Flaming Stage Research Articles

Characteristic visualization of the micro/mesoscale liquid ethanol diffusion flame by using deflection tomography

The micro/mesoscale combustion system of liquid ethanol and the visualization measurement system of flame were designed and constructed on the basis of the microscale combustion theory of liquid fuels. The characteristics of the micro/mesoscale liquid ethanol diffusion flame were investigated experimentally. The flow fields of flame were visualized and measured using deflection tomography. An image sampling system was proposed for deflection tomography to obtain deflectograms and flame images in one shot. The deflection angles of rays were extracted by the wave-front retrieval of deflectograms, which was suitable for micro/mesoscale combustion. The two-dimensional temperature distributions were reconstructed using the deflection angle revision reconstruction algorithm. The three-dimensional distribution of temperature and structure of flame were obtained using a visualization toolkit equipped with the marching cube and ray casting algorithms. At the same diameter of burner, the height, width and peak temperature of flame did not change significantly as the flow rate of fuel increased in the stable combustion stage. In the oscillation combustion stage, the height and width of flame gradually increased, the peak temperature increased firstly and then decreased, and the oscillation cycle of the flame decreased with the increase in the flow rate of fuel. Under equal flow rate of fuel, the height and width of flame were independent of the diameter of burner in the oscillation combustion stage. At the same time, the distribution of temperature and the oscillation period of flame were closely related to the diameter of burner. The pulsating instability of oscillating flame interacted highly with the heat exchange effect between the flame and the liquid fuel.

High Time- and Size-Resolved Measurements of PM and Chemical Composition from Coal Combustion: Implications for the EC Formation Process.

Inefficient coal combustion is a significant source of elemental carbon (EC) air pollution in China, but there is a limited understanding of EC's formation processes. In this study, high time-resolved particle number size distributions (PNSDs) and size-resolved chemical compositions were obtained from the combustion of four bituminous coals burned in a quartz tube furnace at 500 and 800 °C. Based on the distinct characteristics of PNSD, the flaming stage was divided into the first-flaming stage (with a PNSD peak at 0.3-0.4 μm) and the second-flaming stage (with a PNSD peak at 0.1-0.15 μm). For the size-segregated EC and OC measurements, more soot-EC was observed in particles larger than 0.3 μm, whereas the smaller ones possessed more char-EC. The results indicated that gas-phase and direct-conversion EC generation mechanisms dominate different burning stages. The analysis of 16 parent PAHs showed more high-molecular-weight PAHs in the second-flaming stage particles, which supports the idea of different formation processes for char-EC and soot-EC. For all four coals, the PNSD and chemical compositions shared a similar trend, confirming that the different formation processes of EC in different flaming stages were common. This study provides novel information concerning EC formation.

Low-temperature multistage warm diffusion flames

We report on experimental evidence of the existence of a new self-sustaining low-temperature multistage warm diffusion flame, existing between the cool flame and hot flame, at atmospheric pressure in the counterflow geometry. The structure of multistage warm diffusion flames was examined by using thermometry, laser-induced fluorescence, and chemiluminescence measurements. It was found that the warm diffusion flame has a two-staged double flame structure, with a leading diffusion cool flame stage on the fuel side and a second intermediate stage on the oxidizer side, with strong heat release in the second stage that can be comparable to that of the first stage. The results demonstrate that the spatially-distinct multistage character is due to the low-temperature fuel reactivity that allows for the production of reactive intermediates in a leading cool flame. These intermediates are then oxidized, on the oxidizer side, in a second stage via intermediate-temperature chemistry. In the case of dibutyl ether, the low-temperature peroxy branching pathway supports the first cool flame oxidation stage and produces intermediates such as alkyl and carbonyl radicals. The alkyl and carbonyl radicals then react with the hydroperoxyl radical and molecular oxygen to form the second oxidation stage. A detailed analysis revealed that ozone addition in the oxidizer promotes the second stage oxidation by increasing both the radical pool population and the flame temperature, but does not fundamentally change the multistage flame structure. Furthermore, the analysis revealed that with the increase of fuel concentration, a single-stage cool flame can ignite to a warm flame or a hot flame. Moreover, a warm flame can extinguish into a cool flame or ignite to a hot flame when the fuel concentration is substantially reduced or increased, respectively. Finally, under certain conditions, a hot flame can extinguish directly into either a warm flame or a cool flame. Hence, the results suggest that the multistage warm flame can act as a critical bridge between cool flames and hot flames and that it is a fundamental burning mode characteristic of low-temperature non-premixed combustion. The multistage warm diffusion flame is particularly relevant to combustion in highly turbulent flow fields and in microgravity environments, owing to the possibility of long residence times.

