Secondary Flame Research Articles

Secondary Flame of a Candle

Secondary Flame of a Candle

Autoignition and combustion behavior of emulsion droplet under elevated temperature and pressure conditions

In this study, experiments were conducted to investigate the combustion characteristics of an water-in-oil W/O emulsion droplet under elevated temperature and pressure conditions. The base fuel used was n-decane, and total volume ratios of 10, 20, and 30% of distilled water were mixed for producing the emulsion fuel. Span 80 with a volume ratio of 2% was added as a surfactant, and the emulsion fuel was homogeneously mixed via ultrasonication. The combustion process of an emulsion droplet was divided into five stages: droplet heating, classical combustion, puffing, secondary classical combustion, and surfactant combustion. The ignition delay decreased with elevated ambient temperatures, whereas an increase in the ambient pressure and water volume ratio resulted in longer ignition delays. The droplets did not ignite in 500 °C or 600 °C conditions at 1 bar because of the significant Stefan flow of fuel vapor. After droplet ignition, the droplet combustion process, including classical combustion, puffing, and surfactant combustion, followed. The average burning rate increased with ambient pressure, but it was insensitive to ambient temperatures and water volume ratios. After flame extinction, a secondary flame reappeared because of the combustion of surfactant and residues.

Experimental study of flame propagation across a perforated plate

Flame propagation across a single perforated plate was experimentally studied in a square cross-section channel. Experiments were performed in premixed hydrogen-air mixture with different equivalence ratios and initial pressures, aiming at identifying the parametric influence. High-speed schlieren photography and pressure records were used to capture the flame front and obtain the pressure build-up. Four stages for the flame front crossing the perforated plate were obtained, namely, laminar flame, jet flame, turbulent flame and secondary flame front. Following ignition, a laminar flame was obtained, which was nearly not affected by the confinement. This laminar flame was squeezed to pass through the perforated plate, producing the jet flame with a step change on velocity. Turbulent flame was generated by merging the jets, which facilitated the acceleration of the flame front. Secondary flame front induced by Rayleigh-Taylor instability was clearly observed in the process of the turbulent front moving forward. Both velocity and pressure are enhanced in this stage. Parametric studies suggested that the secondary flame front is more obvious in the stoichiometric mixture with higher initial pressure, and characterized by a faster propagation velocity and a bigger pressure rise.

Combined effect of equivalence ratio and velocity gradients on flame stability and emission formation

Premixed flames have shown to behave differently while operating under equivalence ratio or velocity gradients. This distinct behaviour can be used to improve the performance of premixed burners. The present work analysed the effect of both equivalence ratio and velocity gradients on the stability of methane premixed laminar flames. Additionally, the influence of these gradients on CO and NOx emissions was quantified. Results show that using equivalence ratio and velocity gradients can extend the stability limits of the burner. The lifting phenomenon commonly found on peripheral flames was cancelled by using higher equivalence ratios and lower flow velocities in the primary regime. Consequently, stable flames were achieved in the secondary regime for substantially leaner mixtures. However, flame extinction was eventually reached in the inner region of the burner, due to the tip opening of secondary flames. The minimum global equivalence ratio, ϕg, was extended from 0.80 to 0.65, solely by using equivalence ratio gradients. When combined with a velocity gradient, the latter effect was further extended up to ϕg=0.52. Regarding emissions, results show that different combinations of equivalence ratio and velocity gradients result in distinct NOx and CO emissions, even for the same ϕg. Under this stratified arrangement, a reduction of 43% was achieved in NOx formation, when compared with the homogeneous case. Overall, results suggest that introducing richer peripheral flames allows a higher extension of the lower stability limits, whereas leaner peripheral flames lead to lower NOx emissions.

