Ignition Limit Research Articles

E112 UMGT用実機型水素燃料超小型燃焼器の燃焼特性(OS1 マイクロエネルギー変換)

An engineering-model of hydrogen-fueled flat-flame ultra-micro combustor with a combustion chamber volume of 69.4 mm^3 was developed. To obtain high overall heat-insulation, heat-resistant and strength, the engineering-model combustor had a triple layer structure with an advanced ceramic, a heat insulation material and a stainless steel. The burning test of the combustor showed that it was easy to ignite and burn with a stable flame. Both the stable flame region and the ignition limit were enough wide to satisfy the design operation point of the combustor. The engineering-model combustor attained high combustion efficiency and high space heating rate of 7300 MW(m^3・MPa) with pressure loss around 5%. NOx concentration of the combustor was enough low of less than 3 ppm. While the combustor had good combustion characteristics, the heat loss ratio was high of 51 % of the heat release and the combustor had a large heat capacity. The heat transfer at the exit slit must be reduced and the combustor wall must be thinner.

The rules governing hydrogen combustion close to the lower ignition limit in the kinetic region of chain termination

The kinetic scheme of hydrogen combustion near the lower self-ignition limit in the kinetic region can be augmented by the reactions of bimolecular nonlinear termination O + OH → H + O2 and heterogeneous propagation of chains with the participation of adsorbed hydrogen atoms Hs. The literature data on the experimental determination of the probability of hydrogen atom trapping by the surface of quartz were analyzed.

A theoretical study of spontaneous ignition of fuel jets in an oxidizing ambient with emphasis on hydrogen jets

An analysis was performed for the spontaneous ignition of a hydrogen (or other gaseous fuel) jet emanating from a slot into an oxidizing ambient (e.g., air). A similarity solution of the flow field was obtained. This was combined with the species and energy conservation equations, which were solved using activation energy asymptotics. Limits of spontaneous ignition were identified as functions of slot width, flow rate, and temperatures of the hydrogen jet and ambient gas. Two scenarios are examined: a cool jet flowing into a hot ambient and a hot jet flowing into a cool ambient. For both scenarios, ignition is favored with an increase of either the ambient temperature or the hydrogen supply temperature. Moreover, for the hot ambient scenario, a decrease in fuel Lewis number also promotes ignition. The Lewis number of the oxidizer only has a weak effect on ignition. Because spontaneous ignition is very sensitive to temperature, ignition is expected to occur near the edge of the jet if the hydrogen is cooler than the ambient gas and near the centerline if the hydrogen is hotter than the ambient gas.

On the possibility of a thermal explosion initiated by a heterogeneous reaction between H2 and O2

The challenge of the authors of the present paper is to respond to the arguments of the above-published critical review (in what follows, it is indicated by [CR]). In the paper presented, the authors consider surfaceinitiated ignition of the oxyhydrogen gas at the first and third limits; flame propagation, detonation, and combustion of hydrocarbons are not considered. The authors [CR] develop a concept of initiation of the explosion of the oxyhydrogen gas at the third limit by branching chains and consider heat as a secondary factor, which accelerates combustion only at the second stage of the process. The authors of the present paper and [3] develop Semenov’s concept [1, 2], which implies that usually the primary factor acting at the third limit is heat. In this case, the explosion occurs as a result of decomposition of hydrogen peroxide whose main amount is formed above the second limit in the chain reaction with no branching chains [1, 2, 4, 5]. The branching factor φ0 in this region is negative [1, 2], and the fraction of branching chains is smaller than unity. Semenov considered that the chain-branching third limit is theoretically possible but is little probable in practice. Thus, in [6] (see also [2, p. 587]), Semenov criticized the paper of Akulov on the third limit in the hydrogen sulfide oxidation reaction and wrote about the oxyhydrogen gas explosion: “The third chain limit of ignition is sometimes possible, as was demonstrated by Voevodskii. We can see, however, that it arises only as an exception, under extremely specific conditions of the reaction, and only for the reaction of a rarely encountered chain mechanism. The reason for the widespread character of the third limit is the inevitability of the thermal explosion at sufficiently high pressures. The third limit usually has the thermal nature.”