Some relevant parameters for assessing fire hazards of combustible mine materials using laboratory scale experiments

When combustible materials ignite and burn, the potential for fire growth and flame spread represents an obvious hazard, but during these processes of ignition and flaming, other life hazards present themselves and should be included to ensure an effective overall analysis of the relevant fire hazards. In particular, the gases and smoke produced both during the smoldering stages of fires leading to ignition and during the advanced flaming stages of a developing fire serve to contaminate the surrounding atmosphere, potentially producing elevated levels of toxicity and high levels of smoke obscuration that render the environment untenable. In underground mines, these hazards may be exacerbated by the existing forced ventilation that can carry the gases and smoke to locations far-removed from the fire location. Clearly, materials that require high temperatures (above 1400 K) and that exhibit low mass loss during thermal decomposition, or that require high heat fluxes or heat transfer rates to ignite represent less of a hazard than materials that decompose at low temperatures or ignite at low levels of heat flux. In order to define and quantify some possible parameters that can be used to assess these hazards, small-scale laboratory experiments were conducted in a number of configurations to measure: 1) the toxic gases and smoke produced both during non-flaming and flaming combustion; 2) mass loss rates as a function of temperature to determine ease of thermal decomposition; and 3) mass loss rates and times to ignition as a function of incident heat flux. This paper describes the experiments that were conducted, their results, and the development of a set of parameters that could possibly be used to assess the overall fire hazard of combustible materials using small scale laboratory experiments.

Experimental study on the early stage of upward flame spread with cross air flow

In this work, experiments were conducted to study the upward flame spread with cross wind at an early stage (prior to the acceleration of pyrolysis spread rate of the wide slab). An exponential model was fitted to the experimental data of the spread rate of pyrolysis front and the dimensionless cross wind speed, which showed satisfactory results. The pyrolysis front tilt angle showed a decreasing trend with a low cross wind speed. However, at high cross wind speeds, the pyrolysis tilt angle exhibited an increasing trend with the maximum value of 45°. The flame lengths increased with the cross wind for narrower slabs, whereas the phenomenon was most pronounced for the narrowest slab. Additionally, the flame length did not exhibit the lengthening phenomenon for the wider slab (0.1 m). Furthermore, the flame tilt angle did not exhibit significant change over time (even along the pyrolysis length). The correlation of flame tilt angle with the cross wind speed and width was also obtained in this study. The flame tilt angle presented a power-law increase with respect to the dimensionless cross wind speed.

The effect of woody fuel characteristics on fuel ignition and consumption: a case study from a eucalypt forest in south-west Western Australia

Coarse woody debris (>0.6 cm in diameter) is an important component of the fuel complex in Australian eucalypt forests, influencing both fire behaviour, smoke production and post-fire ecological processes. We investigated how physical characteristics of woody fuel affected ignition and consumption during an experimental fire where the fuel complex characteristics, fire weather and fire behaviour varied within a narrow range. Decay status, bark condition, arrangement, suspension and extent of charring were classified for 2866 coarse woody fuel particles. We used generalised linear model (GLM) analysis to explain ignition success and the extent of consumption of individual particles, with a focus on larger diameter fuels (>7.5 cm in diameter), which comprised 83% of the woody fuel load and 94% of the woody fuel consumed during the flaming and smouldering stages of combustion. Ignition success was best explained by a model that included fuel arrangement (a surrogate of fuel proximity), suspension and decay status. The extent of fuel consumption was greater for pieces in advanced stages of decay, but suspension (inversely related) and arrangement (directly related) also affected the outcome. Forest management practices, previous fire history and other natural disturbances are likely to influence the distribution of pre-fire diameters and suspension classes that characterise large woody fuels at a site, and will therefore influence woody fuel consumption. This has practical implications for quantifying heat release and atmospheric emissions from fires burning in forests with different management histories.

