The effect of a shock wave on the ignition behavior of aluminum particles in a shock tube

The effect of a shock wave on the ignition behavior of 5 μm aluminum (Al) particles was studied in a series of experiments by means of a horizontal shock tube with an inner diameter of 70 mm. To isolate the shock effect from other effects, the experiments were conducted in an inert argon (Ar) atmosphere in addition to a few control experiments in air. The use of Ar as driven gas also helps to produce strong shocks. Every aluminum particle is initially covered with a layer of amorphous aluminum oxide (Al₂O₃). The Al₂O₃ passivates the particle, thus playing a key role in the ignition and combustion mechanisms of an Al particle. The experiments showed a strong emission of light originating from the particles immediately after the shock wave has passed them. Spectral analysis revealed strong AlO bands even in experiments in which the volatilization temperature of Al₂O₃ was not exceeded. The emission spectrum of the flame permits the determination of a grey-body temperature. The existence of AlO molecules and the analysis of samples taken after an experiment give a strong evidence of the influence of a shock wave on the ignition and reaction mechanism of Al particle combustion.     

Experimental and numerical investigation of hydrogen gas auto-ignition

This paper describes hydrogen self-ignition as a result of the formation of a shock wave in front of a high-pressure hydrogen gas propagating in the tube and in the semi-confined space, for which the numerical and experimental investigation was done. An increase in the temperature behind the shock wave leads to the ignition on the contact surface of the mixture of combustible gas with air. The required condition of combustible self-ignition is to maintain the high temperature in the mixture for a time long enough for inflammation to take place. Experimental technique was based on a high-pressure chamber inflating with hydrogen, burst disk failure and pressurized hydrogen discharge into tube of round or rectangular cross section filled with air. Two numerical models involving the gas-dynamic transport of a viscous gas, the detailed kinetics of hydrogen oxidation, turbulence model, and heat exchange were used for calculations of the hydrogen self-ignition both in semi-confined space and a tube.     

Formulation reproducing the ignition delays simulated by a detailed mechanism: Application to n-heptane combustion

This article is part of the project to model the kinetics of high-temperature combustions, occurring behind shock waves and in detonation waves. The “conventional” semi-empirical correlations of ignition delays have been reformulated, by keeping the Arrhenius equation form. It is shown how a polynomial with 3^N coefficients (where N∈[1,4] is the number of adjustable kinetic parameters, likely to be simultaneously chosen among the temperature T, the pressure P, the inert fraction X_Ar, and the equivalence ratio Φ) can reproduce the delays predicted by the Curran et al. [H.J. Curran, P. Gaffuri, W.J. Pitz, C.K. Westbrook, Combust. Flame 129 (2002) 253-280] detailed mechanism (565 species and 2538 reactions), over a wide range of conditions (comparable with the validity domain). The deviations between the simulated times and their fits (typically 1%) are definitely lower than the uncertainties related to the mechanism (at least 25%). In addition, using this new formalism to evaluate these durations is about 10⁶ times faster than simulating them with Senkin (Chemkin III package) and only 10 times slower than using the classical correlations. The adaptation of the traditional method for predicting delays is interesting for modeling, because those performances are difficult to obtain simultaneously with other reduction methods (either purely mathematical, chemical, or even mixed). After a physical and mathematical justification of the proposed formalism, some of its potentialities for n-heptane combustion are presented. In particular, the trends of simulated delays and activation energies are shown for , , XAr∈[0,0.7], and Φ∈[0.25,4.0].     

Experimental methodology and heat transfer model for identification of ignition kinetics of powdered fuels

A methodology for investigating and quantifying the thermal processes leading to ignition of rapidly heated metal powders was developed. The simple experiment involves observing ignition of a powder coated on the surface of an electrically heated filament and is well suited for a variety of powdered fuels. In an experimental case study, the ignition temperature of spherical Mg powder was detected optically at different heating rates. To interpret the results, a heat transfer model was developed for a multilayer powder coating on the heated cylindrical filament. The thermal contact resistance between particles was determined from the measured bulk thermal diffusivity of the powder considering the experimental particle size distribution. An Arrhenius type expression was used to describe the exothermic chemical processes leading to ignition with the pre-exponent as an adjustable parameter. For Mg, a pre-exponent value identified by matching the calculations with the experimental data was found to be 10¹⁰ kg/m² s. The match between the experimental and predicted temperatures and times of ignition was good for different heating rates, which validated the proposed heat transfer model and indicated that the developed methodology is practically useful.     

