occurred in diffusion combustion phase of diesel pilot jet. Soot peak value is proportional to the first peak value of ROHR, and is found at around the same crank angle position with the second peak of ROHR. At a given engine speed and diesel content in the fuel, the highest soot peak value is obtained with slightly rich mixture whereas soot concentration in exhaust gas increases monotonically with increasing equivalence ratio. Increasing diesel content in the fuel increases both soot peak value and soot concentration in exhaust gas. At a given equivalence ratio and diesel content in the fuel, engine speed has a moderate effect on soot formation rate but a significant effect on soot combustion rate. Soot concentration in the exhaust gas practically vanished as equivalence ratio under 0.98 and 15% diesel content in the fuel. This is the ideal operation regime of bio-gas diesel dual fuel engine in view of soot emission control.

Soot Emission Analysis in Combustion of Biogas Diesel Dual Fuel Engine

Soot emission in biogas diesel dual fuel engine has been analyzed by numerical simulation with 2-stape soot formation model of Magnussen. The result shows that soot formation mainly occurred in diffusion combustion phase of diesel pilot jet. Soot peak value is proportional to the first peak value of ROHR, and it is found at around the same crank angle position with the second peak of ROHR. At a given engine speed and diesel content in the fuel, the highest soot peak value is obtained with slightly rich mixture whereas soot concentration in exhaust gas increases monotonically with increasing equivalence ratio. Increasing diesel content in the fuel increases both soot peak value and soot concentration in exhaust gas. At a given equivalence ratio and diesel content in the fuel, engine speed has a moderate effect on soot formation rate but a significant effect on soot combustion rate. Soot concentration in the exhaust gas practically vanished as equivalence ratio under 0.98 and 15% diesel content in the fuel. This is the ideal operation regime of biogas diesel dual fuel engine in view of soot emission control.

Sensitivity analyses of biodiesel thermo-physical properties under diesel engine conditions

This reported work investigates the sensitivities of spray and soot developments to the change of thermo-physical properties for coconut and soybean methyl esters, using two-dimensional computational fluid dynamics fuel spray modelling. The choice of test fuels made was due to their contrasting saturation-unsaturation compositions. The sensitivity analyses for non-reacting and reacting sprays were carried out against a total of 12 thermo-physical properties, at an ambient temperature of 900 K and density of 22.8 kg/m3. For the sensitivity analyses, all the thermo-physical properties were set as the baseline case and each property was individually replaced by that of diesel. The significance of individual thermo-physical property was determined based on the deviations found in predictions such as liquid penetration, ignition delay period and peak soot concentration when compared to those of baseline case. Among all the properties, latent heat of vaporisation produced the greatest effect on the spray and soot developments under the tested conditions, as evidenced by a longer liquid penetration of 35.0% and a reduced peak soot concentration of 22.8%. Besides, coupled effects among the thermo-physical properties were also determined. Meanwhile, the effects of thermo-physical properties were found to vary with the addition of unsaturation levels and chemical kinetics in the simulation.

Numerical assessment of flow dynamics for various DI diesel engine designs considering swirl number and uniformity index

Geometrical features of combustion chamber are important factors in subsequent engine’s combustion and emissions. Location and configuration of bowl in diesel engine has been the dynamic field of research especially for optimization procedure. This study considers six different engine patterns with outlined parameters. It follows that different designs are characterized with different swirl motions and tumble flows within the combustion chamber. It was determined that maximum and minimum peak swirl number pertains to “Design5” and “Design1” with 1.59 and 1.1 values, respectively. By using “Design5” case instead of “Design1” (baseline case), uniformity index increased by 25.83% whereby peak soot concentration was reduced over 46.7%. The bigger bowl radius (R1) makes higher swirl ratio and this eventually leads to lower soot emission. Lower bowl depth (T), however, gives way to stronger squish pressure and engine-out power.

An optical study on liquid-phase penetration, flame lift-off location and soot volume fraction distribution of gasoline–diesel blends in a constant volume vessel

Liquid-phase penetration, flame lift-off location and soot volume fraction distribution of gasoline–diesel blended fuel jets (gasoline volume fraction 0%, 20%, 40% and 60%) were measured in a constant volume vessel to investigate the combustion and soot formation processes of the wide-distillation fuel. The test was conducted under a constant ambient condition (Ta=830K, Pa=4MPa) with fixed injection parameters (d=168μm, Pinj=80MPa). Mie-scatter imaging, OH chemiluminescence imaging along with coupled Laser Induced Incandescence (LII) and Laser Extinction Method (LEM) were used to investigate the liquid spray length, lift-off length and quantitative soot concentration, respectively. It was found that the increase of gasoline proportion in gasoline–diesel blends results in the following effects: the liquid spray length decreases with a nearly linear tendency while the flame lift-off length increases non-linearly; the general soot concentration decreases significantly and the initial soot-formation location moves downstream; the peak soot volume fraction decreases and the soot inception time increases; the initial soot point and the peak concentration region of soot shift from the periphery to the center of the jet. It was also found that the lift-off length, first-soot distance, peak soot concentration and soot inception time vary more significantly at higher gasoline proportions.

