Hydrogen-air Combustion Research Articles

Quantitative three-dimensional reconstruction of cellular flame area for spherical hydrogen-air flames

The cellular flame area continues to increase with the development of cells on the spherical flame surface, which will greatly promote the flame burning rate and propagation speed. This work mainly focuses on a new three-dimensional (3D) reconstruction method of the cellular structure on the flame surface, attempting to quantitatively characterize the real flame area. Initially, the visualization investigation of hydrogen-air premixed spherical flames within a constant volume vessel was conducted using the Schlieren optical technique under room temperature and atmospheric pressure conditions. In parallel, the Cellpose 2.0 graphical user interface was used for the preliminary training of the cell segmentation model. Subsequently, this pre-trained model was applied in the image post-processing, enabling the quantitative characteristics extraction of the cellular structure, such as the cells number, area, and the flame radius, etc. Additionally, a concept of peak height h on the flame profile was proposed to characterize the fluctuation degree of flame profile. A new flame equivalent radius ru was defined by the average value of valid distance from flame centroid to flame profile pixel by pixel. Based on the comparison of cell equivalent radius r and average peak height h¯, an innovative 3D reconstruction concept was proposed for the quantitative characterization of flame area. Finally, cellularity factor ξ was introduced to evaluate the cellularization degree on spherical flames surface. Results show that the appearance of secondary cracks marks the formal onset of flame cellularization, accompanied by an increase in the h¯. In the later stages of flame development, cellularization will eventually tend to a stable value of about 0.4, indicating the occurrence of “saturated state”. After 3D reconstruction, the average cell area stable at around 26 mm2 in this stage. The results of this study provide data support for the construction of combustion models in the field of premixed hydrogen-air combustion.

Nitrogen oxides removal from hydrogen flue gas using corona discharge in marine boilers: Application perspective

This paper focuses on the combustion of hydrogen in boilers, as it appears to be a more effective method than using fuel cells for heating purposes due to higher boiler efficiency. One of the main disadvantages of hydrogen combustion in air is NOx formation. Therefore, the authors decided to introduce corona discharge as an innovative technique to clean hydrogen flue gas by effectively reducing NOx levels. The method involves generating positive plasma at atmospheric pressure by applying up to a 23 kV voltage difference between rod and ring-shaped electrodes. Experimental studies have shown that corona discharge can significantly lower the concentrations of NO and NOx in exhaust gases. The maximum DeNOx level was found to be 32.3%, while the plasma generator uses 17.5% of the power contained in burned hydrogen. The findings suggest that this technology holds potential for application in industrial hydrogen combustion systems, offering an environmentally friendly alternative to conventional NOx reduction methods.

Numerical Investigation of a Hydrogen–Air Flame for NOx Prediction

Abstract Sustainable aviation fuels are a major candidate to reduce pollutant emissions in future aeronautical engines. Recently, the use of hydrogen as a fuel has gained a high interest partly because its combustion is free from carbon dioxide, a greenhouse gas, and produces few pollutants, mainly nitrogen oxides (NOx). Over the last decades, efforts on numerical methods for combustion simulation in aero-engines have largely been focused on kerosene-air combustion. However, the current transition may have a significant impact on the computational methodologies for combustor design. Hydrogen defines novel modeling issues and challenges the current state of art on numerical methodologies. The current study presents a numerical investigation of a hydrogen–air burner using large-eddy simulations (LES) with a focus on NOx prediction. The considered configuration is a two-staged combustor, similar to the well-known RQL (Rich-Quench-Lean) technology, supplied by a single coaxial injector characterized experimentally. Two combustion models are investigated: (i) tabulated chemistry based on premixed flamelets (ii) transported chemistry description by using a 21-species chemical scheme. Numerical results are compared with experimental data (NOx concentrations, temperature distributions, pressure losses). A focus on model predictions is carried out. Results show a good agreement to predict the main flow characteristics and the premixed flame position over different operating points and geometries for both frameworks. In contrast, NOx emissions are more sensitive: while the overall trend is well captured, the quantification is more scattered. Finally, an in-depth analysis is proposed to link NOx production with the nonpremixed flame size.

