Reaction Zone Thickness Research Articles

A dynamic droplet breakup model for Eulerian-Lagrangian simulation of liquid-fueled detonation

This study proposes a dynamic model to reflect the physical image of the droplet breakup process in two-phase detonation flows. This breakup model is implemented in a two-phase detonation solver developed based on an open-source computational fluid dynamic platform, OpenFOAM, and compared with three prevalent models (TAB, PilchErdman, and ReitzKH-RT model) under different droplet diameters (30–70 μm) in one- and two-dimensional detonation problems. The simulating results show that the present breakup model well predicts experimentally determined detonation parameters such as detonation velocities and post-wave temperature. In addition, the present model has the advantage of being free of the KH breakup time parameter, which the ReitzKH-RT model needs to fit the experimental data. The one-dimensional detonation simulations indicate that different breakup models slightly impact the detonation wave velocity because the droplet breakup process does not significantly affect the total heat release as long as it is sufficiently fast to sustain the detonation. However, the two-dimensional detonation simulations show that the breakup model and the droplet initial diameter significantly affect the detonation cell size due to the different droplet distributions predicted by various models. The breakup length, which is the distance from the shock wave to the location at which sufficiently small child droplets appear, affects the chemical reaction zone thickness and then the detonation cell size. A longer breakup length will result in a larger detonation cell size.

Effects of methane concentration on flame propagation mechanisms and dynamic characteristics of methane/coal dust explosions

The study explores the flame propagation and explosion dynamic behavior of gas/powder two-phase compound explosions, taking methane and coal dust mixture as the research object. The influence of different methane concentrations on the compound explosion characteristics was researched. The vertical pipeline, high-speed camera, ion current probe, and pressure sensor be used to test the propagation behaviors, microstructure characteristics, and explosion dynamics of compound explosion flames. The function of methane concentration on the propagation behaviors, combustion reaction zone thickness, flame temperature, propagation velocity, and explosion pressure of compound flame was revealed, and the propagation mechanisms of gas/powder compound explosion flames were explained. The experimental research discovered that the compound flame of methane/coal dust explosion presents a white luminous zone with uniform distribution and appears to have an approximate homogeneous combustion state. The explosion dynamics parameters all showed a trend of first increased and then decreased as the methane concentration increased, but the reaction zone thickness of the combustion first reduced and then increased. In addition, the dominant position of homogeneous or heterogeneous combustion in the combustion reaction zone will also switch with the change in methane concentration. The compound flame is similar to premixed gas combustion under optimal methane concentration, the combustion zone thickness (δ) and preheating zone thickness (β) are the smallest, and the compound explosion shows powerful performance. In addition, the flame propagation mechanisms of methane/coal dust compound explosion were elucidated based on the theory of heat.

Multi-material additive manufacturing-functionally graded materials by means of laser remelting during laser powder bed fusion

Many processes may be used for manufacturing functionally graded materials. Among them, additive manufacturing seems to be predestined due to near-net shape manufacturing of complex geometries combined with the possibility of applying different materials in one component. By adjusting the powder composition of the starting material layer by layer, a macroscopic and step-like gradient can be achieved. To further improve the step-like gradient, an enhancement of the in-situ mixing degree, which is limited according to the state of the art, is necessary. In this paper, a novel technique for an enhancement of the in-situ material mixing degree in the melt pool by applying laser remelting (LR) is described. The effect of layer-wise LR on the formation of the interface was investigated using pure copper and low-alloy steel in a laser powder bed fusion process. Subsequent cross-sectional selective electron microscopic analyses were carried out. By applying LR, the mixing degree was enhanced, and the reaction zone thickness between the materials was increased. Moreover, an additional copper and iron-based phase was formed in the interface, resulting in a smoother gradient of the chemical composition than the case without LR. The Marangoni convection flow and thermal diffusion are the driving forces for the observed effect.

