Experimental investigation on the near detonation limits of propane/hydrogen/oxygen mixtures in a rectangular tube

In this paper, an experimental study on the near detonation limits for propane-hydrogen-oxygen is performed. Three mixtures (i.e., 8H₂–C₃H₈–9O₂, 4H₂–C₃H₈–7O₂ and 12H₂–C₃H₈–11O₂) are tested in a rectangular tube (52 mm × 32 mm). Photodiodes with regular intervals are mounted on the tube wall to measure the time of arrival of detonation waves, from which the detonation velocity is determined. Smoked foils are inserted into the tube to obtain the detonation cell pattern. The results indicate that well within the detonation limits, the detonation can propagate at a steady velocity. By reducing the initial pressure, the detonation velocity decreases gradually. Subsequently, the detonation fails as the initial pressure is below a critical pressure. The critical pressures for 8H₂–C₃H₈–9O₂, 4H₂–C₃H₈–7O₂ and 12H₂–C₃H₈–11O₂ mixtures are 4 kPa, 5 kPa and 6 kPa, and the corresponding detonation velocity deficits are 10%, 9%, 10%, respectively. The cellular detonation structures show that the cell size decreases with the decrease of the hydrogen concentration, and the cell structures are very irregular near the detonation limits.     

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.     

Flame acceleration and blast wave of H2–O2–N2–Ar mixtures in unconfined areas

In the present study, the flame acceleration and blast wave of H₂-air and H₂–O₂–N₂–Ar mixtures in nearly unconfined conditions were investigated conducting laboratory-scale and large-scale experiments. In particular, the effect of the volumetric expansion ratio on the flame acceleration examined, because Darrieus-Landau instability is essentially caused by a volumetric expansion of burned gas at flame front. Results demonstrated that the critical Péclet number Pe_c for the onset of flame acceleration decreased with the decrease in the equivalence ratio and the increase in the expansion ratio diluted with Ar. The dimensionless burning velocity increased as the flame radius became larger. The acceleration exponent for H₂-air and H₂–O₂–N₂–Ar mixtures was evaluated. The saturation of the exponent corresponding to the fractal dimension for only very lean H₂-air flame is observed, notwithstanding the value for other mixtures still existed in the transition regime. The blast wave intensity of H₂-air and H₂–O₂–N₂–Ar explosion discussed by the acoustic theory. From the analysis, it is clear the blast wave intensity depends on the expansion ratio, laminar burning velocity and flame acceleration.     

Detonation propagation characteristic of H2–O2–N2 mixture in tube and effect of various initial conditions on it

As for the premixed H₂–O₂–N₂ gas ignited and induced by flame in tube, this paper represents systemic researches on its detonative formation process and flow field changes under different initial conditions (pressure, temperature, component concentration). The conservational Euler equation set with chemical reaction is taken as the basic gas phase equation model and the reduced elementary chemical reaction and shock wave problem are considered available so as to establish a theoretical model of premixed H₂–O₂–N₂ combustible gas detonation process. A unity coupling TVD format with second-order accuracy is adopted to solve the gas phase equation and deduce the two-dimension Riemann invariant, and the TVD format for solution of the polycomponent convection equation with elementary chemical reaction is proposed. Meanwhile, a time splitting format is adopted to perfectly treat with the rigid problem resulted from the higher time difference value between gas phase flow characteristic time and chemical reaction characteristic time. It is shown by the calculation results that the detonation waves form certain angle with relation to the tube wall surface at the initial stage of ignition, so as to incur reflections and form reflection waves; during the propagation of the detonation waves, the reflection wave structures are propagated backwards the back of waves constantly, so the whole flow field is characterized of obvious two-dimension. Besides, the excessive pressure detonation occurs at first before formation of the stable detonation propagation process, then a stable detonation propagation process forms finally. Mixed gas detonation characteristics resulted from different calculated-initially parameters are different. The higher the initial temperature and pressure of flame is, the shorter the induction time for detonation formed due to combustion acceleration of the mixed gas is, but which nearly brings no great influence on the later propagation process of the detonation waves. The initial mixed gas component can influence the detonation characteristic of the mixed gas observably, when the quantity relative ratio is close to 1 and the mixed gas with larger reaction activity, its detonation propagation speed is rapider and the pressure after detonation waves is higher. The simulation result keeps accordant with the calculated result of the typical C–J detonation theory model.     

