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• Graded fuel and oxidizer integrated microspheres improve combustion performance. • Thermal shock induces local cavitation effect leading to microexplosive combustion. • Integrate the three-step thermal decomposition process of AP into a single step. • The reduced heat and mass transfer distance promotes energy release efficiency. The combustion agglomeration of aluminum (Al) and the long heat and mass transfer distance between Al and oxidizer cause serious two-phase flow loss and slag accumulation in the engine of solid propellants. Here, we present an effective strategy by in-situ encapsulating graded nano-aluminum (nAl) and micron-aluminum (μAl) in ammonium perchlorate (AP) simultaneously by using hydroxymethyl cellulose (HMC) as adhesive and constructing a compact coupling fuel oxidizer complex high-energy microsphere (μAl/nAl@AP/HMC) to enhance the energy release efficiency and combustion efficiency. Compared with physical mixture (PM), the ignition delay time and combustion time of μAl/nAl@AP/HMC microspheres are reduced by 69.58 % and 69.87 %, owing to the decomposition process control of AP through the adsorption and spatial constraint effect on the interface between metal fuels and oxidizer. Furthermore, the nAl embedded in the AP surface forms a hot spot around the μAl to promote the combustion reaction, thereby shortening the cavitation threshold and tensile limit time of μAl and accelerating the occurrence of microexplosion. Microexplosion effect in μAl/nAl@AP/HMC microsphere significantly weakens combustion agglomeration and improves combustion efficiency and energy release efficiency by sharp shrinkage of combustion condensation products, which addresses the challenges on energy output and combustion efficiency in high-density solid propellants. The composite energetic microspheres (nAl/μAl@AP/HMC) were prepared using a self-assembly method, where graded fuels(nAl and μAl) were encapsulated within an oxidizer, ammonium perchlorate (AP). High reactive nAl produces thermal shock during combustion, which effects local cavitation effect on the μAl, and then triggers microexplosive combustion of microspheres. The adsorption effect of graded nAl on the gaseous products of AP decomposition modifies the three-step decomposition process of AP into a single-step decomposition, resulting in facilitating the concentrated release of acid and oxidizing gases during AP decomposition. [Display omitted] [ABSTRACT FROM AUTHOR]

Fluorinated graphene (FGO) is a two-dimensional layered compound with thermal conductivity and hydrophobicity. Different percentage of FGO (1.00 wt.%, 3.00 wt.%, 5.00 wt.%) was added to the composite energetic system boron/potassium nitrate (B/KNO 3). Through elemental analysis (EA), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS), the results show that with the increase of fluorination degree, the stretching vibration peak of C-F bond was more obvious, and the contents of fluorocarbon functional groups, especially C-F 2 rise. The results of synchronous thermal analysis and mass spectrometry (DSC-TG-MS) showed that the initial decomposition temperature of fluorinated graphene was around 400 ℃, the enthalpy value was about 7279 J g−1, and a large number of fluorine-containing gas products arising therefrom. The effect of fluorinated graphene on the thermal decomposition performance of energetic system was analyzed by DSC-MS. The results show that fluorinated graphene delayed initial exothermic peak temperature of B/KNO 3 system, and promoted subsequent exothermic reaction. The service performances of B/KNO 3 composites were compared with or without FGO. While adding fluorinated graphene, the moisture resistance and thermal conductivity of the B/KNO 3 composites was improved, flame sensitivity was reduced, thermal safety was improved, gaseous products and combustion heat were increased, flame duration was prolonged, and the ignition delay period was shortened. It is implied that the present of FGO made the composite energetic materials safer under lower heat and more reliable under high heat. FGO plays a role of intelligent regulation and smart responds to different thermal stimuli. [ABSTRACT FROM AUTHOR]

A single ethanol droplet evaporation through laminar methane/air stagnation flame is investigated at lean, stoichiometric and rich conditions experimentally and numerically. For the droplets having initial diameters of 20- 70 μ m , the particle Reynolds number is measured via PIV/PTV, resulting in the slip velocity between the droplet and gas flow being small and the surrounding gas being able to carry without any significant convective effects. Evaporation rates, computed from Abramzon–Sirignano for a moving droplet towards a stagnation flame field, satisfy the ones computed from empirically determined evaporation rates from ILIDS measurements as a function of flame temperature, flame speed and flame thickness. Nevertheless, Abramzon–Sirignano model slightly overestimates the vaporization constant for lean cases. The distance traveled by the droplet after leaving the flame zone determines the average critical diameter. Accordingly, droplets larger than 18 μ m can cross the flame, leading to local modifications in the flame region due to heat loss and vapor diffusion from the droplet. Novelty and Significance Energy conversion in combustion should be carried out by optimizing efficiency, minimizing pollution, and preventing further climate change. The multiphase nature of spray combustion particularly adds further complexity due to the strong coupling of atomization, evaporation, mixing, turbulence, and chemical reactions. Although spray combustion involves a cloud of polydispersed droplets, it is of fundamental importance to understand the dynamics of an isolated droplet and its interactions with the flame front, which can provide indispensable information for spray combustion models. In this study, an experimental and computational framework has been developed to investigate droplet-flame interactions. The dynamics and evaporation characteristics of an isolated droplets interacting with stagnation flames have been determined by coupling several laser diagnostics, while further enhanced with Eulerian–Lagrangian simulations under flame conditions. Thereupon, the outcomes help the design of injectors of spray combustion systems and enhancement of evaporation models. [ABSTRACT FROM AUTHOR]

This study delves into the investigation of multi-scale flow structures and combustion characteristics in a cavity-enhanced circular scramjet operating under a Mach-4.5 freestream flow condition with a total temperature of 2200 K and a total pressure of 164 kPa. To achieve this, a hybrid large-eddy/Reynolds-averaged Navier-Stokes method, coupled with a pressure-corrected Flamelet/Progress Variable model, is employed and validated. The combustion flow field shows abundant small- and medium-scale flow structures induced by compressibility and combustion heat release. Vorticity alteration is found to be mainly caused by these factors, with the volumetric expansion term and baroclinic term serving as the primary driving terms. The whole combustion process is characterized by zoning and multi-scale features in spatial, temporal, heat release and species distribution, arising from differences in fuel-air mixing state and chemical reaction rates in the flow field. The injection-formed bow shock and the cavity ramp-formed compression wave interact intensively with the fuel plume, enriching the turbulent structures and strongly coupling with the chemical reaction process. The cavity not only promotes fuel-air mixing and combustion, but also facilitate the rapid transition from the laminar-like flame to thickened turbulent combustion. Statistical analysis reveals that the combustion heat release is predominantly governed by the scramjet mode and the diffusion flames, and further demonstrates the multi-scale flow and chemical reaction characteristics. Most of the combustion flow field is observed to be governed by fast chemistry, primarily in the corrugated flamelet regime, while some slow chemistry zones are also identified, lying in the distributed reaction zone regime. [ABSTRACT FROM AUTHOR]

This work aims to find the benefits, limitations, and costs (in terms of plasma energy and NO x emissions) of plasma-assisted ignition-stabilized combustion (PAISC). The experiments are performed on a custom-made methane-air jet flame burner with two coaxial flow inlets. Dielectric barrier discharge (DBD) configuration is set up in the outer flow channel which acts as four ignition sources. The DBDs are operated by bursts of positive voltage pulses of 10 ns duration and 24 kV amplitude. In this way, four ignition kernels are created in flow simultaneously. The ignited kernels expand while blowing out with the flow which is followed by new ignition kernel initiation during the next burst. This cycle is repeated continuously and complete burning of the fuel is ensured by proper interactions between ignited kernels. The PAISC dynamics are visualized using OH* chemiluminescence images captured at 10 kHz framerate. Along with visualization measurements, we measure CO and NO x emissions using an industrial gas analyzer. The gas analyzer measurements are complemented by two-photon laser-induced fluorescence (TALIF) measurements of CO. We find the optimal plasma parameters to optimize CO, and NO x emissions and the effective flame length. A parametric study for various flow parameters is performed and combustion operation regimes are identified. The main highlight of this work is quasi-complete combustion at flow speeds of up to 4 times the blowout flow speed, which costs less than 10 ppm@3%O 2 plasma-produced NO x and has a plasma energy consumption of less than 1% of the combustion heat release. Novelty and Significance Statement Various researchers have demonstrated plasma's ability to enhance combustion at near-lean-blowout limits in the past. This work presents a novel plasma-assisted ignition stabilized combustion (PAISC) strategy to achieve quasi-complete combustion at flow speeds much above blowout flow speeds. Pulsed dielectric barrier discharge plasma is used to ignite methane-air flow as it is a fast, repetitive, and energy-efficient ignition source. We study PAISC dynamics using high-speed visualization along with CO, NO x emissions, and plasma energy for various plasma and flow parameters. From these analyses, we optimize plasma parameters first and then discuss the benefits, limitations, and costs of PAISC at optimal plasma conditions. We believe that the presented strategy will motivate future combustor design with a wider range of operability, especially for high-speed propulsion applications. [ABSTRACT FROM AUTHOR]

