Peak Soot Volume Fraction Research Articles

Relationship between soot and scalar dissipation rate in the soot-inception region of turbulent non-premixed jet flames

Abstract In previous work we employed the Krypton Planar Laser-Induced Fluorescence technique combined with simultaneous Laser Induced Incandescence to obtain measurements of mixture fraction, temperature and soot volume fraction in the soot inception region of turbulent non-premixed ethylene-air flames (Park et al., 2017). This paper extends that work and reports the first simultaneous measurements in turbulent flames of the scalar dissipation rate and soot-volume fraction. The specific jet composition was 50% C2H4, 46% N2 and 4% Kr and the jet Reynolds number was 8300. The measurements were made 10 cm downstream of the jet exit where the soot first begins to form. The present study uses the same data set to derive the scalar dissipation rate fields and relate the dissipation to mixture fraction, soot volume fraction and temperature. The peak mean soot volume fraction was found to be well inside of the stoichiometric surface near values of mixture fraction from 0.3 to 0.4, which roughly corresponded to the regions of peak mean scalar dissipation. The polycyclic aromatic hydrocarbon field was also imaged, but not simultaneously with the other quantities. Although, the mean soot location is correlated to the mean dissipation, this result is misleading as shown by joint probability density function (PDF) between soot volume fraction and scalar dissipation. The joint PDF shows that soot is in fact no typically spatially coincident with regions of high scalar dissipation; in fact, higher dissipation is correlated with lower values of soot. These latter observations are consistent with results from recent numerical simulations that soot was preferentially found in the areas of low scalar dissipation rate and soot is locally inhibited by high scalar dissipation rates.

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.

Effect of aromatic fuels and premixing on aromatic species and soot distributions in laminar, co-flow flames at atmospheric pressure

The aim of the present study is to investigate the effects of aromatic fuel structure on the aromatic species and the soot volume fraction in laminar, co-flow flames at atmospheric pressure. Both non-premixed and partially-premixed flames (jet equivalence ratios of 24 and 6) are studied. The four fuels consist of binary mixtures of n-dodecane with one of four aromatic species: n-propylbenzene, toluene, m-xylene, and 1,3,5-trimethylbenzene. For all flame conditions, the total carbon flow rate of the fuel mixture is kept constant. Furthermore, in each binary mixture, the carbon mole fraction from the n-dodecane and the aromatic component is kept constant. Laser-induced incandescence (LII) and laser extinction are used to obtain two-dimensional soot volume fraction in the flames. Laser-induced fluorescence (LIF) is used to obtain the two-dimensional aromatic species distribution in the flames. The experimental results indicate that the 1,3,5-trimethylbenzene/n-dodecane flame has the highest peak soot volume fraction amongst the four fuels; the other three fuel mixtures, within measurement uncertainty, have similar peak soot volume fraction. In addition, premixing changes the spatial distribution of polycyclic aromatic hydrocarbons and soot in the flames. In the non-premixed cases, the peak soot volume fraction is located in an annular region near the flame sheet, whereas the soot field is more uniform across the cross-section of the flame at a jet equivalence ratio of 6. A published aromatic fuel chemical mechanism is used to understand the differences in the pathways to soot precursors among the aromatic fuels. The combined 2D LIF and LII result provide a unique dataset to validate soot and chemical numerical models for the four aromatic fuels: n-propylbenzene, toluene, m-xylene, and 1,3,5-trimethylbenzene.

Soot primary particle size dependence on combustion pressure in laminar ethylene diffusion flames

A multi-probe thermophoretic soot sampling system, installed inside a high pressure combustion chamber, was used to collect soot samples at elevated pressures from ethylene–air laminar diffusion flames. Ethylene was diluted with nitrogen at a ratio of 1/3 by mass, and a constant mass flow rate of ethylene-nitrogen mixture was maintained at pressures of 5, 10, 15, and 20 bar. Selected mass flow rate of ethylene, 0.72 mg/s, provided stable and non-smoking laminar flames with a height of about 16 mm at all pressures considered. Soot samples were collected on transmission electron microscope grids at three heights above the burner rim, 3, 8, and 12 mm. The images of the soot particles were captured by transmission electron microscope and the primary soot particle diameters were determined using an automated edge detection method. The mean primary soot particle diameter increased from the sampling height of 3 mm to 8 mm, which corresponds to the mid-height of the flames where the peak soot volume fractions are observed. The soot diameters decreased from the mid-height of the flame to the sampling location of 12 mm, near the tip of the flame. The mean diameter of the primary soot particles increased with increasing pressure up to 15 bar; at 20 bar, the mean soot diameter seemed to reach a plateau or start decreasing. Measured soot volume fractions at sampling locations of 3 and 8 mm above the burner rim indicate that, in view of the measured mean soot diameters at the same locations, soot number density should be increasing intensely with pressure.