Experimental study of soot and temperature field structure of laminar co-flow ethylene–air diffusion flames with nitrogen dilution at elevated pressures

An experimental study has been conducted to investigate soot formation in laminar co-flow ethylene–air diffusion flames with nitrogen dilution from a co-flow circular nozzle at pressures from 10 to 35 atm. Spectral soot emission (SSE) diagnostic technique was used to determine the radially resolved soot and temperature field structure. Constancy of ethylene and nitrogen flow rates were maintained and the flow rates of ethylene and nitrogen were selected such that no smoke was emitted even at the highest soot loadings. The flame height, marked by visible flame radiation, remained constant at about 5.5 mm and the cross-sectional area of the flame decreased with increasing pressure. At 10 atm, the peak soot concentration of less than 8 ppm, was measured near the flame tip. At 35 atm, the peak soot concentration of about 62 ppm, was measured near the mid-height of the flame. The conversion of carbon in the fuel to soot was strongly dependent on pressure particularly in the lower pressure range. At higher pressure this dependence was weaker. The peak carbon conversion to soot, 6.5%, was observed at 30 atm and remained constant to 35 atm. Temperatures increased along the flame axis and the peak temperature was observed near the flame tip to indicate complete soot oxidation.

Soot formation and temperature field structure in co-flow laminar methane–air diffusion flames at pressures from 10 to 60 atm

The effects of pressure on soot formation and the structure of the temperature field were studied in co-flow methane–air laminar diffusion flames over a wide pressure range, from 10 to 60atm in a high-pressure combustion chamber. The selected fuel mass flow rate provided diffusion flames in which the soot was completely oxidized within the visible flame envelope and the flame was stable at all pressures considered. The spatially resolved soot volume fraction and soot temperature were measured by spectral soot emission as a function of pressure. The visible (luminous) flame height remained almost unchanged from 10 to 100atm. Peak soot concentrations showed a strong dependence on pressure at relatively lower pressures; but this dependence got weaker as the pressure is increased. The maximum conversion of the fuel’s carbon to soot, 12.6%, was observed at 60atm at approximately the mid-height of the flame. Radial temperature gradients within the flame increased with pressure and decreased with flame height above the burner rim. Higher radial temperature gradients near the burner exit at higher pressures mean that the thermal diffusion from the hot regions of the flame towards the flame centerline is enhanced. This leads to higher fuel pyrolysis rates causing accelerated soot nucleation and growth as the pressure increases.

Effect of ceria nanoparticles on soot inception and growth in toluene–oxygen–argon mixtures

Soot formation from the combustion of toluene (C6H5CH3) and of two concentrations of nano-sized-ceria-laden toluene was monitored using a shock tube to observe the effect of the organometallic additive on the formation of soot from its point of inception. Two concentrations of ceria, of chemical composition CeO1.63, were employed to examine the effect on soot production of toluene over the range of temperature 1588–2370K using two levels of inert gas dilution in which reflected-shock pressure was maintained near 1.5atm. The ceria nanoparticles were synthesized using a microemulsion technique which employs sodium dioctyl sulfosuccinate (AOT), a surfactant, to retard agglomeration. Introduction of the nanoparticles into the shock tube is achieved using a novel, two-stage injection procedure. Soot yield measurements reveal that the presence of ceria has no direct implications on peak soot concentration near 1950K. A shift in the parabolic soot profile of toluene in the direction of increased temperature was observed for each concentration of ceria with a larger shift occurring for increased concentration of ceria, although the same effect was exhibited for the toluene–AOT mixtures in absence of ceria, supporting an inefficaciousness of ceria on soot suppression on kinetic timescales. It is evidenced in measured soot delay times that the presence of the surfactant in absence of ceria significantly slows the rate of soot growth for T<2000K, while the presence of ceria has a relatively negligible impact. Under conditions of higher fuel concentration, a remarkable decrease in soot accumulation on the shock tube walls was observed in experiments using the ceria–toluene mixtures over that yielded by pure toluene combustion. In the present paper, the authors report the first measurements of nanoparticle-influenced combustion of a hydrocarbon as performed in a shock tube.