The prevalence and load of Salmonella, and key risk points of Salmonella contamination in a swine slaughterhouse in Jiangsu province, China

We investigated Salmonella contamination in a slaughterhouse in Jiangsu province (China) by analysing prevalences, loads, and serotypes of Salmonella isolates. In total, 480 samples were collected, with total prevalence of 25.4%. High Salmonella prevalence and load were observed at exsanguination and splitting stages (40.6% and 75.0%; 2.50 ± 0.94 and 2.24 ± 0.72 logMPN/m2, respectively), with low prevalence and load at dehairing, flaming, and chilling stages (2.5%, 5.0%, and 15.0%; 1.39 ± 0.42, 1.36 ± 0.31, and 1.38 ± 0.30 logMPN/m2, respectively). The Salmonella prevalence and load increased substantially from flaming to splitting stage. Six serovars were represented among 122 Salmonella isolates; S. Derby and S. Typhimurium were predominant and were subtyped using PFGE and CRISPR typing approaches. Fourteen PFGE clusters were identified, with discrimination indices (DI) of 0.929 and 0.976 for S. Derby and S. Typhimurium, respectively. Clusters A1, A2, B, C1, C3, C4, D1, F, I, J, L1, M2, and N2 indicated the isolates from same sampling visit were distributed in same cluster. Cluster D1 and K showed that the strains isolated from splitter and carcass swab-samples were highly resemble. Salmonella serovars with the same CRISPR type were mostly isolated after splitting (represented by Dercr 1, Dercr 2, Dercr 3, Tycr 1, Tycr 2, Tycr 3, Tycr 4, and Tycr 6). Our findings revealed that introduced Salmonella was the major source of swine carcass contamination; three slaughtering steps (polishing, rectal drilling, and evisceration) between flaming and splitting were important risk points for Salmonella release, post-splitting slaughtering processes were major contamination risk points, and the splitter was a contamination factor. Our data suggest routes for controlling Salmonella contamination in a swine slaughterhouse.

Emission factors from different burning stages of agriculture wastes in Mexico.

Open-air burning of agricultural wastes from crops like corn, rice, sorghum, sugar cane, and wheat is common practice in Mexico, which in spite limiting regulations, is the method to eliminate such wastes, to clear the land for further harvesting, to control grasses, weeds, insects, and pests, and to facilitate nutrient absorption. However, this practice generates air pollution and contributes to the greenhouse effect. Burning of straws derived from the said crops was emulated in a controlled combustion chamber, hence determining emission factors for particles, black carbon, carbon dioxide, carbon monoxide, and nitric oxide throughout the process, which comprised three apparent stages: pre-ignition, flaming, and smoldering. In all cases, maximum particle concentrations were observed during the flaming stage, although the maximum final contributions to the particle emission factors corresponded to the smoldering stage. The comparison between particle size distributions (from laser spectrometer) and black carbon (from an aethalometer) confirmed that finest particles were emitted mainly during the flaming stage. Carbon dioxide emissions were also highest during the flaming stage whereas those of carbon monoxide were highest during the smoldering stage. Comparing the emission factors for each straw type with their chemical analyses (elemental, proximate, and biochemical), some correlations were found between lignin content and particle emissions and either particle emissions or duration of the pre-ignition stage. High ash or lignin containing-straw slowed down the pre-ignition and flaming stages, thus favoring CO oxidation to CO2.

Influence of particle size on the heat release rate and smoke opacity during the burning of dead Cistus leaves and twigs

This experimental study is a contribution to the understanding of the phenomena of the wildfire. This work was realized on the dead twigs of Cistus monspeliensis exposed to the heat flux at 50 kW m2 using a cone calorimeter. The experiments were realized on the shrub particles with different thicknesses (range of leaves and twigs: 1–20 mm) in function of the stage of combustion (pre-ignition, flaming, and glowing phases). For this, heat release rate, smoke production rate, smoke extinction area, the aerosol yield ( Ya) via the light extinction measurements, and modified combustion efficiency were measured. The results show that there is a size limit for which the heat release rate had a different combustion behavior. The smoke production rate was also measured, and it showed that smoke was mainly emitted during the pre-heating phase. Aerosol yield ( Ya) was calculated depending on these combustion phases (pre-ignition and flaming stage) and for the range of thickness of the samples.