New laminar flame speed correlation for lean mixtures of hydrogen combustion with water addition under high pressure conditions

Hydrogen may become a substitute for liquid fossil fuels, contributing to greenhouse gas emissions reductions in internal combustion engines. Numerical simulations play a critical role in the advancement of these engines, with laminar flame speed being the main input. Experimental data of hydrogen flame speed at elevated pressures are scarce, due to the instability of the flames. Nonetheless, stable hydrogen flames can be predicted using chemical kinetics models. Moreover, the injection of water into the hydrogen fuelled engine could offer several benefits to engine combustion and emission performance, as it modulates the laminar flame speed within the combustion chamber and this effect has not been completely understood. Currently, no correlation exists to predict the laminar flame speed of hydrogen-air combustion with water addition under lean mixture engine operating conditions. In this study, we have extended the newly developed laminar flame speed correlation of hydrogen-air combustion to account for the effects of water addition under engine relevant conditions by using chemical kinetic laminar flame speed values. The laminar flame speed correlation was derived for pressures from 10 to 70 bar, temperatures from 400 to 800 K, equivalence ratios from 0.35 to 1 and water addition by mole from 0 to 20%. The hydrogen laminar flame speed correlation was expressed using polynomial forms with reduced order and number of terms with optimized values of coefficients. Additionally, a new exponential term was proposed to the power term α of the laminar flame speed correlation to capture the coupled effects of pressure and temperature on laminar flame speeds under engine-relevant lean burn water-diluted operating conditions.

Large Eddy Simulation of Hydrogen/Air MILD combustion in a cyclonic burner

In this work Large Eddy Simulation (LES) of a Hydrogen/Air cyclonic burner operating in the MILD combustion regime at 1 atm is performed. The cyclonic flow configuration may represent a proper way to enhance the mixing process in a very short time while allowing for residence times long enough to achieve complete oxidation of diluted and preheated mixtures. Accurate molecular transport properties and a reduced chemical mechanism for hydrogen-air combustion, consisting of 18 transported species and 244 elementary reactions are considered. Results show that at the center of the combustor, where MILD combustion occurs, temperature exhibits a statistically steady trend (a mean of 1450 K) with peak values lower than 1700 K. This implies that the Zeldovich mechanism for thermal NO formation is negligible, so NO levels are low and statistically steady with a maximum peak of 20 ppm. These values are lower than those observed in standard combustion, confirming the ability of MILD to reduce NOx pollutants also with hydrogen as fuel. Finally, the simulations outcomes are in very good agreement with the experimental results, in particular radiative heat transfer must be taken into account to better capture the preheating of the reactants and consequently fit the experimental temperature profiles in the combustion chamber.

Early flame development characterization of ultra-lean hydrogen–air flames in an optical spark-ignition engine

Hydrogen-fueled internal combustion engines (H2ICEs) represent a promising technology for decarbonizing the transport sector. In pursuit of this goal, research is needed on lean hydrogen–air combustion under engine conditions aiming to mitigate NOx emissions. However, there is a limited amount of experimental data focusing on flame development at high pressure and temperature, especially in ultra-lean mixtures, which are recently considered to play an important role for future H2ICEs. This study investigates the behavior of very-lean to ultra-lean hydrogen–air flames (0.20 <ϕ< 0.55) in an optical spark-ignition engine, focusing on the early flame development phase. Fuel–air equivalence ratio and engine speed (900 <N< 1500 rpm) effects on hydrogen combustion are addressed with pressure-based heat release analysis and chemiluminescence flame imaging techniques. The findings revealed an intrinsic association between these two approaches. Hydrogen flames development measured before 2% of Mass Fraction Burned (MFB) strongly correlates with pressure-based quantities up to 50% MFB, underscoring the critical role of understanding the early flame phase for predicting total combustion duration and in-cylinder pressure traces. A model for hydrogen turbulent flames in the initial combustion stages is formulated based on the Zimont equation and the experimental data. An equation for 0D/1D combustion duration prediction up to 10% MFB is proposed, demonstrating its validity from the wrinkled flame to the well-stirred reactor premixed combustion regimes. Additionally, flame chemiluminescence images for each zone within the Peters–Borghi diagram are presented, offering valuable insights into the diverse characteristics of these flames.

