Ignition Limit Research Articles

Ignition Delay and Limit of Ignitability for Sodium Pool (Theory and Experimental Comparisons)

Relevant to sodium pool burning, ignition delay and ignitability are exmined in order to obtain appropriate parameters that can fairly correlate dominant system parameters influencing the ignition. For this aim, a theoretical work has first been conducted, being considered that the surface reaction can mainly exert influences. It is found that both the ignition delay time and the limit of ignitability are closely related to the pool size and oxygen concentration, as well as the initial sodium temperature. Specifically, for the ignition delay, a comprehensive parameter defined as the ignition delay time multiplied by oxygen concentration and divided by pool depth has been identified. For the limit of ignitability, it is found another comprehensive parameter defined as a product of oxgen concentration and representative length for the heat transfer. In order to evaluate appropriateness of the theory, experimental comparisons have been conducted, by use of experimental data in the literature. It is demonstrated that there exists a fair degree of agreement between the experimental and analytical results, as far as the trend and/or approximate magnitude are concemed, in spite of differences in experimental conditions and several assumptions done in the present analysis. Because of simple forms of the comprehensive parameters, they are expected to be useful in evaluating ignitability of sodium prior to pool burning in an oxidizing atmosphere. Furthemore, it is found that the increase in volume-to-surface ratio of the sodium pool prolongs the ignition delay, and that the decrease in pool diameter suppresses the ignition, both of which can be attributed to the reduced contribution of the surface reaction that can supply heat in the induction period.

213 HCCI機関の着火・ノック限界に及ぼすDME/天然ガス混合比率の影響(OS2-1,OS2 燃焼の基礎からエンジン燃焼の応用技術まで)

213 HCCI機関の着火・ノック限界に及ぼすDME/天然ガス混合比率の影響(OS2-1,OS2 燃焼の基礎からエンジン燃焼の応用技術まで)

Numerical Calculation of the Ignition Limit on Lean-Burn SI Engines

Numerical Calculation of the Ignition Limit on Lean-Burn SI Engines

Laser Ignition of Methane-Air Mixtures at High Pressures and Diagnostics

Methane-air mixtures at high fill pressures up to 30 bar and high temperatures up to 200°C were ignited in a high-pressure chamber with automated fill control by a 5 ns pulsed Nd:YAG laser at 1064 nm wavelength. Both, the minimum input laser pulse energy for ignition and the transmitted fraction of energy through the generated plasma were measured as a function of the air/fuel-equivalence ratio (λ). The lean-side ignition limit of methane-air mixtures was found to be λ=2.2. However, only λ<2.1 seems to be practically usable. As a comparison, the limit for conventional spark plug ignition of commercial natural gas engines is λ=1.8. Only with excessive efforts λ=2.0 can be spark ignited. The transmitted pulse shape through the laser-generated plasma was determined temporally as well as its dependence on input laser energy and properties of the specific gases interacting. For a first demonstration of the practical applicability of laser ignition, one cylinder of a 1 MW natural gas engine was ignited by a similar 5 ns pulsed Nd:YAG laser at 1064 nm. The engine worked successfully at λ=1.8 for a first test period of 100 hr without any interruption due to window fouling and other disturbances. Lowest values for NOx emission were achieved at λ=2.05 NOx=0.22 g/KWh. Three parameters obtained from accompanying spectroscopic measurements, namely, water absorbance, flame emission, and the gas inhomogeneity index have proven to be powerful tools to judge laser-induced ignition of methane-air mixtures. The following effects were determined by the absorption spectroscopic technique: formation of water in the vicinity of the laser spark (semi-quantitative); characterization of ignition (ignition delay, incomplete ignition, failed ignition); homogeneity of the gas phase in the vicinity of the ignition; and the progress of combustion.