Turbulent flame augmentation using a fluidic jet for Deflagration-to-Detonation

Detonation is a high energetic mode of pressure gain combustion that exploits total pressure rise to augment high flow momentum and thermodynamic efficiencies. Detonation is initiated through flame acceleration driven by turbulence interaction and induction for Deflagration-to-Detonation Transition (DDT). There is a broad desire to unravel the physical mechanisms of turbulence induced DDT. The study examines the role of turbulence induced by a fluidic jet on turbulent accelerating flames. The investigation aims to classify the turbulent flame dynamics and temporal evolution of the flame stages throughout the turbulent regimes. The flame-flow interactions are experimentally studied in a detonation facility using high-speed particle image velocimetry (PIV) and Schlieren imaging. The analysis explores the local turbulence and flame acceleration mechanisms that result from the high level of turbulent transport induced by the jet. Higher flame acceleration is observed for the turbulent flame relative to the laminar flame. The turbulent flame experiences high propagation of turbulence intensities in the lower flame boundary. An increase in vorticity generation, velocity fluctuation, and turbulent strain-rate is experienced throughout the interaction. The turbulent flame regime is characterized showing an evolution between the thin-to-broken reactions.

Conventional Pollutant and Greenhouse Gases Emission from Peatland Fire Based on Peatland Maturity at Flaming Stage

Conventional Pollutant and Greenhouse Gases Emission from Peatland Fire Based on Peatland Maturity at Flaming Stage

Emission factors for PM2.5, CO, CO2, NOx, SO2 and particle size distributions from the combustion of wood species using a new controlled combustion chamber 3CE.

The objective of this research was to determine emission factors (EF) for particulate matter (PM2.5), combustion gases and particle size distribution generated by the combustion of Eucalyptus globulus (EG), Nothofagus obliqua (NO), both hardwoods, and Pinus radiata (PR), softwood, using a controlled combustion chamber (3CE). Additionally, the contribution of the different emissions stages associated with the combustion of these wood samples was also determined. Combustion experiments were performed using shaving size dried wood (0% humidity). The emission samples were collected with a tedlar bag and sampling cartridges containing quartz fiber filters. High reproducibility was achieved between experiment repetitions (CV<10%, n=3). The EF for PM2.5 was 1.06gkg-1 for EG, 1.33gkg-1 for NO, and 0.84gkg-1 for PR. Using a laser aerosol spectrometer (0.25-34μm), the contribution of particle emissions (PM2.5) in each stage of emission process (SEP) was sampled in real time. Particle size of 0.265μm were predominant during all stages, and the percentages emitted were PR (33%), EG (29%), and NO (21%). The distributions of EF for PM2.5 in pre-ignition, flame and smoldering stage varied from predominance of the flame stage for PR (77%) to predominance of the smoldering stage for NO (60%). These results prove that flame phase is not the only stage contributing to emissions and on the contrary, pre-ignition and in especial post-combustion smoldering have also very significant contributions. This demonstrates that particle concentrations measured only in stationary state during flame stage may cause underestimation of emissions.

Measurement of size-dependent single scattering albedo of fresh biomass burning aerosols using the extinction-minus-scattering technique with a combination of cavity ring-down spectroscopy and nephelometry

