Development Of Detonation Research Articles

Detonation development in H2, CH4, and PRF-air mixtures under engine-relevant conditions: From a chemical perspective

The progression of highly boosted internal combustion engines (ICEs) faces impediments due to the super-knock phenomenon primarily associated with detonation development during end gas autoignition. Although it is well known that hot spot potentially triggers super-knock, quantitative prediction of super-knock and detonation development across diverse states and fuels remains a challenge. This study numerically investigates the transient autoignition process induced by a hot spot, considering different initial states and fuels (including alternative fuels H2, CH4, and conventional PRFs), with an emphasis on the chemical effects. It is found that the intense exothermic stage (IES) in a stoichiometric H2/air mixture greatly differs from those in hydrocarbon/air (CH4/air and PRF/air) and lean or diluted H2/air mixtures. Through sensitivity analysis, key elementary reactions dominating the autoignition are identified, attributing their impact on the IES primarily to their influence on the radical pool rather than on the heat release rate. It is found that only reactions H + O2⇌O+OH and H2O2(+M)⇌OH+OH(+M) significantly affect both excitation time and ignition delay time, suggesting that the kinetic pathway controlling the IES differs somewhat from that governing the overall ignition. For the autoignition induced by a hot spot, two types of detonation feature are identified: one corresponds to stoichiometric H2/air with broad detonation regime and short excitation time, while the other is associated with other fuel/air mixtures exhibiting relatively narrow detonation regime and prolonged excitation time. These findings highlight that the excitation time predominantly governs the detonation development. All hydrogen/air mixtures (stoichiometric, lean, and diluted) manifest a suppressed detonation intensity compared to hydrocarbon/air mixtures. Moreover, due to its significantly higher reactivity in the IES, stoichiometric H2/air has an additional normal detonation sub-mode for hot spot with larger temperature gradient. This sub-mode greatly expands the detonation regime and thereby results in a notably higher detonation propensity for the stoichiometric H2/air mixture.

Asymptotic analysis of detonation development at SI engine conditions using computational singular perturbation

The occurrence and intensity of the detonation phenomenon at spark-ignition (SI) engine conditions is investigated, with the objective to successfully predict super-knock and to elucidate the effect of kinetics and transport at the ignition front. The computational singular perturbation (CSP) framework is employed in order to investigate the chemical and transport mechanisms of deflagration and detonation cases in the context of 2D high-fidelity numerical simulations. The analysis revealed that the detonation development is characterised by: (i) stronger explosive dynamics and (ii) enhanced role of convection. The role of chemistry was also found to be pivotal to the detonation development which explained the stronger explosive character of the system, the latter being an indication of the system's reactivity. The role of convection was found to be enhanced at the edge of the detonating front. Moreover, the increased contribution of convection was found to be related mainly to heat convection. Remarkably, the detonation front was mainly characterised by dissipative and not explosive dynamics. Finally, diffusion was found to have negligible role to both examined cases.

Computational diagnostics for flame acceleration and transition to detonation in a hydrogen/air mixture