High-pressure ethylene oxidation behind reflected shock waves

Oxidation of ethylene/air mixtures has been investigated behind reflected shock waves in a shock tube of 76 mm in diameter. Experiments were performed within the temperature range of 1060–1520 K, pressures of 5.9–16.5 atm, and stoichiometries of ϕ = 0.5, 1.0, and 2.0. Emissions of OH (308.9 nm), CH (431.5 nm) and C 2 (516.5 nm) molecules, pressures and ion current records were implemented to measure ignition times of the mixture along the centreline of the tube and in the boundary layer. Empirical correlations for ethylene ignition times have been deduced from the experimental data. Auto-ignition modes (strong, transient and weak) and ignition limits of the mixtures were identified comparing velocities of reflected shock wave and reaction front at different locations from the reflecting wall. Extensive database for validations of high-temperature ethylene reaction mechanism and numerical methods for reaction flow simulations has been obtained from experimental observations.

A numerical study of transient ignition and flame characteristics of diluted hydrogen versus heated air in counterflow

Combined experimental and numerical studies of the transient response of ignition to strained flows require a well-characterized ignition trigger. Laser deposition of a small radical pool provides a reliable method for initiating ignition of mixtures that are near the ignition limit. Two-dimensional direct numerical simulations are used to quantify the sensitivity of ignition kernel formation and subsequent edge-flame propagation to the oxidizer temperature and the initial width and amplitude of O-atom deposition used to trigger ignition in an axisymmetric counterflow of heated air versus ambient hydrogen/nitrogen. The ignition delay and super-equilibrium OH concentration in the nascent ignition kernel are highly sensitive to variations in these initial conditions. The ignition delay decreases as the amplitude of the initial O-atom deposition increases. The spatial distribution and the magnitude of the OH overshoot are governed by multi-dimensional effects. The degree of OH overshoot near the burner centerline increases as the diameter of the initial O-atom deposition region decreases. This result is attributed to preferential diffusion of hydrogen in the highly curved leading portion of the edge flame that is established following thermal runaway. The edge-flame speed and OH overshoot at the leading edge of the edge flame are relatively insensitive to variations in the initial conditions of the ignition. The steady edge-flame speed is approximately twice the corresponding laminar flame speed. The rate at which the edge flame approaches its steady state is insensitive to the initial conditions and depends solely on the diffusion time scale at the edge flame. The edge flame is curved toward the heated oxidizer stream as a result of differences in the chemical kinetics between the leading edge and the trailing diffusion flame. The structure of the highly diluted diffusion flame considered in this study corresponds to Liñán's ‘premixed flame regime’ in which only the oxidizer leaks through the reaction zone such that the flame is located at fuel lean rather than stoichiometric mixture fraction conditions.

Microgravity opposed-flow flame spread in polyvinyl chloride tubes

The effects of gravity on opposed-flow flame spread in a confined geometry were investigated experimentally in the 2.2-s drop tower at the NASA Glenn Research Center. Pure oxygen flowed through samples of 0.64-cm-inner-diameter polyvinyl chloride (PVC) tubing held either horizontally or vertically in a combustion chamber filled with nitrogen. The sample was ignited in normal gravity with a hot wire, and once a flame was established, the apparatus was dropped to observe microgravity effects. Flame spread rate was measured in normal and microgravity at pressures of 1.0 and 0.5 atm. A low-flow ignition limit was observed at an opposed-flow velocity of 1.36 cm/s, at which point the horizontal, vertical, and microgravity flame spread rates were 0.40, 0.30, and 0.16 cm/s, respectively. For flow velocities above approximately 5.2 cm/s, there was no difference in the flame spread rates for normal and microgravity and the flame spread rate increased with a nearly square root dependence with respect to opposed-flow velocity. Buoyant flow velocities of 2.5 and 1.5 cm/s were estimated for horizontal and vertical flames, respectively. Vertical tests conducted at 0.5 atm pressure demonstrated no difference in flame spread rate between normal and microgravity. These results suggest that the fire risk associated with the use of PVC tubes during general anesthesia in either space or ground applications may be reduced if the application of a high-energy surgical tool is prevented during an active phase of the breathing cycle (inhale or exhale).