We present a gas dynamic model for Chapman-Jouguet deflagrations driving a shock in tubes or congested configurations with rear venting. The model extends the previous work of Chue, Clarke and Lee (Proc. Roy. Soc. 1993) to include rear venting. Two models are formulated, one for a perfect gas with constant heat of combustion, and the second by assuming chemical equilibrium in the product gases and temperature dependent specific heats. In the presence of rear venting, much slower flames and shock strengths are generated; these are controlled by the vent ratio, which reduces the post shock strength and speed of the gas convecting the flame forward. Calculation results are presented for small hydrocarbon-air and hydrogen-air deflagrations. The results of the model are found in good agreement with the critical deflagration speed measured experimentally in large scale weakly confined congested experiments for auto-ignition insensitive mixtures. This suggests that flames in these experiments have reached their maximum Chapman-Jouguet deflagration speed prior to transition. We present a time scale estimate for flame acceleration and auto-ignition behind reflected shocks. For sensitive mixtures for which the flame acceleration time is longer than the auto-ignition time (hydrogen, acetylene), limited experimental evidence indicate transition to detonation at critical speeds lower than the CJ deflagration condition. The time scale ratio thus provides the conditions for which reaching the CJ deflagration condition can be taken as a necessary condition for DDT. [ABSTRACT FROM AUTHOR]

Principal component transport-based data-driven reduced-order models (PC-transport ROM) are being increasingly adopted as a combustion model of turbulent reactive flows to mitigate the computational cost associated with incorporating detailed chemical kinetics. Previous studies were mainly limited to replicating relatively-simple chemistry in canonical configurations. The objective of the present study, therefore, is to further explore the accuracy of PC-transport ROM on more complex combustion phenomenon where, for example, large hydrocarbon fuel chemistry spanning a broad range of thermochemical space governs sequential multi-stage compression ignition processes. The cumulative error of PC-transport for this problem, and for others that depend upon sequential highly nonlinear physics, has to be minimal as the combustion phasing and heat release rate in internal combustion engines depends upon accurate predictions of minor ignition species whose concentrations start from ashes and grow orders of magnitude over the course of low- and high-temperature autoignition. Specifically, the PC-transport ROM is applied to predict the compression ignition characteristics of lean n -heptane/air and primary reference fuel (PRF)/air mixtures in a two-dimensional (2-D) constant volume computational domain initialized with a two-dimensional isotropic turbulence spectrum and temperature inhomogeneities. PCA is used to define the low-dimensional manifold that represents the original thermochemical state vector, and artificial neural network (ANN) models are adopted to tabulate chemical kinetics, transport, and thermodynamic properties. A series of 2-D pseudo-turbulent simulations are performed at engine pressures by varying the initial mean and r.m.s. of temperature, turbulence intensity, and the composition of fuel/air mixture. The results show that the PC-transport ROM accurately reproduces the instantaneous and statistical ignition characteristics of the fuel/air mixture, aided by pre-processing techniques including species subsetting, data clustering, and data transformation. It is found that PCs are not properly scaled with a power transformer if reactants are included in the species subset, which leads to a decrease in the accuracy of the PC-transport ROM. A separation of the reactants from the species subset ensures that the temporal evolution of the PCs starts from zero and spans orders of magnitude with time, and as such, this approach is found to effectively redistribute both PCs and their source terms with a power transformer. The computational speed-up factor of the PC-transport ROM ranges between 5.1 and 15.0 for the cases with n -heptane/air mixture and PRF/air mixture, respectively. Moreover, a potential further speed-up is anticipated through a combination of reduction in grid resolution requirements and in the stiffness of the chemical system. As an example, many of the pre-processing methods for inhomogeneous compression ignition may also apply to other complex intermittent combustion phenomena. Novelty and significance statement • The PCA-based reduced-order model (PC-transport ROM) has been applied to the multi-stage compression ignition of large hydrocarbon fuels under HCCI-relevant conditions. The present work presents a systematic procedure to accurately capture the two-stage ignition behavior of lean n -heptane/air or PRF50/air mixture. • The present work demonstrates an advantage of the PC-transport ROM in terms of computational speed-up. The computational speed-up factor for the ROM is up to 15, and moreover, a potential additional speed-up is anticipated through the reduction in the spatial and temporal resolution required. • A series of 2-D PC-transport ROMs are conducted to demonstrate the robustness of the ROM. A limitation of the ROM against different operating conditions is also discussed. [ABSTRACT FROM AUTHOR]

Ammonium perchlorate (AP) is an important oxidant of solid propellant, and its decomposition is of great significance for improving combustion properties of solid propellants. However, it is difficult to balance AP-catalytic effect, ignition and energy level. In this study, lithium hexafluorophosphate (LiPF 6), ferric chloride (FeCl 3) and 1,2,4-triazole were self-assembled on aluminum particles via ionic bonding and metallic coordination. It highly catalyzed the thermal decomposition of AP, which reduced the decomposition temperature of AP by 29.4 °C. The thermal oxidation reactivity of coated-aluminum was much higher than that of raw aluminum powder, and its exothermic enthalpy was 3.45 times that of raw aluminum. The combustion heat was also improved. After ignition delay of 0.2 s, Al/CM-4 was burnt violently at 740 °C, while raw Al could not be ignited even at 950 °C. Our work provides an effective strategy for high-performance self-assembly coating on Al fuels, which addressed big significance in AP-containing propellants and explosives. Besides, such organic/inorganic composite self-assembly system may also be instructive for future design of heterogeneous surface, multifunctional coating, protective interface, and etc. [ABSTRACT FROM AUTHOR]

Burning experiments are analyzed for 59 successfully ignited tests using a gas burner (BRE) to emulate the steady burning of 25 mm diameter flat surface materials. More than half of the tests resulted in long burns of at least 2 and as long as 5 min. Mixtures of ethylene and nitrogen allow for 23.6 and 47.2 kJ/g heats of combustion (LHV). Nominal ambient conditions range from 40% to 21% oxygen and pressures of 1 to about 0.56 bar. NASA human habitat atmospheres have been used. Generally, it has been found that nominal oxygen mole fractions of 26% and above can allow steady flames. The flames are thin blue in color and grow slowly over time for minutes. Some self-extinguish, some had periodic oscillations, and others appear to have become steady and were shut down. Theoretical analyses show the shutdown flames were steady in growth, and all of the test endpoints were more than 94% of steady state in flame heat flux. Flame heat flux for set mass flux allowed the estimation of heats of gasification to relate to steady solid materials that might burn in microgravity. Radiometers allowed the computation of flame radiative fraction that appeared to correlate well with the measured flame height. Results are shown for heat flux, flame height and base radius, radiative fraction as a function of fuel mixture mass flux, and various oxygen and pressure atmospheres. [ABSTRACT FROM AUTHOR]

The incorporation of aluminum (Al) into explosives represents a facility and efficient way to enhance the energy output through the secondary reaction between Al and the detonation products of explosive. The Al/O ratio and microstructure of HMX/Al play critical roles in combustion performance and energy output. Herein, a gradient structured HMX/Al composite has been designed and fabricated via 3D printing technology using well-dispersed HMX/Al-based ink. For HMX/Al lines, the second reaction heat increases from 196.5 J/g to 1644.8 J/g and the burning rate would decrease from 14.6 to 12.0 mm/s with Al content increased from 10 to 30 wt%. For gradient structured HMX/Al, the burning rate could be controlled by varying the component ratio and the burning rate of each component in the gradient structure was higher than that of the normal HMX/Al counterpart. Furthermore, a gradient evolution of pressure output was monitored for gradient structured HMX/Al cylinder ignited by the laser. These results indicate that the gradient structure as an effective approach can tune combustion performance and energy output of HMX/Al. [ABSTRACT FROM AUTHOR]

Polyethylene dust and polypropene dust were selected to measure the minimum explosion concentration in three kinds of gaseous fuel environments. The results show that the MEC of polyethylene dust with higher apparent activation energy decreased more significantly, and the plastic dust showed higher explosion sensitivity for the gaseous fuel with high combustion heat. The explosion mechanism for plastic dust in gaseous fuel environments was proposed with the global reaction kinetics of plastic pyrolysis, and it suggests that the gaseous fuels reduce the apparent activation energy of plastic dust and provide additional heat for the plastic dust explosion. A corresponding prediction model for the minimum explosion concentration of plastic dust was established. The new model has good applicability for the data obtained in this work and the data in the previous reference. [Display omitted] [ABSTRACT FROM AUTHOR]

