Soot Cloud Research Articles

Soot and Flame Structures in Turbulent Partially Premixed Jet Flames of Pre-Evaporated Diesel Surrogates with Admixture of OMEn

In this study, the soot formation and oxidation processes in different turbulent, pre-evaporated and partially premixed diesel surrogate flames are experimentally investigated. For this purpose, a piloted jet flame surrounded by an air co-flow is used. Starting from a defined diesel surrogate mixture, different fuel blends with increasing blending ratios of poly(oxymethylene) dimethyl ether (OME) are studied. The Reynolds number, equivalence ratio, and vaporization temperature are kept constant to ensure the comparability of the different fuel mixtures. The effects of OME addition on flame structures, soot precursors, and soot are investigated, showing soot reduction when OME is added to the diesel surrogate. Using chemiluminescence images of C2 radicals (line of sight) and subsequent Abel-inversion, flame lengths and global flame structure are analyzed. The flame structure is visualized by means of planar laser-induced fluorescence (PLIF) of hydroxyl radicals (OH). The spatial distribution of soot precursors, such as polycyclic aromatic hydrocarbons (PAHs), is simultaneously measured by PLIF using the same excitation wavelength. In particular, aromatic compounds with several benzene rings (e.g., naphthalene or pyrene), which are known to be actively involved in soot formation and growth, have been visualized. Spatially distributed soot particles are detected by using laser-induced incandescence (LII), which allows us to study the onset of soot clouds and its structures qualitatively. Evident soot formation is observed in the pure diesel surrogate flame, whereas a significant soot reduction with changing PAH and soot structures can be identified with increasing OME addition.

Measurement of Gas Temperature Around a Burning Droplet: Thin Filament and Soot Pyrometry

ABSTRACT A common setup for the study of liquid fuel combustion involves hanging a droplet from one or several tiny fibers. This paper explores the capabilities and limitations of thin filament pyrometry (TFP) when applied to the fiber(s) holding the fuel droplet. A 2D, two-color pyrometer is used to obtain the temperature along the fiber with low uncertainty and good spatial resolution. The derivation of the gas temperature profile from that of the fiber, however, entails a number of assumptions regarding the flow pattern around the droplet in the radiation/convection balance and also estimating the heat conduction along the fiber. Both aspects and the associated uncertainties are discussed in detail in a test with a 750 μm eicosane droplet hanging from two 15 µm SiC fibers and burnt in air; the “corrections” needed to estimate the maximum flame temperature sum up to ~ 300 K for a final value around 2000 K. The pyrometer is then applied to the soot cloud above the eicosane droplet, an alternative approach whose uncertainty arises mainly from that regarding the emissivity of soot. A reasonable agreement with the corrected TFP results is found. Finally, TFP is applied in a worst-case scenario, that is the case of a glycerol droplet, with no noticeable soot and a short distance between the flame and the droplet surface.

Explosion Mechanism of Lubricating Oil Droplets in High-Temperature and High-Pressure Combustion Environments

Lubricating oil-induced pre-ignition is a critical issue that requires attention in downsized gasoline engines and marine low-speed two-stroke natural gas engines. As a result, the ignition behavior of lubricating oil at high temperatures and pressures has been extensively studied. In some cases, when studying the ignition of oil droplets using a rapid compression machine, an explosion-like behavior of the oil droplets is observed, producing a soot cloud that can spread throughout the combustion chamber, especially when the ignition delay time of the ambient gas is short. To gain detailed insights into the mechanism of oil droplet explosion, the explosion process under initial pressures from 13 to 31 bar and temperatures from 700 to 1600 K was visualized using high-speed photography and microphotography on a rapid compression machine. The effects of temperature and shock waves were experimentally investigated, and droplet deformation after shock wave impact was calculated using a simple model. The results demonstrated that high temperature does not have a significant effect on droplet explosion under the conditions studied in this paper. The shock wave impact is the primary cause of the droplet’s explosion.

