Overdriven Detonation Research Articles

Evaluating the effect of ignition energy on gaseous hydrogen–oxygen detonations

The effect of ignition energy on the cellular structure of hydrogen-oxygen and hydrogen-oxygen-argon detonations was investigated experimentally over a range of initial conditions. The increase in ignition energy resulted in increased detonation propagation velocities, leading to overdriven detonations for most of the reactive mixtures. The detonation cell width decreased for mixtures subjected to higher ignition energy. A substantial reduction in the detonation cell width was observed for mixtures at a lower initial pressure. The coefficient of variation of the measured detonation cell width decreased with ignition energy, indicating greater regularity and uniformity in cellular structure. The observed reduction in the cell width can be attributed to the propagation of an overdriven detonation in the reactive mixtures due to high ignition energy.

Analysis of Plastic Deformation of Pipes Due to Deflagration to Detonation Transition Using Static Equivalent Pressure

Abstract Explosions of premixed gases with deflagration to detonation transition (DDT) are a risk in the process industry with potentially catastrophic consequences. Robust and simple design methods are required for industrial use. Such a simplified design method based on finite element analysis is proposed to model the dynamic behavior of pipes loaded by gas explosions of premixed gases including DDT. The focus is on so-called long pipes, where the DDT is not affected by reflections at blind flanges. First, it is reviewed how the pressure load of the gas explosion and the resulting material behavior has been modeled in previous publications. The analytical equations are then extended for describing the pressure load as a function of time and location by considering the leading compression wave as it affects the pressure load of the overdriven detonation. The extended analytical equations are parameterized using Chapman-Jouguet conditions and experimental results from publications for both static equivalent pressure and the location of the DDT. To describe the plastic material behavior at high strain rates, the well-known Johnson-Cook plasticity model is used. The material parameters of the model are derived from simple experiments, which are available in an industrial environment. The comparison of a finite element simulation with experimental data shows that the concept of equivalent static pressure can be extended to an finite element analysis, which in the future will allow the sizing of complete pipe systems including tees, bends, and flanges while considering plastic deformation.

Comparative study on the equation of state of detonation products

The equation of state (EOS) for detonation products can predict explosive performance with basic information about density, enthalpy of formation, and elemental composition. However, to achieve higher prediction accuracy, it is necessary to select an appropriate EOS for the specific detonation parameters being calculated. To study the applicability of different equations of states for explosive products, five EOSs, Becker–Kistiakowsky–Wilson, Kihara–Hikita–Tanaka, Virial-Wu (VLW), Lennard-Jones–Devonshire (LJD), and Jacobs–Cowperthwaite–Zwisler (JCZ), were used to calculate the Hugoniot curve of nitrogen and the detonation performance of typical explosives. The calculation results show that all five EOSs can provide reasonably accurate predictions of the Hugoniot curve of nitrogen on the relative volume-pressure plane. Moreover, the EOSs can accurately predict the detonation velocity and pressure of explosives within a wide range of initial densities. When it comes to temperature calculations, LJD and JCZ demonstrate higher prediction accuracy. Additionally, the results indicate that VLW may not be suitable for calculating the overdriven detonation of explosives, particularly at higher pressures.

Experimental study on combustion and explosion characteristics of hydrogen-air premixed gas in rectangular channels with large aspect ratio

To assess the consequences of hydrogen leakage, subsequent combustion, and resulting explosions within confined spaces such as ducts and channels and to reveal the combustion and explosion characteristics and influencing mechanisms of hydrogen, a rectangular test tube with a pressure rating of 10 MPa and a large aspect ratio of 120 was designed. The purpose of this study was to simulate the flame propagation and overpressure characteristics of hydrogen-air mixtures ignited in rectangular channels with large aspect ratios under atmospheric pressure. The effects of the hydrogen volume fraction (20%–45 %) and pipe blockage ratio (0/0.4) on the hydrogen combustion and explosion processes were analyzed. The results show that, under both smooth tube and obstacle installation conditions, increasing the hydrogen volume fraction from 20 % to 25 % resulted in 367.6 % and 63 % increases in explosion overpressure, respectively. The presence of obstacles considerably increases the explosion overpressure, and the position of the highest overpressure moves from downstream to upstream of the explosion outlet. Within the range of 25%–45 % hydrogen volume fraction, the overpressure exceeded the theoretical detonation pressure. At a hydrogen volume fraction of 35 %, the highest overpressure reached 2899 kPa, which was approximately twice that of the smooth tube. At a hydrogen volume fraction of 40 %, the maximum range of overdriven detonation was observed in the tube.