Experimental and kinetic studies of soot formation in methanol-gasoline coflow diffusion flames

Methanol has been considered to be a potential alternative fuel to reduce soot emissions from GDI engine. In order to fully understand the effect of methanol addition on soot formation, the 2-D distribution of soot volume fraction in methanol/gasoline laminar diffusion flames was measured quantitatively with two-color laser induced incandescence (TC-LII) technique. In addition, the Methanol-TRF-PAH mechanism is constructed and used to analyze the formation pathways of soot precursors based on the CHEMKIN PRO 0-D constant pressure reactor. In this experiment, the blending ratio of methanol/gasoline was set as M0/20/40/60/80. Considering the carbon content decreasing due to methanol addition, carbon mass flow rate was remained constant. The experimental results showed that methanol is able to decrease the soot significantly, while the effect of methanol on soot reduction is weakened with the increasing methanol ratio. Compared with pure gasoline, the average soot volume fraction in the M20, M40, M60, and M80 flames decrease by 48.2%, 70.4%, 83.8%, and 97.7%, and the peak soot volume fraction decrease by 41.5%, 64.1%, 75.8% and 91.8% respectively. There is little soot formation in the M80 flame, inferring the pure methanol hardly forms soot. The kinetic analysis showed that mole fraction of A1-A4 decrease with the increasing methanol ratio. For the toluene-containing fuel M0-M80, A1 is mainly formed by C6H5CH3 + H = A1 + CH3 and oxidized by A1 + OH = A1- + H2O. A4 is mainly produced by C6H5CH2 + C9H7 = A4 + 2H2 and oxidized by H-abstraction reaction with H or OH radical. The major reaction pathways of A1 and A4 formation are consistent under different methanol blending ratios. The soot reduction as methanol added mainly attributes to aromatics dilution effect. In addition, the formation process of soot precursors is largely affected by chemical processes of OH, CH3, HO2 radicals.

Measurement of soot distribution in two cross-sections in a gasoline direct injection engine using laser-induced incandescence with the laser extinction method

An approach based on regression was developed to reveal the soot volume fraction (SVF) distribution of a horizontal cross-section in an optically accessible gasoline direct injection engine, based on the quantitative data in vertical images from planar laser-induced incandescence, which was calibrated by the laser extinction method (LEM). The approach used the matching of the corresponding pixels in the vertical and the horizontal images to solve the problem of visible range that limited the use of the LEM in measuring SVFs of the horizontal plane. Local SVFs of as low as 0.05 ppm can be detected. Analysis of both the horizontal and vertical image results showed that the case of ϕa = 0.7 (equivalent air–fuel ratio) resulted in significantly rich soot regions with a peak SVF approximately three times higher than that of the case with ϕa = 0.8.

Experimental and kinetic investigation on soot formation of n-butanol-gasoline blends in laminar coflow diffusion flames

Bio-butanol has been considered to be an important alternative fuel to reduce soot emissions from gasoline engines, and it has the potential to overcome the disadvantages brought by low-carbon alcohols. In this study, the effect of n-butanol addition on soot formation in gasoline laminar diffusion flame is investigated respectively under the same fuel flow and the same carbon flow. The soot volume fraction distributions in the flames are measured using the two-color laser induced incandescence (TC-LII) technique. Furthermore, the But-TRF-PAH mechanism is constructed and used to simulate the formation and oxidation of soot precursors in the CHEMKIN zero-dimensional constant pressure reactor. The experimental results showed that n-Butanol addition can significantly reduce the soot formation, and the measured peak and average soot volume fraction both decrease linearly with the increasing n-butanol ratio. For a given n-butanol ratio, the reduction of soot under the same carbon flow (SCF) condition is only slightly smaller than that under the same fuel flow (SFF) condition, which indicates that the decreasing carbon mass flow due to n-butanol addition has little contribution to soot reduction. The simulated results showed that benzene is mainly produced via C6H5CH3+H=A1+CH3, and is mainly consumed via A1 + OH = A1- + H2O and A1 + CH3 = A1- + CH4. The production pathways of PAH is consistent for the toluene reference fuels (TRF) and n-butanol/TRF blends. The ratio of benzene accounts for toluene consumption is essentially constant, about 1/4. Thus, the n-butanol addition reducing the soot is mainly due to the dilution of the amount of aromatics (toluene in TRF) in gasoline. Furthermore, the amount of OH and HO2 radicals increases with increasing n-butanol ratio, which have an important indirect effect on the soot precursors formation.

