Combustion Wave Speed Research Articles

Radiative heat recuperation in combustion of layered solid fuel

In this work, we investigate the properties and stability of the combustion waves propagating in a system of layered solid fuel material in which the neighbouring fuel sheets can recuperate heat via thermal electromagnetic radiation. The analysis is undertaken within the flame sheet model, which allows to determine the combustion wave speed and structure and derive the dispersion relation characterising the stability of the reaction front. The proper choice of parameters leads to superadiabatic combustion temperature and significant increase of the combustion wave speed. In addition, the stable steady regimes of combustion can be substantially extended in the space of parameters. Such an enhancement of stability and ability to control the characteristics of the combustion wave propagation can be very attractive for the combustion synthesis of new layered materials, similarly to the chemical furnace technology proposed earlier.

Transition to detonation in inhomogeneous hydrogen-air mixtures: The importance of gradients in detonation cell size

This paper presents a numerical study of flame acceleration and deflagration-to-detonation transition (DDT) of hydrogen-air mixtures in a channel with obstacles, where the mixture composition has a spatial gradient perpendicular to the wave propagation direction. The mixture composition gradient corresponds to a detonation cell size gradient, which characterizes the detonability or the detonation sensitivity. We use a recently developed chemical-diffusive model (CDM) to investigate the influence of the cell size gradient on the detonability of an inhomogeneous mixture and examine characteristic behaviors of flame acceleration and DDT. The CDM is a parametric model for chemical reaction and diffusive transport processes in compressible Navier-Stokes simulations. When optimized, the CDM can reproduce the correct flame and detonation properties, particularly the detonation cell size. The simulation results show that the ease with which an inhomogeneous mixture can detonate is bounded by the detonation limits of the most and least detonable homogeneous mixture. That is, the run-up distance to DDT of inhomogeneous mixtures with 0.8<ϕ<1.0 is found to be greater and less than the most and the least detonable mixtures with ϕ=1.0 and ϕ=0.8, respectively. Other macroscopic observables in the inhomogeneous mixture such as the average combustion wave speed and the total flame surface area are also in between those in these two homogeneous mixtures. The simulation data showed that pressure waves generated by a deflagration play an important role to re-distribute the prescribed mixture composition. The combustion process occurs preferentially in mixtures that initially concentrate along the unobstructed channel centerline. The numerical results agree with prior experimental findings that the averaged equivalence ratio cannot adequately estimate the detonability of an inhomogeneous mixture.

Numerical simulations of traveling waves in a counterflow filtration combustion model

We focused on traveling combustion waves that appear in a simplified, one-dimensional combustion model in porous media. The system we consider is a reaction-convection-diffusion system that can be reduced into two-dimension in order to prove traveling waves by phase plane analysis. In previous studies combustion wave velocity was assumed positive and their existence was proven. Also, all possible wave sequences that solve boundary value problems on infinite intervals with constant boundary data were identified. In this study, we generalize the previous work by including the case of negative combustion wave speed and taking the assumption that oxygen is carried faster than temperature. Moreover, we extend the classification of all possible wave sequences by performing numerical simulations using finite difference scheme.

Theoretical analysis of superadiabatic combustion for non-stationary filtration combustion by excess enthalpy function.

The superadiabatic combustion for non-stationary filtration combustion is analytically studied. The non-dimensional excess enthalpy function (H) equation is theoretically derived based on a one-dimensional, two-temperature model. In contrast to the H equation for the stationary filtration combustion, a new term, which takes into account the effect of non-dimensional combustion wave speed, is included in the H equation for transient filtration combustion. The governing equations with boundary conditions are solved by commercial software Fluent. The predictions show that the maximum non-dimensional gas and solid temperatures in the flame zone are greater than 3 for equivalence ratio of 0.15. An examination of the four source terms in the H equation indicates that the thermal conductivity ratio between the solid and gas phases is the dominant one among the four terms and basically determines H distribution. For lean premixed combustion in porous media, the superadiabatic combustion effect is more pronounced for the lower .

Formation of combustion wave in lean propane-air mixture with a non-uniform chemical reactivity initiated by nanosecond streamer discharges in the HCCI engine