Heterogeneous chain propagation in a rarefied oxyhydrogen flame

In the 1950s and 1960s, Semenov’s theory of branched chain processes had become capable of quantitatively describing hydrogen combustion [1, 2], but the theory fit the experimental data only in the depth of the ignition peninsula. As reported in [3], Semenov and colleagues observed [1] a hydrogen combustion rate exceeding the theoretical expectations, and Azatyan and Aleksandrov detected [4] finite “anomalously deep burnups” near the ignition limit, which were tens of times those calculated. In his Nobel lecture in 1956, Semenov formulated the concept of heterogeneous chain propagation [5]. Therefore, Semenov and Azatyan proposed [3] to seek the cause of the discrepancy between the theory and the experimental data among possible heterogeneous reactions. A figure in Semenov and Azatyan’s report [3] shows that the additional contribution of the hypothetical heterogeneous chain propagation to the gas-phase component of the process in terms of finite burnups was only 10‐12%. To explain the discrepancy, a mechanism was put forward in which HO 2 radicals form on the surface and react with hydrogen atoms to produce hydroxyl radicals leaving the surface for the gas phase. However, the case in which the reaction H + HO 2 yields not hydroxyl radicals but a hydrogen peroxide molecule leaving the surface for the gas phase is apparently more advantageous from an energy standpoint; therefore, the assumed processes of heterogeneous branching or noticeable propagation of chains are unlikely. The purpose of this study was to reveal and refine the contributions of various heterogeneous chain propagation mechanisms to the gas-phase component of the process using analysis and targeted experiments. Heterogeneous chain propagation can be experimentally studied only by highly sensitive methods. We used resonance fluorescence spectroscopy of rarefied flames, whose sensitivity threshold ( 10 8 cm —3 ) was suf

TEMPERATURE AND CO CONCENTRATION MEASUREMENTS IN A PARTIALLY PREMIXED CH4/AIR COFLOWING JET FLAME USING COHERENT ANTI-STOKES RAMAN SCATTERING

In a partially premixed methane/air flame, temperature and CO concentration measurements were conducted using coherent anti-Stokes Raman spectroscopy. Additionally, qualitative two-dimensional OH laser-induced fluorescence imaging was applied. Measurements were performed at different positions downstream of the burner's exit for equivalence ratios (φ) between 1.7 and 4.0. A distinct double-flame structure was observed. Accurate temperature and CO concentration results were achieved, even under sooting conditions. Temperature measurements close to the burner (5 mm downstream) indicated a temperature peak of 2000 K while further downstream at 10 and 20 mm a high-temperature plateau could be found. The CO concentration increases with decreasing φ starting below a particular threshold value. It was a maximum at the inner flame due to the formation of CO as a result of partial combustion. The maximum CO is found to be greater than that in a diffusion flame. The maximum CO concentration has been found for φ = 1.7, the upper ignition limit for CH4/air mixtures. OH measurements indicate that OH is mainly formed in the outer flame and is consumed there by reactions with CO and H2 diffusing from the inner flame.

Fundamentals of high-energy spark ignition with lasers

An experimental study of laser-induced spark ignition of flammable, gaseous premixtures is reported, with theoretical interpretations. Ignition was in an explosion bomb, equipped with four variable-speed fans that facilitated the study of quiescent and isotropic turbulent conditions. Good optical access enabled the progress of plasma fronts, shock waves, igniting kernels, and propagating flames to be recorded with high-speed schlieren photography. A focused beam from a Q-switched Nd:YAG laser initiated electrical breakdown, with plasma energies between 85 and 200 mJ. Probabilities of breakdown were found for air and isooctane–air mixtures over ranges of pressures and temperatures. Blast-wave theory applied to shock-wave trajectories enabled initial plasma conditions to be inferred. This suggested electron temperatures of over 10 5 K and very high pressures. Calculated values of the absorption coefficient for the laser beam energy show these plasma properties to be commensurate with the observed energy and size. The ensuing rarefaction wave creates toroidal rings at the leading and trailing edges of the plasma. The former decays more rapidly and a third lobe of the kernel is generated that moves towards the laser. In flammable mixtures this enhances the flame spread. Laminar flame speeds are overdriven by this gasdynamic effect, as well as by the high energy of the plasma, to such an extent that the flame speed decays from elevated values as the flame stretch decreases, contrary to the increases that occur with normal flames with positive Markstein numbers. The extent to which turbulence narrows the ignition limits is found experimentally. For mixtures close to the lean flammability limit, strong gasdynamic flows induced by laser ignition can stretch the flames to extinction and narrow the ignition limits. If a flame becomes established, eventually the third lobe disappears as the initial gas dynamic effects decay and are overwhelmed by the imposed flow fields. Nevertheless, the overdrive effects persist for some time and overdriven flames were observed in regimes where normal flames would have quenched.