Soot Development in an Optical Direct Injection Spark Ignition Engine Fueled with Isooctane

To better understand the formation and evolution processes of soot, the two-color laser induced incandescence diagnostic method was applied on a single cylinder optical direct injection spark ignition engine. Soot volume fraction was measured, and soot distribution was imaged as cyclic fuel quantity changes. The results show that 45.5 mg/cycle generates the most soot at the same measure plane. Pool fire dominates the formation of soot in the tested engine and generates more soot on the top surface of the piston near the injector. In-cylinder soot increases until 42°CA ATDC and then reduces due to oxidation. Pool fire continues through the expansion stroke till 52°CA ATDC, and then soot cloud gathers near the 10 mm plane. After 82°CA ATDC, in-cylinder soot basically in equilibrium, and residual soot moves follow the in-cylinder flow randomly and evenly distributes within the whole combustion chamber. With increasing cyclic fuel quantity, particles number concentration gradually increases and their distribution present dual-peak shape. In detail, 45.5 mg/cycle emits the most accumulation mode particles while 52 mg/cycle emits the most nucleation mode particles.

Coexistence of CH4, CO2, and H2O in exoplanet atmospheres

We propose a classification of exoplanet atmospheres based on their H, C, O, and N element abundances below about 600 K. Chemical equilibrium models were run for all combinations of H, C, O, and N abundances, and three types of solutions were found, which are robust against variations of temperature, pressure, and nitrogen abundance. Type A atmospheres contain H2O, CH4, NH3, and either H2 or N2, but only traces of CO2 and O2. Type B atmospheres contain O2, H2O, CO2, and N2, but only traces of CH4, NH3, and H2. Type C atmospheres contain H2O, CO2, CH4, and N2, but only traces of NH3, H2, and O2. Other molecules are only present in ppb or ppm concentrations in chemical equilibrium, depending on temperature. Type C atmospheres are not found in the Solar System, where atmospheres are generally cold enough for water to condense, but exoplanets may well host such atmospheres. Our models show that graphite (soot) clouds can occur in type C atmospheres in addition to water clouds, which can occur in all types of atmospheres. Full-equilibrium condensation models show that the outgassing from warm rock can naturally provide type C atmospheres. We conclude that type C atmospheres, if they exist, would lead to false positive detections of biosignatures in exoplanets when considering the coexistence of CH4 and CO2, and suggest other, more robust non-equilibrium markers.

Pyrometric imaging of soot processes in a pilot ignited direct injected natural gas engine

This work assesses the soot formation and oxidation processes in a high-pressure direct injection natural gas engine using simultaneous high-speed two-colour pyrometry and OH* chemiluminescence imaging. A parametric investigation considers the effects of fuel injection pressure, injection duration and relative pilot and natural gas injection timings. A typical combustion event consists of natural gas being ignited by diesel pilot combustion products, followed by partially premixed combustion of natural gas near the piston bowl wall. Soot is formed as natural gas is injected into this reaction zone. A toroidal soot cloud forms and grows towards the centre of the piston bowl. For the range of operating conditions tested, the peak apparent heat release rate and the onset of detectable soot were correlated, as were the timing of the peak soot fraction and end of natural gas injection. The latter indicates that the soot formation and transition to a net oxidation process are strongly influenced by the injection process, similar to diesel engines. Changes in the relative timing of the diesel pilot and natural gas injections influenced natural gas premixing times, with increased premixing leading to a higher peak apparent heat release rate and lower peak soot fraction. The injection pressure affected the peak soot fraction though enhanced oxidation was expected to ultimately reduce the engine-out soot for higher injection pressures. Based on this parametric investigation, an existing conceptual model of pilot ignited direct injected natural gas combustion is extended to also describe the soot formation and oxidation processes.

