Reaction Wave Propagation Research Articles

Reaction zone structure of spontaneous reaction wave at steady subsonic and supersonic speeds

The reaction zone structure of spontaneous reaction wave at steady subsonic and supersonic speeds is discussed using numerical simulation and theoretical approaches. The main objective of this study is to develop the mathematical model of the spontaneous reaction wave propagating at a steady speed and to clarify the relationship between the reaction zone structure and non-dimensional parameters. A one-dimensional direct numerical simulation (DNS) of the spontaneous reaction wave propagation is performed, and the modeling assumptions are discussed based on the numerical results. The non-dimensional parameters, the Damköhler number and a parameter related to compressibility defined by the Mach number of the spontaneous reaction wave velocity, are introduced into the modeling. The mathematical model is developed using asymptotic techniques, and the solution of the reaction zone structure is analytically derived using Newton’s method. The relationship between reaction zone structure of spontaneous reaction wave and non-dimensional parameters is discussed based on the analytical solutions.

Effects of diffusion barriers on reaction wave stability in Co/Al reactive multilayers

Bimetallic, reactive multilayers are uniformly structured materials composed of alternating sputter-deposited layers that may be ignited to produce self-propagating mixing and formation reactions. These nanolaminates are most commonly used as rapid-release heat sources. The specific chemical composition at each metal/metal interface determines the rate of mass transport in a mixing and formation reaction. The inclusion of engineered diffusion barriers at each interface will not only inhibit solid-state mixing but also may impede the self-propagating reactions by introducing instabilities to wavefront morphology. This work examines the effect of adding diffusion barriers on the propagation of reaction waves in Co/Al multilayers. The Co/Al system has been shown to exhibit a reaction propagation instability that is dependent on the bilayer thickness, which allows for the occurrence of unstable modes in otherwise stable designs from the inclusion of diffusion barriers. Based on the known stability criteria in the Co/Al multilayer system, the way in which the inclusion of diffusion barriers changes a multilayer's heat of reaction, thermal conductivity, and material mixing mechanisms can be determined. These factors, in aggregate, lead to changes in the wavefront velocity and stability.

Autoignition of DME/DMM Fuel Blends: Part II: Reaction Wave Propagation Modes in the Presence of Temperature Gradients

Autoignition of DME/DMM Fuel Blends: Part II: Reaction Wave Propagation Modes in the Presence of Temperature Gradients

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.

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.

Experimental and Numerical Investigation of Gaseous Detonation in a Narrow Channel with Obstacles

Gaseous detonation propagation in a thin channel with regularly spaced cylindrical obstacles was investigated experimentally and numerically. The wave propagation with substantial velocity deficits is observed and the details of its propagation mechanism are described based on experimental measurements of the luminosity and pressure and on three-dimensional flow fields obtained by numerical simulations. Both experiments and simulations indicate a significant role of shock–shock and shock–obstacle interactions in providing high-temperature conditions necessary to sustain the reaction wave propagation.

Auto-ignition and knocking characteristics of gasoline/ethanol blends in confined space with turbulence

Ethanol as an attractive oxygenate is increasingly applied for blending with gasoline yielding beneficial combustion and emission performance for internal combustion (IC) engines. Due to the complex fuel chemistry and combustion processes, the auto-ignition and knocking mechanism of gasoline-ethanol blends under engine-relevant conditions are not fully understood. In this study, using a novel rapid compressionmachine chamber featured by forced turbulence, three gasoline-ethanol blends (i.e. E10, E20, and E30 on a volume basis) were investigated at the target pressure of 20 ~ 50 atm and temperature of 750 ~ 850 K after compression. With particle image velocimetry for turbulence calibration, auto-ignition initiation and knocking evolutions were synchronously measured using high-speed photography, infrared imaging, and instantaneous pressure acquisition. Aiming at auto-ignition timing and pressure oscillations, the role of fuel reactivity, turbulent mixing, and energy density was comparatively investigated to elucidate the dominant role in strong knocking formation. A detonation peninsula for gasoline-ethanol blends was employed to explore the ethanol blending effect on detonation development. The results show that due to the suppression of ethanol addition on low-temperature reactions, the strong knocking tendency of gasoline-ethanol blends is significantly inhibited, manifesting retarded auto-ignition timing, reduced pressure oscillations, and vanishing fast reaction wave propagation. Turbulent mixing with colder wall boundary layer alleviates knocking intensity through decreasing the thermodynamic states of in-cylinder mixtures, while energy density promotes detonation development owing to the resonance between reaction waves and pressure waves. Nonetheless, fuel reactivity still plays the first-order significance in strong knocking formation and detonation development. The current study highlights the role of key parameters in auto-ignition and knocking characteristics in high-boost/low-temperature engines, and provides useful insights into the gap between RCM platforms and actual IC engines.

