Flame kernel characterization of laser ignition of natural gas–air mixture in a constant volume combustion chamber

In this paper, laser-induced ignition was investigated for compressed natural gas–air mixtures. Experiments were performed in a constant volume combustion chamber, which simulate end of the compression stroke conditions of a SI engine. This chamber simulates the engine combustion chamber conditions except turbulence of air–fuel mixture. It has four optical windows at diametrically opposite locations, which are used for laser ignition and optical diagnostics simultaneously. All experiments were conducted at 10 bar chamber pressure and 373 K chamber temperature. Initial stage of combustion phenomena was visualized by employing Shadowgraphy technique using a high speed CMOS camera. Flame kernel development of the combustible fuel–air mixture was investigated under different relative air–fuel ratios (λ=1.2?1.7) and the images were interrogated for temporal propagation of flame front. Pressure-time history inside the combustion chamber was recorded and analyzed. This data is useful in characterizing the laser ignition of natural gas–air mixture and can be used in developing an appropriate laser ignition system for commercial use in SI engines.     

Flame propagation of nano/micron-sized aluminum particles and ice (ALICE) mixtures

Flame propagation of aluminum–ice (ALICE) mixtures is studied theoretically and experimentally. Both a mono distribution of nano aluminum particles and a bimodal distribution of nano- and micron-sized aluminum particles are considered over a pressure range of 1–10 MPa. A multi-zone theoretical framework is established to predict the burning rate and temperature distribution by solving the energy equation in each zone and matching the temperature and heat flux at the interfacial boundaries. The burning rates are measured experimentally by burning aluminum–ice strands in a constant-volume vessel. For stoichiometric ALICE mixtures with 80 nm particles, the burning rate shows a pressure dependence of r_b = aPⁿ, with an exponent of 0.33. If a portion of 80 nm particles is replaced with 5 and 20 μm particles, the burning rate is not significantly affected for a loading density up to 15–25% and decreases significantly beyond this value. The flame thickness of a bimodal-particle mixture is greater than its counterpart of a mono-dispersed particle mixture. The theoretical and experimental results support the hypothesis that the combustion of aluminum–ice mixtures is controlled by diffusion processes across the oxide layers of particles.     

Experimental investigation of flame spreading over pure methane hydrate in a laminar boundary layer

Flame spreading over pure methane hydrate in a laminar boundary layer is investigated experimentally. The free stream velocity (U_∞) was set constant at 0.4 m/s and the surface temperature of the hydrate at the ignition (T_s) was varied between ?10 and ?80 °C. Hydrate particle sizes were smaller than 0.5 mm. Two types of flame spreading were observed; “low speed flame spreading” and “high speed flame spreading”. The low speed flame spreading was observed at low temperature conditions (T_s = ?80 to ?60 °C) and temperatures in which anomalous self-preservation took place (T_s = ?30 to ?10 °C). In this case, the heat transfer from the leading flame edge to the hydrate surface plays a key role for flame spreading. The high speed flame spreading was observed when T_s = ?50 and ?40 °C. At these temperatures, the dissociation of hydrate took place and the methane gas was released from the hydrate to form a thin mixed layer of methane and air with a high concentration gradient over the hydrate. The leading flame edge spread in this premixed gas at a spread speed much higher than laminar burning velocity, mainly due to the effect of burnt gas expansion.     

Fundamental burning velocities of meso-scale propagating spherical flames with H2, CH4 and C3H8 mixtures

This study is performed to experimentally examine the fundamental burning velocity characteristics of meso-scale outwardly propagating spherical laminar flames in the range of flame radius r_f approximately from 1 to 5 mm for hydrogen, methane and propane mixtures, in order to make clear a method for improving combustion of micro–meso scale flames. Macro-scale laminar flames with r_f > 7 mm are also examined for comparison. The mixtures have nearly the same laminar burning velocity (S_L0 = 25 cm/s) for unstretched flames and different equivalence ratios ?. The radius r_f and the burning velocity S_Ll of meso-scale flames are estimated by using sequential schlieren images recorded under appropriate ignition conditions. It is found that S_Ll of hydrogen and methane premixed meso-scale flames at the same r_f or the Karlovitz number Ka shows a tendency to increase with decreasing ?, whereas S_Ll of propane flames increases with ?. However, S_Ll tends to decrease with the Lewis number Le and the Markstein number Ma, irrespective of the type of fuel and ?. It also becomes clear that the optimum flame size and Ka to improve the burning velocity exist for some mixtures depending on Le and fuel types.     