A new computational diagnostic method for pressure-induced compressibility is proposed by projecting its local contribution to the chemical explosive mode (CEM) in the chemical explosive mode analysis (CEMA) framework. The new method is validated for the study of detonation development during the deflagration-to-detonation transition (DDT) process. The flame characteristics are identified through the quantification of individual CEM contributions of chemical reaction, diffusion, and pressure-induced compressibility. Numerical simulations are performed to investigate the DDT processes in a stoichiometric hydrogen-air mixture. A Godunov algorithm, fifth-order in space, and third-order in time are used to solve the fully compressible Navier-Stokes equations on a dynamically adapting mesh. A single-step, calibrated chemical diffusive model (CDM) described by Arrhenius kinetics is used for energy release and conservation between the fuel and the product. The new diagnostic method is first applied to one-dimensional (1D) canonical flame configurations followed by two-dimensional (2D) simulations of DDT in an obstructed channel where different detonation initiation scenarios are examined using the new CEMA projection formulation. Detailed examinations of the idealized configuration of detonation initiation through shock focusing mechanism at a flame front are also studied using the new formulation. A comparison of the currently proposed CEMA projection and the original formulation by the authors suggests that including the pressure-induced compressibility is essential for the use of CEMA in DDT process. The results also show that the new formulation of CEMA projection can successively capture the detonation initiation through either a gradient mechanism or a direct initiation mechanism, and therefore can be used as an effective local analytical tool for the computational diagnostics of detonation initiation in a DDT process. It was found that detonation development is characterized by a strong contribution of chemistry role to the CEM which is pivotal to the initiation of detonation. The role of compressibility is found enhanced at the edge of the detonation front where diffusion was found to have minimal effects on detonation development. Novelty and Significance StatementA new computational diagnostic method for pressure-induced compressibility is proposed by projecting its local contribution to the chemical explosive mode (CEM) in the chemical explosive mode analysis (CEMA) framework. The proposed method is tested and validated for the study of detonation development during the deflagration to detonation transition (DDT) process. The new method is found to be an effective local analytical tool for the computational diagnostics of detonation initiation in a DDT process. The proposed method is versatile and can be used on various different platforms which makes this study more impactful.

Identification of multi-regime detonation development for hydrocarbon fuels

Recently, a detonation response diagram with multiple nonmonotonic boundaries was identified for syngas/air mixtures (Pan et al., 2022), accounting for wide-ranging operation conditions with thermal inhomogeneities. In this work, we have identified the detonation response diagrams of various fuels, especially large hydrocarbons, initiated by temperature gradients under engine-relevant conditions, in both planar and spherical configurations. Results show significant differences in the multi-regime detonation development for different fuels due to reactivity sensitivities and thermophysical properties. Furthermore, the curvature effect in the spherical configuration can cause substantial degeneration of the detonation development, while cool flame partial oxidation of large hydrocarbons can greatly facilitate robust detonation development, suggesting the significance of the interaction between flow inhomogeneities and chemical kinetics.

Transmission of hydrogen detonation across a curtain of dilute inert particles

Transmission of hydrogen detonation wave (DW) in an inert particle curtain is simulated using the EulerianLagrangian approach. A detailed chemical mechanism is used to predict the detonative combustion of a stoichiometric hydrogen-air mixture and parametric studies are conducted based on a two-dimensional computational domain. A detonation map of propagation and extinction corresponding to various particle sizes, concentrations, and curtain thicknesses is plotted. It is shown that the critical curtain thickness decreases considerably when the particle concentration is less than the critical value. The effects of curtain thickness on the trajectories of peak pressure, shock front speed, and heat release rate are examined. Three propagation modes of the DW in particle curtain are found: detonation transmission, partial extinction and detonation re-initiation, as well as detonation extinction. The first two modes are analyzed with the evolutions of shock frontal structures and chemical explosive mode. The chemical explosive mode analysis confirms that a detonation re-initiation event is caused by a re-initiation point with high pressure and explosive propensity, resulting from transverse shock focusing. The influence of particle dimeter/concentration, and curtain thickness on the DW are also examined with peak pressure trajectories, shock speed, and interphase exchange rates of energy and momentum. In addition, small variations of the curtain thickness will affect the short-term behaviors of detonation development, but the long-term DW behaviors are negligibly affected. Furthermore, the evolutions of curtain morphologies are analyzed by the particle velocity, volume fraction, Stokes drag and Archimedes force. This analysis confirms the importance of drag force in influencing the curtain morphologies. Different curtain evolution regimes are found: quasi-stationary regime, shrinkage regime, constant-thickness regime, and expansion regime. Finally, the influences of the curtain thickness on the characteristic time of curtain evolutions are studied. It is found that all characteristic time become smaller when the curtain thickness increases.