Effect of hydrogen addition on the dynamics of premixed C1–C4 alkane-air flames in a microchannel with a wall temperature gradient

The effect of hydrogen (H₂) addition on the flame dynamics of premixed C₁–C₄ alkane/air mixtures in a microchannel is investigated using a detailed-chemistry model through two-dimensional numerical computations. A detailed computational study have been performed in a 2 mm diameter tube with 120 mm length and a wall temperature gradient along the axial direction of the channel. The numerical simulations are carried out for various stoichiometric hydrocarbon (HC)/H₂ mixtures at 0.15 m/s mixture inlet velocity. Flame repetitive extinction and ignition (FREI) flame pattern has been identified for all the fuel mixtures at these channel wall and mixture flow conditions. CH₄/air mixture shows a higher HRR than C₃–C₄ alkane/air mixtures. Flame residence time in microchannel increases with increase in hydrogen addition percentage for all the three hydrocarbon/air mixtures considered in the present study. A non-monotonic behavior of FREI frequency is identified for CH₄/air mixture, whereas it decreases monotonically for C₃H₈/air and C₄H₁₀/air mixtures with H₂ addition. The amount of HRR and flame propagation velocity decreases with increase in H₂ addition for lower-alkanes/air mixtures. The flame bifurcation effect is observed for CH₄/air mixture, which disappears due to H₂ addition in the mixture. The bifurcation effect is not present for other hydrocarbon/air mixtures investigated in the present study. The addition of H₂ in the mixture enhances the flame stability of hydrocarbon/air mixtures in the microchannel.     

Detonation wave propagation in semi-confined layers of hydrogen–air and hydrogen–oxygen mixtures

This paper presents results of an experimental investigation on detonation wave propagation in semi-confined geometries. Large scale experiments were performed in layers up to 0.6 m filled with uniform and non-uniform hydrogen–air mixtures in a rectangular channel (width 3 m; length 9 m) which is open from below. A semi confined driver section is used to accelerate hydrogen flames from weak ignition to detonation. The detonation propagation was observed in a 7 m long unobstructed part of the channel. Pressure measurements, ionization probes, soot-records and high speed imaging were used to observe the detonation propagation. Critical conditions for detonation propagation in different layer thicknesses are presented for uniform H₂/air-mixtures, as well as experiments with uniform H₂/O₂ mixtures in a down scaled transparent channel. Finally detail investigations on the detonation wave propagation in H₂/air-mixtures with concentration gradients are shown.     

Flame acceleration and transition from deflagration to detonation in hydrogen explosions

Computational Fluid Dynamics solvers are developed for explosion modelling and hazards analysis in Hydrogen air mixtures. The work is presented in two parts. These include firstly a numerical approach to simulate flame acceleration and deflagration to detonation transition (DDT) in hydrogen–air mixture and the second part presents comparisons between two approaches to detonation modelling. The detonation models are coded and the predictions in identical scenarios are compared. The DDT model which is presented here solves fully compressible, multidimensional, transient, reactive Navier–Stokes equations with a chemical reaction mechanism for different stages of flame propagation and acceleration from a laminar flame to a highly turbulent flame and subsequent transition from deflagration to detonation. The model has been used to simulate flame acceleration (FA) and DDT in a 2-D symmetric rectangular channel with 0.04 m height and 1 m length which is filled with obstacles. Comparison has been made between the predictions using a 21-step detailed chemistry as well as a single step reaction mechanism. The effect of initial temperature on the run-up distances to DDT has also been investigated.     

Assessment of H2-CH4-air mixtures oxidation kinetic models used in combustion

Because of a wide number of applications, the potential hazards of H₂-CH₄-air mixtures have to be characterised. For hazard evaluation, an important element is a reliable detailed kinetic scheme. In the present study, three modern kinetic models, those of Konnov, of Dagaut and the GRI-mech 03, have been evaluated with respect to a large set of experimental data, including species profiles obtained in jet-stirred reactor, laminar flame speed, ignition delay time and detonation cell size, for hydrogen-methane-air mixtures. For jet-stirred reactor data, the model of Dagaut provides significantly better results. For flame speed data modeling, the three models are as reliable. For ignition delay times, the model of Dagaut seems the most reliable. For detonation cell size predictions, the model of Konnov is the best. Important chemical reactions are underlined through sensitivity and reaction pathway analysis and are discussed in the frame of rate constant values recommended by Baulch et al.     