Thermal ignition theory applied to diesel engine autoignition

The diesel community has used one-equation models for ignition delay in engines for decades. The inverse Arrhenius forms characterizing many of these expressions thus lump together liquid fuel evaporation, mixing of the vapour with the charge and the chemical delay. As a result, correlations apply only to the injector, engine and operating conditions tested. Additionally, they typically exhibit energies or temperatures of activation that can be as much as ten times lower than those obtained in well-mixed, controlled systems that isolate and study the chemical delay. Here the latter system is modelled first, using classical analysis techniques. The result is then extended to the relatively straightforward fluid mechanic environment of a laminar counterflow burner to clarify how both chemistry and mixing can be included in a more general one-equation model. This new model is validated with ignition limit data for ethane or ethene with O2/N2 either premixed or unmixed in the counterflow arrangement and liquid diesel fuel in a pool configuration. Since the overall mixing time in this burner is known a priori, the data can be manipulated to yield global chemical ignition delay activation temperatures for these fuels. As expected, the results are equivalent to those measured in shock tubes and combustion tunnels where reactants are premixed before ignition

Ignition of β-Azidoethanol Droplets in Air

The behavior of a droplet of β‐azidoethanol, a liquid explosive, in heated air was studied. It was shown that droplet ignition is a gas‐phase process limited by heat release due to the vapor‐phase oxidation of azidoethanol. An increase in the ambient temperature with distance from the ignition limit eads to enhancement of the role of the thermal decomposition of azidoethanol molecules. In the range of ambient temperatures of 560–620 K, the droplet ignition is determined by the kinetics of the parallel reactions — the gas‐phase oxidation and thermal decomposition of azidoethanol.

Influence of the Burnout in Vitiators on Ignition Limits in Scramjet Combustors

The influence of radicals emitted by a vitiator into a model of a hydrogen-fueled scramjet combustor was investigated. The effect of the preheater combustion was analyzed by varying between a flame mode with high radical formation and a flame mode in which the concentration of the OH radicals was reduced below the detection limit of the optical system. The ignition performances of two fuel injectors were compared to estimate the influence of radicals and water molecules from the preheater on the different types of flame stabilization. The shock wave system, the hydrogen air mixing, and the flame stabilization were studied with optical methods and computational fluid dynamics. It was observed that the two injectors react differently with increasing radical content in the incoming air. The injector with high total pressure loss is insensitive to the burnout in the preheater, whereas the injector that achieves low losses is considerably influenced by the quality of the air preheating. Thus, the cases of higher technical interest with a low pressure loss are more difficult to investigate using test rigs with vitiated air. A technical solution for improving the vitiator burnout, which allows the use of vitiators even in these critical cases, is presented.

予混合圧縮着火燃焼に要求される燃料着火性と圧縮比(予混合圧縮着火燃焼と新燃焼(2),予混合圧縮着火燃焼と新燃焼)

Decreasing the compression ratio improves thermal efficiency in a premixed charge compression ignition engine for the range of investigated octane numbers up to the limit of stable ignition. This is because improvement in the degree of constant volume combustion and decreased cooling loss as the ignition timing approached TDC outweigh reductions in combustion efficiency and theoretical efficiency. The indicated thermal efficiency with different octane number fuels is almost unchanged if the compression ratio is optimized. The thermal efficiency benefits of lower compression ratios decreased with higher homogeneity achieved by advancing the injection timing due to a corresponding deterioration in combustion efficiency. However, the fuel ignitability and compression ration dependence of the indicated thermal efficiency and the exhaust gas emissions are nearly unaffected by changes in the mixture homogeneity. In addition, EGR efficiently suppresses premature ignition and reduces cooling losses without deteriorating combustion efficiency.

Spontaneous Ignition of n-Alkane Droplets with Various Volatility

Ignition delays of a cool flame (τ1) and a hot flame (τt) were measured experimentally for single n-decane, n-dodecane, n-tetradecane and n-hexadecane droplets, which have similar volatilities to common commercial hydrocarbon fuels, in hot air. Droplet diameter was 0.7 mm. Ambient pressure was 0.3 and 1.0 MPa. Ambient temperature was 550 K to 1000 K. Results show the similarity of the examined fuels in terms of reactivity of the low- and high-temperature reactions. Values of τ1 and τt were longer for fuels with lower volatility. Values of τ2 (=τt−τ1) were similar for examined fuels. Both the ignition limits of cool and hot flames were similar between fuels, but were slightly lower for less-volatile fuels.