A Comparison Between the Combustion of Natural Gas and Partially Reformed Natural Gas in an Atmospheric Lean Premixed Turbine-Type Combustor

A small-scale combustor was set up to analyze the combustion of natural gas and two mixtures of partially reformed natural gas. The partially reformed mixtures can be formed using biomass to feed the endothermic reforming reactions. Before combusting these mixtures in a gas turbine, experimental work was done on a primary zone combustion chamber to examine the combustor behavior when switching from natural gas to the wet and dry hydrogen-rich mixtures. Temperature profiles, flame location and ignition limits have been investigated for a variety of stoichiometries and several air temperatures. Possible problems concerning blow-off, flashback, increased pollutant products and excessive liner wall temperatures were analyzed. It was concluded that the switch in operation from natural gas to these wet and/or dry partially reformed natural gas mixtures lowers the blow-off limits while maintaining similar liner wall temperature profiles. Furthermore, no significant changes in pollutant production were observed. Flame area, shape and position display considerable differences in combustion regime for the three tested fuel types.

Fundamental Combustion Properties of H2/CO Mixtures: Ignition and Flame Propagation at Elevated Pressures

Because of the current interest in the use of syngas in gas turbine applications, this article provides an overview of the fundamental combustion characteristics of H2/CO mixtures, with emphasis on ignition and flame propagation at high pressures. Specifically, the effects of pressure on the fundamental combustion properties, including auto-ignition delays, forced ignition limits, and laminar flame speeds, are demonstrated and discussed through several recent high-pressure experiments. The controlling reactions in ignition and flame propagation are identified. Through comparison of the newly obtained high-pressure experimental data and the computational results based on existing kinetic models, discrepancies in the rate parameters for the sensitive reactions are also discussed.

H2O2-케로신 로켓을 초기 가속장치로 갖는 새로운 램젯 추진기관

본 연구에서는 RBCC (Rocket Based Combined Cycle)엔진이나 기존 램제트 추진기관의 초기 추력 제공에 과산화수소 가스발생기를 이용하는 새로운 추진시스템을 제안하였고, 기초 연구 수행으로서 촉매 분해된 과산화수소 제트에 케로신을 분사하여 자연발화 및 연소 특성을 연구하였다. 과산화수소는 촉매 베드를 통하여 분해된 후 축소노즐을 통해 연소실로 분사됐으며 이 제트에 인젝터를 통하여 수직으로 케로신을 액상으로 분무하였다. 연소실내에서의 온도와 압력을 측정하여 점화를 확인하고 자연발화 특성을 조사하였다. 400°C의 연소실 온도와 연료와 산화제 혼합비 0.6이상에서 자연발화와 안정적인 연소가 가능하였다. 이 결과를 통하여 램제트의 새로운 초기 가속장치의 가능성을 확인할 수 있었다. New concept ramjet propulsion system with liquid bipropellant rocket using "Green Propellant" hydrogen peroxide for launch stage is proposed. In this novel concept, hydrogen peroxide gas generator produces hot oxygen at launch stage and kerosene injects to this jet in combustor. For basic study of this new concept ramjet system, investigation of auto-ignition characteristics and combustion of decomposed hydrogen peroxide and kerosene was conducted. In various test cases, auto-ignition and stable combustion was verified. The combustion temperature of 400°C and Fuel/Oxidizer mixture ratio of 0.6 were the limit of auto ignition. Through the experiment results, the possibility of novel concept combined propulsion system using hydrogen peroxide gas generator is ascertained.