Temperature dependence of liquid lithium film formation and deuterium retention on hot W samples studied by LID-QMS. Implications for future fusion reactors

Liquid metal (LM) divertor concepts explore an alternative solution to the challenging power/particle exhaust issues in future magnetic fusion reactors. Among them, lithium (Li) is the most promising material. Its use has shown important advantages in terms of improved H-mode plasma confinement and heat handling capabilities. In such scenario, a possible combination of tungsten (W) on the first wall and liquid Li on the divertor could be an acceptable solution, but several issues related to material compatibility remain open. In particular, the co-deposition of Li and hydrogen isotopes on W components could increase the associated tritium retention and represent a safety risk, especially if these co-deposits can uncontrollably grow in remote/plasma shadowed zones of the first wall. In this work, the retention of Li and deuterium (D) on tungsten at different surface temperature (200 °C–400 °C) has been studied by exposing W samples to Li evaporation under several D2 gaseous environments. Deuterium retention in the W–Li films has been quantified by using laser induced desorption-mass spectrometry (LID-QMS). Additional techniques as thermal desorption spectroscopy, secondary ion mass spectrometry, profilemetry and flame atomic emission spectroscopy were implemented to corroborate the retention results and for the qualitative and quantitative characterization of the films. The results showed a negligible (below LID sensibility) D uptake at Tsurface = 225 °C, when the W–Li layer is exposed to simultaneous Li evaporation and D2 gas exposition (0.67 Pa). Pre-lithiated samples were also exposed to higher D2 pressures (133.3 Pa) at different temperatures (200 °C–400 °C). A non-linear drastic reduction in the D retention with increasing temperatures was found on the W–Li films, presenting a D/Li atomic ratio at 400 °C lower than 0.1 at.% on a thin film of ≈100 nm thick. These results bode well (in terms of tritium inventory) for the potential utilization of this material combination in a real reactor scenario.

Experimental and numerical study of cap-like lean limit flames in H2-CH4-air mixtures

Lean limit flames of H2-CH4-air mixtures stabilized inside a tube with an inner diameter of 30 mm in a downward flow are studied experimentally and numerically. A transition from bubble-like flames, with a long decaying skirt, to cap-like flames with a sharp visible flame edge at the bottom is observed as the lean flammability limit is approached. This transition is accompanied by formation of a secondary weak flame front inside the cap-like flame. The CH* chemiluminescence distribution of the studied flames is recorded and the velocity field of the lean limit flames is measured using Particle Image Velocimetry (PIV). The flame temperature field is measured utilizing the Rayleigh scattering method. Numerical prediction with a mixture-averaged transport model and skeletal mechanism for CH4 qualitatively reproduces the above experimentally observed phenomena. The presence of negative flame displacement speed for the entire leading edge of the cap-like flames is numerically predicted and experimentally demonstrated. The secondary weak flame front is located in a region with reverse upward flow of the recirculation zone, which is found to support the propagation of the leading edge with a negative flame displacement speed. Furthermore, radiative heat loss has a significant influence on the lean flammability limit of the cap-like flames.

Effects of operational mode on particle size and number emissions from a biomass gasifier cookstove

Interest in the size distribution of particles emitted from biomass cookstoves stems from the hypothesis that exposure to ultrafine particles is more detrimental to human health than exposure to accumulation mode or other size regimes. Previous studies have reported that gasifier cookstoves emit smaller particles than other cookstove designs under steady operating conditions. In the present study, the number size distribution of particles emitted from a forced-air gasifier cookstove was measured at 1 Hz as the stove transitioned between several steady and transient operating modes. During normal operation, when the stove functioned as a top-lit updraft gasifier, the distribution was bimodal, with peaks at 10 nm and 40 nm, when a pot of water was on the stove. The distribution became unimodal with a peak at 10 nm when the pot was removed. Once the fuel bed had completely gasified and the secondary flame extinguished, the concentration of particles increased and the peak in number concentration shifted to a...