Dilution limits of n-butane/air mixtures under conditions relevant to HCCI combustion

The role of a spark discharge in extending the operating limits of homogeneous change compression ignition (HCCI) combustion has been investigated using engine experiments and computational flame modeling. The flammability limits of ultra-dilute n-butane/air mixtures are calculated over ranges of temperature, pressure, and dilution levels relevant to HCCI operation. The results suggest that with the elevated temperatures required to achieve HCCI combustion the in-cylinder charge is capable of supporting a propagating flame over most of the HCCI operating regime. However, under light-load and idle conditions the dilution levels are too large and the spark has no effect on HCCI combustion. Thus, some other mechanism must be found to control combustion phasing under these conditions. Since the true eigenvalue for the flame propagation calculation is the mass burning rate and not the flame speed, these results demonstrate that using an arbitrary flame speed cut-off criteria for determining the dilution limit significantly underestimates the actual flammability ranges.

Ignition of premixed hydrogen/air by heated counterflow under reduced and elevated pressures

The temperature of an inert jet required to ignite a counterflowing lean premixed hydrogen/air jet was experimentally determined over the pressure range of 0.6 to 7 atm and computationally simulated using detailed chemistry and transport. Results show that, compared to the homogeneous explosion limits, ignition takes place at higher temperatures and exhibits five limits over the pressure range investigated. The first and second ignition limits resemble the corresponding first and second homogeneous explosion limits, except they have steeper slopes in the pressure–temperature response, with the first limit being affected by the significant transport loss of the H radical and the second limit modified by the activation of the otherwise metastable HO2 radicals by the diffusively enriched H2. The third and fifth ignition limits are respectively manifestations of the low- and high-pressure responses of the third homogeneous explosion limit behavior, which is nevertheless punctuated by the fourth ignition limit characterized by the HO2–H reactions. Furthermore, the fourth ignition limit runs fairly parallel to the crossover temperature, but is shifted to lower temperatures. An explicit expression, 2k1={2k10/(k10+k11)}k9[M], was derived and found to describe well this limit as well as the extended second limit observed in previous flow reactor studies. It is further shown that, since transport effects are inherently important for the present premixed system because of the diffusive loss of H to the hot, inert side of the counterflow, the ignition temperature increases substantially with increasing strain rate at all pressures and that such a sensitivity can be moderated by doping the inert flow with a small amount of oxygen.

Sensitivity of dual fuel engine combustion and knocking limits to gaseous fuel composition

Combustion noise, knock and ignition limits data are measured and presented for a dual fuel engine running on dual fuels of Diesel and three gaseous fuels separately. The gaseous fuels used are liquefied petroleum gas, pure methane and compressed natural gas mixture. The maximum pressure rise rate during combustion is presented as a measure of combustion noise, and the knocking and ignition limits are presented as torque output at the onset of knocking and ignition failure. Experimental investigation on the dual fuel engine revealed the noise generated from combustion, knocking and ignition limits for all gases at different design and operating conditions. A Ricardo E6 Diesel version engine is converted to run on dual fuel of Diesel and the tested gaseous fuel and is used throughout the work. The engine is fully computerized, and the cylinder pressure data, crank angle data and engine operating variables are stored in a PC for off line analysis. The effects of engine speeds, loads, pilot injection angle, pilot fuel quantity and compression ratio on combustion noise, knocking torque, thermal efficiency and maximum pressure are examined for the dual engine running on the three gaseous fuels separately. The combustion noise, knocking and ignition limits are found to relate to the type of gaseous fuels and to the engine design and operating parameters.

Ignition delay and limit of ignitability of a single sodium droplet: theory and experimental comparisons

Ignition of sodium is of great importance in the safety aspect of nuclear facilities because of its extensive use of a working fluid. In this paper, an attempt has been made to determine the comprehensive parameters that govern sodium droplet ignition. Comparisons with experimental results have also been made