Investigation on the microstructure, mechanical and electrical properties of Ti3SiC2/Cu joint obtained by Ti25Zr25Ni25Cu25 amorphous high entropy alloy and Ag composite filler

Ti3SiC2/Cu brazed joint has an important application in the pantograph of high-speed trains, but excessively continuous intermetallic compounds (IMC) and thick diffusion reaction zone (DRZ) in the joint are detrimental to both the mechanical and electrical properties of the joint. Therefore, regulating the interfacial microstructure is an effective way to acquire an excellent brazed joint. In this study, Ti25Zr25Ni25Cu25 amorphous high entropy alloy (HEA) was fabricated, and the brazing of Ti3SiC2 to Cu was carried out by the Ti25Zr25Ni25Cu25 HEA and Ag foil composite filler. The hysteresis effect of the HEA effectively slowed down the interfacial atomic diffusion and metallurgical reactions, which is conducive to the formation of relatively thin DRZ. Due to the combination of Ag foil and the HEA, the IMC distribution in the joint obtained at a relatively low temperature is more dispersed and uniform. Under the optimal brazing conditions, the maximum shear strength of the brazed joint obtained with the Ti25Zr25Ni25Cu25 and Ag composite filler is 34.85% higher than that obtained with Ag-26.7Cu-4.5Ti conventional filler. In particular, the electrical resistance of the joint obtained with the composite filler is 20.35% lower than that obtained with the single Ti25Zr25Ni25Cu25 HEA filler.

High temperature performance and interface evolution of a NiCoCrAlY coating with Pt modification and Re-based diffusion barrier

The cyclic and early isothermal oxidation behaviors at 1050 °C of the NiCoCrAlY, Pt-modified NiCoCrAlY and Pt-modified NiCoCrAlY with Re-based diffusion barrier (DB) have been evaluated. The atomic immigration and electron location function (ELF) at the Ni-Re interface were simulated by density function theory (DFT) calculations. After isothermal oxidation for 1 h, island NiCoCrAlY particles with extremely poor Al were observed in the oxide scale of normal NiCoCrAlY coating, and then the oxide scale transformed into a double-layer structure after oxidation for 20 h. The initial “island-oxide” scale model was proposed to illustrate the oxidation mechanisms for normal NiCoCrAlY coating. The oxide scale of Pt-modified NiCoCrAlY coating was single-layer Al2O3 with rapid coverage at the surface, because the modification of Pt changed the initial oxidation mechanism by promoting the uphill diffusion of Al. The Re-based DB reduced the thickness of the secondary reaction zone (SRZ) by inhibiting the element interdiffusion between the NiCoCrAlY coating and the substrate, thus the Pt-modified NiCoCrAlY with Re-based DB exhibited the lowest interdiffusion extent. Interestingly, the thermal stability of Re-based DB in NiCoCrAlY coating system was poorer than that in NiCrAlY system. The DFT simulation shows that the incorporation of Co made the energy difference more negative, which confirmed that Co promoted the diffusion of Re in the NiCoCrAlY coating, thus accelerating the degradation of Re-based DB.

Effects of carbonization on the combustion of rice husks briquettes in a fixed bed

Carbonization of raw materials for making briquettes has been identified as a possible way of improving calorific values and fixed carbon contents of briquettes. In spite of these benefits, the main challenges of carbonization are increased percentage of ash content and energy required to process the fuel. In this study, fuels with mixtures of carbonized and raw rice husks were combusted in a fixed bed to examine the effect of carbonization on their combustion properties with an aim of establishing an optimum ratio. Bed temperature distribution, ignition, flame propagation speed, combustion rates and reaction zone thickness were investigated. All experiments were performed in a fixed bed operated in a reverse counter-current flame propagation mode. Four mixtures consisting of different ratios of carbonized rice husks were used to process briquettes; namely, 25 wt%, 50 wt%, 75 wt% and 100 wt% biochar. Air-mass flux for combustion of each mixture was varied between 0.02 and 0.6 kg/m2s. It was established that carbonization increases ignition time and peak bed temperature but lowers flame propagation speed, reaction zone thickness, and combustion rates. Briquettes with lower percentage of biochar had higher volatiles and higher devolatilization rate that shortened combustion period. Briquettes with carbonized rice husk of between 50 wt% and 75 wt% were found to have optimum combustion properties. Processing of briquettes should be done within these carbonization ratios in order to achieve desirable combustion properties.