High speed turbulent deflagrations and transition to detonation in H2air mixtures

An experimental investigation on flame acceleration and transition to detonation in H₂air mixtures has been carried out in a tube which had a 5 cm cross-sectional diameter and was 11 m long. Obstacles in the form of a spiral coil (6 mm diameter tubing, pitch 5 cm, blockage ratio BR = 0.44) and repeated orifice plates spaced 5 cm apart with blockage ratios of BR = 0.44 and 0.6 were used. The obstacle section was 3 m long. The compositional range of H₂ in air extended from 10 to 45%, the initial pressure of the experiment was 1 atm, and the mixture was at room temperature. The results indicate that steady-state flame (or detonation) speeds are attained over a flame travel of 10–40 tube diameters. For H₂ ? 13% maximum flame speeds are subsonic, typically below 200 m/s. A sharp transition occurs at about 13% H₂ when the flame speed reaches supersonic values. A second transition to the so-called quasi-detonation regime occurs near the stoichiometric composition when the flame speed reaches a critical value of the order of 800 m/s. The maximum value of the averaged pressure is found to be between the normal C-J detonation pressure and the constant volume explosion value. Of particular interest is the observation that at a critical composition of about 17% H₂ transition to normal C-J detonation occurs when the flame exits into the smooth obstacle-free portion of the tube. For compositions below 17% H₂, the high speed turbulent deflagration is observed to decay in this portion of the tube. The detonation cell size for 17% H₂ is about 150 mm and corresponds closely to the value of πD that has been proposed to designate the onset of single-head spinning detonation, in this case for the 5 cm diameter tube used. This supports the limit criterion, namely, that for confined detonations in tubes, the onset of single-head spin gives the limiting composition for stable propagation of a detonation wave.     

Experiments on combustion regimes for hydrogen/air mixtures in a thin layer geometry

A series of experiments in a thin layer geometry performed at the HYKA test site of the KIT. Experiments on different combustion regimes for lean and stoichiometric H₂/air mixtures were performed in a rectangular chamber with dimensions of 200 × 900 x h mm³, where h is the thickness of the layer (h = 1, 2, 4, 6, 8, 10 mm). To model a gap between a fuel cell assembly and a metal housing, three different layer geometries were investigated: (1) a smooth channel without obstructions; (2) a channel with a metal grid filled 25% of chamber length and (3) a metal grid filled 100% of chamber length. The blockage ratio of metal grid has changed from 10 to 60% of cross-section. Detail measurements of H₂/air combustion behavior including flame acceleration (FA) and DDT in closed rectangular channel have been done. Five categories of flame propagation regimes were classified. Special attention was paid to analysis of critical condition for different regimes of flame propagation as function of layer thickness and roughness of the channel. It was found that thinner layer suppresses the detonation onset and even with a roughness, the flame may quench or, in thicker layer, is available to accelerate to speed of sound. The detonation may occur only in a channel thicker than 4 mm.     

Numerical simulation of flame acceleration and deflagration to detonation transition in hydrogen-air mixture

A numerical approach has been developed to simulate flame acceleration and deflagration to detonation transition in hydrogen-air mixture. Fully compressible, multidimensional, transient, reactive Navier–Stokes equations are solved with a chemical reaction mechanism which is tuned to simulate different stages of flame propagation and acceleration from a laminar flame to a turbulent flame and subsequent transition from deflagration to detonation. Since the numerical approach must simulate both deflagrations and detonations correctly, it is initially tested to verify the accuracy of the predicted flame temperature and velocity as well as detonation pressure, velocity and cell size. The model is then used to simulate flame acceleration (FA) and transition from deflagration to detonation (DDT) in a 2-D rectangular channel with 0.08 m height and 2 m length which is filled with obstacles to reproduce the experimental results of Teodorczyk et al.The simulations are carried out using two different initial ignition strengths to investigate the effects and the results are evaluated against the observations and measurements of Teodorczyk et al.     