A pressure-ratio equivalent method for ultra-high pressure hydrogen spontaneous ignition experiment

Increasing hydrogen storage pressure brings high economic benefits and high risks. Pressurized hydrogen leakage spontaneous ignition experiment is an important means to reveal the mechanism of hydrogen leakage spontaneous ignition and improve the safety of hydrogen storage equipment. However, due to the extremely high cost and danger of ultra-high pressure, there is a serious lack of experimental data. In this paper, a pressure-ratio equivalent (PRE) method of experiments is proposed based on the theory of the shock tube problem. By keeping the hydrogen-air pressure ratio constant while reducing the absolute pressure of air and hydrogen, the difficulty of the experiment is greatly reduced. The effectiveness of the PRE method is evaluated theoretically and experimentally. The results show the PRE method retains the ignition characteristics of hydrogen leakage spontaneous ignition largely when the air pressure is within 0.05–0.1 MPa. It is found the pressure ratio of hydrogen to air dominates the leakage spontaneous ignition process. In the experiments of different air pressures, the shock Mach numbers are close to theoretical values. In addition, leakage spontaneous ignition of hydrogen mixed with 30% (vol.) CO is found in experiments using the PRE method, with pressure ratios of up to 250. This indicates that when the storage pressure is high enough, there is also a risk of spontaneous ignition of syngas from high-pressure leakage. The PRE method can widely broaden the pressure scope of experimental research on leakage spontaneous ignition, and it provides a new idea for obtaining the experimental data of gas high-pressure leakage spontaneous ignition.     

Flame acceleration and transition to detonation in ducts

This paper reviews the state of knowledge on flame acceleration and deflagration-to-detonation transition (DDT) in smooth ducts and ducts equipped with turbulence-producing obstacles. The objective is to bring to light the basic understanding of the phenomenon and its application to explosion safety. The scope of the review is restricted to homogeneous gas-phase combustion with emphasis placed on experimental investigation.     

Flame acceleration and DDT of hydrogen-oxygen gaseous mixtures in channels with no-slip walls

Hydrogen-oxygen flame acceleration and transition from deflagration to detonation (DDT) in channels with no-slip walls were studied theoretically and using high resolution simulations of 2D reactive Navier-Stokes equations, including the effects of viscosity, thermal conduction, molecular diffusion, real equation of state and a detailed chemical reaction mechanism. It is shown that in “wide” channels (D > 1 mm) there are three distinctive stages of the combustion wave propagation: the initial short stage of exponential acceleration; the second stage of slower flame acceleration; the third stage of the actual transition to detonation. In a thin channel (D < 1 mm) the flame exponential acceleration is not bounded till the transition to detonation. While velocity of the steady detonation waves formed in wider channels (10, 5, 3, 2 mm) is close to the Chapman-Jouguet velocity, the oscillating detonation waves with velocities slightly below the CJ velocity are formed in thinner channels (D < 1.0 mm). We analyse applicability of the gradient mechanism of detonation ignition for a detailed chemical reaction model to be a mechanism of the deflagration-to-detonation transition. The results of high resolution simulations are fully consistent with experimental observations of flame acceleration and DDT in hydrogen-oxygen gaseous mixtures.     

A shock tube study of iso-octane ignition at elevated pressures: The influence of diluent gases

The ignition of iso-octane/air and iso-octane/O₂/Ar (∼20% O₂) mixtures was studied in a shock tube at temperatures of 868-1300 K, pressures of 7-58 atm, and equivalence ratios Φ=1.0, 0.5, and 0.25. Ignition times were determined using endwall OH^∗ emission and sidewall piezoelectric pressure measurements. Measured iso-octane/air ignition times agreed well with the previously published results. Mixtures with argon as the diluent exhibited ignition times 20% shorter, for most conditions, than those with nitrogen as the diluent (iso-octane/air mixtures). The difference in measured ignition times for mixtures containing argon and nitrogen as the diluent gas can be attributed to the differing heat capacities of the two diluent species and the level of induction period heat release prior to ignition. Kinetic model predictions of ignition time from three mechanisms are compared to the experimental data. The mechanisms overpredict the ignition times but accurately capture the influence of diluent gas on iso-octane ignition time, indicating that the mechanisms predict an appropriate amount of induction period heat release.     