Numerical modelling and simulation of hydrogen and air mixing to prevent ignition delay and flashback

Practically all residential and commercial gas appliances installed within the EU today were designed for operation with natural gas. A clean and efficient solution for heating and hot water generation is the combustion of hydrogen in case of a gas condensing boiler. Safe and stable combustion of hydrogen is a complex issue, and several influence parameters must be understood for the safe design of hydrogen capable gas condensing boilers. In premixed hydrogen-air combustion, there are two physical problems that should be avoided at all costs: flashback through the burner to the premixing duct and ignition delay in the combustion chamber. Well mixing of reactants is therefore very important to achieve stable and efficient hydrogen combustion. To evaluate the influence of commercialized mixing stages (fan-venturi combination), the impact of the rotational velocity on the degree of mixing in case of a venturi-fan combination used in domestic gas condensing boilers is presented in this paper. Transient, three-dimensional simulations with different turbulence modelling approaches were performed to assess the degree of mixing upstream the hydrogen capable multi-hole burner. It is shown that the lower angular velocities produce better mixing. It can also be assumed that a local variation of up to 17% in the adiabatic flame speed can be expected due to the mixing processes as the consequence of the local air-fuel equivalence ratio variation.

The performance of reaction mechanism in prediction of the characteristics of the diffusive-thermal oscillatory instability of methane–hydrogen–air burner-stabilized flames

In this work the performance of various detailed reaction mechanisms of combustion of methane–hydrogen fuel mixtures is studied. The investigation is focused on the burner stabilized flame configuration and is undertaken for the normal pressure conditions, which is a starting point for the analysis of the combustion chemistry at elevated pressures met in rocket engine chambers. The study presents the experimental setup, computational approach and results of comparison, which allow us to access the properties of complex combustion systems in variety of dynamical regimes. The comparison of the predicted and measured characteristics of these regimes is used to test the performance of the reaction models. In particular, the critical behavior, e.g. for blow-up, onset of pulsations and quasi-steady flame front propagation and bifurcation between these regimes may be used to characterize and to study properties of combustion systems with mixed fuels. A neutral stability boundary for onset of pulsations is suggested to be used in this study. A method for automatic identification of the boundary is proposed and implemented with several detailed mechanisms. Although for pure methane the results look acceptable, they show gradual divergence with the increase of percentage of hydrogen addition. Significant quantitative differences between experiments and modeling with respects to frequencies of oscillations are reported even for 2:1 ratio of hydrogen to methane in the unburned mixture. The results of the work indicate that current reaction mechanisms need to be improved in order to gain the quantitative predictability of methane–hydrogen–air combustion at normal pressure before proceeding to the conditions fuel-oxygen high pressure combustion more relevant for rocket chamber.

Effect of the State of the Reactor Surface on the Characteristics of Combustion of Gas Mixtures Containing Halogen-Substituted Hydrocarbons

The influence of the reactor walls on combustion of gas mixtures containing halogenated hydrocarbons at atmospheric pressure has been studied experimentally. When the wall is contaminated with combustion products, the additional amounts of bromine or iodine that passed from it into the volume reduce the efficiency of combustion inhibition of hydrogen–air mixtures by bromine- and iodine-containing hydrocarbons (the effect was more pronounced for iodinated substances) and weakens the self-inhibition of the combustion of ethyl bromide in a mixture with air, leading to the expansion of the concentration region of flame propagation. Based on the analysis of the known kinetic data, the experimentally observed picture was explained by a diminished role of the HI regeneration cycle during the inhibition of hydrogen combustion in air by iodinated hydrocarbon and decreased rate of the reaction of brominated hydrocarbons with atomic hydrogen when additional amounts of I2 and Br2 are supplied from the reactor wall.