A thick reaction zone model for premixed flames in two-dimensional channels

Direct interactions between the flow field and the chemical reaction in premixed flames occur when the reaction zone thickness is comparable to, or greater than flow length scales. To study such interactions, a laminar model is considered that has direct bearings to steadily propagating deflagrations in a Hele-Shaw channel with a background plane Poiseuille flow. The study employs asymptotic analyses, pertaining to large activation energy and lubrication theories and considers a distinguished limit where the channel width is comparable to the reaction zone thickness, with account being taken of thermal-expansion and heat-loss effects. The reaction zone structure and burning rates depend on three parameters, namely, the Peclet number, , the Lewis number, and the ratio of channel half-width to reaction zone thickness, . In particular, when the parameter is small wherein the reaction zone is thick, transport processes are found to be controlled by Taylor's dispersion mechanism and an explicit formula for the effective burning speed is obtained. The formula indicates that for , which interestingly coincides with a recent experimental prediction of the turbulent flame speed in a highly turbulent jet flame. The results suggest that the role played by differential diffusion effects is significant both in the laminar and turbulent cases. The reason for the peculiar dependence can be attributed, at least in our laminar model, to Taylor dispersion. Presumably, this dependence may be attributed to a similar but more general mechanism in the turbulent distributed reaction zone regime, rather than to diffusive-thermal curvature effects. The latter effects play however an important role in determining the effective propagation speed for thinner reaction zones, in particular, when is large in our model. It is found that the magnitude of heat losses at extinction, which directly affects the mixture flammability limits, is multiplied by a factor in comparison with those corresponding to the no-flow case in narrow channels.

Detonation behaviors of stoichiometric H2-O2 mixture diluted with He, N2, CO2 at different initial pressures

In this study, the detonation parameters for stoichiometric H2-O2 mixtures diluted with varying amount of He, N2, CO2 are measured and analyzed in a small bore tube at different initial pressure P0. Signals of flame front and cell patterns are obtained by photodiodes and smoked foil technique, respectively. Experimental results indicate that as dilution mole fraction increases, the average detonation velocity deviates more from the theoretical value, and the detonation velocity deficit increases. The detonation velocity deficit of the H2-O2-N2 mixtures is relatively smaller than that of the H2-O2-He mixtures. The maximum detonation velocity occurs at 65% He dilution fraction. The cell sizes of the H2-O2 mixture diluted with CO2 are most affected by P0. The dilution mole fraction less than 10% has little effect on the cell size at higher initial pressure. The H2-O2-He mixture can make the induction zone length shorter than that of H2-O2 mixture as P0 increases. The detonation instability increases as N2 or CO2 dilution fraction increases, and the cellular instability decreases as He dilution fraction increases. Detonation parameters such as reaction zone thickness are calculated to introduce a cell size predicting formula based on the Ng model. Linear relationships are determined by: λ = 42.457 Δi (H2-O2-N2 mixture), λ = 48.945 Δi (H2-O2-He mixture).

Optimization of combustion parameters of carbonized rice husk briquettes in a fixed bed using RSM technique

This study investigates the combustion of carbonized rice husk briquettes in a fixed bed to establish the optimum briquetting and combustion conditions. The combustion properties, namely, the average flame propagation speed, the average burning rates, the reaction zone thickness, maximum flame temperature and the ignition time, were optimized with respect to air-flow rate, binder ratio and particle size using response surface methodology (RSM). Seventeen experiments with different combinations of air-flow rate, binder ratio, and particle sizes, each at three levels set according to Box-Behnken Design (BBD), an RSM technique, were performed. Mathematical models for predicting the responses were developed by fitting experimental data. It was established that the binder and air-mass flux predominantly affected all fuel samples during processing and combustion. Optimal values of the ignition time and reaction zone thickness were established to be 249.08 s and 102.43 mm when air-mass flux, binder ratio and particle size were 0.31 kg/m2.s, 25% and 2.6 mm, respectively. On the other hand, the optimal peak bed temperature was 1226.25 °C when air-mass flux, binder ratio and particle size were 0.31 kg/m2.s, 25% and 0.3 mm, respectively.