Displacement speed analysis of surface propagation in moderately turbulent premixed reacting waves

The propagation of premixed reacting waves can be characterized by a displacement speed Sd at which the local surface of the reaction progress scalar moves respective to flow. Often, Sd is considered through decomposition into three parts of contribution due to the tangential diffusion of curvature, normal diffusion, and reaction. A set of recently derived transport equations for Sd and three of its decomposed parts provides new diagnostics for better understanding reaction wave propagation in a turbulent environment. In this work, those diagnostics are applied on four similarly setup direct numerical simulation cases studying the propagation of moderately perturbed planar reaction waves into homogeneous turbulence, and the reaction waves differ by the density ratio between fresh and burned gases. The data analysis reveals four self-acceleration behaviors: (i) surfaces propagating at large positive (negative) Sd tend to advance (retreat) faster, (ii) surfaces having large positive (negative) curvature tend to become more curved positively (negatively), (iii) thicken wave zones tend to become thicker, and (iv) surface elements accelerate toward their destruction. The extent of the above accelerations all reduces in the reaction wave having a high density ratio. This can be attributed to the turbulence inhibition due to the flow dilatation and viscosity increase across a thermal-expansion enabled reaction wave. The distribution of curvature for the reaction-zone surface skews toward a negative value, i.e., the curvature center pointing to the burned product.

Natural gas-supported gasification of polyethylene and wood mixtures in a porous medium reactor

Valorisation of plastic waste or forest-agroindustrial residues poses an opportunity to reduce landfilling and to obtain value-added products simultaneously. Through thermal partial oxidation, hydrocarbon fuels can be gasified and converted into syngas. Particularly, hybrid porous media reactors allow gasifying low-grade fuels in an efficient eco-friendly way. In this work, the natural gas-supported gasification of polyethylene and wood mixtures in a porous bed reactor is studied. The reactor is designed and built to operate in semi-continuous mode. The temperature evolution, the reaction wave propagation rate, and the resulting concentration of the main gas species (H2, CO, CH4, CO2, NOx, and UHC) were measured for different polyethylene and wood mixtures, keeping an optimum natural gas-to-air ratio of 0.8. The influence of the gasification conditions on the reaction pathways and the process development is analysed by sampling the system in two representative moments. The maximum syngas production was 23.86 vol%, which was obtained when loading only 100% polyethylene, while the maximum energy return on investment of 49% was registered for the highest biomass fraction in mixture. The semi-continuous operation design was validated based on the thermal behaviour and the combustion wave displacement. This experimental work shows promising results in the search for solutions to gasify solid wastes in continuous mode.

Appearance of the instability from the inhomogeneity in two-phase flow with gas-producing reaction in fractured porous medium

A homogenized model of incompressible two-phase flow accompanied by a gas-producing reaction in a double porosity medium with a chemically active skeleton is derived. The equations of the homogenized model contain non-local in time source terms corresponding to the contribution of the gas-producing chemical reaction in the matrix blocks. The time non-locality, which manifests itself as the appearance of a time delay between the change in reactant concentrations and the reaction rate, is shown to stimulate the instability of the one-dimensional two-phase flow initiated by injection of the acid solution into the double porosity medium with chemically active matrix blocks. The instability results in the development of the self-oscillating mode of the reaction wave propagation.