Effects of turbulence–heat loss interactions on detonation development in end gas and its resulting knock intensity

The main objective of the present work is to investigate the end-gas autoignition and detonation development in a confined space with the presence of wall heat loss by two-dimensional numerical simulations with a hydrogen/air mixture. The effects of turbulence–heat loss interactions, initial temperature, equivalence ratio, and wall temperature on end-gas combustion modes are analyzed in detail. The results show that with the presence of wall heat loss, end-gas autoignition takes place in the hot core regions away from the walls, and the autoignition fronts touching the wall can lead to a much larger wall heat flux than that induced by main flame–wall interactions. In the base cases, increasing the turbulence intensity promotes the end-gas autoignition mode transition from thermal explosion-detonation to thermal explosion-deflagration and finally to no-autoignition, whereas detonation takes place in all cases regardless of the turbulence intensity after the initial temperature or equivalence ratio is raised. However, in these cases with a low equivalence ratio, the detonation propagation is unstable, which can be easily decoupled spontaneously after it encounters the cold flow. It is further found that for the cases with unstable detonation propagation, the burned mass fraction (BMF) dominates the knock intensity, whereas for the cases with stable detonation propagation, the maximum pressure in a chamber will extremely depend on the local and instantaneous interactions between the pressure/shock waves, but the effect of BMF becomes minor.

Detonation peninsula of different stoichiometric ammonia/hydrogen/air mixtures under engine-relevant conditions

This study numerically investigates the detonation development of carbon-free fuels, namely ammonia and hydrogen (NH3 and H2), using one-dimensional (1D) simulations under the end-gas autoignitive conditions relevant to internal combustion (IC) engines. Five stoichiometric NH3/H2/air mixtures with different NH3/H2 blending ratios are studied. A 1D hot spot with varied lengths and temperature gradients is used to induce different ignition modes. The detonation peninsulas are quantitatively identified by two non-dimensional parameters, namely the resonance parameter, ξ, and the reactivity parameter, ε. Increasing the H2 blending ratio up to 80% results in a unique horn-shaped detonation peninsula, i.e., the magnitude of the upper and lower ξ limits, ξu,l, near the leftmost boundaries of the detonation peninsula of the rich-H2 mixtures becomes larger by an order of magnitude as compared to those of the lean-H2 mixtures. Such behavior is attributed primarily to the large heat diffusion of hydrogen, leading to rapid heat dissipation of the hot spot and the significantly decreased transient ξ over time, thus promoting detonation development. The analysis reveals that the characterization of detonation propensity in the rich-H2 mixtures needs to account for the fast heat diffusion of the initial hot spot, in which the initial magnitude of ξ is not representative of its detonability. As such, a correction factor, β, weighted by the ignition Damköhler number, is proposed to resolve the discrepancy of the ξu,l limits between different NH3/H2/air mixtures. With this correction, the transient magnitudes of ξ, ξt, prior to the main ignition are well predicted such that a unified shape of the detonation peninsula for different NH3/H2/air mixture compositions is achieved.

Understanding multi-regime detonation development for hydrogen and syngas fuels

Autoignition and detonation development are foundational events in the combustion community and are fundamentally relevant to engine knocking and detonation propulsion. Autoignition-induced reaction front propagation modes have been extensively investigated, addressing the role of thermal and concentration inhomogeneities. In this work, we have further investigated the nonmonotonic response of detonation development to temperature gradients for low-carbon fuels (hydrogen and syngas) and have found additional detonation regimes, which can depict the panorama of reaction front propagation modes. Results show that separate detonation regimes can be observed when hotspot sizes are below some critical thresholds, with the first corresponding to the known “Bradley detonation peninsula” and the second newly identified featuring broader detonation regions. Despite this, distinct combustion characteristics are observed in the demarcation of detonation regimes between hydrogen and syngas fuels. Specifically, the upper branch of the first detonation regimes for hydrogen is sensitive to temperature gradients at various hotspot sizes, while it exhibits similar behaviors in the lower branch of the second one for syngas, which results in narrower detonation regions. Meanwhile, hydrogen possesses a larger critical hotspot size compared to syngas, and the underlying mechanism is ascribed to the chemical reactivity when hotspot autoignition and the difference of energy density between hotspot interior and exterior. Finally, various detonation regimes are summarized in dimensionless detonation diagrams, in which hydrogen and syngas show similar distributions of detonation peninsula. Despite this, those distinctions in the detonation characteristics between hydrogen and syngas can still be manifested quantitatively. The current work can provide useful insights into knocking inhabitation and detonation promotion.