A shock tube study of ignition delay in the combustion of ethylene

Ethylene combustion was investigated behind reflected shock waves. The experimental conditions covered a temperature range of 1000–1650 K, at pressures of 2, 10 and 18 atm, equivalence ratios of 3 and 1, for several mixture compositions using argon as the diluent (93%, 96% and 98% (vol)). In all experiments, dwell times were kept in the range of 7.55–7.85 ms by using a suitable argon–helium mixture as the driver gas. Ignition delay times were determined from the onset of visible broadband emission observed at the end plate of the shock tube. In selected experiments ignition delay times were also determined by simultaneous measurement of chemiluminescence emissions of CH^* and OH^*. In relatively concentrated ethylene/oxygen mixtures with 93% argon (vol), the results show an indiscernible difference between ignition delay times over the ranges of pressure and equivalence ratio tested. In more dilute mixtures (with 98% and 96% argon), longer ignition delay times were observed and there was a noticeable variation of delay times as a function of pressure; with an increase in pressure having the effect of shortening the delay time and an increase in the apparent activation energy. Modeling results using USC Mech II (Wang et al., 2007 [31]) based kinetic model, SERDP PAH model 0.1, developed by Wang and Colket, show good agreement with experiments under stoichiometric and fuel-rich conditions at low pressures. At high pressures for fuel-rich mixtures, optimized version of USC Mech II model (Wang et al., 2009 [36]) had to be used to produce good agreement between calculated ignition delay times and the experimental results. The results of this study are consistent with literature data. The present work extends the existing ethylene ignition delay experimental data set to high pressure and fuel-rich domain, the conditions that are critical for soot and polycyclic aromatic hydrocarbons (PAHs) formation.     

1-Butanol ignition delay times at low temperatures: An application of the constrained-reaction-volume strategy

Ignition delay times behind reflected shock waves are strongly sensitive to variations in temperature and pressure, yet most current models of reaction kinetics do not properly account for the variations that are often present in shock tube experiments. Particularly at low reaction temperatures with relatively long ignition delay times, substantial increases in pressure and temperature can occur behind the reflected shock even before the main ignition event, and these changes in thermodynamic conditions of the ignition process have proved difficult to interpret and model. To obviate such pressure increases, we applied a new driven-gas loading method that constrains the volume of reactive gases, thereby producing near-constant-pressure test conditions for reflected shock measurements. Using both conventional operation and this new constrained-reaction-volume (CRV) method, we have collected ignition delay times for 1-butanol/O₂/N₂ mixtures over temperatures between 716 and 1121 K and nominal pressures of 20 and 40 atm for equivalence ratios of 0.5, 1.0, and 2.0. The equivalence ratio dependence of 1-butanol ignition delay time was found to be negative when the oxygen concentration was fixed, but positive when the fuel concentration was held constant. Ignition delay times with strong pre-ignition pressure increases in conventional-filling experiments were found to be significantly shorter than those where these pressure increases were mitigated using the CRV strategy. The near-constant-pressure ignition delay times provide a new database for low-temperature 1-butanol mechanism development independent of non-idealities caused by either shock attenuation or pre-ignition perturbations. Comparisons of these near-constant-pressure measurements with predictions using several reaction mechanisms available in the literature were performed. To our knowledge this work is first of its kind that systematically provides accurate near-constant-enthalpy and -pressure target data for chemical kinetic modeling of undiluted fuel/air mixtures at engine relevant conditions.     

Detonation in the hydrogen-oxygen microfoam on the aqueous base

In the paper, the propagation of the detonation wave in the hydrogen-oxygen microfoam on the aqueous base is considered. Microfoam represents a two-phase system containing micron-sized gas bubbles filled with the hydrogen-oxygen mixture. These bubbles are dispersed in the water solution of surfactant (sodium dodecyl sulfate). The dependencies of detonation speed on the equivalence ratio and on the water content in the foam are obtained with the use of high-speed filming. It is found that the detonation speed slightly increases with the decrease in water content in the foam. Based on the pressure measurements, it is established that the detonation propagation in the foam is driven by relatively weak shock waves, which by themselves are not able to induce ignition of the hydrogen-oxygen mixture. To substantiate the fact of detonation existence in the microfoam a hypothesis of the cumulative collapse of gas bubbles under the shock wave action is proposed. The estimation for detonation speed in microfoam is analytically derived on the basis of simple phenomenological representations.     

