Evaluation of the one-step hydrogen–oxygen global reaction rate in a non-premixed mixture to predict auto-ignition limits

Evaluation of the hydrogen–oxygen global reaction rate was conducted to predict auto-ignition onset and delay.     

Asymptotic analysis on autoignition and explosion limits of hydrogen–oxygen mixtures in homogeneous systems

The deducted equations about chemical species and temperature are presented to calculate hydrogen–oxygen ignition delay time. Steady-state assumptions for many of the intermediate species are introduced to derive a new simplified 3-step mechanism. The simplified 3-step mechanism for hydrogen–oxygen leads the steady-state assumptions to linear differential equations. The competition among the full mechanism containing 17-, 8- and the simplified 3-step mechanisms is carried out. The resulting closed form solutions describe the low-, the intermediate- and the high-temperature ignition regimes and obtain an “S-shaped curve”. Finally, the two parameters on ignition and extinction of the continuously stirred flow reactor are discussed in detail and the temperature error analysis is given. It reduces the computational costs and supplies theory and methods for understanding autoignition and explosion limits of hydrogen–oxygen mixtures in homogeneous systems.     

Experimental study of ignited unsteady hydrogen jets into air

In order to simulate an accidental hydrogen release from the low pressure pipe system of a hydrogen vehicle a systematic study on the nature of transient hydrogen jets into air and their combustion behaviour was performed at the FZK hydrogen test site HYKA. Horizontal unsteady hydrogen jets with an amount of hydrogen up to 60 STP dm³ and initial pressures of 5 and 16 bar have been investigated. The hydrogen jets were ignited with different ignition times and positions. The experiments provide new experimental data on pressure loads and heat releases resulting from the deflagration of hydrogen-air clouds formed by unsteady turbulent hydrogen jets released into a free environment. It is shown that the maximum pressure loads occur for ignition in a narrow position and time window. The possible hazard potential arising from an ignited free transient hydrogen jet is described.     

Effects of N2O addition on the ignition of H2–O2 mixtures: Experimental and detailed kinetic modeling study

Ignition delay times of H₂/O₂ mixtures highly diluted with Ar and doped with various amounts of N₂O (100, 400, 1600, 3200 ppm) were measured in a shock tube behind reflected shock waves over a wide range of temperatures (940–1675 K). The pressure range investigated during this work (around 1.6, 13 and 32 atm) allows studying the effect of N₂O on hydrogen ignition at pressure conditions that have never been heretofore investigated. Ignition delay times were decreased when N₂O was added to the mixture only for the higher nitrous oxide concentrations, and some changes in the activation energy were also observed at 1.5 and 32 atm. When it occurred, the decrease in the ignition delay time was proportional to the amount of N₂O added and depended on pressure and temperature conditions. A detailed chemical kinetics model was developed using kinetic mechanisms from the literature. This model predicts well the experimental data obtained during this study and from the literature. The chemical analysis using this model showed that the decrease in the ignition delay time was mainly due to the reaction N₂O + M ? N₂ + O + M which provides O atoms to strengthen the channel O + H₂ ? OH + H.     

Experimental and kinetic study on ignition delay times of lean n-butane/hydrogen/argon mixtures at elevated pressures

Measurements on ignition delay times of n-butane/hydrogen/oxygen mixtures diluted by argon were conducted using the shock tube at pressures of 2, 10 and 20 atm, temperatures from 1000 to 1600 K and hydrogen fractions (${\mathrm{X}}_{{\mathrm{H}}_2}$) from 0 to 98%. It is found that hydrogen addition has a non-linear promoting effect on ignition delay of n-butane. Results also show that for ${\mathrm{X}}_{{\mathrm{H}}_2}$ less than 95%, ignition delay time shows an Arrhenius type dependence and the increase of pressure and temperature lead to shorter ignition delay times. However, for ${\mathrm{X}}_{{\mathrm{H}}_2}$ = 98% and 100% mixtures, non-monotonic pressure dependence of ignition delay time were observed. The performances of the Aramco2.0 model, San Diego 2016 model and USC2.0 model were evaluated against the experimental data. Only the Aramco2.0 model gives a reasonable agreement with all the measurements, which was conducted in this study to interpret the effect of pressure and hydrogen addition on the ignition chemistry of n-butane.     

Another view of the upper and intermediate explosion limits of a H2–O2 system

In the present note we consider the two simultaneous mechanisms that are dominated by gas-phase reactions, namely the upper and intermediate explosion limits, and show that by employing the Le-Chatelier rule, a unified explosion limit is obtained in the form of a single analytical expression.     