Different effects of active minor admixtures on hydrogen and methane ignitions

The methane combustion inhibitor CCl4 exerts no effect on the first ignition limit of hydrogen; therefore, the role of hydrogen atoms in hydrocarbon oxidation consists at least of participating in longer reaction chains than are observed in hydrogen oxidation. The upper limits of the rate constants of the reactions of hydrogen atoms with propylene and isobutylene molecules were estimated by the self-ignition limit method to be (1.0 ± 0.3) × 10−11 exp(−1450 ± 400/T) and (0.8 ± 0.3) × 10−11 exp(−550 ± 200/T) cm3 molecule−1 s−1, respectively, in the temperature range of 840–950 K. These data are evidence that the stronger inductive effect of the two methyl groups in isobutylene lowers the energy barrier to the H + iso-C4H8 reaction. It has been demonstrated experimentally that chemiluminescence in the hydrocarbon flame front at atmospheric pressure precedes heat evolution. Throughout the pressure and temperature ranges examined (5–750 Torr, 298–950 K), the chain mechanism determines the basic laws of combustion.

Evaluation of SCCI Potentials in Comparison to HCCI and Conventional DICI Combustion Using n-Heptane

This work investigates the potential of stratified charge compression ignition (SCCI) to extend the high-load limits of homogeneous charge compression ignition (HCCI) engine. Engine experiments were conducted on a single-cylinder diesel engine equipped with an electrical port fuel injection system. When the n-heptane fuel amount of port injection and in-cylinder direct injection was calibrated, partial fuel stratification was created and three different combustion modes as SCCI, HCCI, and direct injection compression ignition (DICI) were achieved. The premixed ratio (rp) was used to indicate the partial fuel stratification, and the effects of premixed ratio on SCCI combustion were examined. It was found that SCCI could achieve a staged combustion event of reduced pressure-rise rates, thus offering good potential to expand the operating range of the HCCI engine. Also, the engine performance and emission characteristics of SCCI were evaluated, focusing on the comparison with the HCCI and conventional DICI combustion. The results show that, with suitable partial fuel stratification, SCCI has the capability to achieve lower CO and HC emissions in comparison to HCCI and keep the NOx emissions on a low level within 50 ppm while maintaining good efficiency equivalent to DICI.

Ignition and combustion characteristics of the gas turbine slinger combustor

This paper describes the ignition and combustion characteristics of a gas turbine slinger combustor with rotating fuel injection system. An ignition test was performed under various airflow, temperature and pressure conditions with fuel nozzle rotational speed. From the test, there are two major factors influencing the ignition limits: the rotational speed of the fuel nozzle, and the mass flow parameter. Better ignition capability could be attained through increasing the rotational speed and air mass flow. From the spray visualization and drop size measurement, it was verified that there is a strong correlation between ignition performance and drop size distribution. Also, we performed a combustion test to determine the effects of rotational speed by measuring gas temperature and emission. The combustion efficiency was smoothly enhanced from 99% to 99.6% with increasing rotational speed. The measured pattern factor was 15% and profile factor was 3%.

The relative contributions of self-heating and chain branching to the kinetics of hydrogen oxidation outside the ignition region close to the third limit

It is shown that, in hydrogen-oxygen mixtures, under the conditions when the rate of reaction chain termination is higher than the rate of chain branching, the characteristic reaction time is hundreds of seconds even in the immediate vicinity of the third ignition limit. The self-heating of the mixtures therefore does not exceed several degrees and, contrary to the accepted views, cannot result in thermal combustion. Ignition observed in the third limit region is caused by a higher rate of chain branching compared with chain termination; that is, it is a chain reaction. Self-heating only becomes substantial during the development of chain combustion.