Role of temperature-gradient steepness on development of hydrogen-oxygen detonation with ozone sensitization

This work carries out one-dimensional simulations on spontaneous initiation and development of detonations from hot spots with initial temperature gradients embedded with non-autoignitive H2O2 mixtures. Role of temperature-gradient steepness on the development of detonations inside and outside hot spots and the relationship of it to ozone sensitization are examined by changing gradient slope and O3 addition. It is found that steepening temperature gradient has negative effect on initiations and a critical gradient is identified for different O3 additions, below which spontaneous initiation fails. However, the sustenance of a developing detonation is not only related to the steepness, but also to the reactivity of mixtures with O3 additions. High O3 additions permit a detonation to be more likely to sustain outside hot spots; while for low O3 additions, the sustenance depends nonmonotonically on the slope, which cannot be predicted by the idealized reactive flow system with a constant specific heat ratio and the reduced reaction model. The results clarify the suggestions of He & Clavin [1] that temperature gradient may prevent sustenance of a developing detonation and the transmission of detonation to the uniform fresh mixture.

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.

Multi-regime reaction front and detonation initiation by temperature inhomogeneity

Auto-ignition, reaction front propagation, and detonation development are foundational events in combustion, and are relevant to the occurrence of engine knock. It is generally understood that different auto-ignition modes can be initiated by non-uniform initial temperatures, manifesting the transition from supersonic to subsonic combustion modes with increasing temperature gradients. In this work, we have investigated the auto-ignition and reaction front propagation of syngas/air mixtures initiated by wide-ranging temperature gradients, in both spherical and planar coordinates, and have identified a universal detonation response diagram with multiple, non-monotonic boundaries of auto-ignition modes under engine-relevant conditions. Specifically, it is shown that with increasing gradient steepness, in addition to the conventional three regimes of supersonic auto-ignition deflagration, detonation development, and subsonic auto-ignition deflagration, the reaction front propagation speed would first decrease dramatically and then increase, hence inducing additional detonation regimes. Consequently, two detonation peninsulas are identified, with the first corresponding to the well-established Bradley detonation peninsula and the second manifesting a broader detonation regime. Both detonation peninsulas depend on the hotspot size and they can connect together when the hotspot radius becomes sufficiently large. The transient auto-ignition processes and chemical-gas dynamic interactions agree with the typical characteristics of various auto-ignition modes. Finally, auto-ignition modes are summarized in the detonation diagram, in which the Bradley detonation peninsula is well reproduced and the new detonation peninsula is quantitatively determined. The present study demonstrates that auto-ignition modes are significantly affected by the non-monotonic behavior of reaction front propagation, and the use of actual propagation speed is necessary for steeper temperature gradients in order to determine more accurate dimensionless parameters.

Detonation development in hydrogen/air mixtures inside a closed chamber: role of a cold wall

Detonation development from a hot spot has been extensively studied, where ignition occurs earlier than that in the surrounding mixtures. It has also been reported that a cool spot can induce detonation for large hydrocarbon fuels with Negative Temperature Coefficient (NTC) behavior, since ignition could happen earlier at lower temperatures. In this work we find that even for hydrogen/air mixtures without NTC behaviors, a cold wall can still initiate and promote detonation. End-wall reflection of the pressure wave and wall heat loss introduce an exothermic center outside the boundary layer, and then autoignitive reaction fronts on both sides may evolve into detonation waves. The right branch can be further strengthened by appropriate temperature gradient near the cold wall, and exhibits different dynamics at various initial conditions. The small excitation time and the large diffusivity of hydrogen provide the possibility for detonation development within the limited space between the autoignition kernel and the cold wall. Moreover, detonation may also develop near the flame front, which may or may not co-exist with detonation waves from the cold wall. Correspondingly, wall heat flux evolution exhibits different responses to detailed dynamic structures. Finally, we propose a regime diagram describing different combustion modes including normal flame, autoignition, and detonation from the wall and/or the reaction front. The boundary of normal flame regime qualitatively agrees with the prediction by the Livengood-Wu Integral method, while the detonation development from both the end wall and the reaction front observes Zel'dovich mechanism. Compared to hydrocarbons, hydrogen is resistant to knock onset but it is more prone to superknock development. The latter mode becomes more destructive in the presence of wall heat loss. This study isolates and identifies the role of wall heat loss on a potential mechanism for superknock development in hydrogen-fueled spark-ignition engines.