Abstract. Biomass burning (BB) aerosols have a significant effect on regional climate, and represent a significant uncertainty in our understanding of climate change. Using a combination of cavity ring-down spectroscopy and integrating nephelometry, the single scattering albedo (SSA) and Ångstrom absorption exponent (AAE) were measured for several North American biomass fuels. This was done for several particle diameters for the smoldering and flaming stage of white pine, red oak, and cedar combustion. Measurements were done over a wider wavelength range than any previous direct measurement of BB particles. While the offline sampling system used in this work shows promise, some changes in particle size distribution were observed, and a thorough evaluation of this method is required. The uncertainty of SSA was 6 %, with the truncation angle correction of the nephelometer being the largest contributor to error. While scattering and extinction did show wavelength dependence, SSA did not. SSA values ranged from 0.46 to 0.74, and were not uniformly greater for the smoldering stage than the flaming stage. SSA values changed with particle size, and not systematically so, suggesting the proportion of tar balls to fractal black carbon change with fuel type/state and particle size. SSA differences of 0.15–0.4 or greater can be attributed to fuel type or fuel state for fresh soot. AAE values were quite high (1.59–5.57), despite SSA being lower than is typically observed in wildfires. The SSA and AAE values in this work do not fit well with current schemes that relate these factors to the modified combustion efficiency of a burn. Combustion stage, particle size, fuel type, and fuel condition were found to have the most significant effects on the intrinsic optical properties of fresh soot, though additional factors influence aged soot.

Modeling of Low-temperature oxidation and combustion of droplets

Key features of radiation extinguishing of spherical hot flame around a single droplet with its subsequent low-temperature oxidation and combustion under microgravity conditions—a phenomenon discovered in experiments onboard the International Space Station—have been reproduced using the mathematical model of droplet combustion and detailed kinetic mechanism of n-heptane oxidation and combustion. It has been demonstrated that experimentally observed repeated temperature flashes were blue flame flashes, and their duration was determined by the hydrogen peroxide decomposition time. In addition to this phenomenon, calculations predict the existence of new modes of low-temperature oxidation and combustion of droplets without the hot flame stage. In such modes, the basic reaction is concentrated very close to the droplet surface, and fuel vapor reacts in it only partially.

HONO emission and production determined from airborne measurements over the Southeast U.S.

AbstractThe sources and distribution of tropospheric nitrous acid (HONO) were examined using airborne measurements over the Southeast U.S. during the Southeast Nexus Experiment in June and July 2013. HONO was measured once per second using a chemical ionization mass spectrometer on the NOAA WP‐3D aircraft to assess sources that affect HONO abundance throughout the planetary boundary layer (PBL). The aircraft flew at altitudes between 0.12 and 6.4 km above ground level on 18 research flights that were conducted both day and night, sampling emissions from urban and power plant sources that were transported in the PBL. At night, HONO mixing ratios were greatest in plumes from agricultural burning, where they exceeded 4 ppbv and accounted for 2–14% of the reactive nitrogen emitted by the fires. The HONO to carbon monoxide ratio in these plumes from flaming stage fires ranged from 0.13 to 0.52%. Direct HONO emissions from coal‐fired power plants were quantified using measurements at night, when HONO loss by photolysis was absent. These direct emissions were often correlated with total reactive nitrogen with enhancement ratios that ranged from 0 to 0.4%. HONO enhancements in power plant plumes measured during the day were compared with a Lagrangian plume dispersion model, showing that HONO produced solely from the gas phase reaction of OH with NO explained the observations. Outside of recently emitted plumes from known combustion sources, HONO mixing ratios measured several hundred meters above ground level were indistinguishable from zero within the 15 parts per trillion by volume measurement uncertainty. The results reported here do not support the existence of a ubiquitous unknown HONO source that produces significant HONO concentrations in the lower troposphere.

Morphological and chemical characterization of soot emitted during flaming combustion stage of native-wood species used for cooking process in western Mexico

The morphology, microstructure and chemical composition of soot particles emitted directly from biofuel cook stoves have been studied by using high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and ion beam analysis (IBA) techniques. Freshly emitted soot particles were collected using an eight stage cascade impactor that separates particles according to their size. The particulate matter is impacted on thin aluminum foil and copper grids for Transmission Electron Microscope. The analysis of HRTEM micrographs revealed the nanostructure and the particle size of soot chain. Results from the border-based fractal dimension indicated that main morphological differences observed in soot particles perhaps are associated to cook-stove chamber design. XPS analysis shows that particles are mainly composed by carbon and oxygen. The XPS C-1s spectra show carbon with three peaks corresponding to sp2, sp3 hybridization and carbon oxygen bond. Additionally, we determined the areal density (atoms/cm2) of the soot films and the purity of the substrates by means of IBA techniques.