Effects of Fuel Molecular Weight on Emissions in a Jet Flame and a Model Gas Turbine Combustor

The objective of this study is to understand the effects of fuel volatility on soot emissions. This effect is investigated in two experimental configurations: a jet flame and a model gas turbine combustor. The jet flame provides information about the effects of fuel on the spatial development of aromatics and soot in an axisymmetric, co-flow, laminar flame. The data from the model gas turbine combustor illustrate the effect of fuel volatility on net soot production under conditions similar to an actual engine at cruise. Two fuels with different boiling points are investigated: n-heptane/n-dodecane mixture and n-hexadecane/n-dodecane mixture. The jet flames are nonpremixed and rich premixed flames in order to have fuel conditions similar to those in the primary zone of an aircraft engine combustor. The results from the jet flames indicate that the peak soot volume fraction produced in the n-hexadecane fuel is slightly higher as compared to the n-heptane fuel for both nonpremixed and premixed flames. Comparison of aromatics and soot volume fraction in nonpremixed and premixed flames shows significant differences in the spatial development of aromatics and soot along the downstream direction. The results from the model combustor indicate that, within experiment uncertainty, the net soot production is similar in both n-heptane and n-hexadecane fuel mixtures. Finally, we draw conclusions about important processes for soot formation in gas turbine combustor and what can be learned from laboratory-scale flames.

An experimental study on soot distribution characteristics of ethanol-gasoline blends in laminar diffusion flames

In view of the potential of bio-ethanol as an alternative fuel and the particulate matter (PM) issues during gasoline combustion, the soot distribution characteristics of ethanol-gasoline blends in laminar diffusion flames were studied on a Gülder liquid burner using the two-color laser induced incandescence (TC-LII) technique. During the experiments, the ethanol ratio in the blends was varied from 20% to 80% by volume in order to investigate quantitatively the soot reduction potential of ethanol. In order to study the effect of reduction in carbon content due to ethanol addition on soot formation, the experiments were performed under a fixed fuel mass flow rate and a fixed carbon mass flow rate. It was found that both peak and average soot volume fraction in the flame reduced significantly with increasing ethanol content under both fuel supplying modes, however, this effect was progressively less pronounced as ethanol content increased. By comparing the two fueling modes, it was found that the reduction in carbon content due to ethanol addition has little impact on soot reduction. For a given ethanol blending ratio, the soot reduction under the same carbon mass flow rate was only slightly smaller than that under the same fuel mass flow rate. In terms of flame characteristics, the initial height of soot formation increases with increasing ethanol content under both fuel supply modes mainly due to the increased fuel outlet velocity. Radially, the peak soot location moves from the outside towards the center gradually as height increases. However, along the center line of the flame, the initial height of soot formation decreases with increasing ethanol content under the same fuel flow rate, whereas the trend remained similar to that in the whole flame under the same carbon flow rate.

Effects of hydrodynamics and mixing on soot formation and growth in laminar coflow diffusion flames at elevated pressures

The formation, growth, and oxidation of soot are studied in a set of laminar coflow diffusion flames at pressures ranging from 1 to 8 atm. The modeling approach combines detailed finite rate chemical kinetics mechanisms that model the formation of Polycyclic Aromatic Hydrocarbon (PAH) species up to pyrene, and a bivariate method of moments that describes soot particles and aggregates by their volume and surface area. The spatial distribution of soot observed experimentally and that predicted numerically are in good qualitative agreement with the peak soot volume fraction located at the flame tip and soot appearing on the flame wings and closer to the nozzle as pressure increases. A detailed analysis of the effect of hydrodynamics and mixing on soot formation is presented. We show that the scalar dissipation rate is lower for the higher pressure flames, promoting the formation of PAH species and soot. Thus, the observed increase in soot volume fraction across flames with increasing pressure is not due solely to mixture density and kinetics effects, rather is affected by hydrodynamics and mixing processes also. Similarly, our results indicate that the decrease in the scalar dissipation rate contribute to changing the location where soot forms in the flame, with soot formation occurring closer to the nozzle and outward on the flame’s wings as pressure increases. Radiative heat losses are found to lower the flame temperature, inducing a reduction of the PAH species and associated rates of soot formation. However, heat losses are responsible for a slightly longer flame, which allows for more soot. The overall effect is a modest variation of soot volume fraction if radiation is included.