Temperature Stratification Induced Ignition Regimes for Gasoline Surrogates at Engine-Relevant Conditions

ABSTRACT End-gas auto-ignition leading to knocking combustion is one of the major barriers to achieving higher thermal efficiencies in downsized boosted spark-ignition engines. Despite the available framework addressing hotspot-induced ignition (detonation peninsula), a quantitative investigation on hotspot-induced auto-ignition of gasoline surrogates is yet to be done. In particular, the effect of negative temperature coefficient (NTC) chemistry on the distribution of the ignition modes in the detonation peninsula is still missing. Using the established one-dimensional (1D) theoretical and computational framework, the effect of average temperature (including NTC range), initial pressure, and ethanol addition are investigated. Moreover, appearance of NTC chemistry-related events i.e. coolspots, secondary ignition kernels, and off-centered ignition are analyzed using 1D simulations. The results are as follows. 1) NTC chemistry affects the distribution of ignition regimes in detonation peninsula and the dynamics of the front propagation via altering the reactivity gradient. 2) NTC chemistry increases the temperature gradient range associated with the detonation regime. 3) NTC may inhibit detonation development by simultaneously promoting the spontaneous/supersonic ignition modes. 4) An ethanol blend decreases the knock propensity; however, lower ignitability may promote detonation development and the appearance of strong shock waves. 5) Finally, detonation may result in a normal knock at lower initial pressures (20 bar). However, at elevated initial pressures (50 bar), detonation is noted to yield pressure intensities resembling super-knock.

Ethanol blending effects on auto-ignition and reaction wave propagation under engine-relevant conditions

Ethanol as an attractive oxygenated fuel is often blended with gasoline to yield beneficial anti-knocking performance. Due to the distinctions in fuel chemistry, the combustion behavior of gasoline-ethanol blends is not fully understood. This study investigated ethanol blending effects on auto-ignition and reaction wave propagation under engine-relevant conditions covering the negative temperature coefficient (NTC) region. Three ethanol blending levels (i.e., E10, E20, and E30 on a volume basis) were introduced into gasoline surrogates. Characteristic timescale analysis for homogeneous auto-ignition was conducted, and localized hotspot auto-ignition and reaction wave propagation was investigated for different gasoline-ethanol blends. Meanwhile, ethanol blending effects on detonation development were comparatively explored. The results show that ethanol addition significantly suppresses fuel reactivity for homogeneous scenarios, especially in low-temperature regions, manifesting prolonged timescales and diminishing NTC behavior. Despite this, different reaction wave propagation modes can be identified during transient auto-ignition and reaction wave propagation, including deflagration, detonation development, and thermal explosion. The detonation wave is always at an underdeveloped status under engine-relevant conditions, and the induced pressure characteristics are sensitive to initial temperatures, hotspot sizes, and fuel reactivity. Meanwhile, ethanol addition tends to promote transient detonation development by elevating the reaction front propagation speed inside the hotspot and enhancing the deflagration to detonation transition outside the hotspot. Such observations are not contrary to that ethanol yields beneficial anti-knocking performance because only chemical effects of ethanol blending are addressed, without considering vaporization cooling effects. The present work demonstrates that ethanol addition globally suppresses engine knocking, but it may induce stronger knocking intensity once end-gas auto-ignition and detonation development are initiated. The current study provides useful insights into ethanol perturbative effects on auto-ignition and reaction wave propagation from chemical perspectives.