Effect of organic coatings derived from the OH-initiated oxidation of amines on soot morphology and cloud activation

The presence of coatings on soot particles may significantly influence their environmental impacts. In this study, the processing of soot by OH-initiated photooxidation of amines was investigated in a series of chamber experiments. Monodisperse soot aerosol was exposed to oxidation products of monoethanolamine (MEA), triethylamine (TEA) and aniline, and changes in particle size, mass, effective density, shape factor, and activation as cloud condensation nuclei (CCN) were simultaneously measured by a suite of instruments. Different initial concentrations of amine precursor were applied, and in selected experiments, organic coatings were removed by thermal denuding to investigate the change of the soot core structure. Aniline generated the largest amount of low volatility condensable oxidation products, which induced minimal soot restructuring even at a high aniline concentration. On the contrary, the high volatility of MEA oxidation products led to reversible condensation on soot while inducing significant aggregate collapse for a smaller coating mass than with aniline. The products from TEA photooxidation showed volatility intermediate between aniline and MEA and induced the most significant restructuring. It can be concluded that restructuring depended on the physical state of the coating material and its distribution over the aggregate surface. Soot aged with the three different amine precursors produced distinct CCN activity profiles that depended on measured coating mass, and there was an indication that intermediate volatility oxidation products of MEA co-condensed with water during the droplet activation process.

Study of temperature, apparent spectral emissivity, and soot loading of a single burning coal particle using hyper-spectral imaging technique

A hyper-spectral imaging device that can simultaneously capture three-dimensional data information of spectra, space, and time is employed to experimentally investigate the combustion behavior of a single coal particle. Three types of single pulverized coal particles are injected and burned on a Hencken flat-flame burner, and their flames are captured by a high-speed camera and the hyper-spectral imaging device. The camera observations demonstrate that the three single coal particles undergo particle heat-up, volatile release and combustion, and volatile and char oxidation during the combustion process. Furthermore, the image captured by the hyper-spectral imaging device records the spectral and spatial history of the single burning coal particles throughout the combustion process. Then, the time–space temperature and apparent spectral emissivity of the single burning char particles are calculated from the time–space spectra radiative intensity obtained by the hyper-spectral imaging device. The gray characteristic exhibited by the spectral emissivity of the three single burning char particles in the longer wavelength range demonstrates the feasibility of calculating the temperature using the two-color method. Finally, the soot temperature and volume fraction distribution in the volatile flames are obtained to investigate the formation of soot during the combustion of a single coal particle. The results show that an elongated tail-like soot cloud is formed around the two single bituminous coal particles during the combustion process, but a nearly spherical soot cloud is formed around the single lignite coal particle. Moreover, the range of soot volume fractions in the three envelope volatile flames is comparable to the order of magnitude of the values measured by other researchers.

Soot cloud size of a single coal particle in air/oxy combustion under forced convection

Soot cloud size is important in radiative heat transfer and particulate matter emission since it determines the integral effect in the sooty region; meanwhile, study on soot cloud size during single coal particle combustion is essential to develop theories of soot-relevant reaction pathways and mass transport. In this work, a 1-D transient coal combustion model with an empirical soot model was adopted and validated to study the effect of gas physicochemical properties and forced convection on soot cloud size (ls/r0). Tar gasification results in a larger ls/r0 at any xO2, while soot gasification has a limited effect at low xO2 and leads to a larger soot cloud size at high xO2. The effect of tar and soot diffusion becomes more important at higher oxygen mole fraction; a larger heat capacity leads to a larger soot cloud size. In O2/N2 environments, soot cloud size increases when Rep increases from 0 to 10 and decreases when Rep increases from 10 to 16. In O2/CO2 environments, soot cloud size increases dramatically when Rep increases from 0 to 13 and slightly decreases when Rep increases from 13 to 16. The ls/r0 in O2/CO2 is always larger than that in O2/N2. The effect of convection on soot cloud size has two sides. On one hand, the blow away effect tends to increase the ls/r0. On the other hand, convection may increase or decrease total soot mass, leading to an increase or decrease of ls/r0, respectively.