Optical thermometry for high temperature multiphase environments under high-flux irradiation

A non-intrusive optical method to measure gas phase temperature in strongly scattering multiphase environments under high-flux, broad-band irradiation, relevant to conditions in high temperature solar reactors was developed and demonstrated. The high-flux irradiation with a peak flux of 450kW/m2 was provided by a 6kW metal-halide lamp coupled with a reflector and two concentrators. An ethylene/air diffusion flame, which contains fine soot particles, was employed to provide a high temperature reacting flow (approximately 1800K) with strong optical interference from nano particles having a peak soot volume fractions of ∼16ppm (with irradiation) under conditions of relevance to solar reactors. Under this environment, the proposed laser-based thermometry technique, line-wise two-line atomic fluorescence (TLAF) has been successfully demonstrated to measure flame temperature with good spatial resolution of ∼1mm. It was found that the measurement accuracy in the presence of particle and the high-flux external radiation is 65K at a typical flame temperature of ∼1800K, while the measurement precision is 38K. Results reveal that the presence of high-flux irradiation increases the flame temperature by typically 50–100K. This paper presents a thermometry technique that is suitable for temperature measurement within solar reactors, particularly in hybrid solar-thermal receiver-combustor systems. The experimental setup, measurement methodology and data processing are discussed, followed by the temperature measurements.

Experimental characterization of the different nitrogen dilution effects on soot formation in ethylene diffusion flames

This paper addresses the discrimination of dilution and thermal effects on soot production when nitrogen is added to the fuel stream of a steady laminar ethylene diffusion flame. In this context, the mixture-strength flame height Hf and visible flame height Hv are unambiguously documented and exhibit opposite trends within experimental diluted range (N2 volume fraction from 0 to 0.56). Simultaneous soot volume fraction and temperature fields are mapped for different N2 volume fractions by the Modulated Absorption/Emission (MAE) technique. Two characteristic flame heights and the temperature at the peak soot volume fraction are shown to be crucial parameters to assess the extent of the dilution and thermal effects together with their impact on soot formation in the flame. As a result, the mixture-strength flame height is proposed to characterize the soot formation rate within soot growth region, while the visible flame height is more appropriate to describe the overall sooting propensity in the flame. The probed soot temperature is recommended as the characteristic temperature to assess N2 dilution and thermal effects since the thermal effect is overestimated when using the adiabatic temperature.

A Systematic Comparison of Detailed Soot Models and Gas-Phase Chemical Mechanisms in Laminar Premixed Flames

ABSTRACTThis research is part of an ongoing assessment to identify the most promising soot models for applications to laboratory turbulent flames, and ultimately to practical combustion systems. To that end, it is of interest to compare soot model simulation results with experiments over a broad range of conditions. Here five steady, one-dimensional, laminar, premixed ethylene flames have been targeted that cover a range of C/O ratios (0.68 to 0.88), pressures (1 atm to 10 bar), and peak soot levels (0.03 to 3.2 ppm). Seven different chemical mechanisms have been considered. For each mechanism, results from several soot model variants are compared. Two different approaches for soot aerosol dynamics are used: a discrete sectional method (DSM), and a method of moments with interpolative closure (MOMIC). DSM and MOMIC results are also compared with those from a widely used semi-empirical two-equation soot model. The main figure-of-merit is the ratio of computed-to-measured peak soot volume fraction. The computed soot volume fraction is most sensitive to variations in the surface chemistry scheme. Sensitivities to variations in model configurations are reduced at high pressure. The DSM-based models yield slightly lower soot volume fraction and slightly larger particle size compared to the corresponding MOMIC-based models. A modified semi-empirical two-equation model produced a good match to experiment for all five flames. Particle sizes and number densities from the models are discussed for a low-sooting flame where such data are available. The DSM-based models produce bimodal particle size distributions that are qualitatively consistent with those observed experimentally. The best results overall are obtained using an underlying chemical mechanism that incorporates recent understanding of polycyclic aromatic hydrocarbons kinetics. There is no clear advantage of DSM over MOMIC in predicting global quantities, such as soot volume fraction, particle number density, and average particle size, while the computational cost of DSM is significantly higher than that of MOMIC.