Laser ignition of CL-20 (hexanitrohexaazaisowurtzitane) cocrystals

Energetic cocrystals are a new class of materials that consist of two or more energetic molecules in a crystal structure. The multicomponent cocrystals can possess significantly different properties than either component, and therefore it is possible that their ignition behavior could be different than a physical mixture. In this paper, we report the time to ignition and reaction dynamics of various energetic materials, including select cocrystal materials under CO2 laser heating. An effort was made to minimize the amount of material used in comparison to previous laser ignition studies due to the limited availability of materials. The 1:1 molar Trinitrotoluene (TNT): Hexanitrohexaazaisowurtzitane (CL-20) produced by slurry or precipitation methods and 1:2 molar cyclotetramethylene-tetranitramine (HMX):CL-20 produced by slurry or resonant mixing methods were investigated along with the individual constituents and equivalent molar physical mixes. Pressed cylindrical pellets with diameters of 3.2 mm and heights between 1 and 2 mm were ignited at irradiances ranging from 310 to 1446 W/cm2. Visual imaging was performed with a high speed camera and ultraviolet (UV) spectral data were collected with a spectrometer coupled to a streak camera. Additionally, high speed schlieren imaging was performed to investigate the ignition dynamics prior to light generation. Specific species identified with the spectrometer include OH and CN, which coincide with the observation of the secondary flame as indicated from schlieren and first-light in the visual video record. All the cocrystallized materials ignited in a similar manner to CL-20 with comparable times to secondary flame formation, but the cocrystallized materials were found to have a shorter onset to gasification than the constituents or physical mixtures.

Different combustion modes caused by flame-shock interactions in a confined chamber with a perforated plate

The present work investigates the interaction of the turbulent flame and shock wave as well as the end-gas autoignition in a newly designed constant volume combustion chamber equipped with a perforated plate using a stoichiometric hydrogen-air mixture. Detailed high speed schlieren photography is used to track the turbulent flame fronts and shock waves which are generated by the laminar flame passing through the perforated plate. The different propagation speeds of the turbulent flames and shock waves can be obtained by controlling the initial pressures and the hole size of the perforated plate. In this work, three combustion modes were obtained clearly by experiment, depending on the interactions of the turbulent flame and shock wave, such as normal combustion, oscillating combustion and end-gas autoignition. The normal combustion is a weak turbulent flame propagation without an obvious shock wave in the confined chamber. The oscillating flame propagation is generated by the interaction of the reflected shock wave and flame front and this process can be clearly visualized in the present work. The end-gas autoignition is induced by the combined effect of the supersonic flame and the shock waves. The accelerating combustion in the confined chamber could produce the primary shock wave and the subsequent secondary shock wave is induced by the secondary flame occurring between the primary flame and primary shock wave. It is found that the secondary shock wave with speed of 780m/s is faster than the primary one, which is the source of the end-gas autoignition. It is also observed that quasi-detonation wave produced by the end-gas autoignition can reach the speed of 1700m/s. This wave is accompanied by a strong pressure oscillation which can explain the mechanism of engine knock.

Synthesis of Nanoparticles through Flame Spray Pyrolysis: Experimental Apparatus and Preliminary Results

Nanostructured materials represent nowadays a wide, and probably still largely unexplored, field of potential applications. In fact, this is a research topic in high and rapid development, both at a basic level and under the point of view of possible practical applications, leaving large space for a thorough scientific analysis, which requires with no doubt long time for ultimate conclusions. This paper deals with the preliminary work performed in the field of FSP (Flame Spray Pyrolysis) synthesis for nanoparticles, using an external mixing gas assisted nozzle. Particularly, an experimental apparatus has been designed, realized and characterized for the controlled synthesis of nanoparticles by the flame spray pyrolysis technique. The apparatus consists of a gas-assisted spray for droplet generation and dispersion in a secondary pilot flame. By dissolving suitable precursors in a liquid fuel, different types of nanoparticles have been produced. In the preliminary tests SiO2 and TiO2 have been synthesized and characterized by TEM analysis and XRD, respectively, as a function of the main operating conditions (for instance, precursor concentration) of the experimental apparatus. The designed set-up shows good stability and reproducibility of the reaction flame and, therefore, of the material produced and the obtained results, even if preliminary, encourage the use of the experimental apparatus for the controlled synthesis of nanostructured materials.