The flame propagation characteristics and detonation parameters of ammonia/oxygen in a large-scale horizontal tube: As a carbon-free fuel and hydrogen-energy carrier

As a carbon-free fuel and a hydrogen-energy carrier, ammonia is a potential candidate for future energy utilization. Therefore, in order to promote the application of ammonia in detonation engines and to evaluate the safety of ammonia related industrial process, DDT experiments for ammonia/oxygen mixtures with different ERs were carried out in a large-scale horizontal tube. Moreover, pressure transducers and self-developed temperature sensors were applied to record the overpressure and the instantaneous flame temperature during DDT process. The results show that the DDT process in ammonia/oxygen mixtures contains four stages: Slow propagation stage, Flame and pressure wave acceleration stage, Fast propagation and detonation wave formation stage, Detonation wave self-sustained propagation stage. For stoichiometric ammonia/oxygen mixtures, flame front and the leading shock wave propagate one after another with different velocity, until they closely coupled and propagated together with one steady velocity. At the same time, it is found that an interesting retonation wave propagates backward. The peak overpressure, detonation velocity, and flame temperature of the self-sustained detonation are 2 MPa, 2000 m/s and 3500 K, respectively. With the ER increased from 0.6 to 1.6, the detonation velocities and peak overpressures ranged from 2310 m/s to 2480 m/s and 25.6 bar–28.7 bar, respectively. In addition, the detonation parameters of ammonia were compared with those of methane and hydrogen to evaluate the detonation performance and destructiveness of ammonia.     

Numerical analysis on influence of hydrogen peroxide (H2O2) addition on the combustion and emissions characteristics of NH3/CH4-air (O2/N2)/ H2O2 mixture

In this work, extensive chemical kinetic modeling is performed to analyze the combustion and emissions characteristics of premixed NH₃/CH₄–O₂/N₂/H₂O₂ mixtures at different replacement percentages of air with hydrogen peroxide (H₂O₂). This work is comprehensively discusses the ignition delay time, flame speed, heat release rate, and NOx & CO emissions of premixed NH₃/CH₄–O₂/N₂/H₂O₂ mixtures. Important intermediate crucial radicals such as OH, HO₂, HCO, and HNO effect on the above-mentioned parameters is also discussed in detail. Furthermore, correlations were obtained for the laminar flame speed, NO, and CO emissions with important radicals such as OH, HO₂, HCO, and HNO. The replacement of air with H₂O₂ increases flame speed and decreases the ignition delay time of the mixture significantly. Also, increases the CO and NOx concentration in the products. The CO and NOx emissions can be controlled by regulating the H₂O₂ concentration and equivalence ratios. Air replacement with H₂O₂ enhances the reactions rate and concentration of intermediate radicals such as O/H, HO₂, and HCO in the mixture. These intermediate radicals closely govern the combustion chemistry of the NH₃/CH₄– O₂/N₂/H₂O₂ mixture. A linear correlation is observed between the flame speed and peak mole fraction of OH + HO₂ radicals, and 2nd degree polynomial correlation is observed for the peak mole fraction of NO and CO with HNO + OH and HCO + OH radicals, respectively.     

Effect of hydrogen and syngas addition on the ignition of iso-octane/air mixtures

There is worldwide interest in using renewable fuels within the existing infrastructure. Hydrogen and syngas have shown significant potential as renewable fuels, which can be produced from a variety of biomass sources, and used in various transportation and power generation systems, especially as blends with hydrocarbon fuels. In the present study, a reduced mechanism containing 38 species and 74 reactions is developed to examine the ignition behavior of iso-octane/H₂ and iso-octane/syngas blends at engine relevant conditions. The mechanism is extensively validated using the shock tube and RCM ignition data, as well as three detailed mechanisms, for iso-octane/air, H₂/air and syngas/air mixtures. Simulations are performed to characterize the effects of H₂ and syngas on the ignition of iso-octane/air mixtures using the closed homogenous reactor model in CHEMKIN software. The effect of H₂ (or syngas) is found to be small for blends containing less than 50% H₂ (or syngas) by volume. However, for H₂ mole fractions above 50%, it increases and decreases the ignition delay at low (T < 900 K) and high temperatures (T > 1000 K), respectively. For H₂ fractions above 80%, the ignition is influenced more strongly by H₂ chemistry rather than by i-C₈H₁₈ chemistry, and does not exhibit the NTC behavior. Nevertheless, the addition of a relatively small amount of i-C₈H₁₈ (a low cetane number fuel) can significantly enhance the ignitability of H₂-air mixtures at NTC temperatures, which are relevant for HCCI and PCCI dual fuel engines. The CO addition seems to have a negligible effect on the ignition of i-C₈H₁₈/H₂/air mixtures, indicating that the ignition of i-C₈H₁₈/syngas blends is essentially determined by i-C₈H₁₈ and H₂ oxidation chemistries. The sensitivity and reaction path analysis indicates that i-C₈H₁₈ oxidation is initiated with the production of alkyl radical by H abstraction through reaction: i-C₈H₁₈ + O₂ = C₈H₁₇ + HO₂. Subsequently, the ignition chemistry in the NTC region is characterized by a competition between two paths represented by reactions R2 (C₈H₁₇ + O₂ = C₈H₁₇O₂) and R8 (C₈H₁₇ + O₂ = C₈H₁₆ + HO₂), with the R8 path dominating, and increasing the ignition delay. As the amount of H₂ in the blend becomes significant, it opens up another path for the consumption of OH through reaction R36 (H₂ + OH = H₂O + H), which slows down the ignition process. However, for T > 1100 K, the presence of H₂ decreases ignition delay primarily due to reactions R31 (O₂ + H = OH + O) and R35 (H₂O₂ + M = OH + OH + M).     