Fabrication and characterization of high-performance acetone-assisted polyamide thin-film composite membranes for fluoridated saline water treatment

Nanofiltration (NF) and reverse osmosis (RO) membrane show promising potential for treating fluoridated brackish water and wastewater. We assessed the water flux and rejection performance of a self-fabricated membrane with various acetone concentrations as co-solvent for the treatment of fluoridated brackish water. The membrane performance was studied at different pH and pressure levels using a crossflow RO test cell with a single fluoride feed solution, brackish water composition, and real seawater. The addition of acetone to the organic and aqueous solutions during interfacial polymerization reduced the solubility difference and formed a narrower miscibility zone, resulting in a thicker reaction zone and a multilayer polyamide structure with larger pores. The water flux performance improved as the acetone concentration rose, with just a minor loss in fluoride solute rejection. Additionally, the fluoride removal efficiency increased with pH increase as the membrane became highly negatively charged. The performance of acetone added membrane was compared with commercial RO/NF membranes under the same conditions, and the acetone-added membrane exhibited higher flux and rejection. This work illustrates the utility of an acetone-assisted membrane as a suitable stand for fluoride removal and brackish water treatment.

Preparation and Enhanced Isothermal Oxidation Resistance of a Low Diffusivity NiRePtAl Single-Phase Coating

A low diffusivity NiRePtAl single-phase coating was formed on a Ni3Al-based SC superalloy by electroplating and aluminizing treatments, in which the electroplating consisted of depositing Ni-Re and Pt layer. The isothermal oxidation test of the sample was evaluated at 1100 and 1200 °C; the results indicated that the low diffusivity NiRePtAl single-phase sample promoted the oxidation resistance due to a greater β-NiAl phases-enriched outer layer. The Re-base diffusion barrier could effectively refrain the outward diffusion of Mo. Low diffusivity NiRePtAl single-phase coating has a lower inter-diffusion rate to the superalloy, where the thickness of the secondary reaction zone decreased by 35%. Mechanisms responsible for improved oxidation resistance and decreased extent for the formation of secondary reaction zones are discussed in the present study.

Ni-Pb-Te phase equilibria and interfacial reactions in Ni/PbTe couples

BackgroundPbTe is an important thermoelectric compound. Ni is commonly used as a barrier layer. To provide fundamental information, an Ni-Pb-Te phase equilibria isothermal section at 650 °C and Ni/PbTe interfacial reactions at 650 oC, 500 oC and 400 oC are determined. MethodsThe isothermal section is proposed based on results of equilibrated alloys and related phase diagrams. The interfacial reactions are examined using reaction couples. Significant FindingsThere is no ternary compound. At 650 oC, the liquid phase extending from the Pb corner has significant symmetrical solubilities of Ni and Te, and displays a peculiar deep and narrow shape. No interfacial reactions are observed in the Ni/PbTe couple heat-treated at 500 oC and 400 oC for a month. However, significant reactions are observed when reacted at 650 oC. Reaction zone thickness is 120 μm when reacted for 30 min. The reaction zone has many phases, and yet has clear phase region boundaries. Liquid and β-Ni3Te2 phases are formed, and the reaction path is Ni/β-Ni3Te2+Liquid (I)/Liquid (I)/PbTe. Many phases were formed from solidification when the couple was removed from a furnace. Ni is the dominating diffusion species in the Ni/PbTe couples. Ni penetrates into the PbTe substrate, reacts with Te to form Ni3Te2, and the liquid phase is formed with excess Pb.