Experimental study of the influence of the fuel and boundary conditions over the soot formation in multi-hole diesel injectors using high-speed color diffused back-illumination technique

Two prototypes multi-hole diesel injector from a high-pressure common rail system were used to operated with diesel and n-Heptane fuels in a high-pressure and high-temperature test rig able to reach 1100 K and 15 MPa under different oxygen concentrations with the goal to study the influence of the boundary conditions on the combustion process evaluating parameters as ignition delay, lift-off length and soot formation. A novel optical set-up was implemented to visualize the soot cloud evolution in the fuel jet between 30 and 85 mm from the nozzle exit using the high-speed color diffused back-illumination technique, it was used as a result of the insertion of a high-pressure window in the injector holder opposite to the frontal window of the vessel allowing an optical arrangement in line-of-sight. The experiments performed in this work used a LED with a wavelength of 460 nm to provide information about physical of the soot properties, the results of varying the operational conditions show the reduction of soot formation with an increase in injection pressure, a reduction in ambient temperature, a reduction in oxygen concentration or a reduction in ambient density. Furthermore, fuel effect on soot formation also has been studied showing a big difference between both fuels, diesel is known to be more prone to produce soot than n-Heptane reducing the ability of the technique implemented, especially in conditions with high temperature and high density.

Refinement of the two-color pyrometry method for application in a direct injection diesel and natural gas compression-ignition engine

The soot emissions from internal combustion engines have significant health and environmental impacts and, as such, are subject to increasingly stringent regulations. Two-color pyrometry provides the in-cylinder soot cloud temperature and soot volume fraction and can provide insight to the in-cylinder soot formation and oxidation processes to guide research for reducing engine-out soot emissions. This work demonstrates improvements to the two-color pyrometry methodology, with a focus on low-temperature, low-soot regimes such as low-temperature combustion or combustion of direct injected natural gas. Through selection of a fast and robust numerical algorithm, characterizing and increasing the detection envelope, performing static and dynamic perspective adjustments, accounting for non-uniform and non-linear system response, as well as localized signal-to-noise ratio enhancement through image filtering, the performance of the pyrometric method was improved by a 40% increase in the resolved signal fraction. The refined two-color method was evaluated for both direct injected diesel and natural gas fueling strategies using a pilot-ignited direct injected natural gas fuel system and facilitated evaluation of local temperatures and soot concentrations in pilot-ignited direct injected natural gas combustion, despite the generally low soot levels in this combustion strategy.

Ignition of a Single Lubricating Oil Droplet in Combustible Ambient Gaseous Mixture under High-Temperature and High-Pressure Conditions

ABSTRACT An experimental investigation on the ignition of a single lubricating oil droplet in combustible ambient gaseous mixture was performed on a rapid compression machine, under high-temperature (1060–1090 K) and high-pressure (13.0–36.5 bar) conditions. The lubricating oil droplet was suspended on a thin quartz fiber tip in the combustion chamber. Under the conditions presented in this study, the lubricating oil droplet almost has no distinct influence on the ignition delay of the ambient gaseous mixture with an initial radius of 600 μm due to the low evaporation rate. After the ignition of the ambient gaseous mixture, the droplet exploded severely, forming a flame ball-like soot cloud. With the increase of ambient pressure, the time for droplet explosion was advanced and the explosion get more intensive. Within the middle range of effective pressure (27–31 bar), the droplet explosion seemed to have a positive effect on the amplitude of the pressure oscillation after the autoignition of the end-gas of ambient mixture. Furthermore, the evaporation and its cooling effect of the lubricating oil droplet were investigated by a numerical method, so as to analyze the effect of the droplet size on the ignition order between the oil droplet and the ambient gaseous mixture. The results show that as for a bigger droplet, the cooling effect was more distinct and the evaporation mass ratio would be lower, which made the ignition around the droplet more difficult to occur.