A Computational Study of Soot Formation in Methane Air Co-Flow Diffusion Flame Under Microgravity Conditions

An in-house developed code has been used to predict soot formation in a methane air co flow diffusion flame at normal gravity and at lower gravity levels of 0.5 G, and 0.0001 G (microgravity). There is an augmentation of soot formation at lower gravity levels because of lower buoyancy induced acceleration leading to an increased residence time. The peak temperature at microgravity is reduced by about 50 K than that at normal gravity level. The axial velocity under normal gravity and reduced gravity show negative values (relatively small in magnitude) near the wall at axial height beyond 15 cm; but axial velocity is never negative in microgravity condition. Peak value of soot volume fraction at 0.5 G and microgravity multiplies by a factor of ∼3 and ∼7, respectively of that at normal gravity. The zone of peak soot volume fraction shifts away from the axis towards the wings, as gravity level is lowered. In comparison to soot volume fraction, the factors of amplification of soot number density at reduced gravity and at microgravity are comparatively lower at 1.2 and 1.5 of that at normal gravity respectively. On the other hand, mean soot particle sizes at reduced gravity and microgravity increase to 1.5 and 2 times of that at normal gravity respectively.

Improvement of precision and accuracy of temperature imaging in sooting flames using two-line atomic fluorescence (TLAF)

Severe interference to the fluorescence signal from seeded indium has previously limited the application of current non-linear excitation regime two-line atomic fluorescence (NTLAF) to low sooting flames. The present paper reports new details of these interferences, from spatially- and temporally-resolved spectral measurements in a laminar, non-premixed sooting C2H4 flame burning in air. Three types of interference to the indium LIF signal have been identified: laser-induced fluorescence (LIF) from polycyclic aromatic hydrocarbons (PAH), elastic laser scattering (ELS) and laser-induced incandescence (LII) from soot particles. This knowledge was used to propose new methods to suppress interferences and select customized narrow-band filters for the signal collection, with a narrow bandwidth of ∼1.2 nm (full-width at half-maximum, FWHM) and a high peak transmission of ∼95%. The efficacy of the narrow-band filters for interference suppression in planar NTLAF measurements was assessed by comparing these measurements with those obtained with a set of conventional band-pass filters (FWHM = 10 nm), in both a soot-free flame and a sooting flame. The application of the narrow-band filters was found to increase both the accuracy and the precision of the temperature measurement, so that measurement accuracy that is due to the presence of interferences has been improved from 198 K to 10 K at a location where the soot volume fraction (fv) is 1.2 ppm. An average reduction of 40% in the standard deviation of measured temperature in single-shot measurements has also been achieved with the use of high transmission filters. With the use of narrow-band filters, NTLAF can be reliably applied in laboratory target sooting flames with peak soot volume fraction of several ppm.

Experimental and numerical study of soot formation in laminar coflow diffusion flames of gasoline/ethanol blends

This paper reports the experimental and numerical results of soot formation in laminar coflow diffusion flames of vaporized gasoline/ethanol blends at atmospheric pressure to gain improved understanding of ethanol addition to gasoline on soot formation. Four gasoline/ethanol blends of different ethanol mole fractions in the fuel stream ranging from 0% up to 85%, E0, E20, E50, and E85, were investigated to quantify how soot loading varies with the amount of ethanol blending in the fuel. The laminar coflow diffusion flames were generated using a burner system designed for vaporized liquid fuels. The fuel stream was heavily diluted with nitrogen to lower the boiling points and to prevent the flames from smoking. The soot volume fraction distributions were measured using a 2D line-of-sight attenuation technique. These flames were also modeled numerically using the extensively validated CoFlame code and a recently developed reaction mechanism for gasoline surrogates including PAH formation. The results of experiment and numerical modeling agree quite well in terms of the levels of soot volume fraction and they both show a decrease in the soot loading as more ethanol is added in the fuel stream. The measured peak soot volume fraction occurs in the flame centerline region. The soot model used in this study is unable to capture this experimental feature and predicts the peak soot volume fraction in the co-annular region. Ethanol addition reduces the soot loading primarily by lowering soot inception and PAH condensation rates through decreasing the concentrations of aromatics. Additional measurements in gasoline surrogate/ethanol blend flames indicate that the gasoline surrogate model emulates well the sooting propensity of the real gasoline fuel.