Experimental study of upward flame spread of an inclined fuel surface

A thermally thick slab of polymethyl methacrylate was used to study the effects of the inclination angle of a fuel surface on upward flame spread. While investigation of upward spread over solid fuels has typically been restricted to an upright orientation, inclination of the fuel surface from the vertical is a common occurrence that has not yet been adequately addressed. By performing experiments on 10 cm wide by 20 cm tall fuel samples it was found that the maximum flame-spread rate, occurring nearly in a vertical configuration, does not correspond to the maximum fuel mass-loss rate, which occurs closer to a horizontal configuration. A detailed study of both flame spread and steady burning at different angles of inclination revealed the influence of buoyancy-induced flows in modifying heat-flux profiles ahead of the flame front, which control flame spread, and in affecting the heat flux to the burning surface of the fuel, which controls fuel mass-loss rates.     

The diffusion flame structure of an ammonium perchlorate based composite propellant at elevated pressures

Examination of the surface behavior and flame structure of a bimodal ammonium perchlorate (AP) composite propellant at elevated pressure was performed using high speed (5 kHz) planar laser-induced fluorescence (PLIF) from 1 to 12 atm and visible surface imaging spanning 1–20 atm. The dynamics of the combustion of single, coarse AP crystals were resolved using these techniques. It was found that the ignition delay time for individual AP crystals contributed significant to the particle lifetime only at pressures below about 6 atm. In situ AP crystal burning rates were found to be higher than rates reported for pure AP deflagration studies. The flame structure was studied by exciting OH molecules in the gas phase. Two types of diffusion flames were observed above the composite propellant: jet-like flames and v-shaped, inverted, overventilated, flames (IOF) lifted off the surface. While jet-like diffusion flames have been imaged at low pressures and simulated by models, the lifted IOFs have not been previously reported or predicted. The causes for the observed flame structures are explained by drawing on an understanding of the surface topography and disparities in the burning rates of the fuel and oxidizer.     

Experimental study of burning rates of cardboard box fires near sea level and at high altitude

To study the difference of solid fuel fire characteristics at different altitudes, two series of fire experiments of cardboard boxes filled with shredded office paper were conducted separately in Lhasa (altitude: 3650 m; air pressure: 65 kPa) and Hefei (altitude: 24 m, air pressure: 100.8 kPa), using a specially designed igniter. The measured parameters in the experiments include mass loss and flame axis temperature. Fuel load quantity and configuration were varied in the experiments. The results of the study indicate a likelihood that an ignition will result in smoldering fire at the high altitude. There is also likelihood that incipient phase may occur before the onset of full flaming combustion regardless of the altitude. The fuel mass loss fraction of flaming fires was found to follow a simple form of correlation with time when normalized over the half fuel consumption time, which was found to be inversely proportional to four-third power of the ambient pressure.     

Reaction propagation modes in millimeter-scale tubes for ethylene/oxygen mixtures

Effects of tube diameter and equivalence ratio on reaction front propagations of ethylene/oxygen mixtures in capillary tubes were experimentally analyzed using high speed cinematography. The inner diameters of the tubes investigated were 0.5, 1, 2 and 3 mm. The flame was ignited at the center of the 1.5 m long smooth tube under ambient pressure and temperature before propagated towards the exits in the opposite directions. A total of five reaction propagation scenarios, including deflagration-to-detonation transition followed by steady detonation wave transmission (DDT/C–J detonation), oscillating flame, steady deflagration, galloping detonation and quenching flame, were identified. DDT/C–J detonation mode was observed for all tubes for equivalence ratios in the vicinity of stoichiometry. The velocity for the steady detonation wave propagation was approximately Chapman–Jouguet velocity for 1, 2, and 3 mm I.D. tubes; however, a velocity deficit of 5% was found for the case in 0.5 mm I.D. tube. For leaner mixtures, an oscillating flame mode was found for tubes with diameters of 1 to 3 mm, and the reaction front travelled in a steady deflagrative flame mode with velocities around 2–3 m/s when the mixture equivalence ratio becomes even leaner. Galloping detonation wave propagation was the dominant mode for the fuel lean regime in the 0.5 mm I.D. tube. For rich mixtures beyond the detonation limits, a fast flame followed by flame quenching was observed.     