Shock tube evaluation on C2H4 ignition delay differences among N2, Ar,He, CO2 diluent gases

To reveal the fuel ignition delay differences among diluent gases, non-reactive gas Ar, He, N₂ and reactive gas CO₂ four gases were selected to compare the ethylene ignition delay difference on a shock tube rig. The tested condition temperature and pressure range is 1074–1832 K and 2 atm. Comparison results show that the ethylene ignition delay discrepancy was not pronounced among Ar, He and N₂ these non-reactive dilution gases. In terms of CO₂ dilution, a critical CO₂ concentration was observed. When the CO₂ concentration was lower than this critical value, no distinguished ignition delay difference was found. Conversely, the ignition delay difference was pronounced once the CO₂ concentration was above this critical value. Particularly, the difference was specially apparent in low temperature conditions. Reaction dynamic analysis shows that the heat effect is predominant in low temperature while the chemical effect is predominant in high temperature in the reaction of CO₂ participation. Additionally, the CH* and OH* radical emission signal intensity turns smaller and profile curve turns irregular as the CO₂ ratio increases. The CO₂ shows a strong inhibition effect which is closely related with H radical consumption reaction CO₂+HCO + OH.     

The evolution and cellular structure of a detonation subsequent to a head-on interaction with a shock wave

This paper analyzes the results of a head-on collision between a detonation and a planar shock wave. The evolution of the detonation cellular structure subsequent to the frontal collision was examined through smoked foil experiments. It is shown that a large reduction in cell size is observed following the frontal collision, and that the detonation cell widths are correlated well with the chemical kinetic calculations from the ZND model. From chemical kinetic calculations, the density increase caused by shock compression appears to be the main factor leading to the significant reduction in cell size. It was found that depending on the initial conditions, the transition to the final cellular pattern can be either smooth or spotty. This phenomenon appears to be equivalent to Oppenheim's strong and mild reflected shock ignition experiments. The difference between these two transitions is, however, more related to the stability of the incident detonation and the strength of the perturbation generated by the incident shock.     

Effects of the geometry of downstream pipes with different angles on the shock ignition of high-pressure hydrogen during its sudden expansion

To investigate the effects of the geometry of downstream pipes on the shock ignition and the formation of the shock waves during high-pressure hydrogen sudden expansion, a series of bench-mark experiments were designed and high-pressure hydrogen were released into five types of pipes with different angles (60, 90, 120, 150 and 180°). It was found that the geometry of downstream pipes had a significant influence on the shock ignition of hydrogen. The incident shock wave would be reflected at the corner of the pipes with angles of 60, 90, 120 and 150°. The intensity of the reflected shock wave is higher if the angle is smaller. In addition, the average velocity of the leading incident shock wave would decrease when it passed the corner of the pipe. Using a pipe with smaller angle significantly increases the likelihood of shock ignition and lowers the minimal required burst pressure for shock ignition. The overpressure of the incident shock waves inside the exhaust chamber (for the cases with the angles of 60, 90, 120 and 150°) decreases sharply. There are three flame propagation behaviors inside the exhaust chamber: flame quenching, flame separation and no flame separation. The results of this study have implications concerning designs for storage safety of hydrogen energy and may help get better understanding of shock ignition mechanism of high pressure hydrogen and effect of pipeline geometry on ignition.     

An experimental and kinetic modeling study of n-propanol and i-propanol ignition at high temperatures

Ignition delay times of n- and i-propanol mixtures in argon-diluted oxygen were measured behind reflected shocks. Experimental conditions are: temperatures from 1100 and 1500 K, pressures from 1.2 to 16.0 atm, fuel concentrations of 0.5%, 0.75%, 1.0%, and equivalence ratios of 0.5, 1.0 and 2.0. A detailed kinetic model consisting of 238 species and 1448 reactions was developed to simulate the ignition of the two propanol isomers, with the computed ignition delay times agreeing well with the present measured results as well as the literature data at other conditions. Further validation of the kinetic mechanism was conducted by comparing the simulated results with measured JSR data and laminar flame speeds, and reasonable agreements were achieved for all test conditions. Moreover, reaction pathway analysis indicated that n-propanol mainly produces ethenol, ethene and propene, while i-propanol primarily produces acetone and propene. Finally, sensitivity analysis demonstrated that some fuel-species reactions can be found in the most important reactions for both propanols, and these are mainly the H-abstraction reactions.     