Cycle-resolved visualization of pre-ignition and abnormal combustion phenomena in a GDI engine

Downsized spark ignition engines are highly efficient but promote abnormal combustion phenomena like pre-ignition and knocking. This limits the maximum load of SI engines and consequently the scale of downsizing. In this paper, cycle resolved visualization and thermodynamic analysis were used to investigate abnormal combustion phenomena such as pre-ignition at localized surfaces and due to moving hot-particles, as well as post-spark discharge secondary flames near the walls and knock. Tests were performed on an optically accessible single cylinder ‘wall guided’ GDI engine equipped with self-lubricant rings in order to avoid oil residuals in the combustion chamber and to exclude them as cause of pre-ignitions. Cycle resolved visualization allowed obtaining information on hot-particles trajectory and provided insight into the morphology and speed of flame fronts (both spark-ignited and secondary ones). Optical data were correlated to in-cylinder pressure and rate of heat release curves. Secondary flames were detected both in normal and knocking cycles. Under heavy knocking conditions, when secondary flames were identified, their occurrence was correlated to the onset of knock; more advanced start of pressure oscillations corresponded to prior evidence of non-spark ignited sites near the walls. A distribution map of secondary flames showed higher probability of occurrence near the cylinder walls and towards the injector location, suggesting that fuel deposits play a major role in the initiation of abnormal combustion phenomena.

Spatio-temporal structure of vertically spreading flame over non-planar PMMA surfaces

Burning of PMMA slabs with non-planar surfaces is presented in this paper. Non-planar surfaces have been produced using steps fabricated in transparent thermally thick PMMA slabs. The step size is kept constant and three step locations have been used in this study. Either the plate portion or the step portion of the slab is ignited, but keeping the ignition location at the bottom. Concurrent flame spread over stepped slabs is analysed in detail. Time-dependent %mass loss, surface regression, and OH*, CH* chemiluminescence, and planar laser induced incandescence of soot and planar laser induced fluorescence of polyaromatic hydrocarbons (PAH) are determined. The results indicate that the main influence of the step at any location has been to increase the mass loss rate and therefore, the burning rate. The flow fields are quite different between the step ignition and the plate ignition cases. As a result, secondary flame zones are observed in the wake zone downstream of the step for step ignition cases and the flames influenced by stagnation point flow field are observed for plate ignition cases. The influences of step size for step ignition and location of step for plate ignition cases are also studied. The OH*/CH* maps show their overlap region as that of high heat release rates. The PAH precursor to soot is spatially mapped relative to the latter. Although the plate-injection cases are sooty, LII indicates otherwise because of poor mixing in a forward-facing step leading to substantial sooting above the sample outside the field of view. On the other hand, the effect of step recirculation zone in promoting mixing and suppressing soot is evident from the results.

Secondary atomization and spray flame characteristics of carbonated W/O emulsified fuel

The secondary atomization behavior and spray flame characteristics of carbonated W/O emulsified fuel were studied. The dissolved CO2 in the carbonated emulsified fuel was able to reduce the bubble nucleation energy needed for secondary atomization. First, the spray characteristics of carbonated fuels were studied in a N2 environment at 923K, using high-magnification shadow imaging of the spray droplets. This allowed the spray droplets to be clearly visualized during secondary atomization. The dissolved CO2 was shown to enhance secondary atomization by decreasing the bubble nucleation energy, resulting in the production of finer spray droplets. The spray flame characteristics of n-dodecane, emulsified fuel, and carbonated emulsified fuel were then studied in a laminar counterflow burner. Direct photography of the flame using a color high-speed video camera and magnified shadow imaging using a monochrome high-speed video camera were used. Compared the ignition point of luminous flames in n-dodecane, an ignition delay was found in the emulsified fuel, which was not found in the carbonated emulsified fuel. Compact luminous flames were also formed in the carbonated emulsified fuel. In the magnified flame images, bright spots of higher luminosity than the surrounding area were observed in both the emulsified fuels. These proved to be a useful marker for secondary atomization. In the carbonated emulsified fuel, the bright spots appeared earlier, indicating that the dissolved CO2 reduced the droplet temperature at which secondary atomization occurred. This effect countered the ignition delay caused by water addition in the emulsified fuel. A higher frequency of bright spot occurrence was also found in the carbonated emulsified fuel. It was demonstrated that carbonation of emulsified fuel improved the spray flame characteristics through the enhancement of secondary atomization.