Combustion characteristics for a hydrogen-fuelled low-pressure-ratio split-cycle engine

Investigation of hydrogen-air combustion characteristics for low-pressure-ratio split-cycle engines is performed for the first time. Ignition delay and combustion duration data are presented for low initial pressures below 5 bars and temperatures from ambient level till 550 °C. A simplified constant-volume combustion chamber experiment with glow-plug ignition and timed intake and exhaust valves is constructed in order to simulate the real operation. Ignition delay is found to be inversely proportional to the temperature, and unexpectedly directly proportional with pressure. Equivalence ratio has a weak influence on the ignition delay. Combustion duration exhibits the same behaviour as the ignition delay and represents an average 40% portion of the total combustion delay. Ignition delay results are bounded to the selected glow plug surface temperature range and complements with data in literature. Based on the results, the engine rotational speed would be limited to a maximum of 4500 RPM for an intake duration of 30° at 550 °C and 3.5 bars pre-combustion conditions.     

Ignition limits of rapidly expanding diffusion layers: Application to unsteady hydrogen jets

The present paper addresses the ignition problem of a one-dimensional unsteady diffusion layer of fuel and oxidizer, undergoing volumetric expansion. The problem is applied to shock induced diffusion-ignition of pressurized fuel jets that are released into an oxidizing atmosphere. Upon the sudden release of a pressurized gaseous fuel into the ambient atmosphere through a hole, a strong shock wave forms, driven by rapid expansion of the forming jet. The model follows the thin diffusion layer at the head of the jet in Lagrangian coordinates, with its rate of expansion dictated by the local pressure evolution of the surrounding gasdynamic flow. Following the analysis of Radulescu and Law, the latter can be calculated a priori before the ignition event. Hence, the expansion rate is prescribed as a source term in our calculations of the diffusion layer. The calculations, which are performed for hydrogen and air with realistic thermo-chemical data and transport properties of the chemical species, revealed the transient events leading to ignition in this unsteady diffusion layer. Furthermore, the calculations showed that when the rate of expansion was sufficiently strong, which may occur for releases through sufficiently small holes, ignition can be prevented. This illustrates the important role that gasdynamic expansion plays on ignition phenomena. The results of the present model are found to be in very good agreement with previous numerical and experimental results of transient jet release ignition.     

The role of instabilities on ignition of unsteady hydrogen jets flowing into an oxidizer

The current study is a series of experiments designed to examine the role of turbulent instabilities on the ignition process of pressurized hydrogen jets which are released into oxidizer environments. The experiments were conducted in a shock tube where hydrogen gas is shock-accelerated into a partly confined oxygen environment across a perforated plate. Although the considered scenario differs from most practical applications where high pressure hydrogen is released into air, the results may be applicable toward cases where hydrogen leaks are shock assisted through holes in fuel cell membranes. Schlieren visualization permitted the reconstruction of the gas dynamic evolution of the release while time resolved self-luminosity records permitted us to record whether ignition was achieved. Despite the presence of confinement in the experiments, the ignition limits determined experimentally were found to be relatively agreeable with trends predicted by a previously developed 1-D numerical model (Maxwell and Radulescu, 2011), which assumes a release into an unconfined environments. However, the role of confinement in the experiments not only influence ignition at lower limits compared to the 1-D ignition model, but was also found to promote turbulent mixing through shock reflections and flow instabilities. Turbulent mixing thus influences how the ignition spots interact to ignite the entire jet.     

Shock tube and kinetic study of C2H6/H2/O2/Ar mixtures at elevated pressures

Using a high-pressure shock tube facility, the ignition delay times of stoichiometric C₂H₆/H₂/O₂ diluted in argon were obtained behind reflected shock wave at elevated pressures (p = 1.2, 4.0 and 16.0 atm) with ethane blending ratios from 0 to 100%. The measured ignition delay times were compared to the previous correlations, and the results show that the ignition delay times of ethane from different studies exhibit an obvious difference. Meanwhile, numerical studies were conducted with three generally accepted kinetic mechanisms and the results show that only NUIG Aramco Mech 1.3 agrees well with the measurements under all test conditions. Sensitivity analysis was made to interpret the poor prediction of the other two mechanisms. Furthermore, the effect of ethane blending ratio on the ignition delay times of the mixtures was analyzed and the results show that ethane blending ratio gives a non-linear effect on the auto-ignition of hydrogen. Finally, chemical interpretations on this non-linear effect were made from the reaction pathway analysis and normalized H radical consumption analysis.     

Turbulent hydrogen deflagration induced by ostacles in real confined environment

The CVE (Chambre View Explosion) is an experimental facility owned by the DIMNP at the University of Pisa “Scalbatraio” laboratory to study the confined vented explosion phenomena in real environments. The CVE is a cubic apparatus of 25 m³, equipped with two glass walls that enable to video record the explosion and the flame expansion, and the relevant path. The experimental campaign and the results here presented were focused on the investigation of the turbulence induced by obstacles on the vented deflagration for homogeneous conditions of concentration. For this purpose two sets of obstacles constituted by plates were designed and put inside the apparatus.     