Influence of EGR and ethanol blending on soot formation in a DISI engine

Soot formation and in-cylinder soot oxidation in an optically accessible DISI-engine is analysed for gasoline–ethanol mixtures under part load operating conditions. A volumetric extinction measurement technique was used both for the determination of the liquid fuel distribution and quantitative soot measurements. Toliso, a mixture containing isooctane and toluene (65 vol% isooctane and 35 vol% toluene) was utilised as a surrogate gasoline fuel. The EGR (EGR-exhaust gas recirculation) rate-dependence and the effect of ethanol blending (0, 20 and 40% ethanol, E0–E40) were studied at part load operation with two different injection timings. Operating point 1 (OP1) represents an early injection with piston wetting and pool fire, operating point 2 (OP2) represents a late injection timing with reduced time for mixture formation. Soot formation is more pronounced for OP1 and for E20 and E40 as compared to E0. Here the pool fire due to the fuel-dependent liquid wall film formation plays a major role in soot formation and oxidation. Late injection timing leads to increased soot formation for E20 compared to Toliso, while E40 shows lowest soot formation. Here the fuel–wall wetting is less pronounced, but mixture inhomogeneities exist, and the soot cloud covers a larger region in the cylinder. An EGR addition leads to a higher soot formation for Toliso for both operating points. The ethanol-blends show a reduction of the soot formation with EGR, which is explained by reduced combustion temperatures and the chemically bound oxygen for E20 and E40 leading to locally leaner mixtures at constant global air–fuel ratio. The study shows optimisation potentials of injection strategies for ethanol-blended gasoline in combination with EGR in DISI-engine applications.

Evaluation of a Lagrangian Soot Tracking Method for the prediction of primary soot particle size under engine-like conditions

This paper reports the implementation and evaluation of a Lagrangian soot tracking (LST) method for the modeling of soot in diesel engines. The LST model employed here has the tracking capability of a Lagrangian method and the ability to predict primary soot particle sizing. The Moss-Brookes soot model is used here as the Eulerian method to simulate soot formation and oxidation processes. The inception, surface growth and oxidation models are adopted and modified such that the associated reaction rates can be computed using the Lagrangian approach. The soot nuclei are treated as Lagrangian particles when the mass of incipient soot exceeds a designated threshold value. Their trajectories are then computed using the particle momentum equation. The change of primary soot particle size is dependent on the modified Lagrangian surface growth and soot oxidation models. Performance of the LST model in predicting temporal soot cloud development, mean soot diameter and primary soot size distribution is evaluated using measurements of n-heptane and n-dodecane spray combustion obtained under diesel engine-like conditions. In addition, sensitivity studies are carried out to investigate the influence of soot surface ageing and oxidation rates on the primary soot particle size distribution. With the use of surface ageing, the predicted maximum primary soot particle sizes are closer to the experimentally measured maximum primary soot sizes. Also, the associated particle size distribution shows a lognormal shape. A higher rate of soot oxidation due to OH causes the soot particles to be fully oxidized downstream of the flame. In general, the LST model performs better than the Eulerian method in terms of predicting soot sizing and accessing information of individual soot particles, both of which are shortcomings of the Eulerian method.

Re-ignition of multi-species soot clouds in building fires

The re-ignition potential of multi-species soot clouds in building fires were investigated based on their extinction characteristics. The investigation was carried out theoretically using the adaptation of Semenov`s thermal explotion theory. The critical sizes of soot particles in the cloud were found to be strongly effected by the particle temperature., shape, and reactivity, as the mass fraction of each species, and ambient conditions. The clous shape, cloud particle number density, fuel mass fraction and soot reactivity were identified as the major parameters impacting upon the cloud extinction potential. Our analysis indicate that blending of a base soot with a less reactive soot generally increases extinction potential of the cloud ( i.e. likelihood of extinction) while addition of a more reactive secondary soot to the base one minimizes the probability of cloud extinction.Keywords: extinction, clouds, re-ignition, soot

The combustion of solid paraffin wax and of liquid glycerol in a fluidised bed

Two fuels were burned in electrically heated beds of alumina sand, fluidised by air. The fuels were: (i) paraffin wax, which is a solid containing 100% volatile matter and (ii) glycerol, a liquid, whose potential as a fuel needs assessing. The bubbling fluidised beds were held in the range 400–900°C. Pieces of paraffin wax burned like a plastic, so when fed on top of a bed, the wax floated and generated clouds of soot. Soon, it then sank into the bed. When the sand was below ∼800°C, combustion occurred noisily in exploding bubbles leaving the bed. In beds hotter than ∼800°C, combustion proceeded in bubbles fairly low in the bed and was controlled by the mixing of hydrocarbon vapours (from the wax) with the fluidising air. If wax were fed half way up a bed, bubbles of hydrocarbon vapours were quickly produced; they ascended and mixed with the fluidising air. In a bed below 800°C, combustion mainly occurred noisily in bubbles just after leaving the bed, but in a hotter bed, there was quieter burning in smaller bubbles, before they reached the top of the bed. Glycerol behaved similarly, when fed into the middle of a bed. Thus bubbles of glycerol vapour were formed; they mixed with air ascending the bed as either bubbles or percolating between particles. Again bubbles exploded noisily at the top of a bed below 800°C. With the bed above 800°C, glycerol burned inside smaller bubbles below the bed’s upper surface. No soot was observed when burning glycerol in such a hot bed, yielding CO and CO2 as the only products of combustion. It appears that burning glycerol cleanly in a hot fluidised bed is a feasible proposition.