Processes of chemoionization in the course of inflammation of a methane-oxygen mixture by a high-current gliding surface discharge in a closed chamber

Results are presented from experiments on the inflammation of a stoichiometric methane-oxygen mixture by a high-current multielectrode spark-gap in a closed cylindrical chamber. It is shown that, in both the preflame and well-developed flame stages, the gas medium is characterized by a high degree of ionization (n e ≈ 1012 cm−3) due to chemoionization processes and a high electron-neutral collision frequency (ν e0 ≈ 1012 s−1).

Characteristics of flame kernal radius in a spark ignition engine according to the electric spark ignition energy

To study flame propagation in a spark ignition engine, it must concentrate on the initial combustion stage, i.e., the formation and development of flame. Therefore, we must study the theoretical calculation of minimum flame kernel radius which affects the formation and development of flame kernel. To calculate the minimum flame kernel radius, we must know the thermal conductivity, flame temperature, laminar burning velocity, etc. To evaluate the accuracy of the minimum flame kernel radius based on the theory, authors compared with those from calculation and those from tests. According to the increase of number of sparks and spark intervals, the flame kernel radii become to increase and the expected lean limits are extended. It is considered that is due to the stability of combustion as increase of flame kernel size according as high ignition energy supplies in initial period and discharge energy period lengthens.

Gaseous and particulate emission profiles during controlled rice straw burning

Burning of rice straw can emit considerable amounts of atmospheric pollutants. We evaluated the effect of rice straw moisture content (5%, 10%, and 20%) on the emission of carbon dioxide (CO2) and on the organic and inorganic constituents of released particulate matter (PM): dioxins, heavy metals, and polycyclic aromatic hydrocarbons (PAHs). Four burning tests were conducted per moisture treatment using the open chamber method. Additionally, combustion characteristics, including burning stages, durations, temperature, and relative humidity, were recorded. Burning tests showed flaming and smoldering stages were significantly longer in 20% moisture treatment (P < 0.05) compared with the rest. The amount of burned straw and ashes decreased with increasing straw moisture content (P < 0.001). Carbon dioxide was the main product obtained during combustion with emission values ranging from 692 g CO2 kg dry straw−1 (10% moisture content) to 835 g CO2 kg dry straw−1 (20% moisture content). Emission factors for PM were the highest in 20% moisture treatment (P < 0.005). Fine PM (PM2.5) accounted for more than 60% of total PM mass. Emission factors for dioxins increased with straw moisture content, being the highest in 20% moisture treatment, although showing a wide variability among burning tests (P > 0.05). Emissions factors for heavy metals were low and similar among moisture treatments (P > 0.05). Emission factors for individual PAHs were generally higher in 20% moisture treatment. Overall, emission factors of atmospheric pollutants measured in our study were higher in the 20% moisture content. This difference could be attributed to the incomplete combustion at higher levels of rice straw moisture content. According to our results, rice straw burning should be done after straw drying and under minimal moisture conditions to lower pollutant emission levels.

Novel application of a combustion chamber for experimental assessment of biomass burning emission

Biomass burning is an important ecological factor in the Mediterranean ecosystem and a significant source of several atmospheric gases and particles. This paper demonstrates the performance of a recently developed combustion chamber, showing its capability in estimating the emission from wildland fire through a case study with dried leaf litter of Quercus robur. The combustion chamber was equipped with a thermocouple, a high resolution balance, an epiradiometer, two different sampling lines to collect volatile organic compounds (VOCs) and particles, and a portable analyzer to measure carbon monoxide (CO) and carbon dioxide (CO2) emission. VOCs were determined by gas chromatography–mass spectrometry (GC–MS) after enrichment on adsorption traps, but also monitored on-line with a proton-transfer-reaction mass spectrometer (PTR-MS). Preliminary qualitative analyses of emissions from burning dried leaf litter of Q. robur found CO and CO2 as the main gaseous species emitted during the flaming and smoldering stages. Aromatic VOCs, such as benzene and toluene, were detected together with several oxygenated VOCs, like acetaldehyde and methanol. Moreover, a clear picture of the carbon balance during the biomass combustion was obtained with the chamber used. The combustion chamber will allow to distinguish the contribution of different plant tissues to the emissions occurring during different combustion phases.