Analysis of the sooting propensity of C-4 and C-5 oxygenates: Comparison of sooting indexes issued from laser-based experiments and group additivity approaches

Oxygenated biofuels have received a particular attention during the last few years due to their ability to reduce soot emissions. Numerous experimental and numerical studies focusing on the sooting propensity of such alternative fuels especially revealed that the ability of oxygenates to limit the soot production was strongly related to their molecular structure in addition to their oxygen content. The objective of the present work is thus to analyze and quantify the potential of soot reduction of several C-4 and C-5 oxygenated molecules used as additives in a Diesel surrogate (the IDEA fuel composed of 70vol% n-decane and 30vol% α-methylnaphthalene). To do so, soot concentration measurements have been carried out by Laser-Induced Incandescence (LII) in 33 turbulent spray flames burning mixtures of IDEA with 12.5, 25 and 50vol% of various oxygenated additives including alcohols, aldehydes, ketones and esters. The different oxygenates have been selected to assess the impact of the position of the oxygenated functional group, the number of oxygen atoms and the type of CO link (single or double) on the production of soot. Based on the peak soot volume fractions measured in standardized jet flames, a Fuel Equivalent Sooting Index (FESI) has been inferred for each considered oxygenated additive and compared with corresponding Yield Sooting Index (YSI), Threshold Sooting Index (TSI) and Oxygen Extended Sooting Index (OESI) values measured and/or estimated based on structural group contribution methods. Obtained results show that all the tested oxygenates reduce the sooting propensity of the base fuel in which they are added. For a given number of carbon atom in the molecule, the sooting tendency of oxygenated compounds increases in the following order: esters<aldehydes<ketones<alcohols. Reasons explaining such trends have been discussed considering the different effects that may impact the soot formation (i.e. the chemical effect induced by the presence of oxygenated functional groups in the fuel mixtures and the dilution of the base fuel by the additives which remains the most important factor for all the considered molecules). Finally, FESI values derived from a previous database obtained by analyzing flames burning gasoline-surrogate/ethanol blends [1] have been confronted with recently published OESI values proposed in [2] for gasoline/ethanol blends.

Effects of a JP-8 surrogate and its components on soot in laminar, N2-diluted ethylene co-flow diffusion flames from 1 to 5 atm

Experimental results are presented for changes in soot volume fraction resulting from the addition of a JP-8 surrogate and each of its components to a N2-diluted, C2H4 co-flow diffusion flame. The surrogate, which consists of 77% n-dodecane and 23% m-xylene by volume, was designed to match the threshold soot index of a nominal JP-8 fuel. Total carbon flow rate was constant for all experiments; pre-vaporized liquid fuel was added at two different levels: 2.5% and 5% of the total carbon flow rate. Tests were conducted at pressures from 1 to 5atm. The use of relatively small amounts of carbon from the liquid fuel resulted in a linear relationship between the peak soot volume fraction and the amount of carbon from the liquid fuel. In these experiments, the peak soot volume fraction was found to be vary with pressure according to a power-law relationship, consistent with prior work on pressure effects on soot. The surrogate fuel showed very similar trends to the JP-8, but yielded lower soot volume fractions. Simulation results for the flames with m-xylene capture the trends of increasing soot volume fraction with increasing carbon from the liquid fuel and with increasing pressure. However, the simulations show smaller increases than were observed in the experiments.

The influence of gravity levels on soot formation for the combustion of ethylene-air mixture

The reduced mechanism coupled with 2D flame code using CHEMKIN II to investigate the effect of gravity on flame structure and soot formation in diffusion flames. The results show that the gravity has a rather significant effect on flame structure and soot formation. The visible flame height and peak soot volume fraction in general increases with the gravity from 1g decreased to 0g. The peak flame temperature decreases with decreasing gravity level. Comparing the calculated results from 1g to 0g, the flame shape becomes wider, the high temperature zone becomes shorter, the mixture velocity has a sharp decrease, the soot volume fraction has a sharp increase and CO and unprovided species distribution becomes wider along radial direction. At normal and half gravity, the flame is buoyancy controlled and the axial velocity is largely independent of the coflow air velocity. At microgravity (0g), the flame is momentum controlled.

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.