Propagation of high-speed hydrogen-air combustion waves through inert gases

The propagation characteristics of high-speed hydrogen-air combustion waves through different inert gases (i.e., nitrogen, argon, and carbon dioxide) are experimentally investigated. Three inlet combustion waves of the transonic deflagration wave, supersonic deflagration wave, and stable detonation are considered. The variation of flame velocity, shock wave pressure and transient flame structures are presented and analyzed. Results indicate that a deflagration-to-detonation transition followed by a velocity decay tends to occur after the high-speed deflagration propagates into the inert gas. The deflagration-to-detonation transition is more likely to occur when the inlet combustion wave is enhanced from a transonic deflagration wave to a supersonic deflagration wave. For the inlet detonation wave case, the velocity decay process can be divided into two stages successively with the sound speed in the corresponding inert gas as the boundary value: nearly linearly decay and slowly decay accompanied by small fluctuations. For all cases, the velocity decay is closely coupled with the flame-inert gas interaction. Carbon dioxide shows the most significant dilution and suppression effects due to its considerable molecular weight and specific heat capacity compared with nitrogen and argon.

Minimum ignition energy of hydrogen-air mixtures at ambient and cryogenic temperatures

The ignition and combustion of hydrogen in air is considered more hazardous compared to other fuels due to the lower minimum ignition energy (MIE) and the wider flammability range. Spark discharge is the most common type of electrostatic ignition hazard. There is a need in validated safety engineering tools to accurately calculate MIE in a wide range of temperatures from atmospheric to cryogenic which are characteristic for hydrogen systems and infrastructure. Current MIE assessment methodologies rely on the availability of experimental data on quenching distance and/or laminar burning velocity and thus are mostly empirical correlations. This prevents their application beyond the limited number of experimental data, i.e. to arbitrary composition of the hydrogen-air mixture at arbitrary temperatures including cryogenic. This work aims at the development of a model able to accurately predict MIE for hydrogen-air mixtures with arbitrary initial composition and temperature. Cantera and Chemkin software are used to calculate the properties and unstretched laminar burning velocity of hydrogen-air mixtures. The flame thickness is found to well represent the critical flame kernel in the suggested model. The model is validated against experimental data on MIE for mixtures at ambient and cryogenic (down to 123 K) temperatures. Results show that the effect of flame stretch and preferential diffusion shall be considered to accurately predict MIE for lean hydrogen-air mixtures, which was not possible for previous models.

A Data-Driven Framework for Computationally Efficient Integration of Chemical Kinetics Using Neural Ordinary Differential Equations

AbstractA data-driven methodology is introduced for computationally efficient integration of systems of stiff rate equations in chemical kinetics using neural ordinary differential equations (NODE). A systematic algorithm is developed for training data generation and sampling. Subsequently, a novel transformation technique for sampled training data is designed to regularize the neural network parameters, leading to a stable training process. Finally, the NODE network is iteratively trained to learn the accurate neural network representation of chemical kinetics source terms by minimizing the mean absolute error between the true and predicted solutions. The computational efficiency and accuracy of the NODE network are evaluated by simulating the evolution of the thermochemical state of a constant pressure homogeneous hydrogen-air reactor. The combustion of hydrogen in air is described by a finite-rate mechanism including 9 chemical species and 21 reaction steps. The NODE network shows excellent multi-step prediction accuracy for a wide range of initial temperatures and equivalence ratios, spanning the composition space of real flames. The NODE also exhibit a significant reduction in numerical stiffness of the system, enabling the utilization of explicit solvers for integration. The present simulation results using NODE demonstrate up to 70% speed up in computation time compared to direct integration of the chemical mechanism with at most 3.16% relative error in ignition delay time.

Performance Assessment of Chemical Kinetics Neural Ordinary Differential Equations in Pairwise Mixing Stirred Reactor

Abstract The present study aims to assess the potential of the neural ordinary differential equations (NODE) network for reliable and computationally efficient implementation of chemistry in combustion simulations. Investigations are performed using a hydrogen-air pairwise mixing stirred reactor (PMSR). The PMSR is a zero-dimensional case affordable to study combustion chemistry entailing a similar numerical solution procedure as probability density function methods for turbulent combustion simulations. A systematic approach is presented to apply the NODE, solely trained on canonical constant pressure homogeneous reactor data, to predict complex chemistry and mixing interactions in PMSR. The reactor involves combustion of hydrogen in air described by a finite-rate mechanism with 9 chemical species and 21 reaction steps. The NODE network is shown to accurately capture the evolution of thermochemical variables for different mixing and chemical timescales. It also exhibits a significant reduction in numerical stiffness resulting in improving the computational efficiency and enabling the use of explicit solvers for the integration of chemical kinetics. The assessment results based on PMSR show that compared to direct integration of detailed kinetics, the NODE can achieve significant computational time speedup for a comparable accuracy.

