Decrease In Ignition Delay Time Research Articles

G060041 非予混合火炎の着火遅れに対するスカラー散逸率の影響([G06004]熱工学部門一般セッション(4):着火)

The objective of this study is to suggest a new method to predict the stability limit of high temperature air combustion. For the first stage of this study, the flamelet equation is solved numerically to investigate the effects of the scalar dissipation rate and the dilution with burned gas on ignition delay. Methane was used as fuel. Ignition time is defined as the time at which a temporal increase of maximum temperature becomes maximum. The ignition delay time increases with the scalar dissipation rate in the higher range of x. However, in the lower range of x, the time decreases with x. The ignition delay time based on the maximum temperature includes durations of radical pool growth and of movement of most reactive region. In the lower range of x, the increase in the movement speed decreases the ignition delay time with x. With the increase in the dilution ratio K, the ignition delay time decreases, and then increases. This trend is attributed to the increase in temperature and the decrease in species concentration with K. The introduction of chemical characteristics time provides the generalization of ignition delay time for all dilution ratios.

Kinetics of ignition of saturated hydrocarbons by nonequilibrium plasma: CH 4-containing mixtures

Kinetics of ignition in a CH 4:O 2:Ar mixture under the action of a high-voltage nanosecond discharge has been studied experimentally and numerically. Ignition delay time behind a reflected shock wave was measured with and without the discharge. It was shown that the initiation of the gas discharge with a specific deposited energy of 10–30 mJ/cm 3 leads to an orders-of-magnitude decrease in ignition delay time. Discharge processes and following chain chemical reactions with energy release were simulated separately because of an essential difference in their time scales. A kinetic scheme was developed to simulate the production of active particles during the discharge and in its afterglow. The generation of atoms, radicals, and excited and charged particles was numerically simulated using measured time-resolved discharge current and electric field in the discharge phase. The calculated densities of active particles were used as input data to simulate plasma-assisted ignition of methane. The calculated ignition delay time agrees well with experimental data. Analysis of the simulation results showed that the effect of nonequilibrium plasma on ignition delay is primarily associated with faster development of chain reactions due to O and H atoms produced by electron impact dissociation of molecules in the discharge phase.

Effect of Hydrogen Addition to Hydrocarbon Fuels on Combustion Characteristics

Effect of hydrogen addition to hydrocarbon fuels on combustion characteristics were investigated using chemical full kinetics. The changes of ignition delay time and laminar flame velocity when hydrogen is added to methane or propane fuels were calculated. The result shows that for ambient pressure condition, ignition delay time decreases corresponding to the amount of hydrogen addition. However for high pressure condition, when 10% hydrogen is added to hydrocarbon fuel, ignition delay time decreases as short as that of hydrogen itself. Laminar flame velocity increases gradually with adding hydrogen, and when more than 70% hydrogen is added, laminar flame velocity rapidly increases. Furthermore, the production rate of OH is an important factor for the change of ignition delay time and laminar flame velocity, when hydrogen is added.

SHOCK-TUBE STUDY OF THE IGNITION OF PROPANE AT INTERMEDIATE TEMPERATURES AND HIGH PRESSURES

The ignition delay times of lean (Φ = 0.5) propane–air mixtures were measured in the temperature range 900 K ≤T ≤ 1300 K at pressures of 10 and 30 bar, respectively. The results obtained are in very good agreement with measurements of Cadman, Thomas, and Butler (Phys. Chem. Chem. Phys., Vol. 2, p. 5411, 2000). The experiments confirm that the activation energy of the ignition delay time decreases at around 1050 K and a linear extrapolation is not possible. Reaction schemes used in the literature cannot predict the ignition delay times at these low temperatures. CCD pictures show that the ignition occurs in the middle of the shock tube.