Mathematical modeling of the formation and decomposition of a metastable product during MSHS

Synthesis and decomposition of nonequilibrium phases are studied in a thin-layer gas-free system that forms metastable products, using a developed mathematical model and numerical methods. The model uses the chemical transformation equation, which takes into account the possibility of the formation and decomposition of a metastable product, as well as the energy equation with the possibility of external heating and cooling of the reaction sample. The development of a chemical reaction is considered under two extremely different conditions: wave synthesis and thermal explosion. Characteristic modes of the process are found. The possibility of multiple synthesis of a metastable product, determined by external heat input and heat removal in a reacting condensed system, is shown. It is revealed that for the reaction wave propagation, three modes of formation and decomposition of metastable phases can take place: breaking, merging, and quenching.

Forced turbulence affected auto-ignition and combustion modes under engine-relevant conditions

Auto-ignition phenomena are paid more attention due to their close relation with abnormal combustions in advanced internal combustion engines. Previous investigations on intensive auto-ignition and knocking phenomena have been mainly conducted under standard flow conditions, whereas the role of turbulence is not fully clarified under engine-relevant conditions. In this study, forced turbulence affected auto-ignition and combustion modes were investigated in a novel rapid compression facility fueled with isooctane/air mixtures. In combination with instantaneous pressure acquisition, high-speed photography and infrared imaging were employed for auto-ignition initiation and reaction wave propagation. The combustion modes were comparatively investigated under standard flow and forced turbulence conditions. Experimental results show that standard flow scenarios are dominated by a thermo-chemical mechanism. With the elevation of intake pressure and thereby energy density, sequential auto-ignition induced normal combustion, end-gas auto-ignition induced mild knocking, and super-knock are observed in sequence, with obvious cool flame temperature rise and two-stage auto-ignition. Under forced turbulence conditions, the adiabatic core starts to be destroyed by the colder fluid from wall boundary layers, leading to lower temperatures and larger thermal stratifications. Such that prolonged auto-ignition timing, reduced burning rate, and attenuated cool flame are observed. Energy density plays an important role in strong knocking initiation, and there are some increases in the demarcation under forced turbulence conditions. However, mixture reactivity still plays the first-order significance in intensive auto-ignition and knocking formation. The current work helps to provide useful insights into the nature of auto-ignition and knocking combustion and the gaps between fundamental platforms and practical engines.

Numerical Simulations on Autoignition Propagation Modes under Reciprocating Engine-relevant Conditions

ABSTRACT Previous studies on autoignition propagation modes were often performed based on constant-volume configuration. However, the reactant mixture in reciprocating engines always experiences significant variable volume and ever-changing thermodynamic conditions, which may affect autoignition initiation and subsequent development during knocking combustion. In this study, the autoignition reaction wave propagation induced by thermal stratifications was investigated numerically, with addressing the role of reciprocating piston motion and primary flame compression. Compression heating was considered to emulate the compression and expansion caused by reciprocating piston motion, and different combustion boundary conditions and fuel properties were performed to investigate the impact on autoignition propagation modes. The results of hydrogen cases show that similar to constant-volume configurations, various autoignition propagation modes (including thermal explosion, detonation, and deflagration) can be observed. However, the normalized temperature gradients demarcating different autoignition propagation modes change significantly under variable thermodynamic conditions of reciprocating engines. Such an influence can also be embodied in engine combustion phasing. It is found that the intense autoignition involving detonation development prefers to occurring around the Top Dead Center with higher chemical reactivity and energy density. Furthermore, similar studies were further carried out for isooctane and the significant influence from reciprocating piston motion is still observed. Besides, it is found that almost all the autoignition events induced by thermal stratifications develop into deflagration rather than detonation for isooctane. The underlying reasons can be elucidated through the detonation peninsular diagrams for different fuels.