End-gas autoignition and knocking combustion of ammonia/hydrogen/air mixtures in a confined reactor

End-gas autoignition and detonation development in ammonia/hydrogen/air mixtures in a confined reactor is studied through detailed numerical simulations, to understand the knocking characteristics under IC engine relevant conditions. One-dimensional planar confined chamber filled with ammonia/hydrogen/air mixtures is considered. Various initial end-gas temperature and hydrogen concentration in the binary fuels are considered. Homogeneous ignition of stochiometric ammonia/hydrogen/air mixtures is firstly calculated. It is found that H₂ addition significantly promotes autoignition, even if the amount of addition is small. For ammonia/air mixtures and ammonia/hydrogen/air mixtures with low hydrogen mole ratios, it is found from chemical explosive mode analysis results that NH₂ and H₂NO are most important nitrogen-containing species, and R49 (NH₂+NO<=>N₂+H₂O) is a crucial reaction during thermal runaway process. When the hydrogen mole ratio is high, the nitrogen-containing species and reactions on chemical explosive mode becomes less important. Moreover, a series of one-dimensional simulations are carried out. Three end-gas autoignition and combustion modes are observed, which includes forcibly ignited flame propagation, autoignition (no detonation), and developing detonation. These modes are identified within wide ranges of hydrogen contents and initial end-gas temperatures. Furthermore, chemical kinetics at the reaction front and autoignition initiation locations are also studied with chemical explosive mode analysis. Finally, different thermochemical conditions on knocking intensity and timing are investigated. It is found that a higher initial temperature or a higher H₂ content does not always lead to a higher knocking intensity, and the knocking timing decreases with the reactivity of end-gas.     

Influence of longitudinal obstacle spacing on the deflagration-to-detonation transition through a bank of obstacles in a hydrogen-air mixture

Flame acceleration and deflagration-to-detonation transition (DDT) in a channel containing an array of staggered cylindrical obstacles and a stoichiometric hydrogen-air mixture were studied by solving the fully-compressible reactive Navier-Stokes equations using a high-order numerical algorithm and adaptive mesh refinement. Four different longitudinal spacings (ls) of the neighboring obstacle rows (i.e., ls = 15.28, 19.1, 25.4, and 38.2 mm, corresponding to 1.2, 1.5, 2 and 3 times of obstacle diameter, respectively) were used to examine the effect of obstacle spacing on flame acceleration and DDT. The results show that the main mechanisms of flame acceleration and transition to detonation in all the cases studied are consistent. While the flame acceleration is caused by the growth of flame surface area in the initial stage, it is governed by shock-flame interactions in the later stage when shock waves are generated. The focusing of strong shocks at flame front is responsible for the initiation of detonation. It was found that the flame propagation speed and the DDT run-up distance and time are highly dependent on ls. Specifically, the flame acceleration declines as ls increases, since a larger ls leads to less disturbance of flow by obstacles per unit channel length. For detonation initiation, both the run-up distance and time increase with the increase of ls. It is interesting to note that the DDT distance and time increase significantly as ls increases from 19.1 mm to 25.4 mm. This is related to the slowdown of the increase rate of energy release over a period before DDT occurs under large ls condition, because every time the flame passes over an obstacle row the shock-flame interaction is delayed and numerous isolated pockets of unburned gas material are formed.     