Chemical heating of the working gas in a short-duration high-enthalpy facility

Operation of a short-duration high-enthalpy facility with products of reactions of propane, hydrogen, and nitrous oxide with air and oxygen is studied. The mixtures are ignited either by an electric arc or by an igniter with a “chamber-plume” principle of operation, which involves local ignition of the mixture from one spot located at the center of the settling chamber. It is confirmed that the use of a combined (electrical and chemical) heating of the working gas completely resolves the problem of reproduction of real values of stagnation pressure and enthalpy for flight conditions of a vehicle with Mach numbers of 4 to 7. The lean flammability limits of ignition by an electric arc turn out to be lower than the data reported in the literature for standard conditions. Combustion of three-species (fuel + oxygen + air) mixtures with hydrogen (mole fraction of hydrogen $$r_{H_2 } > 0.1$$ ) and mixtures with propane $$(r_{C_3 H_8 } > 0.027)$$ is completed in 5–8 msec with combustion efficiency close to unity. The time needed for chemical reactions initiated by an igniter to be completed is significantly greater than that with ignition by an electric arc. The time of combustion with the use of an igniter can be substantially reduced by increasing the number of ignition spots, by using mixtures with a greater fraction of the fuel than that required for reproduction of natural enthalpy, or by replacing a certain part of the fuel in the original mixture by nitrous oxide.

Spark ignition of turbulent nonpremixed bluff-body flames

Spark ignition of turbulent nonpremixed bluff-body flames with radial fuel injection for different fuel and air bulk velocities and swirl conditions has been investigated in terms of stability and ignitability limits and ignition probability ( P ign , defined as the probability of igniting the whole flame from a localized 2-mm spark). The velocity field and the mixture fraction distributions have been measured with laser Doppler velocimetry and planar laser-induced fluorescence (PLIF) of acetone, respectively. Successful and failed ignition events have been visualized by high-speed and OH PLIF imaging. The results show that the stability limits are much wider than the ignitability limits, but with swirl, they become closer. The mixture fraction distribution shows high sensitivity to any change in the fuel and air velocities, with the mixture inside the central recirculation zone (CRZ) tending to be relatively well-mixed and rich. Visualization of different spark realizations shows that successful ignition events manage to initiate a flame at the separation corner of the bluff body and that the direction of initial flame propagation depends more on the ignition location than on the flow conditions due to the convection of the flame kernel by the mean flow. With successful spark events inside the CRZ the flame expands in all directions, up to a certain point, when it starts forming the shape of the stable flame. The highest P ign was close to the CRZ boundaries with low P ign or no ignition inside CRZ when the mixture there was rich. Under conditions where the CRZ is not close to the rich flammability limit, P ign is substantial. Downstream of the CRZ, P ign is uniform but low due to the adverse velocity. The probability of just initiating a kernel is higher than P ign , especially, in the area above the CRZ. With swirl, the highest P ign is at the burner exit, with no ignition inside the CRZ because the mixture is too lean. The data highlight the difference between P ign and the probability of kernel initiation and the flammability factor F (the probability of finding a mixture within the nominal flammability limits) and emphasize that flame propagation in inhomogeneous mixtures must be captured by any model that aims to simulate ignition of nonpremixed flame burners.

The effect of nonequilibrium excitation on the ignition of hydrogen-oxygen mixtures