Reflected Shock Ignition of SiH4/H2/O2/Ar andSiH4/CH4/O2/Ar Mixtures

High-temperature experiments were performed behind reflected shock waves with H 2 /O 2 , SiH 4 /H 2 /O 2 , CH 4 /O 2 , and SiH 4 /CH 4 /O 2 mixtures highly diluted in argon. Reflected-shock temperatures ranged from 1000-2250 K at a pressure near 1 atm. Reaction progress was monitored by observation of the time histories of several species by the use of emission techniques, including OH*, SiH4, and CH*. The oxidation and ignition data are reported in the form of species concentration profiles and plots of characteristic times as a function of temperature. The presence of silane in the fuel/O 2 mixtures markedly reduced the ignition delay time of all H 2 /O 2 and CH 4 /O 2 mixtures, usually by a factor of two or more, even at molar SiH 4 concentrations as low as 1% of the fuel concentration. Although the decrease in ignition delay time was, in some cases, quite significant, the activation energy of ignition when plotted on an Arrhenius diagram remained virtually unchanged; this result indicates that the chain-branching behavior that takes place when silane is present, although faster and through different elementary reactions, is similar to the behavior when silane is not present. The addition of silane to H 2 /O 2 mixtures also appears to extend the chain-branching kinetics to lower temperatures, eliminating the classic chain termination seen near the second explosion limit of H 2 /O 2 ignition.

Influence of Radical Concentration and Fuel Decomposition on Ignition of Propane/Air Mixture

A study of hydrocarbon fuel (propane) thermal decomposition was conducted by varying the temperature and pressure of air/fuel mixtures and determining their effect on the degree of fuel decomposition and its attendant product composition. Based on the composition data, the concentrations of low molecular weight components and active radicals were found. Intensie cation of the combustion process due to an increase in fuel reaction activity resulting from fuel thermal decomposition is demonstrated, and the effect of thermal decomposition resulting in a decrease in ignition delay time is shown. It is also shown that intensie cation of the fuel/air reaction is possible through the addition of active radicals such as O, OH, and H. The character of the extraequilibrium radical concentrations change, dependent on the conditions of the combustion processes parameters, is determined.

Ignition of a solid fuel by thermal radiation

Ignition characteristics of a vertical solid fuel plate with block have been investigated experimentally. For low radiant heat flux, ignition does not occur in a vertical solid fuel plate without block. In the case with the block on a vertical fuel plate, however, ignition can occur by increasing the residence time and the time to absorb the incident radiation flux by fuel vapor in gas phase. The ignition occurs below block and the point varies according to the block location and the block height. As the block height increases, the block locates at higher position, and the hot wall temperature increases, the ignition delay time decreases. Also as the initial temperature of fuel plate rises, the ignition delay time of the solid fuel plate decreases. The temperature distribution of solid fuel plate with block is nearly proportional to the radiant heat flux distribution. Therefore, the effect temperature by natural convection heat transfer is of the same order as that of inhibition of temperature increase by pyrolysis.

Ignition of composite propellants in a stagnation region under rapid pressure loading

Ignition of AP-based composite solid propellants located at the tip of an inert crack wasinvestigated both experimentally and theoretically. The ignition process was observed by simultaneously using a high-speed (≈40,000 pictures/s) camera and a fast-response photodiode system. Heat flux to the propellant surface was measured with a thin-film heat-flux gage. Effects of pressurization rate, crack-gap width, and igniter flame temperature on the ignition process were studied experimentally. Experimental results indicate that the ignition-delay time decreases and the heat flux to the propellant surface increases as the pressurization rate is increased. Results of the theoretical analysis which employed separate solid-phase energy equations for fuel and oxidizer and the measured heat flux to the propellant surface are in good agreement with experimental data. The decrease in ignition delay with increasing pressurization rate is caused by enhanced heat feedback to the propellant surface at higher pressurization rates. This augmentation in heat feedback to the propellant at higher pressurization is believed to be a result of a combination of the following mechanisms: heating due to compression-wave reflection at the closed end; heat release due to burning of unreacted igniter species near the tip, behind the compression wave; and enhanced heat transfer due to recirculating hot gases near the tip.