Acceleration of laminar hydrogen/oxygen flames in a tube and the possible onset of detonation

The possibility is analysed of a laminar flame accelerating along a cylindrical tube, closed at one end, and inducing a deflagration to detonation transition in a stoichiometric H₂/O₂ mixture. The pressure and temperature ratios at the ensuing shock wave increase, as do laminar burning velocities, while autoignition delay times decrease. Combined with appreciable elongation of the flame, these enhance the strength of the shock. The conditions necessary for delay times of 0.05, 0.1, 1.0 and 5.0 ms, at an unburned mixture critical Reynolds number of 2300, are computed for different tube diameters. Probable consequences of the different delay times and hot spot reactivity gradients, including detonation, are all considered. The probability of a purely laminar propagation leading to a detonation is marginal. Only when the initial temperature is raised to 375 K, do purely laminar detonations become possible in tubes of between about 0.5 and 1.35 mm diameter.     

Experimental evaluation of aluminum powder fuel in a hydrogen/oxygen detonation tube

Solid powder detonation technology was investigated in this study. Several problems related to the controllable and uniform injection of solid powder fuels were examined. This study demonstrated the feasibility of solid powder detonation technology by designing fluidization devices and evaluating the solid powder supply system. A stable supply of powder was achieved, and ignition tests were conducted using Al/H₂/O₂ and H₂/O₂ fuels. The maximum detonation wave velocity obtained using Al/H₂/O₂ was 2950 m/s, which was 40% higher than that obtained using H₂/O₂. We observed a mixed two-phase double-front detonation structure, with hydrogen/oxygen being detonated in the first front and aluminum powder/oxygen being detonated in the second wave. At a relatively low equivalence ratio, the aluminum powder/oxygen wave surface pressure was higher, whereas the hydrogen/oxygen wave surface pressure was higher at a higher equivalence.     

Initiation and formation of the corrugated structure leading to the self-turbulization of downward propagating flames in a combustion tube with external laser absorption

Combustion tube (length 45 cm, inner diameter 5 cm) experiments with flames of premixed gas of C₂H₄/CO₂–O₂ (Le < 1) were conducted. The flame fronts propagated downward to the closed bottom of an open-ended tube. An initially steadily propagating flat flame was deformed by an external laser irradiation method to investigate its evolution under the interaction with acoustic vibration. Results showed that the locally deformed flame evolved into a corrugated structure at the flame front followed by self-turbulization. The process to form this corrugated structure was investigated in detail based on the images captured using high-speed cameras. From the observations, a possible mechanism for the initiation of the corrugated structure, explained mainly by periodic acoustic acceleration, was proposed. Then, according to the mechanism an alternative definition for the inverse Froude number is proposed in this work and used as criterion for the initiation of the corrugated flame structure. To prove the validity of the criterion two mixtures having different flame speed were tested and it was confirmed that the criterion provided transition condition very well for both tested mixtures.     

Effects of hydrogen peroxide addition on two-stage ignition characteristics of n-heptane/air mixtures

Rising fuel cost and environmental concerns of greenhouse emissions have driven the development of advanced engine technology with optimal fuel strategy that can simultaneously yield high thermal efficiency and low emissions. Due to its strong reactivity and extra oxygen atom serving as an oxidizer, hydrogen peroxide (H₂O₂) has been used along with other hydrocarbons to promote overall combustion process. To explore the potential benefits of H₂O₂ in clean combustion technology, a numerical study with detailed chemistry is conducted to investigate the effects of H₂O₂ addition on the two-stage ignition characteristics of n-heptane/air mixtures at low-to-intermediate temperatures (below 1000 K), with due emphasis on how the negative temperature coefficient (NTC) behavior is affected. The results show that H₂O₂ addition shortens both the first-stage and total ignition delay times of n-heptane/air mixtures and suppresses the NTC behavior by reducing the upper turnover temperature. With increasing H₂O₂ addition, the lower turnover temperature, corresponding to the first-stage ignition delay minimum, is found to increase first and then decrease. Chemical kinetic analyses show that the addition of H₂O₂ promotes both first- and second-stage ignition reactivity by enhancing OH production through H₂O₂ decomposition. Furthermore, low-temperature chemistry controls the first-stage ignition, while H₂O₂ chemistry dominates the second-stage ignition.