Detonation wave structure and thrust variation of a ram accelerator with different projectile velocities

A study was conducted to investigate the effect of velocity on the detonation wave structure and thrust of a ram accelerator using a simulation of a premixed, reacting flow over a conical projectile. The simulation involved solving the compressible reactive Navier-Stokes equations with a detailed chemical model in a channel with a supersonic stoichiometric hydrogen-oxygen inflow. Results showed that at projectile velocities lower than the Chapman-Jouguet detonation velocity, a thermally choked unstart occurs. As the projectile velocity increases, the overdriven detonation mode, trans-detonative mode, and super-detonative mode successively form. The maximum thrust was achieved at the lower velocity limit under the overdriven normal detonation condition, but decreased continuously as an oblique detonation wave formed. The super-detonative mode was distinguished by the smoothness of the tail thrust derived from the recirculation zone and ignition before the conical ramp led to an unstart at the upper velocity limit.

Influence of overdriven detonation on the energy release of aluminized explosives in underwater explosion

An underwater explosion experiment was designed for the cyclotrimethylenetrinitramine-based aluminized explosives to study the influence of detonation conditions on the explosion performance and the afterburn reaction of aluminum (Al) particles. The research results showed that the performance of shock wave and bubble pulsation grew stronger under the influence of overdriven detonation (ODD), and the contribution of ODD to the bubble energy was associated with the proportion of the inner core and the outer coat. Considering the propagation characteristics of the detonation wave in the ordinary charge and the inner/outer charge, the attenuation of ODD and its role in the initial pressure of the bubble were investigated, and this relationship was used to establish a numerical model for the bubble dynamic behaviors of aluminized explosives. According to the experimental results, the correctness of the established model for the bubble dynamics under ODD was verified, and the variation laws of the afterburn reaction during the bubble pulsation were investigated. It was found that the initial pressure of the bubble under ODD was higher than that under steady detonation. The improvement on the initial pressure promoted the afterburn reaction during the accelerating expansion and enhanced the bubble expansion capacity to support the formation and propagation of shock waves. In the subsequent bubble pulsation, the duration of the decelerating expansion was significantly longer than that of the accelerating expansion, and the ODD had little effect on the afterburn reaction in this stage.

Propagation characteristics of unstable detonation waves in a round tube with annular perturbation

The detonation propagation characteristics of the mixtures, 2H2 + O2+3Ar and CH4 + 2O2, were investigated. Accordingly, the mixtures were tested in round tube with inner diameter of D = 80 mm and annular tubes with widths of w = 25 mm, 15 mm, and 5 mm. The two mixtures represent stable mixture with regular cell pattern and unstable mixture with irregular cell pattern, respectively. Smoked foils were utilized to record cellular structure under various initial conditions. Subsequently, the length scale Ldsc was measured, which represents the length from the start of the test section to the position where the cellular structure changes drastically (the cell size obviously increases or the structure disappears). The results reveal that both mixtures can successfully propagate in round tube and annular tubes of 25 mm and 15 mm, but fail in 5 mm annular tube. The Ldsc value of 2H2 + O2 + 3Ar is higher than that of CH4 + 2O2 in 80 mm and 15 mm tubes, but it is opposite in 25 mm tube. Moreover, the relationship between Ldsc and hydraulic diameter DH was analysed. For a given tube, the values of Ldsc and Ldsc/DH increased when the initial pressure increased. And the variation trend of Ldsc and Ldsc/DH of CH4 + 2O2 is steeper. Furthermore, the mixtures 2H2 + O2 + 3Ar and CH4 + 2O2 resulted in over-driven detonation in 15 mm and 25 mm annular tubes, respectively. The ratio between the total reaction length (sum of the induction length and exothermic length) and the hydraulic diameter (DH/(Δi + Δe)) correspond to critical values of 18 for hydrogen-oxygen-argon and 6 for methane-oxygen, below which the detonation will fail.