Role of substrate,temperature, and hydrogen on the flame synthesis of graphene films

Graphene films are grown in open-atmosphere on metal substrates using a multiple inverse-diffusion flame burner with methane as fuel. Substrate material (i.e. copper, nickel, cobalt, iron, and copper–nickel alloy), along with its temperature and hydrogen treatment, strongly impacts the quality and uniformity of the graphene films. The growth of few-layer graphene (FLG) occurs in the temperature range 750–950 °C for copper and 600–850 °C for nickel and cobalt. For iron, the growth of graphene is not exclusively observed. The variation of graphene quality for different substrates is believed to be due primarily to the difference in carbon solubility between the metals.     

Evolution of flame to surface heat flux during upward flame spread on poly(methyl methacrylate)

The heat feedback profile across 5 cm wide and 15 cm tall samples of poly(methyl methacrylate) was studied from ignition until total sample involvement as a flame spread vertically upward. Incident heat flux to a water-cooled gauge was measured at 1 cm intervals. At 6–15 cm above the bottom edge of the flame, the maximum heat flux value was found to be on the order of 35 kW m^?2. Lower in the sample, 2–5 cm above the flame bottom, where the flame is thinner and thus closer to the sample’s surface, the maximum heat flux is slightly higher, about 40 kW m^?2. Using these results and finely resolved measurements of sample burning rate recorded throughout the length of experiments, an analytical model that accurately predicts the measured heat flux profile along the vertical dimension of samples solely as a function of the burning rate was developed. Coupling this model with an accurate pyrolysis solver, which predicts material burning rate based on incident heat flux, is expected to enable highly accurate simulations of the flame spread dynamics.     

Reduction of flammability of ultrahigh-molecular-weight polyethylene by using triphenyl phosphate additives

The mechanism of reducing the flammability of ultrahigh-molecular-weight polyethylene (UHMWPE) with triphenyl phosphate (TPP) additives was investigated, using the methods of molecular-beam mass spectrometry (MBMS), differential mass spectrometric thermal analysis (DMSTA), thermocouple, thermogravimetry (TGA), and gas chromatography mass spectrometry (GC/MS). Kinetics of thermal degradation of pure UHMWPE and of that mixed with TPP was studied at high (～150 K/s) and low (0.17 K/s) heating rates at atmospheric pressure. Effective values of the rate constants of the thermal degradation reaction were determined. Times of ignition delay, the limiting oxygen index, the burning rates of UHMWPE and UHMWPE + TPP and their temperature profiles in the flames were measured. The flame structure was investigated and the composition of the combustion products in the flame zone adjacent to the specimen’s combustion surface. TPP vapors in flame were found. Addition of TPP to UHMWPE was found to result in reduction of polymer flammability. TPP was shown to act as flame retardant both in the condensed and gas phases.     

Experimental investigation of partially premixed,highly-diluted dimethyl ether flames at low temperatures

Atmospheric-pressure highly-diluted laminar dimethyl ether (DME)–oxygen flames with temperatures below 1500 K were stabilized for the first time on a specially designed burner which allows preheating of the gas streams of fuel and oxidizer. With regard to the partially premixed structure of these flames which contain up to 90% argon in the unburnt gases, molecular-beam mass spectrometry (MBMS) with electron ionization (EI) was used to investigate chemical species profiles of reactants, intermediates, and products at a series of lateral positions and as function of distance from the burner. The flame structure reveals a near one-dimensional behavior at the flame front and beyond, towards the burnt gas. In a systematic approach, combustion parameters including stoichiometry, dilution, and gas preheating temperature were varied. The partial premixing effects upon the flame were revealed by comparing the distribution of flame species in a full two-dimensional concentration field above the burner, which is a starting point to model such flames in further studies. Formaldehyde and the methyl radical as two prominent species in the combustion process of DME were used to discuss characteristics of both high- and low-temperature kinetics.     

Extinguishment of propane/air co-flowing diffusion flames by fine water droplets

In the present study, extinguishment of propane/air co-flowing diffusion flame by fine water droplets was investigated experimentally. Water droplets are generated by piezoelectric atomizers with the maximum droplets flow rate of 1500 ml/h. When the fuel injection velocity U_f is low, an attached laminar diffusion flame with a premixed flame at the base is stabilized. At some distance from the burner rim, a transition from laminar to turbulent diffusion flame occurs, and a turbulent diffusion flame is formed in the downstream region. When the fuel injector rim is thin (δ = 0.5 mm), the flame stability deteriorates with increase of the co-flowing air stream velocity U_a and the water droplets flow rate Q_m. The stability mechanism can be explained by the balance of the gas velocity and the burning velocity of premixed flame formed at the base. However, when the injector rim is thick (δ = 5 mm), a recirculation zone is produced downstream of the injector rim. The dependence of the quenching distance H_q on U_f and Q_m is relatively weak, and the stability diagram shows curious features. It was shown that U_a is crucially important since it determines flow residence time; if U_a < 0.4 m/s, water droplets can evaporate when they go by the recirculation zone, and the water vapor can diffuse into the recirculation zone. However, if U_a > 0.4 m/s, the water droplets should pass by the recirculation zone without sufficiently evaporated and are not so effective to extinguish the flame. The supply velocity of droplet-laden air should be low enough so that water droplets can evaporate and water vapor can diffuse into the premixed region at the base to obtain sufficient effectiveness of water droplets for fire suppression.     