Interfacial reactions in Cu/PbTe and Cu/PbSe couples

PbTe and PbSe are important thermoelectric compounds. Cu is commonly used as an electrode material. To provide fundamental information for the devices using these materials, the interfacial reactions in the Cu/PbTe and Cu/PbSe couples are examined. Similar interfacial reactions are observed in these two couples. No interfacial reactions are found in both kinds of couples reacted at 400 °C and 500 °C. There are very significant interfacial reactions in both kinds of couples reacted at 650 °C. The phase equilibria isothermal sections of Cu-Pb-Te and Cu-Pb-Se at 650 °C are proposed for better understanding the respective interfacial reactions. The reaction zone thickness is 160 µm in the Cu/PbTe couple reacted at 650 °C for 3 min, and its reaction path is Cu/Liquid (I)+Cu2Te/Cu2Te/Liquid (II)/PbTe. The reaction zone thickness is 155 µm in the Cu/PbSe couple reacted at 650 °C for 2 min, and its reaction path is Cu/ Liquid (I)+ (Cu)+Cu2Se/Cu2Se+Liquid (I)/Liquid (I)/PbSe. For both kinds of couples, Cu is the dominating diffusion species.

Interfacial reactions in Ag/Se, Ag/Se-30at%Te and Ag2Te/Se couples and the phase equilibria of the Ag-Se-Te ternary system

Phase equilibria information of the Ag-Se-Te ternary system is important as this system is used in thermoelectric applications. The Se and Te elements are the major constituents of many thermoelectric compounds, whilst the Ag element is widely used in the thermoelectric modules as an electrode. This study examines the reaction couples of Ag/Se, Ag/Se-30at%Te and Ag2Te/Se at 350 °C, as well as the isothermal section of the Ag-Se-Te system at 350 °C. There are five three-phase regions and no ternary compounds. The Ag/Se reaction couple shows the formation of the Ag2Se reaction layer with this phase having a very homogeneous composition. The reaction is very fast. The reaction layer is 550 µm thick in the couple reacting for 2 min. The Ag2Te/Se reaction couple shows the formation of Ag2Se, liquid, and Te-rich phases. The reaction path in the Ag2Te/Se reaction couple is Ag2Te/(Se,Te)/Liquid/Ag2Se/Se. It is found that the reaction layer is 576 µm thick in the couple reacting for 2.5 min. The Ag/Se-30at%Te reaction couple shows the formation of Ag2(Se,Te) and liquid phases. A very thick reaction zone of 2600 µm is found after reacting for 2.5 min. The related reaction paths and mass balances of the reaction couples are well explained using the determined isothermal section of the Ag-Se-Te system at 350 °C.

Microscopic Imaging and Optical Pressure Measurements of Detonations

AbstractThe microstructure of solid high explosives can affect their safety and performance. However, the extreme conditions in the vicinity of detonating explosives make high‐resolution imaging and data acquisition challenges. A new diagnostic method is presented here that allows microscopic imaging of self‐supported detonations while optically measuring pressures in an arbitrary confiner material nearby using an imaging fiber and an image intensifier at a safe distance. Tests with pressed pellets of HMX and PBX 9501 revealed features during pore collapse in engineered holes and detonation reaction zone morphology and thickness. Interface experiments allowed the measurement of pressure at the boundary between the explosive and a silicone polymer and the determination of the specific heat ratio for the gaseous products. The same methodology can be used to fit any form of equation‐of‐state and Hugoniot curves for arbitrary materials using few simple experiments.

Contributions of flame thickening and extinctions to a heat release rate marker of intensely turbulent premixed hydrogen-enriched methane-air flames