Eulerian–Lagrangian modeling of deflagration to detonation transition in n-decane/oxygen/nitrogen mixtures

In this work, to promote deflagration to detonation transition (DDT), a designed hot jet in a pre-detonator is produced to initiate detonations in the main detonation tube. We perform two-dimensional simulations of the DDT process for low-volatile fuel (n-decane) mixed with nitrogen and oxygen based on the Eulerian–Lagrangian approach. The effects of fuel atomization, vaporization, and shock focusing on the flame acceleration and DDT are discussed under different nitrogen dilution ratio and droplet size conditions. The results show that the flame acceleration process can be divided into slow and fast deflagration stages. Additionally, initiation times are mainly determined by the fuel atomization and evaporation in the slow deflagration stage, which dominates the entire DDT time. Furthermore, there are different intensities of hot jets rather than stable detonation waves formed at the pre-detonator exit. Moreover, local decoupling and re-initiation events are detected near the internal wall of the U-bend, inducing the overdriven detonation decaying into stable detonation waves in the smooth tube. The results also demonstrate that the detonation pressure and velocity decrease by 13.56% and 12.55%, respectively, as the nitrogen dilution ratio increases from 0.5 to 2. In particular, as the nitrogen dilution ratio continued to increase to 2.25, the development in DDT is similar, but the jet intensity is significantly weakened. While as the droplet size increases from 10 to 40 μm, the detonation pressure and velocity decrease only by 2.69% and 1.49%, respectively.

Overdriven Detonation Velocity Dependence on Front Curvature

Overdriven detonation is a forced detonation where the shock state is stronger than the CJ state of the explosive. Overdriven detonations are usually not steady but decaying towards the CJ state. It is however possible to have a steady overdriven detonation wave when the wave forcing agent is steady. For sub-CJ curved and steady detonations it is known that detonation velocity (D) is a decreasing function of front curvature (k). This D(k) relation is often used in an approximate and an efficient procedure, known as Detonation Shock Dynamics (DSD), to calculate the propagation of a quasi-steady curved detonation front, without having to solve the entire flow field behind it. In real situations, a divergent front going around an obstacle may become convergent (negative curvature). To tackle this part of the front propagation, people using DSD extrapolate the D(k) curve into the negative curvature region. Matignon et al. [2] performed a test in which they created a steady overdriven detonation with a negative curvature in an explosive rod. The forcing agent was a stronger explosive cylinder wrapped around the tested rod. In this way they were able to measure a point on the D(k) plane above the CJ velocity, to which they extrapolated the usual D(k) curve. Here we raise the question of the uniqueness of this extrapolated curve. It is hard to answer this question experimentally, as candidates to replace the outer explosive are hard to find. But as we show here, it is quite easy to answer this question computationally. We use our reactant temperature dependent reactive flow model TDRR [3-5], which we have calibrated and validated from many test results. As a forcing agent we put a travelling pressure boundary condition on the test rod. We change independently the pressure P and the travelling wave speed D, and in this way, we are able to obtain points on the D(k) plane for negative curvatures and above the CJ state. We show that these points do not fall on a single D(k) curve, and conclude that there is no unique D(k) relation for overdriven steady detonations.

Detonation development in PRF/air mixtures under engine-relevant conditions

The development of advanced boosted internal combustion engines (ICEs) is constrained by super-knock which is closely associated with end gas autoignition and detonation development. The present study numerically investigates the transient autoignition and detonation development processes under engine-relevant conditions for primary reference fuel (PRF) consisting of n-heptane and isooctane. The effects of PRF composition are systematically examined. By considering the transient local sound speed rather than its initial value, a new non-dimensional parameter is proposed to assess the transient chemical-acoustic interaction and to quantify the autoignition modes. Two detonation sub-modes, normal and over-driven detonation, are identified and the corresponding mechanisms are interpreted. For the over-driven detonation, there exist two developing regimes with weak/strong chemical-acoustic coupling and slow/rapid pressure enhancement. It is found that the maximum pressure caused by autoignition decreases with the blending ratio of isooctane, mainly due to the increase in excitation time. Besides, the strongest detonation induced by hot spot usually occurs within the over-driven detonation sub-regime. Its condition can be well quantified by the new non-dimensional parameter proposed in work and its strength is determined by the ratio of hot spot acoustic time to excitation time. The deviation of transient autoignition front propagation from prediction based on homogenous ignition is mainly attributed to the non-uniform compression effect caused by gradually enhanced pressure wave, while the influence of heat conduction and mass diffusion is negligible. The initial expansion stage dominating the induction period of local autoignition is greatly influenced by the compression of pressure wave. Therefore, the continuously enhanced pressure wave non-uniformly changes the local ignition delay (i.e. reduces its spatial gradient) within the hot spot and thereby accelerates the autoignition front propagation. The relationship among the parameters quantifying the detonation propensity is assessed and interpreted. The present study provides helpful understanding of detonation development under engine conditions.