Numerical Simulations of Constant-Volume Spray Combustion of n-Heptane with Chemical Kinetics

Objectives: A reduced toluene reference fuel (TRF) mechanism of multi-component nature from the literature is utilized to simulate constant-volume spray combustion of n-heptane. The approach allows a preliminary assessment of fuel kinetic model and computational fluid dynamics (CFD) formulations in a simplified computational domain before integrating them in complex engine simulations. Methods: The operating conditions vary in ambient densities between 14.8 kg/m 3 and 30 kg/m 3 with initial oxygen concentrations ranging from 10% to 21%. The CFD models are first calibrated to replicate spray penetration lengths of the non-reacting condition. The tuned numerical models are then applied to simulate the combustion and soot formation events of reacting sprays. The soot model employed is the multi-step Moss-Brookes model with updated oxidation models. Findings: The relative errors for ignition delay and lift-off length predictions are within 35% and 22% respectively. Furthermore, simulated soot volume fraction contours agree qualitatively with the experimental soot clouds. Computed peak soot locations, however, are found to be further downstream axially as compared to the experimental results across all test cases. Application: Good agreement with experimental spatial soot distributions allows the incorporation of both fuel and soot models in engine configurations.

Optical soot measurement of bio-butanol upstream product, ABE (Acetone–Butanol–Ethanol), under diesel-like conditions

ABE (Acetone–Butanol–Ethanol), the upstream product of bio-butanol, is considered as a promising alternative fuel for IC engine. Two-dimensional soot distribution and soot mass of spray combustion of ABE and diesel was measured by forward illumination light extinction (FILE) technology. The ambient temperature and oxygen concentration of injection was varied from 1200K to 900K and 21% to 11% to represent normal temperature combustion and low temperature combustion modes respectively. It was found that soot distribution area and intensity of ABE, especially ABE with a high acetone fraction, is much lower than that of diesel. The soot clouds area and intensity increase with increasing ambient temperature and decreasing oxygen concentration, but the increased scale of soot for ABE is much lower than that of diesel. Quantitative results of soot mass for ABE show that the soot increases rapidly with increasing ambient temperature at lower oxygen concentration conditions because the soot producing rate is largely increased when elevating temperature, while the soot oxidation rate remained slow due to the insufficient oxygen. Based on analysis, it was found that clean combustion of ABE benefited from a lean air–fuel mixture because of the great volatility and low stoichiometric ratio of the fuel, and the competition of Carbon atom between CO and C2H2 in the pyrolysis reactions that reduced the soot source materials. Overall, as an alternative IC fuel, ABE is able to achieve much lower soot than diesel and is able to maintain clean combustion in a large range of environmental conditions.

New field-based agricultural biomass burning trace gas, PM2.5, and black carbon emission ratios and factors measured in situ at crop residue fires in Eastern China