Metal fuel with high energy and reactivity is an urgent need for the development of solid propellant, explosive and pyrotechnics. Herein, spherical B/Al/PTFE fuel is designed based on synergistic reaction effect between B/F and Al/F and prepared through the confinement-emulsion-templated method. The combustion reaction performance and pressure output performance of B/Al/PTFE microspheres are much better than their physically mixed counterparts. The synergistic reaction effect between B/F and Al/F on the reaction heat, flame propagation, and pressure output were systematically investigated through adjusting the B/Al mass ratio (30/0∼0/30). The DSC results show that the highest heat of combustion of 4904 J/g can be achieved when the B/Al mass ratio is 21/9. Furthermore, the combustion duration, peak pressure, and pressurization rate are monotonically accelerated from 653 ms, 352.15 kPa, and 305.67 MPa/s to 200 ms, 821.25 kPa, and 1831.18 MPa/s, respectively. This work provides a promising method for the design and preparation of highly composited metal fuel for practical application. [ABSTRACT FROM AUTHOR]

A set of 58 group additivity values (GAV) for the calculation of the heat of formation is derived from an extensive and accurate database of 192 ab initio heats of formation. The ab initio values are from companion calculations at the CCSD(T)–F12/cc-pVTZ-F12//B2PLYPD3/cc-pVTZ level of theory employing second order connectivity based hierarchy reference reactions with ANL energies for the reference species. This database of 192 species consists of alkanes (RH), alkyl radicals (Ṙ), alkyl hydroperoxides (RO 2 H), alkyl-peroxy (RȮ 2) radicals, and hydroperoxy-alkyl (Q ˙ OOH) radicals of all of the isomers of C 2 –C 5 alkane fuels and a select set of C 6 –C 8 isomers. The GAV and ab initio based enthalpies for this dataset show excellent agreement, with an estimated 2σ uncertainty of 0.9 kcal mol–1. Among the 58 GAV terms, 40 are refinements of previously reported terms, while 18 are newly developed. These new GAV terms are mainly non-next-nearest neighbor interactions (NNI) and β -corrections. The inclusion of these new groups significantly improves the accuracy of the GAV estimates of the heats of formation. The updated GAVs can be used with increased confidence to estimate the heats of formation of combustion relevant hydrocarbons and oxygenated hydrocarbons and their corresponding radicals, which are important in predicting low-temperature chemistry and are critical in the development of accurate chemical kinetic models. [ABSTRACT FROM AUTHOR]

Burning rate modulation of solid fuel can improve the performance of solid fuel ramjet over a wider altitude or Mach number envelopes. To better control the burning rate of a solid fuel, the effect of magnetic fields on the regression rate of solid fuel was experimentally and numerically investigated. In this experiment, the regression rates of polymethyl methacrylate (PMMA) rods under different types of magnetic field gradients were measured. Meanwhile, the morphological changes in the flame during combustion were recorded by a complementary metal-oxide semiconductor (CMOS) camera. The experimental results indicated that the regression rate for PMMA varies regularly with various magnetic field gradients, with rates in the range from -32.5% to 10.8%. Simultaneously, the flame structure also changes. The numerical simulation that considered both gas-phase combustion and gas-solid heat transfer produced results that were in good agreement with the experiment. The results show that the change in the regression rate is a result of the magnetic force acting on the paramagnetic and diamagnetic materials. By changing the flow field, the magnetic force can change the flame shape and temperature distribution. This alters the heat exchanged between the gas and the fuel surface and thus, changes the burning rate. It was found that the burning rate of solid fuel can be adjusted regularly by applying different types of non-uniform magnetic fields, and it may be used in the solid fuel ramjet burning rate modulation. [ABSTRACT FROM AUTHOR]

Engineered smouldering applications require self-sustaining conditions where the fuel is consumed without any addition of energy beyond ignition. In practical applications, it is preferred to create scenarios that are robust (i.e., far from extinction). In this work, smouldering column experiments were conducted, spanning conditions from robust self-sustaining through weak self-sustaining and to extinction by systematically lowering the applied air flux and the fuel concentration. A previously developed one-dimensional numerical model, which was validated under robust smouldering conditions, was used to simulate the experiments. Experiments showed front deformation in the weak and extinction cases due to multi-dimensional effects. This deformation resulted in a redistribution of the air flow, altering the global energy balance at the centre-line. The model was found to be unable to accurately reproduce these weak and extinction cases with the original validation parameters. A sensitivity analysis revealed that treating the heat of combustion as a fitting parameter, and letting it vary beyond its realistic maximum, enabled the model to create a condition of sufficient energy at the centre-line and was the best way to quantitatively simulate the weak and extinction cases for this specific scenario. This reveals that the model assumptions, particularly the assumption of one-dimensionality, are sufficient for robust smouldering but are insufficient for weakly self-sustaining scenarios. [ABSTRACT FROM AUTHOR]

• Get the influence of aluminum powder on mixture fuel explosion pressure. • Get the influence of aluminum powder on mixture fuel explosion maximum pressure rise rate. • Get the influence of aluminum powder on mixture fuel combustion duration. • Get the influence of aluminum powder on mixture fuel flammability mass concentration limits. • Get the influence of aluminum powder on mixture fuel explosive products. The impact of aluminum powder on the explosive characteristics of solid-liquid mixed explosives under low fuel concentration conditions was studied using a 20 L spherical explosion container. In particular, measurements and analyses of an aluminum powder/ hexogen (RDX) powder/mist nitromethane–air mixture were studied in terms of explosion pressure, maximum pressure rise rate, combustion duration and flammability mass concentration low limits for different aluminum powder mass fractions. As the aluminum powder mass fraction increases for fuel concentrations at 300 g m−3, due to the increase in the concentration of aluminum powder with high combustion heat. The interaction between components leads to the maximum pressure increase rate first increases and then decreases, and the combustion duration first decreases and then increases. The change in the combustion duration is the complete opposite of the maximum pressure rise rate. At the same time, as the mass fraction of aluminum powder increases, the flammability mass concentration limit continues to decrease. [ABSTRACT FROM AUTHOR]

Thermo-physical properties play an important role in combustion and heat transfer of propulsion system. The thermo-physical properties (thermal diffusivity, specific heat and thermal conductivity) aspects were studied on incorporation of RDX by partial replacement of AP in low aluminized propellant. These properties were experimentally determined by Flash technique at ambient temperature up to 100 °C. The finding reveals that on increasing the ratio of RDX to AP, there is an increase in specific heat and thermal conductivity while thermal diffusivity decreases, accordingly. The effect of RDX on combustion mechanism is evident in combustion wave, which was clearly visible in high-speed images of propellant burning surface at ambient conditions. [ABSTRACT FROM AUTHOR]

In turbulent premixed flames at low Karlovitz number, combustion heat release can have a significant impact on turbulence. Thermal expansion in flame induces dilatation, and the corresponding pressure–dilatation correlation acts as a primary source of turbulent kinetic energy (TKE). As a consequence, the flame-normal component of the normal Reynolds stresses significantly increases. Additionally, for sheared flames, typical of jet flames, the shear component of the Reynolds stresses exhibits counter-Boussinesq behavior. For flames at low Karlovitz number, where these effects dominate, no models have successfully predicted all Reynolds stress components. To develop more complete turbulence models, heat release effects on the evolution of all Reynolds stress components need to be analyzed. In this work, Reynolds stress budgets are evaluated from Direct Numerical Simulation (DNS) databases of spatially-evolving turbulent premixed planar jet flames at low and high Karlovitz numbers. In the Reynolds stress budgets, the velocity–pressure gradient correlation term is important at both Karlovitz numbers but serves fundamentally different roles in each case. In the budgets for the normal components, the velocity–pressure gradient correlation term is decomposed into a redistribution term and an isotropic term, where the redistribution term acts to redistribute energy between the Reynolds stress components and the isotropic term is the pressure–dilatation source term of turbulent kinetic energy. At high Karlovitz number, the isotropic term is negligible, and the redistribution term acts to isotropize the turbulence as in non-reacting flows. Conversely, at low Karlovitz number, the isotropic term acts as a large source, and the redistribution term preferentially injects energy into the flame-normal component at the expense of other components which acts to make the turbulence less isotropic. In the budget for the shear component, the shear production term dominates the velocity–pressure gradient correlation term at high Karlovitz number, but the opposite is observed at low Karlovitz number. The dominance of the velocity–pressure gradient correlation term at low Karlovitz number is primarily induced by the flame-generated mean pressure gradient and ultimately leads to the counter-Boussinesq behavior of the shear component. The overall analysis indicates that any turbulence model that relies on small-scale isotropy and/or rapid isotropization will fail to capture the heat release effects on turbulence at low Karlovitz number. [ABSTRACT FROM AUTHOR]