Influence of Variable Blocking Ratio on DDT Process

The influence of a variable blocking ratio on the DDT process is investigated through numerical simulation and experiment. Two dimensionless parameters, the average blocking ratio (BR¯) and the blocking ratio change rate (α), are specified to characterize the blocking ratio of obstacles. Four arrangements are utilized to describe the variation trend of the blocking ratio in a combustor. The main results are as follows: The obstacles can stretch the flame surface and facilitate reflection and diffraction of shock waves, which causes the acceleration of the flame front. The “hot spot” plays a vital role in the formation process of detonation waves. The overdriven detonation waves generated from “hot spots” promote the energy in the primary reaction zone, stimulating the formation of self-sustaining detonation. Compared with the fixed blocking ratio arrangement, the variable blocking ratio of obstacles can shorten the DDT distance. When BR¯=0.43, |α| = 0.03, and the variation trend (0.52–0.34–0.49) is adopted, the minimum DDT distance is obtained in numerical and experimental results. This paper can help with the design of detonation combustors in the future.

Shock-induced chemical decomposition and overdriven detonation in hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) single crystals

Understanding the differences in the shock compression and detonation response of insensitive high explosives (IHEs) and conventional HEs (CHEs) is a long-standing need in HE science and technology. Having previously examined 1,1-diamino-2,2-dinitroethene (FOX-7) IHE single crystals [Winey et al., J. Appl. Phys. 130, 015902 (2021)], the shock and detonation response of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX)—a widely used CHE—was determined using wave profile measurements in ∼250 μm thick single crystals shock compressed to 63 GPa. In marked contrast to FOX-7, RDX single crystals shocked along the [100] and [111] orientations showed wave profile features consistent with chemical decomposition onset at 15 GPa. These features were more pronounced for [100] RDX, suggesting a higher decomposition rate compared to [111] RDX. At 51 GPa and above, flat-topped single waves were observed for both orientations, showing the classic Chapman–Jouget (C–J) detonation response in which the decomposition is completed within the detonation front. The Hugoniot states and sound speeds determined for the detonation products were similar for both [100] and [111] orientations, showing that the overdriven detonation response for RDX does not depend on crystal orientation. The C–J pressure for RDX single crystals (35 GPa)—determined experimentally—is comparable to that of FOX-7. However, compared to FOX-7, chemical decomposition onset for RDX occurs at much lower pressures and the overdriven C–J detonation response occurs at higher pressures. The present findings constitute the first experimental comparison of the shock and detonation response of conventional and insensitive HE single crystals over a broad pressure range below and above the C–J pressure.

Parametric study of hydrogen concentration on detonation initiation by shock focusing

Previous studies indicated that shock focusing is an effective way leading to detonation initiation. Hydrogen concentration as an operating condition is crucial to detonation initiation when the combustible gas is a mixture of hydrogen-air. To discuss the effect of hydrogen concentration, simulations of initiation by shock focusing with different hydrogen concentrations are performed. This paper provides an analysis of initiation by fuel in different amounts (in rich, stoichiometric, and lean). The initiation behaviors of shock focusing with different hydrogen concentrations (range from 20 vol% to 60 vol%) are discussed. The results show that high hydrogen concentration on initiation is significantly effective in accelerating the attenuation of overdriven detonation, which is conducive to initiating a quick and stable detonation initiation. An examination also shows that detonation failure is more likely caused by lean fuel mixtures. As to rich and stoichiometric fuel, there is the same physical mechanism for detonation initiation.