Effect of Fuel Spray Angle on Soot Formation in Turbulent Spray Flames

Results are presented from a combined experimental and modeling study undertaken to understand the effect of fuel spray angle on soot production in turbulent liquid spray flames. The experimental work was conducted in a cylindrical laboratory furnace at fuel spray cone angle of 30o, 45o and 60o. Soot concentrations inside the combustor are measured by filter paper technique. The soot concentration is modeled by using the soot particle number density and the mass density based acetylene concentrations. Soot oxidation occurred by both hydroxide radicals and oxygen molecules. The comparison of calculated results against experimental measurements shows good agreement. Both the numerical and experimental results show that the peak value of soot and its location in the furnace depend on fuel spray cone angle. An increase in spray angle enhances the evaporating rate and peak temperature near the nozzle. Although peak soot concentration increase with enhance of fuel spray angle but soot emission from the furnace decreases. Keywords—Soot, spray angle, turbulent flames, liquid fuel.

Field measurements of soot volume fractions in laminar partially premixed coflow ethylene/air flames

The conditions under which soot is formed vary widely and depend upon several factors, including pressure, temperature, fuel type, combustor geometry, and extent of premixing. Although it is known that partially premixed flames (PPFs) can become either more or less sooting than their nonpremixed or premixed counterparts, the impact of partial premixing on soot formation across a large equivalence ratio and flow range is still inadequately understood. Comprehensive experimental data are relatively sparse for this important configuration. Herein, we report on soot formation in various ethylene/air PPFs utilizing full-field light extinction. The dimensionless extinction coefficient K ext is an important calibrated constant for the determination of the soot volume fraction for this measurement technique. We find that a value of K ext = 7.1 provides results that are in good agreement with benchmark literature data for a nonpremixed flame. We examined the soot microstructures for two flames established at ϕ = ∞ (i.e., nonpremixed) and 5. In both cases, the primary particles were found to be nearly spherical. In case of the nonpremixed flame the average primary soot particle diameter was ∼35 nm, but for the ϕ = 5 flame it was ∼20 nm. However, the parameter responsible for the value of K ext is the average aggregate size and not that of the primary particles. The aggregate sizes are similar for the two flames. We consider this as verification of a constant K ext value over the entire equivalence ratio range. The addition of air to the fuel stream produces an initial increase in the flame height. Further air addition gradually decreases the flame height, which is followed by a more rapid decrease with larger premixing. Likewise, the peak soot concentration first increases with small amounts of air addition (or partial premixing of the fuel stream) and reaches a maximum value at ϕ ∼ 24 . With further air addition, as ϕ decreases below a value of 20, the soot volume fraction considerably decreases.

Prediction and measurement of soot particulate formation in a confined diesel fuel spray-flame at 2.1 MPa

A high-pressure combustion system able to provide stable continuous combustion of a liquid fuel spray under closely controlled reproducible steady conditions at pressures approaching those of diesel engine operation is described. The system provides ready access and time for gaseous and particulate samples to be taken for analysis from the exhaust and selected locations inside the flame. Measurements provide data for comparison with available mathematical models. A relatively fast and efficient spray combustion model, including soot formation and oxidation, has been used to compare the predicted values with the results of the experimental tests at a gauge pressure of 2 MPa. The computer programme qualitatively predicted the influence of pressure and input equivalence ratio on the formation and distribution of soot inside the combustion chamber. Increasing peak soot concentration at higher pressure and higher input equivalence ratio was measured and predicted, with off-axis maximum values in an annulus around the axial peak value. Measurements of primary-particle diameters in soot agglomerates sampled at different operating conditions and locations in the spray flame gave number-mean primary-particle diameters in the range 20 to 50 nm. Larger values were measured at the highest chamber pressures and input equivalence ratios.

Structure and Soot Properties of Nonbuoyant Ethylene/Air Laminar Jet Diffusion Flames