Reactive materials of aluminum (Al) and polytetrafluoroethylene (PTFE) are attracting intense attention, which have been extensively developed owing to their high energy density (21 kJ/cm3) and practical applications. However, the combustion kinetics and heat of reaction are limited by the slow mass diffusion and incomplete reaction. Herein, an effective and simple way was proposed to enhance the heat of reaction, combustion and pressurization rates of PTFE/Al system by introducing ammonium perchlorate (AP) as the gas generator and oxidizer. The gaseous products from the AP decomposition can not only increase the pressure but also can further react with the produced carbon from the PTFE/Al to release more energy and gas. The significantly enhanced energy output, pressurization rate and burn rate (8863 J/g, 340 MPa/ms and 1626 m/s) were achieved for PTFE/Al by adding with 9 wt% of AP, which were much higher than that for pure PTFE/Al (2019 J/g, 29.3 MPa/ms and 260 m/s, respectively). More interestingly, the deflagration phenomenon was observed due to the high energy and pressure output. The main decomposition compounds and reaction products were further studied to understand the reaction mechanism. The strategy of adding AP in the PTFE/Al can provide efficient approach to enhance the energetic performance and promote the practical applications in propellant, heterogeneous explosives and gas generator. [ABSTRACT FROM AUTHOR]

A quantitative understanding of the thermal transport within a growing intumescent char layer remains largely unsolved. The development of improved techniques to analyze charring and intumescent materials is necessary to investigate the physics of heat transfer within the char. To aid in this endeavor, a systematic methodology to parameterize comprehensive pyrolysis models for charring and intumescent materials is presented. Thermogravimetric analysis, differential scanning calorimetry and microscale combustion calorimetry were conducted on 4–7 mg samples to analyze the kinetics and thermodynamics of thermal decomposition and determine heats of complete combustion of gaseous pyrolyzates. A multi-step reaction mechanism, consisting of sequential first-order reactions, was constructed to capture the physical transformations and chemical reactions observed in the milligram-scale experiments. 0.07 m diameter disk-shaped samples were gasified in the Controlled Atmosphere Pyrolysis Apparatus II to characterize the thermal transport within the condensed phase material and evolving char layer. ThermaKin2Ds was employed to interpret the experimental data using inverse analysis techniques. Bisphenol A polycarbonate and poly(ether ether ketone), widely used intumescent materials, were analyzed within this study. The resulting two-dimensional models of bisphenol A polycarbonate and poly(ether ether ketone) were capable of predicting the experimental gasification mass loss rates with a mean error of 17.8% and 16.9%, respectively. An analysis of the char pore structure was also conducted from which quantitative relationships were subsequently developed between relevant thermal transport quantities and physical descriptors of the char's physical structure. Two prominent linear correlations were discovered: the char's effective thermal conductivity increased as a function of increasing volume-weighted mean pore diameter and the product of density and thermal conductivity increased with an increasing char porosity based on image analysis. Finally, a brief sensitivity analysis of the polycarbonate and poly(ether ether ketone) burning rates was conducted to determine which key material properties were responsible for the observed differences in flammability. [ABSTRACT FROM AUTHOR]

Combustion characteristics of ethanol-water (EW) droplets laden with ceria nanoparticles are investigated. The present experimental study focuses on three facets of droplet combustion (i) burning time scale of droplets with and without NPs, (ii) pathways of secondary atomisation due to interface deformations and (iii) coupling of droplet shape deformations and flame heat release. A theoretical vaporisation timescale is advocated which considers natural convection-based evaporation, mass loss due to daughter droplet ejections, and flow through porous media. Droplets seeded with ceria nanoparticles, exhibit arrested surface undulations although internal ebullition is discernibly enhanced as compared to EW droplets without NPs. Deformations and formation of surface craters in EW droplets are traced to the imbalance between local vapour recoil (due to rapid ethanol vaporisation) and surface tension. Such craters collapse and form high-speed ligaments which eventually break at the tip through Rayleigh–Plateau mechanism. This pathway of secondary atomisation of EW droplets has been elucidated using a modified local weber number. On the contrary, for nanofuels, bubble rupture is the mechanism behind the surface crater formation. Proper orthogonal decomposition (POD) technique is utilised for investigating the droplet shape and flame heat release coupling. EW droplet shape and HR are found to be a synced system with a phase lag arising from the flame response timescale. However, a weak coupling is detected for nanofuel droplets. [ABSTRACT FROM AUTHOR]

Molecular simulations of reacting shock-bubble interaction were performed to study viscous effects on the supersonic premixed combustion, using the Direct Simulation Monte Carlo (DSMC) method. Different bubble diameters that are comparable to the thickness of the normal shock wave (Mach=2) were taken into account. The evolution of bubble morphology indicates that viscous effects (diffusion and dissipation) leads to the disappearance of some typical phenomena, including the shock focusing, positive viscosity deposition and wake-like cross-section. Meanwhile, the viscous dissipation is mainly responsible for the enstrophy degeneration, which is enhanced by reducing the bubble scale and heat release in combustion. The observed combustion process, which is prominent near the bubble core, is more evident with a larger bubble diameter and unlikely to occur with the smallest bubble diameter. The corresponding reason is that the strong viscous diffusion reduces the collision events between fuel and oxygen molecules. In addition, a time-scale analysis was conducted to illustrate the viscous diffusion in the hydrodynamic and chemical reaction time-scales with different bubble diameters. As a consequence, a changing dimensionless Damkohler number was presented to understand the dominant diffusion effect in microscale supersonic premixed combustion. [ABSTRACT FROM AUTHOR]

This article presents a numerical study of a jet-in-hot-coflow (JHC) burner which emulates Moderate or Intense Low-oxygen Dilution (MILD) conditions. Such combustion regime offers reduction in pollutant emissions and improvements in efficiency. However, some phenomena like the relations between auto-ignition and flame propagation, local extinction and re-ignition are not easily detected by experimental analysis or through the inspection of CFD calculations. The advanced post-processing tools based on the theories of computational singular perturbation (CSP) and tangential stretching rate (TSR) are adopted to investigate the Large Eddy Simulation (LES) results of the JHC burner with different coflow oxygen levels. A topological characterization of the flowfield is achieved employing the local number of chemically exhausted modes, highlighting regions that share similar dynamical features. Strong chemical activity, denoted by a small number of exhausted modes, is found in the fuel/coflow mixing layer and, to a minor extent, in the coflow/air mixing layer, exhibiting correlation with the higher heat release rate zones. The analysis of the reactive layers with TSR suggests that the flame under MILD condition is initiated by auto-ignition. Moreover, the investigation of the TSR participation indices (PIs) mark the local extinction and re-ignition zone for the low oxygen level case, indicating that the lack of oxygen in the coflow suppresses the path to produce final combustion products and heat—thus reducing the reactivity of the whole system. [ABSTRACT FROM AUTHOR]

Autoignition of n -heptane droplet/vapor/air mixtures behind an oblique shock wave are studied, through Eulerian-Lagrangian method and a skeletal chemical mechanism. The effects of gas/liquid equivalence ratio (ER), droplet diameter, flight altitude, and Mach number on the ignition transient and chemical timescales are investigated. The results show that the ratio of chemical excitation time to ignition delay time can be used to predict the oblique detonation wave (ODW) transition mode. When the ratio is relatively high, the combustion heat release is slow and smooth transition is more likely to occur. In heterogeneous ignition, there are direct interactions between the evaporating droplets and the induction/ignition process, and the chemical explosive propensity changes accordingly. The energy absorption of evaporating droplets significantly retards the ignition of n -heptane vapor. In the two-phase n -heptane mixture autoignition process, the ignition delay time decreases exponentially with flight Mach number and increases first and then decreases with the flight altitude. As the liquid ER increases, both ignition delay time and droplet evaporation time increase. With increased droplet diameter, the ignition delay time decreases, and the evaporation time increases. Besides, when Mach number is less than 10, the ratio of the chemical excitation time to ignition delay time generally increases with the flight altitude or Mach number. It increases when the liquid ER decreases or droplet diameter increases. When the Mach number is sufficiently high, it shows limited change with fuel and inflow conditions. The ODW is more likely to be initiated with a smooth transition at high altitude or Mach number. Abrupt transition mode tends to happen when fine fuel droplets are loaded. The results from this work can provide insights into spray ODW initiation. [ABSTRACT FROM AUTHOR]