Experimental observation on the end-gas autoignition and detonation affected by chemical reactivity in confined space

The deflagration-to-detonation transition remains one of the most interesting and mysterious physical phenomena in the combustion of energetic materials, which contains substantial complicated and nonlinear characteristics. In the present work, the effect of the chemical reactivity of different fuels and diluent gases on the end-gas autoignition and detonation development in a confined space was investigated. Five fuels (hydrogen, methane, iso-octane, n-heptane, and PRF50) and three diluent gases (argon, nitrogen, and carbon dioxide) were used to change the chemical reactivity. The results showed that both the chemical reactivity and shock wave had a significant influence on the end-gas autoignition and detonation development. For mixtures with different diluent gases, it was observed that the transition thresholds (denoted by critical oxygen fraction) increased in the order of argon, nitrogen, and carbon dioxide. Different detonation modes with varying shock compressions were observed under different diluents for n-heptane. Although the flame propagation of different fuels differs at 21% oxygen fraction, end-gas autoignition and detonation development processes can still be observed in all kinds of fuels when the oxygen fraction was elevated to a certain value. The transition thresholds increased in the order of hydrogen, n-heptane, PRF50, iso-octane, and methane. Further analysis revealed that the fuel with a shorter ignition delay usually required a lower flame tip velocity, accomplished with a delayed occurrence of detonation. In addition, the transition threshold was determined by the chemical reactivity and flame speed.

Ignition and deflagration-to-detonation transition modes in ethylene/air mixtures behind a reflected shock

Dynamics of ethylene autoignition and deflagration-to-detonation transition (DDT) are first numerically investigated in a one-dimensional shock tube using a reduced chemistry including 10 species and 10 reactions. Different combustion modes are investigated through considering various premixed gas equivalence ratios (0.2 − 2.0) and incident shock wave Mach numbers (1.8–3.2). Four ignition and DDT modes are observed from the studied cases, i.e., no ignition, deflagration combustion, detonation after reflected shock, and deflagration behind the incident shock. For detonation development behind the reflected shock, three autoignition hot spots are formed. The first one occurs at the wall surface after the re-compression of the reflected shock and contact surface, which further develops to a reaction shock because of “the explosion in the explosion” regime. The other two are off the wall, respectively, caused by the reflected shock/rarefaction wave interaction and reaction induction in the compressed mixture. The last hot spot develops to a reaction wave and couples with the reflected shock after a DDT process, which eventually leads to detonation combustion. For deflagration development behind the reflected shock, the wave interactions, wall surface autoignition hot spot as well as its induction of reaction shock are qualitatively similar to the mode of detonation after incident shock reflection, before the reflected shock/rarefaction wave collision point. However, only one hot spot is induced after the collision, which also develops to a reaction wave but cannot catch up with the reflected shock. For deflagration behind the incident shock, deflagration combustion is induced by the incident shock compression whereas detonation occurs after the shock reflection. The chemical timescale increases after the reflected shock/contact surface collision, whereas decreases behind the incident and reflected shocks, as well as after the reflected shock/rarefaction wave interaction. Therefore, mixture reactivity behind the reflected shock is weakened by the contact surface, but is intensified by the rarefaction wave. The multi-dimensionality characteristics, including reflected shock/boundary layer interactions, reflected shock bifurcation, destabilization, and detonation, are further present in a two-dimensional configuration. Planar autoignition occurs because of reflected shock compression and detonation combustion is formed first in the central region due to the collision of the reflected shock wave/reflected compression wave. The left and right bifurcations of the separation region in the wall boundary layer are then sequentially ignited.

Effects of thermal stratification and turbulent intensity on auto-ignition and combustion mode transition