Influences of the reaction zone thickening and extinctions on a heat release rate marker of extremely turbulent hydrogen-enriched methane-air flames are investigated using simultaneous planar laser-induced fluorescence of formaldehyde molecule and hydroxyl radical as well as separate stereoscopic particle image velocimetry techniques. The fuel-air equivalence ratio is set to 0.7, and the amount of hydrogen-enrichment varies from 0% to 70% by volume. Mean bulk flow velocities ranging from 5 to 35 m/s with Reynolds and Karlovitz numbers of 18 to 2729 and 0.1 to 76.0, respectively, are examined. It is shown that, by increasing the turbulence intensity, the preheat and reaction zone thicknesses can increase to values that are, respectively, 6.3 and 4.9 of the corresponding laminar flames, which were measured experimentally. Broadening of these zones for intensely turbulent hydrogen-enriched methane-air flames is shown experimentally for the first time. Broadening of the reaction zone suggests that the flamelet assumption used for development of the turbulent flame speed formulations may not hold. Thus, a new formulation, which does not utilize the flamelet assumption, is developed and used to calculate a heat release rate marker of the tested flames. It is shown that, at small turbulence intensities, the heat release rate marker values follow those of the local consumption speed, which is developed in the literature based on the flamelet assumption. However, at large turbulence intensities, the estimated heat release rate marker features large values, and the ratio of this parameter to the local consumption speed is consistent with the ratio of the global and local consumption speeds reported in the literature. It is shown that the ratio of the normalized heat release rate marker to the normalized local consumption speed is correlated with the broadening of reaction zone, suggesting that the disparity between the values of the heat release rate marker and the local consumption speed is linked to the reaction zone thickening. It is shown, although the flame thickening increases the heat release rate marker, extinctions decrease this parameter, which may contribute to the characterized bending behavior. It is argued that the amount of bending is related to the inverse of the Damköhler number. Using this, a mathematical formulation that allows for estimation of the heat release rate marker is developed. The predictions of the proposed formulation agree well with the normalized global consumption speeds of past experimental investigations.

Inhibition of turbulent methane/air premixed bunsen flames by dimethyl methylphosphonate

There is little knowledge about the inhibition efficiency of dimethyl methylphosphonate (DMMP) against turbulent flames, in which vortices of different sizes corrugate and stretch the flame front. This study is to investigate the performance and the mechanism of DMMP in inhibiting turbulent methane/air premixed Bunsen flames. Several important parameters are chosen to characterize the combustion of the flames, including flame brush height, flame brush thickness and turbulent burning velocity. It is found that the flame brush expands both horizontally and vertically with the increase of turbulent intensity. For all the flames, the centerline mean flame brush thickness increases linearly with flame height. Turbulent burning velocity decreases monotonically with the increasing of DMMP addition. However, the normalized turbulent burning velocity increases with the normalized turbulent intensity. Compared with the performance in laminar premixed flames, the inhibition efficiency depends on the turbulent intensity. For low-intensity turbulent flames, the inhibition efficiency is higher than that in laminar premixed flames, but the relative inhibition efficiency decreases with DMMP addition. This can be attributed to the relative size of the vortices and the flame front thickness: when the Kolmogorov length scale is less than the preheat zone thickness but greater than the reaction zone thickness, vortices reduce the burning velocity; when the Kolmogorov length scale is less than the reaction zone thickness, vortices facilitate the combustion. The trend is opposite in high-intensity turbulent flames, which is mainly attributed to the generating of the flame pockets in the flame tips.

Application of machine learning for filtered density function closure in MILD combustion

Abstract A machine learning algorithm, the deep neural network (DNN) 1 , is trained using a comprehensive direct numerical simulation (DNS) dataset to predict joint filtered density functions (FDFs) of mixture fraction and reaction progress variable in Moderate or Intense Low-oxygen Dilution (MILD) combustion. The important features of the DNS cases include mixture fraction variations, turbulent mixing lengths, exhaust gas recirculation (EGR) dilution levels, etc., posing a great challenge for data-driven modelling. The DNN architecture is built and optimised with extreme care to achieve high robustness and accuracy, resorting to dimensionality reduction techniques such as principal component analysis (PCA) to identify and remove the outliers in the training data. To better interpret the predictive ability of the DNN, two analytical joint FDF models respectively using two independent β and copula distributions, are also employed for a detailed comparison with the DNS data. The FDFs in MILD combustion behave differently compared to those in conventional flames because the reaction zones are more distributed. They generally exhibit non-regular (neither Gaussian nor bi-modal) distributions and strong cross correlations, which cannot be captured adequately by the analytical models. However, the DNN is well suited for this physico-chemically complex problem and its predictions are in excellent agreement with the DNS data for a broad range of mixture conditions and filter sizes. Furthermore, a priori assessment is conducted for filtered reaction rate closure. It is found that the DNN model significantly outperforms the analytical models for all cases showing very good predictions for the filtered reaction rate for a range of filter sizes. The DNN prediction improves as the filter size becomes larger than the characteristic reaction zone thickness while the analytical models works relatively better for smaller filter sizes. This is a clear advantage for the DNN to be used in practical LES applications.