Experimental investigation and comparison of flame acceleration, hot spot ignition, and initiation of detonation in curved and straight channels

An experimental investigation of the deflagration to detonation transition (DDT) in four types of channels with rectangular cross sections and various boundary conditions is presented. The experimental channels included curved and straight channels with closed and open outlets. Experimental tests were performed using a stoichiometric mixture of ethylene and oxygen with initial pressures of 10–60 kPa. Flame shape was observed by a high-speed camera. Local velocity at high spatial resolution was calculated as the ratio of the distance between the flame fronts in sequential frames to the time interval between them. The amplification of a shock wave ahead of the flame and evolution of a series of shock waves after hot spot ignition were monitored using pressure transducers. In addition, a soot-foil technique was used to confirm the exact location of the onset of the overdriven detonation wave. The experiments showed that for a curved channel, three flame shapes (spherical, tulip, and tongue) were observed before hot spot ignition. Notably, the flame was tongue-shaped for about half the time required for the DDT. A funnel of unreacted gas formed between the tongue-shaped flame and the outer wall, at which hot spots were most frequently triggered. The locations of hot spots strongly depended on initial pressure in curved channels with closed outlets. When the pressure was relatively low (p0 < 22 kPa), hot spots always ignited at the intersection of the tongue-shaped flame and the outer wall (the root of the tongue-shaped flame), while relatively high initial pressure (p0 > 33 kPa) caused the hot spots to explode at the surface of the outer wall near the head of the tongue-shaped flame. At intermediate pressures, i.e., 22 ≤ p0 ≤ 33, hot spots ignited at both locations. The above results were very reproducible. Finally, the DDT for closed and open outlets in curved channels were compared with two types of straight channels, and it was found that the time required from ignition to onset of an overdriven detonation in the four types were, in order of magnitude, open curved, closed curved, open straight, and closed straight channels.

The behaviors of supersonic combustion wave through a perforated plate in a stoichiometric mixtures of H2/CH4/O2 and H2/O2

In this study, we reported the supersonic combustion wave behaviors as it propagated through a perforated plate in stoichiometric mixtures of H2-CH4-2.5O2, H2-CH4-4O2 and 2H2-O2. The perforated plate maintained an identical thickness of 10 mm with different hole diameters of 4 mm, 8 mm and 10 mm, respectively. Explosion pressures were recorded using pressure transducers flushed-mounted on the top wall, based on which the average velocity was calculated. Meanwhile, the cellular structures of the supersonic combustion waves were recorded by soot foils. According to the average velocity and soot foils, the propagation modes were classified into four regimes including Fast-flame to fast-flame, Fast-flame to detonation, Detonation to fast-flame, Detonation to re-initiation and Overdriven detonation to detonation. The perforated plate exerts a positive effort on fast-flame by disturbing the flow flied, therefore the fast-flame does not decelerate downstream of the perforated plate. However, the effect of the perforated plate is opposite for detonation, and it weakens the detonation and even causes detonation failure. And this effect is weakened with the hole diameter increasing, which is proven by the limit of re-initiation decreases from d/λ = 0.98(d = 4 mm) to d/λ = 0.71(d = 10 mm). However, the re-initiation mechanisms are different at various initial pressures. At higher initial pressure, the detonation front is elongated dramatically but not failed and it recovers downstream. However, the failed detonation needs the aid of the shock refection off the tube wall to re-initiate at lower initial pressure. With increasing the sensitivity of the mixture, the overdriven detonation characterized by d/λ > 1 was found to transmit the holes with no failure. By comparing with the failing cases owning small cells, d/λ > 1 is not the sole condition to ensure a detonation traveling through with no failure.

Influence of perforated plate thickness on supersonic combustion wave propagation mechanism in a stoichiometric H2-O2 mixture

The behaviors of a supersonic combustion wave through the perforated plate were studied based on velocity and cellular structures. The perforated plate with the thicknesses of Δ = 40 mm, 70 mm, and 100 mm was placed perpendicular to the direction of combustion wave propagation. Stoichiometric hydrogen-oxygen was used in all tests with initial pressure ranging from 11 kPa to 55 kPa. Soot foils were employed to record the triple-point trajectory of a detonation through the perforated plate. Pressure transducers were used to measure the shock time-of-arrival, based on which average velocity was derived. It was found that the fast flame, entering the perforated plate, either maintains fast flame or accelerates to detonation. For CJ detonation, the diffraction kills the detonation directly, and the decoupled detonation may re-initiate or not, which is up to the initial pressure and the geometry of the perforated plate. However, for an overdriven detonation, it can transmit the perforated plate successfully with no failure. The critical condition for a detonation go or no go requires d/λ (d is the hole diameter and λ is the cell size) to be larger than 2.29.