Experimental and kinetic study on ignition delay times of DME/H2/O2/Ar mixtures

Ignition delay times of dimethyl ether (DME)/hydrogen/oxygen/argon mixtures (hydrogen blending ratio ranging from 0% to 100%) were measured behind reflected shock waves at pressures of 1.2–10 atm, temperature range of 900–1700 K, and for the lean (? = 0.5), stoichiometric (? = 1.0) and rich (? = 2.0) mixtures. For more understanding the effect of initial parameters, correlations of ignition delay times for the lean mixtures were obtained on the basis of the measured data (X_H2 ? 95%) through multiple linear regression. Ignition delay times of the DME/H₂ mixtures demonstrate three ignition regimes. For X_H2 ? 80%, the ignition is dominated by the DME chemistry and ignition delay times show a typical Arrhenius dependence on temperature and pressure. For 80% ? X_H2 ? 98%, the ignition is dominated by the combined chemistries of DME and hydrogen, and ignition delay times at higher pressures give higher ignition activation energy. However, for X_H2 ? 98%, the transition in activation energy for the mixture was found as decreasing the temperature, indicating that the ignition is dominated by the hydrogen chemistry. Simulations were made using two available models and different results were presented. Thus, sensitivity analysis was performed to illustrate the causes of different simulation results of the two models. Subsequently, chemically interpreting on the effect of hydrogen blending ratio on ignition delay times was made using small radical mole fraction and reaction pathway analysis. Finally, high-pressure simulations were performed, serving as a starting point for the future work.     

Modeling and simulation of hydrogen combustion in engines

Hydrogen being an ecological fuel is very attractive now for engines designers. It is already actively used in rocket engines. There exist plans to use hydrogen in pulse detonation engines. However, peculiarities of hydrogen combustion kinetics, the presence of zones of inverse dependence of reaction rate on pressure, etc. prevent from wide use of hydrogen engines. Computer aided design of new effective and clean hydrogen engines needs mathematical tools for supercomputer modeling of hydrogen–oxygen components mixing and combustion gas dynamics.     

Effect of initial turbulence on vented explosion overpressures from lean hydrogen–air deflagrations

To examine the effect of initial turbulence on vented explosions, experiments were performed for lean hydrogen–air mixtures, with hydrogen concentrations ranging from 12 to 15% vol., at elevated initial turbulence. As expected, it was found that an increase in initial turbulence increased the overall flame propagation speed and this increased flame propagation speed translated into higher peak overpressures during the external explosion. The peak pressures generated by flame–acoustic interactions, however, did not vary significantly with initial turbulence. When flame speeds measurements were examined, it was found that the burning velocity increased with flame radius as a power function of radius with a relatively constant exponent over the range of weak initial turbulence studied and did not vary systematically with initial turbulence. Instead, the elevated initial turbulence increased the initial flame propagation velocities of the various mixtures. The initial turbulence thus appears to act primarily by generating higher initial flame wrinkling while having a minimal effect on the growth rate of the wrinkles. For practical purposes of modeling flame propagation and pressure generation in vented explosions, the increase in burning velocity due to turbulence is suggested to be approximated by a single constant factor that increases the effective burning velocity of the mixture. When this approach is applied to a previously developed vent sizing correlation, the correlation performs well for almost all of the peaks. It was found, however, that in certain situations, this approach significantly under predicts the flame–acoustic peak. This suggests that further research may be necessary to better understand the influence of initial turbulence on the development of flame–acoustic peaks in vented explosions.     

Computational fluid dynamics modeling of hydrogen ignition in a rapid compression machine

In modeling a rapid compression machine (RCM) experiment, a zero-dimensional code is commonly used along with an associated heat loss model. However, the applicability of such a zero-dimensional modeling needs to be assessed over a range of accessible experimental conditions. It is expected that when there exists significant influence of the multidimensional effects, including boundary layer, vortex roll-up, and nonuniform heat release, the zero-dimensional modeling may not be adequate. In this work, we simulate ignition of hydrogen in an RCM by employing computational fluid dynamics (CFD) studies with detailed chemistry. Through the comparison of CFD simulations with zero-dimensional results, the validity of a zero-dimensional modeling for simulating RCM experiments is assessed. Results show that the zero-dimensional modeling based on the approach of “adiabatic volume expansion” generally performs very well in adequately predicting the ignition delay of hydrogen, especially when a well-defined homogeneous core is retained within an RCM. As expected, the performance of this zero-dimensional modeling deteriorates with increasing temperature nonuniformity within the reaction chamber. Implications for the species sampling experiments in an RCM are further discussed. Proper interpretation of the measured species concentrations is emphasized and the validity of simulating RCM species sampling results with a zero-dimensional model is assessed.