A high pressure ignition delay time study of 2-methylfuran and tetrahydrofuran in shock tubes

Ignition delay time studies for tetrahydrofuran (THF) and 2-methylfuran (2MF) as well as optical investigations of combustion for 2MF have been carried out using two shock tubes. The experiments with undiluted THF/air mixtures were performed at 20 and 40 bar in a high pressure shock tube (HPST) at an equivalence ratio of Ф = 1 covering an overall temperature range of 780–1100 K and 691–1006 K, respectively. Undiluted 2MF/air mixtures (Ф = 1) were also investigated in the HPST at 40 bar in the temperature range of 820–1215 K. The experimental data of 2MF obtained at 40 bar were supported with kinetic simulations of existing models from literature. Additionally, sensitivity analyses of 2MF at several temperatures were performed for finding out the most sensitive reactions. Schlieren imaging was employed in a rectangular shock tube (RST) utilizing a high speed video camera through which the ignition process was captured for a stoichiometric 2MF/O₂/Ar mixture at pressures of about 10 bar and in the temperature range of 871–1098 K.     

Visualization of spontaneous ignition under controlled burst pressure

A high-pressure hydrogen jet released into the air has the possibility of igniting in a tube without any ignition source. The mechanism of this phenomenon, called spontaneous ignition, is considered to be that hydrogen diffuses into the hot air caused by the shock wave from diaphragm rupture and the hydrogen-oxidizer mixed region is formed enough to start chemical reaction. Recently, flow visualization studies on the spontaneous ignition process have been conducted to understand its detailed mechanism, but such ignition has not yet been well clarified. In this study, the spontaneous ignition phenomenon was observed in a rectangular tube. The results confirm the presence of a flame at the wall of the tube when the shock wave pressure reaches 1.2–1.5 MPa in more than 9 MPa burst pressure and that ignition occurs near the wall, followed by multiple ignitions as the shock wave propagates, with the ignitions eventually combining to form a flame.     

Measurements and kinetic study on ignition delay times of propane/hydrogen in argon diluted oxygen

Measurements on ignition delay times of propane/hydrogen mixtures in argon diluted oxygen were conducted for hydrogen fractions in the fuel mixtures (XH₂)$\left(X_{{\text{H}}_2}\right)$ from 0 to 100%, pressures of 1.2, 4.0 and 10 atm, and temperatures from 1000 to 1600 K using the shock-tube. Results show that for XH₂$X_{{\text{H}}_2}$ less than 70%, ignition delay time shows a strong Arrhenius temperature dependence and it decreases with the increase of pressure, while for XH₂$X_{{\text{H}}_2}$ larger than 90%, there is a crossover pressure dependence of the ignition delay time with increasing temperature. Numerical studies were made using the selected kinetic mechanisms and results show that the predicted ignition delay time gives a reasonable agreement with the measurements. Both measurements and predictions show that for XH₂$X_{{\text{H}}_2}$ less than 70%, the ignition delay time is only moderately decreased with the increase of XH₂$X_{{\text{H}}_2}$, indicating that hydrogen addition has weak effect on ignition enhancement. Sensitivity analysis reveals the key reactions that control the simulation of ignition delay time. Kinetic study is made to interpret the ignition delay time dependence on pressure and XH₂$X_{{\text{H}}_2}$.     

Using rapid compression machines for chemical kinetics studies

Rapid compression machines (RCMs) are used to simulate a single compression stroke of an internal combustion engine without some of the complicated swirl bowl geometry, cycle-to-cycle variation, residual gas, and other complications associated with engine operating conditions. RCMs are primarily used to measure ignition delay times as a function of temperature, pressure, and fuel/oxygen/diluent ratio; further they can be equipped with diagnostics to determine the temperature and flow fields inside the reaction chamber and to measure the concentrations of reactant, intermediate, and product species produced during combustion.This paper first discusses the operational principles and design features of RCMs, including the use of creviced pistons, which is an important feature in order to suppress the boundary layer, preventing it from becoming entrained into the reaction chamber via a roll-up vortex. The paper then discusses methods by which experiments performed in RCMs are interpreted and simulated. Furthermore, differences in measured ignition delays from RCMs and shock tube facilities are discussed, with the apparent initial gross disagreement being explained by facility effects in both types of experiments. Finally, future directions for using RCMs in chemical kinetics studies are also discussed.