Characterization of inverse diffusion flames in vitiated cross flows via two-photon planar laser-induced fluorescence of CO and 2-D thermometry

Two-photon, planar laser-induced fluorescence (PLIF) of carbon-monoxide (CO) and two-dimensional thermometry employing two-color, hydroxyl radical (OH) PLIF are used to characterize atmospheric-pressure inverse diffusion flames. These flames are important tools to aid the understanding of secondary reaction zones that may form in gas turbine engines when film-cooling air reacts with fuel-rich packets from the combustor. For the experiments performed in the present study, exhaust from a propane–air well-stirred reactor is channeled to a test section where three different film-cooling geometries are used to create inverse diffusion flames: (1) a single row of normal cooling holes, (2) a slot cut at an angle of 30° with respect to the wall, and (3) an 5×11 array of cooling holes. It is found that CO and H2 concentrations of a few percent can lead to secondary reaction zones and that different cooling-hole geometries can produce dramatically different secondary reaction-zone shapes. These secondary reaction zone flames have Damköhler numbers greater than unity and are diffusion limited. The PLIF measurements show regions where CO is consumed, OH produced, and the temperature perturbed. For film-cooling flows that remain attached to the wall, the secondary reaction zone is also close to the wall and can cover a relatively long axial length. For film-cooling flows that separate from the wall, the secondary reaction zones protrude farther into the cross flow then quickly mix with the cross flow. By comparing the CO, OH, and temperature fields, three characteristic regions of flows with secondary reaction zones are identified: the injection region where cooling air displaces the vitiated cross flow, the secondary reaction zone region, and the mix-out region where all of the oxygen has been consumed and mixing with the vitiated cross flow controls the local composition and temperature.

Near-surface flame structure characterization of simplified ammonium perchlorate/hydroxyl-terminated polybutadiene compositions

Simplified model propellant configurations, such as monomodal propellants, can be valuable in the development and validation of predictive numerical tools. These idealized experiments also yield insight into the effect of diffusion length scales on combustion, but comprehensive data covering a large range of diffusional length scales do not currently exist. Here, monomodal propellants with ammonium perchlorate (AP) particle sizes under 800 µm and AP pellets ported and filled with hydroxyl-terminated polybutadiene (HTPB) were used to systematically study the effect of diffusion length scales, or AP equivalent particle sizes) of up to 4.1 mm on flame structure. In general, burning rates increased with pressure and decreasing particle size, as expected. Burning rates for samples with particle sizes greater than 400 µm converged with AP monopropellant burning rate data above approximately 2 MPa, the AP low-pressure deflagration limit (LPDL). For a given pressure above the LPDL, burning rates eventually became constant for both increasing and decreasing particle sizes. Conversely, for a given pressure below the LPDL burning rate was shown to be a function of particle diameter. Flame structures above the composite propellants were observed using 5 kHz OH planar laser-induced fluorescence (PLIF). The transient flames were underventilated (jet-like) over the AP particles at 1 atm while lifted, inverted, and overventilated at 5 atm. Distinct diffusion flame structures were observed visually above the ported samples at 1 atm. Very luminous flames were observed at the interface between the AP and binder. The effect of strain rate on sample combustion was examined using an opposed flow burner; at 1 atm, sample burning rate was not affected by strain rate. At the largest strain rate, the sample self-extinguished after igniter shutoff, indicating that secondary diffusion flames are important in the opposed flow configuration.

Burners for Rotary Furnace - Proposal and Comparison

In the paper are presented the possibility of using axial and radial diffusion burner in rotary furnace. Both types of burners allow secondary air combustion, combustion quality measure is the ratio of primary and secondary flame. Experimental comparison of both types of burners has been performed with the high-revolution rotary furnace. On this basis, benefits have been identified and their use possibilities.

F232 CO_2溶解エマルジョン燃料の二次微粒化挙動と噴霧火炎特性(OS-03:革新的技術のための燃焼研究(5))

Secondary atomization behavior and spray flame characteristics of carbonated W/O emulsified fuel were studied. An emulsified fuel or carbonated emulsified fuel was injected into a flame stabilized in a laminar counterflow field. Direct photography of the flame using a color high-speed video camera and magnified shadow imaging of spray droplets using a monochrome high-speed video camera were used. As a result, tip position of spray flame became shorter and spray flame was compact when carbonated emulsified fuel was used. This was because dissolved CO_2 enhanced secondary atomization, leading to improvement of fuel vapor ignition at upstream region. Secondary atomization behavior of carbonated emulsified fuel droplet was also observed at upstream region.