Hydrogen engines with zero-carbon emissions have recently drawn attention while they often suffer from pre-ignition and knocking issues. It is generally understood that knocking intensity is closely associated with auto-ignition development modes, which are affected by multiple variables and their interactions. In this work, the role of thermal stratification and turbulent intensity in auto-ignition development modes were numerically investigated, addressing combustion mode transition. Representative end-gas auto-ignition conditions were employed for hydrogen/air mixtures. Meanwhile, the support vector machine (SVM) algorithm was first applied in the predictions of detonation development. The results show that thermal stratification plays a decisive role in auto-ignition development modes, and supersonic auto-ignition deflagration, direct detonation, and subsonic auto-ignition deflagration can be sequentially observed with the increase of temperature gradient. However, affected by multiple hotspot interactions and pressure wave disturbance, deflagration to detonation transition arising from secondary hotspot auto-ignition becomes prevalent. It is noted that, unlike direct detonation that evolves at the entire circumferential directions, deflagration to detonation transition develops only along certain directions with intermediate temperature gradient, and such gradient steepness is much higher than previous results based on simplified models. When turbulence fluctuation is introduced, the distribution of thermal stratification is modified significantly and the local temperature field is broken into multiple smaller regions filled with mixed temperature gradients. Consequently, deflagration to detonation transition is promoted both temporally and spatially, especially at high turbulence intensity. Besides, the SVM classifier is trained using the above results of numerical simulations for detonation predictions. A diagram consisting of the most energetic length scale and temperature fluctuation is proposed, which shows good performance in predicting detonation occurrence. The present study demonstrates that thermal stratification and turbulent intensity can significantly affect the transition behavior of auto-ignition development mode, and machine learning approaches have the potential to achieve good predictions for detonation occurrence in multiple physical fields.

Study on the Flame Acceleration and Transition from Deflagration to Detonation of N2O/C2H4/CO2

AbstractThe application of nitrous oxide (N2O) as a candidate oxidant provides new opportunities for green propellants and propulsion systems. In this study, flame propagation, acceleration, and transition to detonation characteristics of stoichiometric N2O/C2H4 diluted with CO2 were investigated in 2 m cylindrical channels at 1 atm and 293 K. The flame propagation and acceleration were monitored by a high‐speed video. Pressure transducers were located at different positions along the channel to collect data concerning DDT and detonation development. The influence rules of various mass fractions of CO2 on the above performance were evaluated. The results indicated that the process of flame propagation for N2O/C2H4 was divided into three stages, ignition stage, DDT and detonation stage. Both the process of flame acceleration (FA) and peak overpressure for all the test mixtures decreased with an increase of CO2 concentration in the initial stage, and detonation wave propagation was stabilized with the increase of CO2, then the corresponding propagation velocity was close to the C−J velocity.

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 H 2 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 NH 2 and H 2 NO are most important nitrogen-containing species, and R49 (NH 2 +NO<=>N 2 +H 2 O) 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 H 2 content does not always lead to a higher knocking intensity, and the knocking timing decreases with the reactivity of end-gas. • Detailed simulations on end-gas knocking combustion are performed. • Ammonia and hydrogen binary fuel blends with different ratios are considered. • Different end-gas combustion modes are observed from parametric studies. • Evolutions of thermochemical states in end-gas combustion are discussed. • Knocking intensity and timing in ammonia/hydrogen blending fuel are studied.

Hotspot auto-ignition induced detonation development: emphasis on energy density and chemical reactivity

Previous work shows that auto-ignition propagation modes for strong knocking in engines are dominated by both chemical reactivity and energy density. To clarify the unique role of energy density and chemical reactivity, hotspot auto-ignition induced reaction wave propagation was investigated using direct numerical simulations, addressing the detonation regime in the detonation peninsula. Different operating conditions and mixture components were employed to design specific chemical reactivity and energy density under engine-relevant conditions. Various auto-ignition propagation modes were quantitatively investigated. The results show that unified standards for detonation development and high-accuracy excitation time are necessary for constructing a detonation peninsula. Energy density promotes detonation development and broadens the detonation regime, especially at large hotspot sizes. Meanwhile, there are positive correlations between peak pressure and energy density during detonation development, and the pressure peak can reach an amplitude beyond detonation equilibrium pressure. Furthermore, the improvement in chemical reactivity suppresses detonation development at small hotspot sizes while promoting detonation development at large hotspot sizes. But the variations in chemical reactivity caused by thermal effects show a greater significance than by fuel properties. Besides, obvious distinctions in peak pressure can be identified with the variations of reactivity gradient, and the amplitude always attains the maximum level just when detonation occurs at the edge of the hotspot. Underlying reasons are ascribed to the initial non-uniform distribution of energy density within the hotspot. Meanwhile, the reactivity progress of the remaining mixtures also affects energy density outside the hotspot.