Overdriven detonation

Shock pressure of overdriven detonation is above the CJ pressure. We use here our reactive flow model TDRR (=Temperature Dependent Reaction Rate) to calculate examples of overdriven detonation, including details of the reaction zone. The explosive is PBX-9502, and the detonation shock pressure for different examples is below and above the intersection pressure of the reactant and products Hugoniot curves. We find that: 1) below that intersection, as well as above it, the reaction zone is on the corresponding Rayleigh line, like for a CJ detonation; 2) below the intersection, pressure decreases along the reaction zone, as for a CJ detonation, but above the intersection, pressure increases along the reaction zone; 3) for a shock pressure above the intersection point, the reaction zone is continuous, smooth, and without any peculiarities.

Propagation Characteristics of Gas Explosion in Linked Vessels Based on DDT Criteria

To study the occurrence conditions and propagation characteristics of deflagration to detonation transition (DDT) in linked vessels, two typical linked vessels were investigated in this study. The DDT of the methane–air mixture under different pipe lengths and inner diameters was studied. Results showed that the CJ detonation pressure of the methane–air mixture was 1.86 MPa, and the CJ detonation velocity was 1987.4 m/s. Compared with a single pipe, the induced distance of DDT is relatively short in the linked vessels. With the increase in pipeline length, DDT is more likely to occur. Under the same pipe diameter, the DDT induction distance in the vessel–pipe–vessel structure is shorter than that in the vessel–pipe structure. With the increase in pipeline diameter, the length of the pipe required to form the DDT is reduced. For linked vessels in which detonation formed, four stages, namely, slow combustion, deflagration, deflagration to detonation, and stable detonation, occurred in the vessels. Moreover, for a pipe diameter of 60 mm and a length of 8 m, overdriven detonation occurred in the vessel–pipe–vessel structure.

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An experimental investigation was performed to study the pressure limits and mechanism of the detonation re-initiation behind the perforated plate with various thicknesses and hole diameters in H2–O2-Ar mixture by schlieren and soot track measurement. The Chapman-Jouguet (CJ) detonation and overdriven detonation were used to collide with the perforated plate. For the CJ detonation transmission, the detonation re-initiation distance is larger than 3.9 times the tube hydraulic diameter. Both the thickness and the hole size have significant effects on the critical pressure and the re-initiation distance. The ratios of the hole hydraulic diameter and the critical cell size (dH/λc) are less than 1. The cellular structure can be observed near the perforated plate on the smoked foils due to the collision of the arc-sharp shock waves via multi-jet structure. For the overdriven detonation transmission, the detonation can be re-initiated quickly, and the re-initiation distances are all less than twice the tube hydraulic diameter. The re-initiation distance increases with the hole size while the thickness has little effect on the re-initiation distance. The “abrupt” mode of the detonation re-initiation is transformed into the “gradual” mode with the increase of the initial pressure. The pressure limit of the re-initiation mode transformation also increases with the decrease of the hole size. Two mechanisms of the multi-jet initiation: rapid turbulent mixing initiation and the induction by the collisions of transverse waves were verified experimentally, corresponding to the “abrupt” and “gradual” modes, respectively. The critical condition of detonation propagation can be quantified as dH/λ∗ > 3.37 and 3.77 for the 3 mm-hole and 2 mm-hole perforated plates, respectively, where dH is the hydraulic diameter and λ is the detonation cell size.

Role of overdriven state of ignition wave in pre-detonators on detonation transition modes in a flat chamber

Using the small-diameter pre-detonator is an important method to provide reliable and fast detonation to the denotation engines (i.e., pulse detonation engines or rotating detonation engines). This work examines the role of the overdriven state of ignition waves generated in pre-detonators on detonation transition modes in a flat chamber. For this purpose, a series of experiments are performed using different lengths of pre-detonators (Lp) with different mixtures to initiate the detonation in the main chamber. Results show that maintaining an overdriven state at the pre-detonator exit can significantly enhance the probability of fast detonation initiation (i.e., supercritical/critical mode) in the main chamber. Especially for the high nitrogen diluting ratio (β) mixtures, forming the overdriven detonation at the pre-detonator exit becomes a necessary condition for the fast initiation. As a result, the fast detonation initiation mode distributes in an upper-right shrinking ‘canyon’ in the Lp-β map. It is illustrated that the increased overdriven degree required for transitioning detonation wave under the higher β cases is responsible for this observation.