Thick reaction zones in non-flamelet turbulent premixed combustion

Internal structure of extremely turbulent flames is investigated experimentally using simultaneous planar laser-induced fluorescence of formaldehyde molecule and hydroxyl radical as well as stereoscopic particle image velocimetry. The mean bulk flow velocity is changed from 5 to 35 m/s. The fuel-air equivalence ratio is 0.7 for all tested conditions. Three different turbulence generating mechanisms leading to a wide range of turbulence intensity with the corresponding Reynolds and Karlovitz numbers ranging from 19 to 2729 and 0.3 to 76.0, respectively, are examined. Preheat and reaction zone thicknesses for the tested flames are calculated and compared with those of the laminar flame to quantify deviation from flamelet behavior. It is shown that the preheat and reaction zone thicknesses increase to values that are about 6.2 and 3.9 times the laminar flame counterparts, respectively. While broadening of the preheat zone is reported in the literature, broadening of the reaction zone for a relatively large diameter Bunsen burner is reported for the first time in this study. The broadening of the reaction zones is related to non-flamelet behavior of the tested flames, and a parameter proposed earlier by the authors is used to quantify this non-flamelet behavior. Swirling strength contours are overlaid on the cold reactants and preheat zones to study the reason for the reported broadening. The results show that positive correlations exist between the preheat/reaction zone thicknesses and the mean value of eddy swirling strength inside the reactants and preheat zone. These correlations as well as the estimated turbulent kinetic energy of these eddies suggest that energetic eddies may potentially penetrate into the preheat and reaction zones causing broadening of these zones.

Role of instability on the limits of laterally strained detonation waves

The present work examines the role of instability and diffusive phenomena in controlling the limits of detonations subject to lateral strain. Experiments were conducted in mixtures with varying levels of cellular instability, i.e., stoichiometric methane–oxygen (CH4/2O2), ethylene–oxygen (C2H4/3O2), and ethane–oxygen (C2H6/3.5O2). These detonations were propagated in channels with exponentially enlarging cross-sections, following the recent works of Radulescu and Borzou (2018) and Xiao and Radulescu (2020). Steady detonation waves were obtained at the macro-scale, with the near-limit reaction zone structures characterized by significant unreacted gas pockets. The turbulent flame burning velocity of these pockets was evaluated to be 30 m/s to 70 m/s, which is larger than the theoretical laminar value by a factor of 2 to 7 and smaller than the CJ deflagration speed by a factor of 2 to 3. For all the mixtures tested, the characteristic D−K relationships, relating the detonation mean propagation speed with lateral flow divergence, were obtained directly from experiments and as well from the generalized ZND model with lateral strain rates using detailed chemical kinetics. The results showed that the degree of departure between experiments and the theoretical predictions increases significantly with the detonation instability level. As compared to the laminar ZND wave, the more unstable detonations are much more detonable than the more stable detonations, with substantially larger limiting divergence rates and maximum velocity deficits. Such enhanced detonability with detonation instability can be manifested in the significantly enhanced global rates of energy release with the notably suppressed thermal character of ignition for the more unstable detonations. This globally enhanced burning mechanism is found to be realized by the intensified auto-ignition assisted by the turbulent diffusive burning of the unreacted gas pockets, substantially shortening the characteristic reaction zone lengths. Finally, the relatively good universality of the D/DCJ−Keffλ relations implicitly suggests that cell size is also a function of the detonation wave stability, since it is controlled by the reaction zone thickness of unstable detonations.