The effects of unburned-gas temperature and pressure on the unstable behavior of cellular-flame fronts generated by intrinsic instability in hydrogen-air lean premixed flames under adiabatic and non-adiabatic conditions: numerical simulation based on the detailed chemical reaction model

ABSTRACT The effects of unburned-gas temperature and pressure on the unstable behavior of cellular-flame fronts in hydrogen-air lean premixed flames are elucidated by numerical simulations. The detailed chemical reaction mechanism with seventeen reversible reactions of eight reactive species and a diluent is adopted for hydrogen-air combustion and heat loss is taken into account. The burning velocity of a planar flame increases as the unburned-gas temperature increases, and it decreases as the unburned-gas pressure and heat loss increase. In the dispersion relation, the growth rate increases and the unstable range widens when the unburned-gas temperature increases. The former decreases and the later narrows when the heat loss becomes larger. The normalized growth rates become large under the non-adiabatic conditions. The normalized burning velocity of a cellular flame is large when the pressure is high and heat loss is large, and it decreases as the temperature increases at high pressure. This shows that the high unburned-gas temperature at high pressure weakens the instability in hydrogen-air lean premixed flames. With increasing the domain size, the burning velocity of a cellular flame increases, and the cellular-flame front becomes more unbalance. This is an aspect of the long-wavelength disturbance to the unstable behavior of cellular-flame front.

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.

DNS predictions of NOx production in developing turbulent mixing layers with non-premixed hydrogen–air combustion

ABSTRACT Direct numerical simulations of three-dimensional compressible mixing layers with non-premixed hydrogen–air combustion were performed using a detailed chemical reaction mechanism with production. Flow fields with three types of initial disturbances were simulated to investigate the relationship between developing vortical structures and formation. The amounts of and produced in the simple shear layer were smaller than those in the two- and three-dimensional mixing layers with vortical structures. In the mixing layers, the formation and expansion of the combustion region by the roller vortices and the baroclinic torque had a significant impact on production, while the relatively low-temperature combustion region formed by the three-dimensional developed rib vortices in the blade regions between the roller vortices had a large effect on the production. It was found that a two-dimensional simulation can estimate the production, while the information on a three-dimensional mixing layer is necessary to predict the production.

Modulation of wall turbulence by propagating flame of premixed hydrogen–air combustion

The mechanism of modulation of turbulent structures in the vicinity of a combustion engine wall is not yet clarified. Thus, we conducted direct numerical simulations of wall turbulence based on premixed hydrogen–air combustion using a detailed chemical reaction mechanism to clarify the mechanism of modulation of turbulence structures such as quasi-streamwise vortices by a flame propagating toward a wall. In particular, the turbulence modulation can alter the momentum and heat fluxes through the wall surface. In the simulations, we solved the fundamental equations for the compressible flow of multiple chemical species under combustion. The flame propagation speed was affected by the turbulence and was 2.04 times larger than that of the laminar flame. Moreover, upon examining the variation in the flow field, we determined that the turbulence kinetic energy in the flow field increased temporarily owing to the newly produced vortices along the flame surface including the existing turbulence vortices. In contrast, the turbulence vortices along the wall were suppressed as the flame approached the wall, and the turbulent flow tended to become laminar. In terms of vorticity transport equation, the suppression of the turbulence vortices was caused by the thermal expansion of the combustion rather than the increased viscosity. Furthermore, we determined that the vortex structure slightly influenced the chemical reaction process, but it strongly influenced the heat release rate distribution and concentration distribution of the chemical species. The current findings on the stated mechanism will aid in improving the accuracy of practical prediction and control methods for wall turbulence with combustion.