Numerical analysis of knocking characteristics and heat release under different turbulence intensities in a gasoline engine

In this work, different knocking characteristics and heat release have been numerically studied in a downsized spark-ignition engine, with addressing turbulence intensity and spark-ignition timing. The results show that knock intensity mainly depends on the heat release of hot-spots, and the heat release is not only associated with unburned mass fraction but also closely related to subsequent autoignition development processes, e.g. end-gas autoignition mode. Under low turbulence intensity, local mixture autoignition is induced by a single hot-spot, accompanied by an autoignition reaction wave propagation; whereas under high turbulence intensity, a nearly homogeneous autoignition event characterizing quasi constant-volume combustion is observed. Meanwhile, compared with the single hot-spot autoignition, the homogeneous one can lead to more intensive local overpressure and thus stronger knock intensity. Further analysis on the underlying reasons shows that the higher heat release rate under homogeneous autoignition conditions is the main reason for the stronger knock intensity. Besides, the dependency of knock intensity and heat release on unburned mass fraction is found effective only for the same autoignition scenarios. Current study can provide insights into knocking suppression with consideration of intake system optimization in modern downsized gasoline engines.

Temperature gradient induced detonation development inside and outside a hotspot for different fuels

With the dimensionless parameters obtained for syngas/air mixture, Bradley detonation peninsula is often used to determine the detonation development for hotspot autoignition (AI) in reactive flows. In this work, similar numerical simulations were carried out in order to identify the characteristics of detonation peninsula when considering other fuels. Three alternative C0-1 fuels with detailed chemistry and transport were employed in a 1-D reaction wave propagation induced by temperature gradients, and different critical temperature gradients and hotspot sizes were considered. Meanwhile, the role of detonation parameters in the detonation development outside hotspot was addressed. First, the results show that different AI propagation modes can be well depicted using the dimensionless parameters for individual fuel at various critical temperature gradients. However, the quantitative difference in detonation development regime is significantly observed between different fuels with distinct physical–chemical properties even though similar regime distribution is observed. Second, the evolutions of AI reaction wave propagation outside hotspot were further studied, and combustion mode transitions involving detonation termination and formation were observed. The evolutions of the thermodynamic state of different flow particles show that detonation development is found to switch from constant-pressure to constant-volume combustion. Meanwhile, scaling analysis on combustion mode transitions indicates that besides the early-stage propagation controlled by reactivity gradient in hotspot interior, the reactivity of the mixture outside hotspot also plays an important role in detonation development. This can provide great insights into proposing integrated dimensionless parameters for determining detonation development in the whole reactive flows.

Current Trends in Cellulose Assisted Combustion Synthesis of Catalytically Active Nanoparticles

Cellulose assisted combustion synthesis (CACS) has emerged as a novel variation of traditional solution combustion synthesis (SCS) over the past decade for the synthesis of catalytically active nanomaterials. This report targets to summarize the fundamental developments in the synthesis technique seeking correlations between synthesis conditions, observed chemo-physical properties. Molecular level interactions among the metal precursors and reducing agents that lead to combustion reaction and thermal wave propagation in CACS are expected to be different from the SCS method due presence of cellulose fibrils affecting the thermo-physical properties and energy density of the system. In addition, the void spaces and pores may generate a complex network of micro and nanoreactors as opposed to a continuous volume of reacting species in SCS. The goal of this report is to gather the essential developments so far and provide an outlook for future directions.

Laboratory study of oscillatory multiphase flow in porous medium with chemically active skeleton

A laboratory study is conducted of high-viscosity mineral oil displacement by an acid solution in a synthetic porous medium with a chemically active skeleton. The skeleton is made of a mixture of glass balls and particles of sodium bicarbonate. The displacement is driven by the pressure difference between the inlet and outlet sides of the working cell and is accompanied by a gas-producing chemical reaction. The study is focused on the self-oscillating mode of the reaction wave propagation. This mode manifests itself in the appearance of periodically generated secondary waves and is accompanied by pressure oscillations. The experiments show that the amplitude of the pressure oscillations at the reaction front is directly proportional to the driving pressure difference, and that the frequency of the oscillations does not depend on it.