2D soot concentration and burning rate of a vertical PMMA slab using Laser-Induced Incandescence

In the field of fire studies, it is interesting to provide useful data for the validation of soot production and radiation models. 2D soot concentration in the flame and burning rate of the solid surface have been determined in the case of the combustion of a vertical PMMA slab. The local soot concentration has been measured with the Laser-Induced Incandescence method. This one has been calibrated with in situ extinction measurements performed simultaneously (at 1064 nm). The interference signals of LII caused by laser scattering and Laser-Induced fluorescence have been considered and eliminated by a well suited detection. The flat field effect caused by the ICCD camera has also been corrected. The trapping effect on the LII signal has also been considered. The flame grows on the slab after the ignition, and after 1500 s a steady state of combustion appears. During this period, the soot profiles in the boundary layer have been measured at two heights in the flame and their main features will be discussed. It has been possible to determine the burning rate of the PMMA slab from the observation of the displacement of soot profiles in the camera field. The values at both heights are respectively 5.55 and 6.95 g/s/m². These values will be compared with results obtained in other studies.     

Study of the competing chemical reactions in the initiation and spread of smouldering combustion in peat

Smouldering combustion of natural fuel layers such as peatlands leads to the largest fires on Earth and posses a possible positive feedback mechanism to climate change. In this paper, we use an experimental methodology to study the smouldering combustion of samples of peat under a wide range burning conditions. Vertical samples (30 mm deep and 125 mm in diameter) are ignited by radiation on the top free surface and the smouldering front propagates downward against a forced flow of oxidizer. By varying the oxygen concentration ([O₂]) and the ignition conditions we investigate the competing pyrolysis and oxidation reactions. A reaction framework with two regimes is consistently observed. The measurements show that a char species is formed by the competing pyrolysis and oxidation reactions in the first regime resulting in net char production and in the second regime char oxidation results in conversion of the char to ash. Lower mass loss rates and the larger residual mass at lower [O₂] suggest that a wider smouldering front is required to sustain combustion as the [O₂] is decreased. These results improve our understanding of smouldering phenomena and the role of the competing chemical reactions.     

Effect of pressure on structure and extinction of near-limit hydrogen counterflow diffusion flames

Results of measurements of critical conditions for extinction and of temperature profiles in counterflow diffusion flames are reported. The fuel was a hydrogen–nitrogen mixture with 14 mole percent hydrogen, and the oxidizer was air. Pressures ranged from 0.1 MPa to 1.5 MPa; measurements were made in a facility especially constructed for carrying out counterflow combustion experiments at high pressures. With increasing pressure, the strain rate at extinction first increases and then decreases, in qualitative agreement with predictions, but there are observable quantitative differences. Temperature profiles, obtained employing an R-type thermocouple at a fixed strain rate of 100/s, agree well with predictions, within experimental uncertainty. The results may help to improve knowledge of underlying chemical-kinetic and transport parameters at elevated pressures.     

A high temperature and atmospheric pressure experimental and detailed chemical kinetic modelling study of 2-methyl furan oxidation

An experimental ignition delay time study for the promising biofuel 2-methyl furan (2MF) was performed at equivalence ratios of 0.5, 1.0 and 2.0 for mixtures of 1% fuel in argon in the temperature range 1200–1800 K at atmospheric pressure. Laminar burning velocities were determined using the heat-flux method for mixtures of 2MF in air at equivalence ratios of 0.55–1.65, initial temperatures of 298–398 K and atmospheric pressure. A detailed chemical kinetic mechanism consisting of 2059 reactions and 391 species has been constructed to describe the oxidation of 2MF and is used to simulate experiment. Accurate reproduction of the experimental data has been obtained over all conditions with the developed mechanism. Rate of production and sensitivity analyses have been carried out to identify important consumption pathways of the fuel and key kinetic parameters under these conditions. The reactions of hydrogen atom with the fuel are highlighted as important under all experimental conditions studied, with abstraction by the hydrogen atom promoting reactivity and hydrogen atom addition to the furan ring inhibiting reactivity. This work, to the authors knowledge, is the first to combine theoretical and experimental work to describe the oxidation of any of the alkylated furans. The mechanism developed herein to describe 2MF combustion should also function as a sub-mechanism to describe the oxidation of 2,5-dimethyl furan whilst also providing key insights into the oxidation of this similar biofuel candidate.     