To design and prepare multifunctional metastable intermixed composites (MICs) with excellent properties, the enhanced "litchi-like" core-shell structure Al/Ti/CuO MICs were prepared by high-energy ball milling (Al/Ti/CuO-B). The morphology, structure, composition, thermal reactivity, energy release and combustion performance of the prepared MICs were investigated by scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and synchronous thermal analysis (DSC-TG), closed system constant volume combustion experiment, open system CO 2 laser ignition experiment and high-energy laser-induced ablation experiment. This study reveals that, the obtained Al/Ti/CuO-B composites have a "litchi-like" core-shell structure, the components are evenly distributed and closely contacted. The exothermic reaction of Al/Ti/CuO-B should be divided into two stages. Due to the introduction of Ti and the special structure of litchi, Al/Ti/CuO-B has superior reactivity (E a =178.15 kJ/mol), energy release (Q = 6443 J/g, P max =164.90KPa), shock wave propagation (v = 1028.33 m/s) and combustion performance than Al/Ti/CuO-P (simple physical mixing). The thrown Al/Ti/CuO composite particles will undergo secondary micro-reactions during the combustion, and a continuous multi-point "micro-explosion" phenomenon will occur, then self-sustainable combustion can be achieved through multi-level energy superposition release, which will enhance its reaction, combustion and energy release. Al/Ti/CuO-B undergoes an ultrafast reaction to generate plasma under high-energy laser induction, which expands and pushes the surrounding air to form a shock wave. The application of Al/Ti/CuO-B shows that the density, heat of detonation, shock wave overpressure, fireball diameter and temperature of composite structural explosive charge containing Al/Ti/CuO ring are all significantly improved. [ABSTRACT FROM AUTHOR]

Increasing the performance of an energetic material can be achieved by changing its composition or using additives, such as metal powders. Aluminum hydride (AlH 3) exhibits attractive properties, such as high hydrogen/energy storage, relatively good stability, and low dehydrogenation temperature. AlH 3 has excellent prospects for use as a component in solid propellants for increasing the specific impulse of rocket engines and effectively reducing the erosion of engine nozzles. In this work, the effect of AlH 3 on the energy performance, ignition characteristics, and combustion characteristics of solid propellants, as well as the agglomeration characteristics of metal particles was investigated. It was found that with increasing AlH 3 content, the theoretical mass heat of combustion and the theoretical volumetric heat of combustion of the propellants increased, and the combustion efficiency also gradually improved. When the content of AlH 3 in the propellant increased from 0.00% to 17.00%, the temperature of the propellant combustion chamber was reduced from 3314.59 to 2957.49 K and the theoretical specific impulse increased from 2584.2 to 2641.7 N‧s/kg. The average molecular mass of the gas was reduced from 25.54 to 21.54 due to the large amount of hydrogen gas released by the combustion of AlH 3. The maximum combustion temperature decreased and the burning rate increased with the gradual replacement of aluminum (Al) in the propellant with AlH 3. The maximum combustion temperatures of the pure Al propellant and the pure AlH 3 propellant were 2510.5 and 2232.5 K, respectively. The linear burning rates were 0.29 and 0.46 cm/s at 1.0 MPa, respectively. The agglomeration process of AlH 3 included three main stages, namely accumulation, aggregation, and agglomeration. With the increase in the AlH 3 content, the number of agglomerated particles in the propellant gas-phase flame region decreased continuously. [ABSTRACT FROM AUTHOR]

Large eddy simulations of a partially premixed flame are performed with the purpose of predicting the reacting flow in a swirl-stabilised, low emissions industrial gas turbine combustor. The corresponding sub-grid scale turbulence–chemistry interactions are modelled using a probability density function transport equation, which is solved by the stochastic fields method. A 15-step reduced, but accurate, methane mechanism including 19 species is employed for the description of all chemical reactions. The test case involves a combustor with complex geometry and simulations are carried out for two different combustor operating conditions. Overall, results of the velocity, temperature and species mass fractions (including carbon monoxide) as well as the instantaneous thermochemical properties are shown to be in good agreement with experimental data, demonstrating the capabilities of the applied stochastic fields method. The inclusion of wall heat transfer in the combustion chamber is found to improve temperature and species predictions, especially in the near-wall regions. Comparisons between an oscillating and a 'stable' flame case furthermore highlight the influence of experimentally observed thermo-acoustic instabilities on the scalar fluctuations near the combustor centreline. None of the default model parameters were adjusted and the results showcase the accuracy and flexibility of the present large eddy simulation method for an application to complex, partially premixed combustion problems; this being particularly important for the designers of new generation low emission gas turbine combustors. [ABSTRACT FROM AUTHOR]

Aluminum based energetic nanomaterials attract significant attention for various applications owing to their ultrahigh energy density. The main obstacle in the application of nano-Al based energetic materials is the slow combustion reaction kinetics and reduced energy output resulting from the inert Al 2 O 3 shell. In this paper, an efficient surface self-activation strategy is proposed to significantly improve the combustion performance and energy output of nano-Al based energetic materials. A porous AlF 3 shell is formed on the surface of the nano-Al particle by an etching reaction between perfluorododecanoic acid and the Al 2 O 3 dense layer. The porous AlF 3 shell provides a new reaction channel for the reaction of Al and the oxidizer, thus significantly improving the energy output and combustion reaction kinetics. The energy output and combustion reaction speed of polytetrafluoroethylene (PTFE)/nano-Al coated with C 11 F 23 COOH are 6304 J/g and 670 m/s, which are 3.0 and 2.6 times higher than those of PTFE/nano-Al, respectively. The mechanism of the self-activating process is proposed to explain the enhanced combustion reaction kinetics and energy output of the nano-Al based energetic materials. The proposed surface self-activation strategy for nano-Al particles can efficiently to enhance the reactivity and energy output and promote the development of the nano-Al based energetic materials. [ABSTRACT FROM AUTHOR]

A gas fuel supplied burner (BRE) was used to emulate burning of real solid and liquid fuels in microgravity to address the fire safety concerns of spacecrafts. The analytical solution of these flames was utilized to predict the steady flame temperature, and a critical temperature range of 1100–1200 K, was chosen to distinguish between self-extinguished and steady flames. Steady flames are examined on a flammability diagram composed of the emulated heat of combustion and heat of gasification. It is shown that for a given heat of combustion, there is an upper and a lower limit value of heat of gasification that will allow steady burning. The theory and the boundary of extinction experimental data support the limits. The upper limit is classed as radiation extinction with the lower like the fire-point in Earth gravity and is called small flame extinction limit. A theoretical prediction for steady burning is demonstrated to within 10% for a given atmospheric condition of pressure and oxygen, material diameter, and external radiative heat flux. The material properties of Δ h c , heat of combustion; L, heat of gasification; and T b , burning temperature allow for the prediction of burning in microgravity, and can serve as a method to evaluate fire safety of spacecrafts. [ABSTRACT FROM AUTHOR]

Well-known thermite reactions, which are strong exothermic reactions typically between a metal and an oxide, offer diverse important applications, such as the synthesis and processing of materials, detonators, and propulsion. A fascinating family of nitrogen-containing analog solid-state metal carbodiimides, with the general formula M x (NCN) y considered to be pseudo-chalcogenide or even pseudo-oxide, have drawn much attention owing to their unique physico-chemical properties. Herein, metal carbodiimides such as CuNCN are investigated as novel oxidizers for thermite reactions. We studied the Al/CuNCN thermite reaction, including thermal transformation, energy release and combustion performance, by means of complementary techniques, such as thermogravimetric-mass spectrometry (TGA-MS), differential scanning calorimetry (DSC), pressure evolution measurements and high-speed cameras. Compared to conventional Al/CuO thermite, the novel Al/CuNCN thermite system not only exhibited a strong combustion phenomenon with large energy release but also generated gases such as N 2 and (CN) 2. Additionally, the burning flame of Al/CuNCN presented a mushroom-cloud shape and was accompanied by a large number of hot spots due to the unique combustion characteristics of blasting and gas production. Even more encouraging was that the Al/CuNCN system was able to prepare AlN materials following a self-propagating route within one second. This work sheds light on the novel application of CuNCN in thermite reactions and potential routes to prepare gas generator and AlN. [ABSTRACT FROM AUTHOR]

In this work, a three-layers Mallard-Le Chatelier inspired theoretical model is developed to fully characterise the steps occurring during the flame propagation of combustible dusts/air. The model is based on the hypothesis that the dust flame propagation follows a homogeneous path: the dust-air mixture is pre-heated up to the volatile point (VP), at which production of volatiles occurs, thanks to the back-diffusion of heat from the combustion zone of the flame to the colder zones. The volatiles produced are then heated up to the ignition temperature and enter in the combustion zone. The flame burning velocity is the results of the coupling between heating rate, pyrolysis and/or evaporation/sublimation rate and volatiles combustion rate. The rate of formation of volatiles was measured by means of TG/DSC analysis. The laminar burning velocity of gases was computed by simulating the gas flame propagation in a tube starting from the measured gas compositions (by literature data or FTIR analysis). [ABSTRACT FROM AUTHOR]