A Study on Energy Conversion Efficiency of Direct Flame Fuel Cell Supported by Clustered Diffusion Microflames

The micro power generator based on clustered diffusion microflames and a direct flame fuel cell (DFFC) is proposed and its energy conversion efficiency is evaluated to investigate the optimum state of combustion. The clustered diffusion microflames are established on the 2.5 mm pitch 3-by-3 array of fuel jets with a diameter of 0.07 mm and a velocity of 24 m/s at the outlet by the interaction with other microflames around. The clustered diffusion microflames is more suitable for micro power generator than microflame array because of suppression of soot formation without air supply system. Although the conversion efficiency of the system with clustered microflames is quite sensitive to the separation distance between the burner and the cell, compared with the microflame array, it has attained the same level of conversion efficiency with microflame array, i.e. 0.45 %. The results of cell temperature suggest that the rapid decrease of conversion efficiency with increasing separation distance is caused by the decrease of species supply such as CO and H2 rather than the decrease of heat supply. The observation of the secondary flame suggests that the reason of high sensitivity to the separation distance is because the cell position changes the flowrate of entrained ambient air and hence the flame equivalence ratio.

Flame behaviors and pressure characteristics of vented dust explosions at elevated static activation overpressures

Dust explosion venting experiments were performed using a 20-L spherical chamber at elevated static activation overpressures larger than 1 bar. Lycopodium dust samples with mean diameter of 70 μm and electric igniters with 0.5 KJ ignition energy were used in the experiments. Explosion overpressures in the chamber and flame appearances near the vent were recorded simultaneously. The results indicated that the flame appeared as the under-expanded free jet with shock diamonds, when the overpressure in the chamber was larger than the critical pressure during the venting process. The flame appeared as the normal constant-pressure combustion when the pressure venting process finished. Three types of venting processes were concluded in the experiments: no secondary flame and no secondary explosion, secondary flame, secondary explosion. The occurrence of the secondary explosions near the vent was related to the vent diameter and the static activation overpressure. Larger diameters and lower static activation overpressures were beneficial to the occurrence of the secondary explosions. In current experiments, the secondary explosions only occurred at the following combinations of the vent diameter and the static activation overpressure: 40 mm and 1.2 bar, 60 mm and 1.2 bar, 60 mm and 1.8 bar.

Premixed Combustion of Kapok (ceiba pentandra) seed oil on Perforated Burner

Availability of fossil fuels in the world decrease gradually due to excessive fuel exploitation. This situations push researcher to look for alternative fuels as a source of renewable energy, one of them is kapok (ceiba pentandra) seed oil. The aim this study was to know the behavior of laminar burning velocity, secondary Bunsen flame with open tip, cellular and triple flame. Premixed combustion of kapok seed oil was studied experimentally on perforated burner with equivalence ratio (φ) varied from 0.30 until 1.07. The results showed that combustion of glycerol requires a large amount of air so that laminar burning velocity (SL) is the highest at very lean mixture (φ =0.36) in the form of individual Bunsen flame on each of the perforated plate hole. Perforated and secondary Bunsen flame both reached maximum SL similar with that of ethanol and higher than that of hexadecane. Slight increase of φ decreases drastically SL of perforated and secondary Bunsen flame. When the mixture was enriched, secondary Bunsen and perforated flame disappears, and then the flame becomes Bunsen flame with open tip and triple flame (φ = 0.62 to 1.07). Flame was getting stable until the mixture above the stoichiometry. Being isolated from ambient air, the SL of perforated flame, as well as secondary Bunsen flame, becomes equal with non-isolated flame. This shows the decreasing trend of laminar burning velocity while φ is increasing. When the mixture was enriched island (φ = 0.44 to 0.48) and petal (φ = 0.53 to 0.62) cellular flame take place. Flame becomes more unstable when the mixture was changed toward stoichiometry.