The part played by various radicals and excited particles in reducing the induction period is discussed when analyzing the ignition of combustible mixtures under highly nonequilibrium conditions. A review is made of experimental and theoretical investigations of the effect made by atoms of hydrogen and oxygen, electron-excited molecules of O2(a 1Δg), and vibrationally excited molecules of H2 and O2 on the times of induction and on the shift of the limits of ignition of hydrogen-oxygen mixtures. The addition of atoms of oxygen and hydrogen to a combustible mixture may cause a significant reduction of the times of induction and the lowering of the temperature limit of ignition of combustible mixtures. However, the latter effect is observed only in the vicinity of the limit of ignition. In the zone of relatively low initial temperatures, the shift of the ignition limit is largely associated with the self-heating of mixture owing to recombination of atomic particles being added. In so doing, the nonequilibrium pattern of the impact hardly shows up. Molecules of singlet oxygen (SO) may be involved in both chain initiation reactions and chain branching reactions. The rates of these processes increase significantly with the temperature of the mixture. However, the temperature increase further causes an increase in the rate of reaction of hydrogen quenching of singlet oxygen O2(a 1Δg). The acceleration of deactivation of O2(a 1Δg) with increasing temperature, which was previously ignored by researchers, causes an abrupt reduction of the effectiveness of the impact of SO on the times of induction. In the low-temperature region, where the quenching is relatively weak, the effect of O2(a 1Δg) on the ignition of H2 : O2 mixtures may be significant in the case of a sufficiently high (in excess of 10−2) level of relative concentrations of SO. Vibrationally excited molecules of H2(v) may likewise be involved in chain initiation and branching reactions. The available results lead one to assume the possibility of lowering the temperature limit of ignition of hydrogen-oxygen mixtures in the case of intensive excitation of vibrational degrees of freedom of H2(v) molecules. However, in the case of electric-discharge excitation (even in H2 : O2 mixtures, in which a significant fraction of discharge energy may be delivered to the vibrational subsystem of H2(v)), it is quite difficult to attain high levels of vibrational excitation (T v H2 ≥ 2000 K). In a stoichiometric mixture of H2:air, where the bulk of discharge energy is spent for vibrational excitation of nitrogen molecules, this problem becomes even more complicated.

Numerical investigation of wall heat conduction effects on catalytic combustion in split and continuous monolith tubes

The optimum length of a monolith tube is one for which near-hundred percent conversion is attained, and at the same time, the catalyst over the entire length of the tube is utilized. In practice, the length is adjusted by stacking monolith plugs end-to-end. In this study, the repercussions of such a practice are investigated numerically with the goal to determine if a tube of length 2 L demonstrates the same behavior as two tubes of length L each, stacked end-to-end. Catalytic combustion of methane–air mixture on a platinum catalyst is considered. The studies are conducted using a multi-step reaction mechanism involving 24 surface reactions between 19 species. Two different materials are considered for the walls of the monolith tube, namely silicon carbide and cordierite. Both steady state and transient simulations are performed. Results indicate that the ignition and blowout limits can be significantly different between split and continuous tubes when the wall is made up of a high thermal conductivity material, such as silicon carbide. For steady state combustion, for both wall materials, the point of attachment of the flame to the wall is altered by splitting the tube—the effect being more pronounced for silicon carbide and at relatively high Reynolds numbers. These results imply that axial heat conduction, or lack thereof due to thermal contact resistance, through the walls of the monolith results in thermal non-equilibrium between the solid and fluid phase, and subsequently affects ignition and flame stability in catalytic combustion.

Heterogeneous stages in the below-ignition-limit oxidation of hydrogen and carbon monoxide

It was demonstrated experimentally that, during the oxidation of hydrogen and carbon monoxide under stationary conditions (i.e., below the first ignition limit), the state of the reaction vessel surface experiences reversible changes, which, in turn, influence the kinetics of the gas-phase process. It was concluded that the reaction of molecular oxygen with hydrogen adatoms plays an important role in chain reactions below the first ignition limit.

Modelling of the effect of fuel reforming and EGR on the acceptable operating range of an ethanol HCCI engine

Homogeneous Charge Compression Ignition (HCCI) engines have great potential in ultra-low NOx emissions, high efficiency and low particulates. However, HCCI also has a narrow operating range with low power output. We therefore investigated the expansion of the Acceptable Operating Range (AOR) using fuel reforming complemented by Exhaust Gas Recirculation (EGR). The modelled HCCI engine is fuelled with ethanol of equivalence ratio (Φ) of 0.2, 0.4 and 0.5. The AOR must meet both the complete combustion and the maximum NOx limit (10 ppm). It is found that fuel reforming enhances combustion and extends the complete combustion limit to lower initial temperatures, but also increases NOx emissions. Since the NOx limit can be greatly extended by the complementary use of EGR, fuel reforming is not as effective as EGR in widening the operating range at the Φ's studied. Therefore, fuel reforming is most beneficial when used at lower initial temperatures near the ignition limit.