The combustion characteristics of micro- and nano-sized boron carbide (B4C) powders were investigated to exploit the role of carbon atoms on combustion. Various combustion parameters had been evaluated for B4C powders. Results demonstrated a direct correlation between the gas oxide of carbon atoms with the morphology of condensed combustion products. The thrust generated when the gas diffused from the B4C matrix to the environment could not only disperse the burning B4C powder, but also break through the liquid oxide layer and leave holes on the surface of micro-sized B4C. The formed holes on the surface were considered oxygen diffusion channels to the interior of B4C particle, which accelerated the mass transfer rate and improved the oxidation efficiency. The evolution images of confined combustion and the characterizations of condensed combustion products suggested that the B4C powder released most of energy potential after stable combustion and transformed into boron oxide. Given the combustion process, the B4C powders exhibited the higher combustion heat and the larger peak area of combustion pressure than boron powders with similar diameter distribution, and the B4C powders in nano-sized generated strongest energy output among all samples. [ABSTRACT FROM AUTHOR]

B/JP-10 suspension fuel has very high energy density and high application prospects, but energy release during its actual combustion is incomplete. In this study, the surfaces of boron nanoparticles were modified by coating the particles with LiF and AP using a two-solvent method. Thermogravimetric experiments show that the ignition temperature of boron decreases from 772°C to 599°C when the LiF coating amount is 20 wt%. In a laser ignition experiment, the AP-modified B/JP-10 suspension fuel displayed a relatively obvious "microburst" phenomenon at the end of the combustion process. The "microburst" intensifies as the amount of AP coating increases; this behavior has positive implications for the release of energy from the boron in the fuel. The oxidation of boron in 10 wt% LiF-modified fuel increased from 26.35% to 36.22% without an increase in the ambient oxygen partial pressure. AP modification increases not only the oxidation rate of boron in the fuel but also the amount of oxidizing gas produced by decomposition, and this increases the burning intensity of JP-10. LiF and AP modification can destroy the shell layer of the agglomerates formed by the combustion of suspended fuels. LiF makes it difficult to close the shell layer by reacting with B 2 O 3 to form easily sublimated substances and flaky crystals. AP breaks the shell structure due to the high pressure generated by the "microburst", thus forming a porous configuration on the surface of the agglomerate. [ABSTRACT FROM AUTHOR]

Theoretical scaling arguments for turbulent premixed combustion indicate that the pressure-dilatation source of turbulent kinetic energy becomes significant at low Karlovitz numbers, leading to potential invalidation of commonly-used turbulence models developed for non-reacting flow. Based on these arguments, a critical Karlovitz number is defined, below which dilatation effects are expected to become significant. Velocity and scalar statistics are obtained from Direct Numerical Simulation (DNS) calculations of low Mach number spatially-evolving turbulent premixed planar jet flames. At fixed bulk Reynolds number and stoichiometric equivalence ratio, two simulations are performed at Karlovitz numbers above and below the critical Karlovitz number. Hydrogen combustion with detailed transport is modeled using a detailed nine-species chemical kinetic mechanism, and coflows of combustion products are used to ensure flame stability at uniform equivalence ratio. The analysis of these statistics focuses on three key areas. First, the influence of the velocity-pressure gradient source of turbulent kinetic energy is confirmed at a low Karlovitz number, and the unimportance of these effects is confirmed at a high Karlovitz number. Similar effects are observed for the chemical source term in the scalar variance budgets. Second, the degree of alignment between the Reynolds stress tensor (scalar flux) and the strain-rate tensor (scalar gradient), the foundation of a majority of the turbulence models used in reacting flows, is assessed with the DNS databases. Additionally, consistency of anisotropic Reynolds stress and strain-rate tensor invariants is assessed using invariant maps. While good alignment and consistency are obtained for statistics and invariants at a high Karlovitz number, both alignment and consistency degrade at a low Karlovitz number. Third, turbulence models formulated for non-reacting flow are modified algebraically in the Bray–Moss–Libby (BML) formalism for turbulent premixed combustion. A variable efficiency function is defined to capture the regime dependence of heat release effects in these models. Model performance is evaluated at Karlovitz numbers above and below the critical Karlovitz number using the DNS databases, and satisfactory prediction of counter-gradient transport in the flame-normal direction is obtained. However, heat release effects are also observed in the flame-parallel directions in the low-Karlovitz number simulation, and the models developed in the formalism for statistically planar flames fail to capture these effects. Furthermore, in the low-Karlovitz number case, redistributive effects are active on the shear components of the Reynolds stress, which are not considered in the BML formalism. More advanced turbulence models are therefore necessary for turbulent premixed jet flames below the critical Karlovitz number. [ABSTRACT FROM AUTHOR]

This communication describes and demonstrates an approach to making simultaneous multi-scalar measurements with reduced equipment requirements. Specifically, this letter describes and demonstrates the ability to simultaneously acquire high-quality planar laser-induced fluorescence (PLIF) images of CH 2 O and either CH, OH, or a combination of CH and OH using a single Nd:YAG-pumped dye-laser system and two intensified cameras. Acquiring these images with common diagnostic equipment was facilitated by exciting strong transitions in the C–X (0,0) band of CH (near 314 nm) and in the A–X (0,0) and (1,1) bands of OH, which are spectrally adjacent. Additionally, Rayleigh scattering images were acquired simultaneously with these PLIF images using a second Nd:YAG laser and an un-intensified camera. However, it would be possible to conduct these measurements with a single, sufficiently energetic Nd:YAG laser. [ABSTRACT FROM AUTHOR]

Lignocellulosic biomass is a promising alternative fuel source which can promote energy security, reduce greenhouse gas emissions, and minimize fuel consumption when paired with advanced combustion strategies. Pyrolysis is used to convert lignocellulosic biomass into a complex mixture of phenolic-rich species that can be used in a transportation fuel. Anisole (or methoxybenzene) can be used as a surrogate to represent these phenolic-rich species. Anisole also has attractive properties as a fuel component for use in advanced spark-ignition engines because of its high blending research octane number of 120. Presented in the current work are new measurements of laminar burning velocities, jet-stirred reactor (JSR) speciation of anisole/O 2 /N 2 mixtures, and the development and validation of a detailed chemical kinetic mechanism for anisole. Homogeneous, steady state, fixed gas temperature, perfectly stirred reactor CHEMKIN simulations were used to validate the mechanism against the current JSR measurements and published JSR experiments from CNRS-Nancy. Pyrolysis and oxidation simulations were based on the experimental reactant compositions and thermodynamic state conditions including P  = 1 bar and T  = 675–1275 K. The oxidation compositions studied in this work span fuel-lean (ϕ = 0.5), stoichiometric, and fuel rich (ϕ = 2.0) equivalence ratios. Laminar burning velocities were measured on a heat flux stabilized burner at an unburnt T  = 358 K, P  = 1 bar and simulated using the CHEMKIN premixed laminar flame speed module. Ignition delay times of anisole were then simulated at conditions relevant to advanced combustion strategies. Current laminar burning velocity measurements and predicted ignition delay times were compared to gasoline components (e.g., n-heptane, iso-octane, and toluene) and gasoline surrogates to highlight differences and similarities in behavior. Reaction path analysis and sensitivity analysis were used to explain the pathways relevant to the current studies. Under pyrolysis and oxidative conditions, unimolecular decomposition of anisole to phenoxy radicals and methyl radicals was found to be important due to the relatively low bond strength between the oxygen and methyl group, ∼65 kcal/mol. Reactions of these abundant phenoxy radicals with O 2 were found to be critical to accurately reproduce anisole's reactivity. [ABSTRACT FROM AUTHOR]

The combustion of a carbon-free nitrogen-based monofuel consisting of an aqueous ammonium hydroxide/nitrate (AAN) solution was studied at different equivalence ratios ranging between 0.6 and 12. The main oxidizing and reducing agents in AAN are nitric acid and ammonia, respectively. A combined differential thermal/barometric analysis (DTA/DBA) study was used to investigate the effect of the equivalence ratio on the autoignition temperature (AIT) of this monofuel. Increasing the equivalence ratio was found to increase the AIT value and reduce the energy generated during the ignition. Kinetic gas-phase simulations were used to calculate the theoretical AIT values. Good agreement was found at equivalence ratios close to unity. Rate of production and sensitivity analyses were performed to explore the reaction kinetics leading to the thermal auto-ignition. These indicated that reactions reducing nitrogen dioxide to nitrogen oxide and molecular oxygen inhibit the ignition, while reactions between ammonia and high oxidation state NO x producing amidogen promote it. The results of this study shed light on the influence of non-stoichiometric conditions on the thermal auto-ignition of AAN. [ABSTRACT FROM AUTHOR]

The first-stage ignition delay of n -heptane/air mixtures is computationally studied using detailed mechanism and theoretically studied using eigenvalue analysis of simplified systems. Results show that the delay has a turnover behavior as temperature increases, being dominated by the competition of low-temperature branching and termination channels as well as the competition of forward and reverse reaction channels. As temperature increases to the intermediate range, the termination and reverse pathways result in a minimum in the delay, the state of which is theoretically derived. Simple analytical solutions for the delay as well as the species evolutions are presented to identify the rate constants that control the first-stage ignition and quantify the influence of the mixture composition, initial temperature and system pressure. It is further demonstrated that the above results also hold for n -octane/air and iso -octane/air mixtures. [ABSTRACT FROM AUTHOR]