Combustion wave propagation in mixtures of JSC-1A lunar regolith simulant with magnesium

Combustion of lunar regolith mixed with energetic additives is a potential method for production of construction materials in future moon missions. Recently, self-sustained combustion in the mixtures of JSC-1A lunar regolith and magnesium has been demonstrated. However, the concentration of magnesium in those mixtures was as high as 26 wt%. Note that magnesium must be either transported from Earth or recovered from lunar minerals or used structures. The present paper focuses on the minimization of magnesium content in JSC-1A/Mg mixtures. The mixtures were compacted into pellets and ignited in argon environment. Initial attempts to decrease magnesium concentration resulted in the observations of a spinning combustion wave at 23 wt% Mg. The observed spin combustion involved periodical motion of two counterpropagating hot spots along a helical path on the sample surface. These observations, including features such as formation of a faster hot spot after collision of the counterpropagating spots, confirm theoretical predictions for spin combustion in solid–solid mixtures. High-energy mechanical milling of JSC-1A in a planetary ball mill significantly increased its reactivity and improved combustion of its mixtures with magnesium. Mixtures of the obtained powder (the median diameter of about 3 μm) with 26 wt% Mg exhibit easy ignition and vigorous combustion. The minimum concentration of magnesium required for self-sustained propagation of a planar combustion front is as low as 13 wt%.     

Flame spread along a thin solid randomly distributed combustible and noncombustible areas

Flame spread route in fire strongly depends on distribution of combustible materials. Two types of scenario are considered in flame spread when combustible materials randomly distributed; one case is that flame spreads and combustible materials burn out, and the other case is that flame self-extinguishes on the way. The threshold of burning out or self-extinguishing may be determined by quantity of combustible materials and their placement in space. Our objectives are to clarify the characteristics and threshold of flame spread. In this paper, we examine non-uniform flame spread in open air along a thin combustible solid with randomly distributed pores, which are considered as noncombustible space. Experimental results show that the flame spread rate for S ≦ 1 (S ≡ d/L_h, S: scale ratio, d: pore-scale, L_h: pre-heat length ahead of flame leading edge measured by using a shadowgraph method) increases with increasing the porosity and reaches maximum value approximately at 20–30% of porosity, while the flame spread rate for S > 1 is almost constant. Over 40% of porosity, the flame spread rate for both S ≦ 1 and S > 1 decreases. The flame cannot spread and completely self-extinguish over 60% of porosity independently with pore-scale and shape. The threshold of flame spread is related with the average-number of slit, N_s, which is made by connecting each pores. The N_s as the threshold of flame spread is unity for S > 1, while the modified average-number of slit (N_s × S) becomes two for S ≦ 1.     

Flame chemistry of tetrahydropyran as a model heteroatomic biofuel

The flame chemistry of tetrahydropyran (THP), a cyclic ether, has been examined using vacuum-ultraviolet (VUV)-photoionization molecular-beam mass spectrometry (PI-MBMS) and flame modeling, motivated by the need to understand and predict the combustion of oxygen-containing, biomass-derived fuels. Species identifications and mole-fraction profiles are presented for a fuel-rich (Φ = 1.75), laminar premixed THP–oxygen–argon flame at 2.66 kPa (20.0 Torr). Flame species with up to six heavy atoms have been detected. A detailed reaction set was developed for THP combustion that captures relevant features of the THP flame quite well, allowing analysis of the dominant kinetic pathways for THP combustion. Necessary rate coefficients and transport parameters were calculated or were estimated by analogies with a recent reaction set [Li et al., Combust. Flame 158 (2011) 2077–2089], and necessary thermochemical properties were computed using the CBS-QB3 method. Our results show that under the low-pressure conditions, THP destruction is dominated by H-abstraction, and the three resulting THP-yl radicals decompose primarily by β-scissions to two- and four-heavy-atom species that are generally destroyed by β-scission, abstraction, or oxidation.