Despite policy attempts to limit or prevent agricultural burning, its use to remove crop residues either immediately after harvest (e.g. field burning of wheat stubble) or after subsequent crop processing (e.g. “bonfires” of rice straw and rapeseed residues) appears to remain widespread across parts of China. Emission factors for these types of small but highly numerous fire are therefore required to fully assess their impact on atmospheric composition and air pollution. Here we describe the design and deployment of a new smoke measurement system for the close-range sampling of key gases and particles within smoke from crop residue fires, using it to assess instantaneous mixing ratios of CO and CO2 and mass concentrations of black carbon (BC) and PM2.5 from wheat stubble, rice straw, and rapeseed residue fires. Using data of our new smoke sampling system, we find a strong linear correlation between the PM2.5 mass and BC, with very high PM2.5 to BC emission ratios found in the smouldering phase (up to 80.7 mg m−3.(mg m−3)−1) compared to the flaming phase (2.0 mg m−3.(mg m−3)−1). We conclude that the contribution of BC to PM2.5 mass was as high as 50% in the flaming phase of some burns, whilst during smouldering it sometimes decreased to little over one percent. A linear mixing model is used to quantify the relative contribution of each combustion phase to the overall measured smoke composition, and we find that flaming combustion dominated the total emission of most species assessed. Using time series of trace gas concentrations from different fire cases, we calculated ‘fire integrated’ trace gas emission factors (EFs) for wheat, rice and rapeseed residue burns as 1739 ± 19 g kg−1, 1761 ± 30 g kg−1and 1704 ± 27 g kg−1 respectively for CO2, and 60 ± 12 g kg−1, 47 ± 19 g kg−1 and 82 ± 17 g kg−1 respectively for CO. Where comparisons were possible, our EFs agreed well with those derived via a simultaneously-deployed open path Fourier transform infrared (OP-FTIR) spectrometer. These EFs, and the linear best fit relationships between both PM2.5 and BC mass and the CO2 and CO measurements, were used to generate particulate EFs, which varied over the 5.8–20.3 g kg−1 and 0.25–2.89 g kg−1 range respectively. We note a particularly high 2.89 g kg−1 BC emission factor for the rapeseed bonfires, reflective of intense flaming combustion that gave off visible clouds of soot. These field-measured EFs offer a different perspective than is obtained when burning in laboratory combustion chambers, and are suitable for combining with landscape-scale fuel consumption estimates to provide atmospheric modelling inputs of emissions from these types of crop residue fires.

Spray combustion of Jet-A and diesel fuels in a constant volume combustion chamber

This work investigates the spray combustion of Jet-A fuel in an optical constant-volume combustion chamber under different ambient initial conditions. Ambient temperature was varied at 800K, 1000K, and 1200K and five different ambient O2 concentrations were used, spanning 10–21%. These ambient conditions can be used to mimic practical diesel engine working conditions under different fuel injection timings and exhaust gas recirculation (EGR) levels. Both transient and quasi-steady state analyses were conducted. The transient analysis focused on the flame development from the beginning to the end of the combustion process, illustrating how the flame structure evolves with time. The quasi-steady state analysis concentrated on the stable flame structure and compared the flame emissions in terms of spatially integrated intensity, flame effective area, and intensity per pixel. The transient analysis was based on measurements using high-speed imaging of both OH∗ chemiluminescence and broadband natural luminosity (NL). For the quasi-steady state analysis, three flame narrow-band emissions (OH∗ at 310nm, Band A at 430nm and Band B at 470nm) were captured using an ICCD camera. Based on the current Jet-A data and diesel data obtained from previous experiments, a comparison between Jet-A and diesel was made in terms of flame development during the transient state and spatially integrated intensity, flame effective area, and intensity per pixel during the quasi-steady state. For the transient results, Jet-A shares a similar flame development trend to diesel, but featuring a narrower region of NL and a wider region of OH∗ with the increase of ambient temperature and O2 concentration. The soot cloud is oxidized more quickly for Jet-A than diesel at the end of combustion, evident by comparing the area of NL, especially under high O2 concentration. The quasi-steady state results suggest that soot is oxidized effectively under high O2 concentration conditions by the wider region of OH∗ in the downstream locations where only OH∗ emission is observed. The intensity of OH∗ is higher for Jet-A than diesel under low O2 concentration but lower under high O2 concentration. The intensity of NL is higher for Jet-A for all the conditions investigated. However, the intensities of Band A and Band B are lower for Jet-A for all these conditions. Based on the imaging of multiple-band flame emissions, the spray flame structures were further analyzed for the two fuels under both low temperature and conventional combustion modes. Conceptual flame structures were proposed to complement the previous conceptual models for spray combustion under different combustion modes.