The concept of heterogeneous multiscale method (HMM) is applied into combustion chemistry integration in reactive flows in this study. It adopts a macro solver to integrate the slow variables with global time steps and a micro solver to integrate the fast variables with smaller inner time steps. Explicit or implicit schemes with different orders of accuracy could be applied in the macro and micro solvers. In this study, we formulated HMM with a first-order Euler forward scheme in the micro solver and a second-order trapezoidal rule in the macro solver. An adaptive time step method was proposed to better evaluate a proper time step and control the error in this HMM. This formulation of HMM with the adaptive time step method has been implemented and tested in homogeneous constant volume auto-ignition in hydrogen, methane and butane mixture under various initial temperatures, pressures and equivalence ratios. The speed-up factor of 100–700 has been achieved compared with the implicit solver DASAC. In addition, the numerical accuracy and stability of this HMM formulation have been discussed. It was also applied into multi-dimensional Reynolds-averaged Navier–Stokes (RANS) engine simulations and good efficiency and accuracy was accomplished. In order to better optimize its efficiency performance in multi-dimensional reactive flow simulations, a hybrid scheme with VODE and HMM was proposed and tested in non-homogeneous charged engine simulations. In sum, the concept of HMM in stiff combustion chemistry integration is numerically stable, accurate and efficient. [ABSTRACT FROM AUTHOR]

In turbulent premixed combustion, turbulence is affected by both the influence of combustion heat release and turbulent shear effects. At low Karlovitz number, heat release effects are more dominant than turbulent shear effects, but, at high Karlovitz number, turbulent shear effects dominate the turbulence dynamics. Closure models for the velocity and scalar fields that only account for one of those effects are incapable of generally describing combustion-turbulence interactions at any finite Karlovitz number. A manifold-based turbulence model that solves the conditional mean momentum transport equation conditioned on a progress variable has the potential to provide such a general description, but closure models for the equation are required. Among the unclosed terms, the conditional mean pressure gradient is significant at both low and high Karlovitz numbers but has fundamentally different behaviors in each regime. In this work, a general closure model is developed to capture the conditional pressure gradient in both Karlovitz number regimes by considering heat release and turbulent shear effects. The deviation of the conditional pressure gradient from the unconditional pressure gradient is modeled with two components. One represents the pressure decrease across the flame due to combustion heat release, and the other represents the hydrodynamic pressure fluctuations due to turbulent shear. The heat release component depends on the heat release rate and the flame-normal vector, and the turbulent shear component depends on the conditional Reynolds stresses and the gradient of the progress variable probability density function. The model is validated against Direct Numerical Simulations of spatially-evolving turbulent premixed hydrogen/air planar jet flames at low and high Karlovitz numbers. The proposed model well captures the conditional pressure gradient at both Karlovitz numbers and shows a natural transition of its dominant term: the heat release component at low Karlovitz number and the turbulent shear component at high Karlovitz number. [ABSTRACT FROM AUTHOR]

Recently fluoropolymers are of great interest as an oxidative binder and modifier in Al-based metastable intermixed composites (MICs) due to the high heat of reaction between Al-F and gas production characteristics. In this paper, Polytetrafluoroethylene (PTFE) and Polyvinylidene Fluoride (PVDF) have been incorporated into three typical Al-based MICs independently using CuO, Fe 2 O 3 , and Bi 2 O 3 as oxidizers, where the contents of Al to metal oxide are maintained optimum stoichiometric ratios. Heats of reaction have been measured by a bomb calorimeter for mechanically mixed MICs samples, from which the optimum fluoropolymer content has been determined using the maximum calorific values as the criteria. After that, the optimized formulations have been prepared by a spray drying method. The thermal behavior, combustion characteristics and pressurization features of the studied Al-based MICs were evaluated and compared with those without fluoropolymers. The experimental results showed that with a minor inclusion of fluoropolymers, the combustion heat was improved by 3.8% to 8.6% depending on the type of oxidizers, and the onset temperature of the exothermic reaction was observed to be decreased by 250 °C for Al/CuO, 200 °C for Al/Fe 2 O 3 and 16 °C for Al/Bi 2 O 3 with a higher reaction heat release, showing a higher reactivity. Besides, the maximum pressure and pressurization rate were both increased for the fluorine-containing MICs, whereas the burn rates under confinement were found to drop significantly by 72% to 91%. The AlF 3 , Cu-Al alloy, and Fe-Al alloy are found in the condensed combustion products (CCPs), and it tends to have smaller and more uniform particles. It probably results from the large difference in the energy and mass transfer process on the combustion surface when the gaseous phase products are generated. [ABSTRACT FROM AUTHOR]

The existence of an oxide (B 2 O 3) film covering the boron (B) surface affects its energy release during combustion. Fluoropolymer modification can help to solve the problem. The electrospinning and solvent anti-solvent methods were utilized to modify boron nanoparticles (n-B) using polyvinylidene fluoride (PVDF) as a modified material, which can effectively coat B particles. The thermal behavior, combustion characteristics, and condensed combustion products of the B/PVDF composites were investigated and evaluated. The agglomeration size of B particles can be decreased while its reaction heat and combustion intensity are increased. Significantly, the combustion performance of the B/PVDF composites prepared by the electrospinning method is better than that of the B/PVDF composite prepared by the solvent anti-solvent method. These results can be helpful in promoting the practical application of B. [ABSTRACT FROM AUTHOR]

In this work, experimental measurements of flame heat flux and sample mass loss rate are obtained as a diffusion flame spreads vertically upward (in the direction opposed to the vector of gravity) over the surface of seven commonly used polymeric materials, two of which are glass reinforced composites. Using these measurements, a previously developed empirical flame model specific to poly(methyl methacrylate) is generalized such that it can predict (flame to material surface) heat feedback from 3 to 20 cm tall flames supported by a wide range of materials. Model generalization is accomplished through scaling on the basis of a material's gaseous pyrolyzate heat of combustion, which can be measured using mg-sized material samples in a microscale combustion calorimeter. For all seven materials tested in this work, which represent diverse chemical compositions and burning behaviors including polymer melt flow, sample burnout, and heavy soot and solid residue formation, model-predicted flame heat flux (to a water-cooled heat flux gauge) is shown to match experimental measurements taken across the full length of the flame with an average absolute error of 3.8 kW m −2 (approximately 10–15% of peak measured flame heat flux). Coupled with a numerical pyrolysis solver, this generalized wall flame model provides the framework to quantitatively study material propensity to ignite and support early fire growth in a range of common scenarios with a level of accuracy and reduced computational cost unmatched by other currently available modeling tools. [ABSTRACT FROM AUTHOR]

In the current work, we utilize a recently developed staged Livengood–Wu (L–W) correlation to evaluate and predict the two-stage ignition behaviors of different primary reference fuels (PRFs) under homogeneous reciprocating engine conditions, to demonstrate its capability in capturing detailed ignition kinetics in combustion simulation. Based on the understandings of the global and detailed kinetics of low-temperature chemistry, simplified Arrhenius-based global expressions were developed for both ignition stages, and a linear correlation was applied for the cool flame temperature rise at the end of the first stage, for PRFs with various octane number (ON) 0, 20, 60 and 87 under constant volume conditions. For their performance under homogeneous charge compression ignition (HCCI) conditions, it is shown that for the same engine speed, the inlet temperature range allowing for two-stage ignition shrinks with increased ON numbers, and eventually two-stage ignition turns into single-stage ignition with sufficiently high ON numbers. Based on the ignition database, the combustion phasing for all PRFs under a wide HCCI operation range are predicted with the staged L–W integral method with satisfactory accuracy, including the first- and second-stage ignition timing as well as the cool flame temperature rise. The sensitivities of each of the engine operation parameters are successfully captured, further demonstrating the applicability of the staged L–W method for realistic fuels with low temperature chemistry and its potential applications for engine combustion control. This study also provides an alternative approach of accommodating low temperature ignition kinetics in combustion simulation, especially useful for those under variable thermodynamic conditions. [ABSTRACT FROM AUTHOR]