Assessing diffusion model impacts on enstrophy and flame structure in turbulent lean premixed flames

Diffusive transport of mass occurs at small scales in turbulent premixed flames. As a result, multicomponent mass diffusion, which is often neglected in direct numerical simulations (DNS) of premixed combustion, has the potential to impact both turbulence and flame characteristics at small scales. In this study, we evaluate these impacts by examining enstrophy dynamics and the internal structure of the flame for lean premixed hydrogen-air combustion, neglecting secondary Soret and Dufour effects. We performed three-dimensional DNS of these flames by implementing the Stefan–Maxwell equations in the code NGA to represent multicomponent mass transport, and we simulated statistically planar lean premixed hydrogen-air flames using both mixture-averaged and multicomponent models. The mixture-averaged model underpredicts the peak enstrophy in the multicomponent simulation by up to 13% in the flame front. Comparing the enstrophy budgets of these flames, the multicomponent simulation yields larger peak magnitudes compared to the mixture-averaged simulation in the reaction zone, showing differences of 17% and 14% in the normalised vortex stretching and viscous effects terms. In the super-adiabatic regions of the flame, the mixture-averaged model overpredicts the viscous effects by up to 13%. To assess the effect of these differences on flame structure, we reconstructed the average local internal structure of the turbulent flame through statistical analysis of the scalar gradient field. Based on this analysis, we show that large differences in viscous effects contribute to significant differences in the average local flame structure between the two models.

Flame acceleration and deflagration-to-detonation transition in narrow channels filled with stoichiometric hydrogen-air mixture

Understanding flame acceleration (FA) and deflagration-to-detonation transition (DDT) is important for combustion applications. However, DDT is a high-speed, nonlinear and complex process and the exact underlying mechanisms remain unclear, especially for narrow channels with a width smaller than detonation cell size. In this paper, numerical simulations were carried out to study the FA and DDT in a stoichiometric hydrogen-air mixture in narrow channels with width on the order of magnitude comparable to laminar flame thickness. The fully-compressible reactive Navier-Stokes equations were solved by using a high-order numerical method. Hydrogen-air combustion was taken into account using a calibrated simplified chemical-diffusion model. Four different channel widths, i.e., d = 2.0 mm (∼6 δl), 1.0 mm (∼3 δl), 0.5 mm (∼1.5 δl), 0.25 mm (∼0.7 δl), were considered in the simulations to explore the size effect, where δl is the laminar flame thickness. The results show that the FA is mainly determined by boundary layer effects for all the channels considered. In the early stages, the flame keeps accelerating due to the stretching effect of boundary layer that leads to a significant increase in flame surface area. In the later stages of acceleration, an ultra-fast flame develops in the boundary layer ahead of the flame front due to the viscous heating effect associated with wall friction. It was found that the DDT mechanism is essentially consistent in all the channels. Detonation arises from the contact of two neighboring flame fronts in the shocked region that is vulnerable to a mild temperature gradient or fluctuation. The contact of the two high-temperature reaction fronts instantly causes a very high concentration of heat flux and consequently a detonation wave emerges from the contact spot. The detonation propagation after DDT, however, depends on the channel width. When d ≥ 1.5 δl, a quasi-steady or a single-head detonation forms and sustains until the end of the combustion. For the case of d < 1.5 δl, the detonation decays into a high-speed flame. When the channel width is sufficiently small, e.g., d ∼0.7 δl, detonation initiation can occur occasionally, but soon decouples into a shock and a flame.

Effects of vitiation on the shock-induced combustion of hydrogen-air mixtures

In order to achieve a more thorough understanding of the impacts of vitiation on supersonic combustion, a series of numerical simulations are performed to investigate the shock-induced combustion of hydrogen-air mixtures. Direct comparisons are made between electric heating, in which the mixture is brought up to the designated temperature without modifying its composition, and vitiated heating, in which the mixture is heated by the products of hydrogen-air combustion. While the cases with electric heating have much higher thermicities than the cases with vitiation at a given condition, vitiated heating tends to have smaller induction lengths than electric heating for lower temperatures and richer mixtures. At reasonable residence times, this decrease in induction length is primarily caused by the presence of nitric oxide in the vitiated mixture and not by hydroxyl or hydrogen radicals, which implies that nitrogen chemistry cannot easily be neglected in studies of supersonic ignition and detonation.