A microscale explosive initiator integrated with a reactive thin-filmed metallic bridge of boron/titanium laminated layers

Aided by the advancement of microfabrication technologies, the electro-pyrotechnical initiators are being miniaturized into micron sizes to gain higher safety and reliability as well as lower mass manufacturing cost. One of the most important innovations proposed for the explosive initiator design in recent years would be the device integration with reactive thin-film metallic bridge which is composed of many laminated layers of different metals. The nanoscale boron and titanium (B/Ti) multilayers would be a feasible candidate for substantially improving the thermal initiation performance due to their fast reaction speed and high exothermic heat release. This study introduces the numerical modeling of ohmic heating in thin-film metallic bridge and the self-propagating intermetallic reaction in B/Ti laminated layers coupled with pulse-forming circuit in order to investigate the feasibility of such an initiator design. The computational results show that the ignition of intermetallic reaction and propagation of reaction wave in B/Ti multilayers of reactive initiator bridge are successfully modeled and simulated. The effects of several reactive bridge design parameters are investigated such as B/Ti layer thickness and its set number on bridge, electrical energy input in pulse form, and the existence of pre-mixing zone at bimetallic interface.

Thin reaction zones in highly turbulent medium

A big database (23 cases characterized by Damköhler number less than unity) created recently in 3D Direct Numerical Simulation (DNS) of propagation of a statistically one-dimensional and planar, dynamically passive reaction wave in statistically stationary, homogeneous, isotropic turbulence is analyzed. On the one hand, the DNS data well support the classical Damköhler expression, i.e., square-root dependence of a ratio of turbulent and laminar consumption velocities on the turbulent Reynolds number. On the other hand, contrary to the common interpretation of the Damköhler theory and, in particular, to the concept of distributed burning, the DNS data show that the reaction is still localized to thin zones even at Da as low as 0.01, with the aforementioned ratio of the consumption velocities being mainly controlled by the reaction-zone-surface area. To reconcile these apparently inconsistent numerical findings, an alternative regime of propagation of reaction waves in a highly turbulent medium is analyzed, i.e., propagation of an infinitely thin reaction sheet is theoretically studied, with molecular mixing of the reactant and product being allowed in wide layers. In this limiting case, an increase in the consumption velocity by turbulence is solely controlled by an increase in the reaction-sheet area. Based on physical reasoning and estimates, the area is hypothesized to be close to the mean area of an inert iso-scalar surface at the same turbulent Reynolds number. This hypothesis leads to the aforementioned square-root dependence. Thus, both the DNS data and the developed theory show that a widely accepted hypothesis on penetration of small-scale turbulent eddies into reaction zones is not necessary to obtain the classical Damköhler scaling for turbulent consumption velocity.

Influence of chemical kinetics on detonation initiating by temperature gradients in methane/air

Different simplified and detailed chemical models and their impact on simulations of combustion regimes initiating by the initial temperature gradient in methane/air mixtures are studied. The limits of the regimes of reaction wave propagation depend upon the spontaneous wave speed and the characteristic velocities of the problem. The present study mainly focus to identify conditions required for the development a detonation and to compare the difference between simplified chemical models and detailed chemistry. It is shown that a widely used simplified chemical schemes, such as one-step, two-step and other simplified models, do not reproduce correctly the ignition process in methane/air mixtures. The ignition delay times calculated using simplified models are in orders of magnitude shorter than the ignition delay times calculated using detailed chemical models and measured experimentally. This results in considerably different times when the exothermic reaction affects significantly the ignition, evolution, and coupling of the spontaneous reaction wave and pressure waves. We show that the temperature gradient capable to trigger detonation calculated using detailed chemical models is much shallower (the size of the hot spot is much larger) than that, predicted by simulations with simplified chemical models. These findings suggest that the scenario leading to the deflagration to detonation transition (DDT) may depend greatly on the chemical model used in simulations and that the Zel'dovich gradient mechanism is not necessary a universal mechanism triggering DDT. The obtained results indicate that the conclusions derived from the simulations of DDT with simplified chemical models should be viewed with great caution.