Observations of the structure and soot properties of round, soot-emitting, nonbuoyant, laminar jet diffusion flames are described, based on long-duration (175-230 s) experiments at microgravity carried out on orbit in the Space Shuttle Columbia. Experimental conditions included ethylene-fueled flames burning in still air at nominal pressures of 50 and 100 kPa and an ambient temperature of 300 K with luminous flame lengths of 49-64 mm. Measurements included luminous flame shapes using color video imaging, soot concentration (volume fraction) distributions using deconvoluted laser extinction imaging, soot temperature distributions using deconvoluted multiline emission imaging, gas temperature distributions at fuel-lean (plume) conditions using thermocouple probes, soot structure distributions using thermophoretic sampling and analysis by transmission electron microscopy (TEM) and flame radiation using a radiometer. After an initial 20s flame stabilization period (caused by effects of ignitor disturbances, fuel flow rate adjustments and transient development of flame structure), the flames reached steady-state conditions aside from slow (quasisteady) changes due to pressure increases and ambient oxygen consumption within the test chamber caused by combustion. The present flames were larger, and emitted soot more readily, than comparable flames observed during ground-based microgravity experiments due to closer approach to truly steady conditions resulting from the longer test times and the reduced gravitational disturbances of the space-based experiments. Increasing the pressure from 50 to 100 kPa for soot-emitting flames of similar length caused maximum soot volume fractions to increase from 2 to 32 ppm and average primary soot particle diameters to increase from 24 to 40 nm, showing that soot emissions are the result of the relative rates of soot formation and oxidation and do not correlate closely with peak soot concentrations and primary particle sizes within the flames. In addition, comparable sootemitting buoyant laminar diffusion flames at normal gravity and 100 kPa have significantly smaller maximum primary soot particles (32 nm diameter implying roughly 50 percent less mass) than the nonbuoyant flames. It was also found that the tipopening phenomena associated with nonbuoyant sootemitting flames is caused by extinction of the flame near its tip due to radiative heat losses, which means that emissions of unburned fuel are associated with emissions of soot in the present nonbuoyant flames. Finally, soot production properties (characterized by maximum soot concentrations) are similar for various paths through the 50 kPa flame where effects of radiative extinction and soot particle thermophoresis are small, suggesting potential for a simple state relationship between soot concentrations and mixing level (mixture fraction) at flame conditions representative of many practical applications. This behavior follows because flame residence times are relatively independent of path for nonbuoyant laminar jet diffusion flames, and may help to explain the universality of many properties of soot emitted from practical flames (which generally are relatively nonbuoyant).

Laser-induced incandescence measurements of soot production in steady and flickering methane, propane, and ethylene diffusion flames

Quantitative experimental measurements of soot concentrations and soot scattering are presented for a series of steady and flickering coflowing methane, propane, and ethylene flames burning at atmospheric pressure. Flickering diffusion flames exhibit a wide range of time-dependent, vortex-flame sheet interactions, and thus they serve as an important testing ground for assessing the applicability of chemical models derived from steady flames. Acoustic forcing of the fuel flow rate is used to phase lock the periodic flame flicker close to the natural flame flicker frequency caused by buoyancy-induced instabilities. For conditions in which flame clip-off occurs, the peak soot concentrations in the methane flickering flames are 5.5 to 6 times larger than measured in a steady flame burning with the same mean fuel flow rate, whereas the enhancement for the flickering propane and ethylene flames is only 35 to 60%, independent of the flicker intensity. Soot concentration profiles and full Mie analysis of the soot volume fraction/scattering results reveal significant differences in the structure of the soot fields and in the roles of soot inception, growth, and oxidation for the different hydrocarbon fuels. The soot concentrations have been measured using laser-induced incandescence (LII). Since this is the only technique currently available for making time- and spatially-resolved soot concentration measurements in time-varying flow fields, considerable effort has been devoted to developing LII for quantitative applications. Important considerations include (1) proper calibration measurements, (2) signal detection which minimizes interferences from C 2 Swan-band emission and broadband molecular fluorescence, (3) correction for the laser beam focus/spatial averaging effect in line image measurements, and (4) correction for LII signal extinction within the flame.

Soot formation and burnout during the combustion of dispersed pulverized coal particles

The formation and burnout of soot during the combustion of three high volatile bituminous coals were studied using a combination of high-speed photography and two-color optical pyrometry. The soot is observed to form in a nearly spherical shell displaced from the particle surface by one to four particle radii. The overall soot cloud diameter is approximately constant with the shell thickness increasing with decreasing oxygen concentration. The soot concentration builds to a peak value and then decays over intervals of 5 ms at 100% O2 to 8 ms at 15% O2. The peak soot concentration ranges from 0.5% of the mass of coal at the higher oxygen concentrations to 3% at the lower. The location and duration of the soot flame are roughly correlated with values calculated from a simple model of the formation and burnout of coal volatiles.

Experimental studies on the flame structure in the wake of a burning droplet

Droplets burn in envelope flames at low approach velocities and in wake flames at high velocities. Detailed measurements on pentane burning on a porous sphere have shown that the wake portion of the envelope flame is quite different from the wake flame. The former has a near wake of a few diameters, which resembles a gaseous laminar diffusion flame, followed by a long far wake in which soot burns. The latter has a short near wake which is similar to the flame zone behind a flame holder, except for the lack of a recirculation zone, followed by a short zone of soot combustion. The burning rate for the envelope flame is about three times greater than the wake flame at a slightly higher approach velocity. Peak temperatures are about 1400°–1500°K, located near the droplet in the wake flame, but in the far wake in the envelope flame. Radiation from envelope flames is far greater than from wake flames; peak emittances are in the ratio of about 3.5 to 1. The contribution of soot to droplet flame radiation is negligible in the near wake, and approaches gas radiation in the far wake. Peak soot concentrations in the envelope flame are more than three times greater than in the wake flame.