A model that accounts for the volatile matter release and its subsequent combustion in the surroundings of an isolated coal particle is presented and used to analyze the characteristics and relevance of the flame so generated in a broad range of conditions. The devolatilization kinetics were experimentally determined in a previous work; the volatile composition is given by the chemical percolation model; and the GRI 3.0 mechanism accounts for the homogeneous reactions. Both conventional and oxy-fuel combustion conditions are considered. In all the cases, three consecutive (although not fully independent) flames are identifiable around the particle, corresponding to the consumption of CH 4 and C 2 H 2 , first, and the oxidation of CO, further on. A more intense chemistry is generally observed in N 2 than in CO 2 -rich environments; in any case, those reactions result in significant heat transfer towards the particle, and thus in an acceleration of the devolatilization, more marked for higher bulk oxygen concentrations. The predictions of the model regarding devolatilization duration and peak flame temperature and position are in good qualitative agreement with the (rather scarce) experimental data available in the literature. Finally, simulations with different particle diameters from 30 to 120 µm show that only for the finest particles an overlapping of the devolatilization and the char oxidation steps during combustion is foreseeable. [ABSTRACT FROM AUTHOR]

Total pressure gain is one of the most significant performance metrics for rotating detonation engines. In this study, total pressure gains of kerosene-hot air(523K∼553 K) RDE for different combinations of A 8 /A 3.2 and A 3.1 /A 3.2 are measured by EAP(Equivalent Available Pressure).It is found that total pressure gain tends to increase with the decrease of A 8 /A 3.1. The highest total pressure gain of -15% occurs at A 8 /A 3.1 = 0.87.And the highest total pressure gain of each configuration approximately increases linearly with the decrease of A 8 /A 3.1 with the maximum deviation of 3.3% for the same A 8 /A 3.1 under different combustor widths. The total pressure gain climbs with the decrease of P t2 /P 3.2. And there appears a clear demarcation point at P t2 /P 3.2 of 1.89 for the effects of P t2 /P 3.2 on total pressure gain because of different total pressure loss mechanisms. The equation of total pressure gain is rewritten to separate inlet losses from other losses. Positive total pressure gain is obtained when inlet losses are excluded for some cases. In order to take the effects of equivalence ratios, types of fuel, combustion efficiency and widths of combustion chamber on total pressure gains into account, Qv is introduced which is defined as the actual heat released by the combustion of fuel entering the combustion chamber for 1 hour in unit pressure and unit combustion chamber volume. There appears to be an approximately linear trend in pressure gain as a function of Qv for each A 8 /A 3.1 A simple pressure gain model is developed to describe the variation of total pressure gain with A 8 /A 3.1 and Q v. Higher Q v and lower A 8 /A 3.1 are required to achieve positive total pressure gain which is not easy to be realized at the same time. As an important parameter for calculation of Q v , the average pressure of combustion chamber is approximately equal to the product of velocity of detonation waves and mass flux of nozzle outlet. A comparison is made among total pressure gain measured by EAP, (Mach-Corrected Static Pressure)and MFF(Mass Flow Function). They are basically linear with relative errors falling within +/-10%. [ABSTRACT FROM AUTHOR]

We report the effects of the Lewis number on the turbulent kinetic energy, Reynolds stresses, and scalar fluxes in premixed jet flames with strong mean shear. The direct numerical simulation (DNS) of piloted turbulent premixed jet flames is performed under the same condition except for different Lewis numbers to isolate the Lewis number effects. The turbulence intensity within the mean flame brush is found to decay more rapidly along the axial distance with a smaller Lewis number. This Lewis number dependence results from the coupling of the mean shear and the combustion heat release, where the mean shear is the primary source of turbulence production. We show that, under the strain imposed by the mean shear and turbulence fluctuation, the heat release of premixed jet flames is significantly enhanced at the Lewis number smaller than unity, which is represented by the rising flamelet consumption speed and flame surface area. As a consequence, the mean shear within the flame brush is suppressed at small Lewis numbers via the dilatation effect, and the weakened mean shear production causes the fast turbulence suppression along the axial direction. The enhanced heat release at small Lewis numbers also increases the flame-normal Reynolds stress, and leads to the counter-Boussinesq behavior of the shear Reynolds stress and the counter-gradient transport of scalar fluxes. We model the Lewis number effects on local turbulence and scalar transport using the stretched laminar flame speed and thickness, and achieve overall good estimations of the flame-normal Reynolds stress and scalar transport at small Lewis numbers. [ABSTRACT FROM AUTHOR]

The development of energy efficient combustion systems is a critical technical objective in the engine and power generation industries. This pursuit often prompts using conditions at the limits of capability, dictating the need to expand the envelope of robust and reliable operation. Nanosecond-pulsed high-frequency discharges (NPHFD) have proven to be effective in extending ignition limits in both quiescent and flowing environments. However, limited research has been conducted comparing the kernel growth behavior in a flowing environment of a train of discrete nanosecond discharges to that of a conventional DC arc discharge. Therefore, this work focused on comparing these two types of discharges by matching the total energy deposited, total deposition time, and average power. For each system, three average power conditions (100, 180, and 400 W) were tested, each including five total discharge times (0.05, 0.15, 0.25, 0.50, and 1.00 ms) at two inter-electrode gap distances of 1 and 2 mm. The 1 mm gap distance results showed that the NPHFD system produced larger ignition kernel areas than the conventional system at the highest average power (therefore, highest pulsation frequency) setting. At all other average power conditions, the NPHFD system produced equivalent or smaller ignition kernels than the conventional system due to a lack of pulse-to-pulse coupling. At the 2 mm inter-electrode gap distance, the NPHFD ignition system produced larger kernels at all conditions except at the lowest average power and discharge duration setting. At the highest average power and longest discharge duration setting, the NPHFD kernel was nearly twice as large in area as the kernel produced by the conventional system at 5 ms after discharge. The larger kernel area created by the NPHFD system at the larger gap distance, higher average power, and longer total discharge duration conditions could be explained by the high level of pulse-to-pulse coupling and discharge-induced jetting behavior. The jetting behavior was a function of electrode polarity and mainly extended the reactive region of the kernel in the spanwise direction. [ABSTRACT FROM AUTHOR]

Overcoming the severe particle agglomeration of unreacted fuel boron (B) in combustion process to enhance the heat release efficiency of B-based fuel-rich propellants is still a great challenge. Glycidyl azide polyether (GAP) was introduced to synthesize GAP-based energetic thermoplastic elastomer (GAP-ETPE) binder, which was used to fabricate the B/GAP-ETPE propellant. High GAP content (85 wt%) can be used to synthesize 85%GAP-ETPE as the binder. With the increase of GAP content, the decomposition conversion rate of GAP-ETPE was increased and the apparent activation energy (E a) of B/GAP-ETPE propellant was decreased. The heat of explosion and combustion of B/85%GAP-ETPE were increased from 4004.1 kJ/kg to 4546.2 kJ/kg and 24,056.5 kJ/kg to 24,938.3 kJ/kg, respectively. The combustion results showed that the burning rate of B/85%GAP-ETPE (3.1 mm/s) was higher than that of B/TPE (1.8 mm/s) using a thermoplastic elastomer (TPE) without GAP, and the particle injection effect was significantly improved. The pressurization rate of B/85%GAP-ETPE (85 wt% is maximal GAP content in binder in current study) was 2.6 MPa/s during combustion in a closed bomb, which was higher than that of other prepared B/GAP-ETPE, showing the propellant using GAP-ETPE with high GAP content exhibited better gas formation and higher heat release, which provided good dispersion and less particle agglomeration of B particles when burning. This strategy enables B to be ejected more effectively in the combustion process to achieve a better combustion performance and heat release. [ABSTRACT FROM AUTHOR]

To improve and adjust the combustion and energy output of HMX/Al, graphite fluoride (CF) as oxidizer was introduced in HMX/Al and gradient structured HMX/Al/CF was prepared by additive manufacturing technology. For HMX/Al added with CF, a significantly exothermic peak is observed ranging from 860 to 950 °C due to the reaction of Al and fluorine derived from CF decomposition, and the heat release increases by 106.63% compared to that of HMX/Al. The fourth exothermic heat increases from 161.48 to 955.25 J/g with the increased Al content from 10 to 30 wt%. Moreover, the combustion rate and pressure output of HMX/Al have been improved by introducing CF. For HMX/Al added with CF, the combustion time decreases from 10.3 to 2.5 s, which is dependent on the Al content from 10 to 30 wt%. And the highest peak pressure is 1712.2 kPa when Al content is 20 wt% in HMX/Al/CF composites. Furthermore, gradient structured HMX/Al/CF consisting of three layers with Al content of 10, 20 and 30 wt% was prepared to improve combustion and pressure output. The combustion rate and peak pressure of gradient structured HMX/Al/CF are higher than those of single structured HMX/Al/CF. The highest peak pressure (∼2.24 MPa) is obtained when the Al content of the middle layer and outer layer is 30 wt% and 20 wt% in the gradient structured HMX/Al/CF cylinder, respectively. Finally, the reaction intermediates and combustion reaction products were fully characterized to understand the enhanced effect of CF in HMX/Al. This work provides a promoting way to improve and regulate the combustion reaction performance and pressure output of HMX/Al. [ABSTRACT FROM AUTHOR]