Effects of turbulence on nonpremixed ignition of hydrogen in heated counterflow

Experiments were conducted to determine the effects of turbulence on the temperature of a heated air jet required to ignite a counterflowing cold hydrogen/nitrogen jet. In contrast to pseudo-turbulent flows, where turbulence was generated by only a perforated plate on the fuel side, resulting in little effect on ignition in a hydrogen system, fully turbulent flows with perforated plates on both sides of the flow were found to produce noticeable effects. The difference was attributed to the fact that in fully turbulent flows, a significantly larger range of turbulent eddies extend to smaller scales than in pseudo-turbulent flows. At atmospheric pressure, the lowest turbulence intensity studied had ignition temperatures notably lower than laminar ones, while further increases in turbulence intensity resulted in rising ignition temperatures. As a result, optimal conditions for nonpremixed hydrogen ignition exist in weakly turbulent flows where the ignition temperature is lower than can be obtained in other laminar or turbulent flows at the same pressure. Similar trends were seen for all fuel concentrations and at all pressures in the second ignition limit (below 3–4 atm). At higher pressures, turbulent flows caused the ignition temperatures to continue to follow the second limit resulting in ignition temperatures higher than the laminar values. The extension of the second limit ends at the highest pressures (7 to 8 atm) where evidence of third limit behavior appears. Three mechanisms were noted to explain the experimental results. First, turbulent eddies similar in size to the ignition kernel can promote discrete mixing of otherwise isolated pockets of gas. Second, this mixing can promote HO 2 chain branching pathways, which can account for the enhanced ignition noted in the second limit where reaction is governed by crossover temperature chemistry. Third, turbulence limits the excursion times available for reaction, inordinately affecting the slower HO 2 reactions. This is responsible for the increasing ignition temperature with turbulence intensity and pressure.

揮発性の異なる正アルカン液滴の自発点火

Ignition delays of cool flame (τ1) and of hot flame (τt) are experimentally measured for single n-decane, n-dodecane, n-tetradecane and n-hexadecane droplets, which have corresponding volatility to prevailing commercial hydrocarbon fuels, in hot air. Droplet diameter is 0.7 mm. Ambient pressure is 0.3 and 1.0 MPa. Ambient temperature is 550–1000 K. Results show the similarity of the examined fuels in terms of reactivity of the low- and high-temperature reactions. τ1 and τt are longer for the fuels with less volatility. τ2(=τt-τ1) is similar for the examined fuels. Ignition limits for cool flame and for hot flame are also similar, while they are slightly lower for the fuels with less volatility.

Superiority of oxygen as feedstock for a plasma jet igniter in supersonic methane/air streams

Ignition tests by plasma jets were conducted with N2, O2, and air feedstocks to clarify what radicals play a more important role in methane/air reaction. Under test conditions of flow Mach 2 and total temperature of 400 K, ignition limits were measured by changing the electrical input power and feedstock flow rates. The experimental results indicated that oxygen plasma jets were superior to nitrogen or air plasma jets, and when the oxygen feedstock flow rate was 60L/min, the input power at ignition limits was about 0.7 kW, only 13% of the value using air as the feedstock. Spectroscopy measurements revealed the presence of radicals such as nitrogen and oxygen atoms in the torch exhausts and identified the chemical species produced by the reactions of methane/air and radicals by plasma jets. The chemical intermediate products were greatly changed depending on whether the added radical was an oxygen atom or a nitrogen atom; oxygen atoms generated H and OH, important species for augmentation of the reaction rates. These results show that the superiority of oxygen plasma jets for ignition is due to the chemical effect of radicals produced in the jet, and accordingly that oxygen atoms are particularly effective in methane/air from reaction standpoints.

Effects of unsteady scalar dissipation rate on ignition of non-premixed hydrogen/air mixtures in counterflow

As a step toward understanding the effects of a turbulent environment on ignition delay, the effect of impulsive strain forcing on the autoignition of non-premixed hydrogen/air mixtures is studied numerically using a one-dimensional unsteady opposed-flow code with detailed chemistry. The sensitivity of ignition kernel growth to changes in the scalar dissipation rate during the ignition process is studied at conditions in the second and third ignition limits. The sensitivity of the kernel growth is quantified by examining the time evolution of key radical species as well as their reaction and flow flux balances over a range of impulse amplitudes and times. Results show that transient ignition in both the second and the third limits is sensitive to changes in scalar dissipation rate. Increases in ignition delay of as much as five times are observed, depending upon the impulsive forcing amplitude and timing. For a given impulse amplitude, kernels that have accumulated more radicals at a given time during induction are found to ignite much sooner, indicating that the time history of the kernel radical pool relative to the impulse time is important. Furthermore, kernels are found to be able to survive excursions in the scalar dissipation rate to values that far exceed the steady ignition state. The increase in ignition delay in both limits is attributed to a shorter residence time of radicals in the kernel as measured by an instantaneous Damkohler number. A new ignition criterion based on the instantaneous Damkohler is found to be an accurate measure of predicting the ignitability under highly transient conditions.