The propagation of combustion wave, which is formed as a result of impact of a filamentary pulsed periodic non-equilibrium discharge on a mixture that is not ignited by only compression in the HCCI engine, has been simulated. The discharge for a short time locally activated the mixture in the engine cylinder at a certain crankshaft rotation angle at the compression stage. The approach to evaluation of the temperature and composition in the region activated by high-frequency corona discharge has been proposed. Multi-pulse and multi-channel nature of discharge was accounted for in our model. The densities of chemically active species produced during streamer propagation were found using two-dimensional axially symmetric fluid model. The obtained composition and temperature in the activated zone were the initial conditions for modeling the propagation of combustion wave along the cylinder radius in 1D approximation. Special terms were included in equations responsible for the compression along the axis of cylinder to mimic the real change in pressure in cylinder. The calculations were performed for a lean C3H8-air mixture, which is characterized by the presence of low-temperature heat release (LTHR) stage, and intermediate and high temperatures heat release (ITHR and HTHR) stages. It was shown that the ignition timing in activated zone depends on the specific energy input into the streamer channel, the radius of activated region coupled with the volume fraction of activated region treated by discharge, and the moment of discharge initiation relative to the top dead center (TDC). The speed of combustion wave, the occurrence of auto-ignition in the end gas and transition to combustion, accompanied by “knocking” or strong pressure fluctuations are largely determined by the moment of discharge initiation and the specific energy input. The significant effect of discharge is explained by the stimulation of oxidation kinetic mechanisms at LTHR and ITHR stages.

Effect of azobis-isobutyronitrile propellant loading and nanoscaling of CuO on combustion performance of CuO/Al nanoenergetic materials

This paper reports on the combustion performance of nanoenergetic composites of CuO/Al as a function of nanoscale size of CuO oxidizer and various weight percent loading of gas-generating solid chemical propellant, azobis-isobutyronitrile (AIBN). CuO nanorods with short (sub-100 nm) and long (1.5 μm) nanoscale size were prepared by simple solid-state reaction process using poly-ethylene glycol, PEG400 and sonoemulsion process using PEG8000 respectively. The combustion performance was experimentally characterized in terms of combustion wave speed measurement in an instrumented burn-tube experiment and dynamic pressure-time characteristics measurement in a fully-confined instrumented pressure-cell. The reactivity and peak pressure of PEG8000 assisted synthesized long CuO nanorods based nanoenergetic composites were observed to be remarkably enhanced due to possible consequence of high rate of oxygen release from its increased surface area. The loading of a few percentages (15%) of AIBN has positive effect on enhancing the peak pressure at the sacrifice of combustion wave speed, pressurization rate and heat of exothermic reaction.

Grain size effects on Ni/Al nanolaminate combustion

Abstract

Influence of vacancy defect concentration on the combustion of reactive Ni/Al nanolaminates

Self-propagating reactions in Ni/Al nanolaminates have been widely studied for their high combustion temperatures surpassing 1900 K and rapid combustion wave speeds exceeding 10 m/s. These combustion characteristics have motivated unique industrial applications, such as soldering of electrical components, and possible military applications. Unfortunately, there is a limited understanding of the effect of lattice defects on combustion characteristics. This work explores the effect of vacancy concentration on the combustion rate and peak temperature of reactive Ni/Al nanolaminates. Increasing vacancy concentration increases both reaction rates and peak reaction temperatures. For the reaction rate, vacancy concentration effects are shown to be interdependent with bilayer thickness, initial temperature, and hydrostatic pressure. The effects on reaction peak temperature are independent of these other system parameters. A new method for mapping vacancy and composition profiles is presented to demonstrate the formation and migration of vacancies during the self-propagating reaction.

Self-propagating high-temperature synthesis and thermoelectric performances of Cu2SnSe3

Recently, a self-propagating high-temperature synthesis (SHS) technique has been discovered for the low-cost and ultra-fast preparation of Cu2SnSe3. In this research, the reaction mechanism of the ultra-fast SHS process during the synthesis of Cu2SnSe3 was investigated in detail, and the thermoelectric properties of the compacted bulk by PAS (Plasma Activated Sintering) were discussed. Nearly single phased Cu2SnSe3 could be prepared by the SHS reaction in 1.5 s, and a high combustion wave speed of 6.7 mm/s was found in this process. The quenching experiment and the multi-step experiments verified that, the SHS synthesis of Cu2SnSe3 from raw elements was majorly composed of three individual SHS processes from Cu-Se, Sn-Se and CuSe-SnSe, in which the speed of combustion wave was estimated to be 5 mm/s, 18 mm/s and 3 mm/s, respectively. The SHS-PAS prepared Cu2SnSe3 possessed superior electrical properties and thermal properties as compared to samples prepared by other methods, and thus possessed a relatively higher thermoelectric figure of merit ZT across the entire measuring temperature range. The Cu2SnSe3 prepared by SHS-PAS acquired the highest ZT of 0.53 at 750 K. A very low lattice thermal conductivity of 0.61 Wm−1K−1 at 750 K was achieved in the SHS-PAS synthesized Cu2SnSe3, which was due to the structure distortion caused by different cations and the strengthened phonon scattering introduced by in-situ formed nano-precipitates. The discovery of the SHS reaction mechanism would be instructive for the reliable and mass production of Cu2SnSe3.

Analytical solutions of superadiabatic filtration combustion

Analytical solutions for superadiabatic filtration combustion of lean methane-air mixtures in a monolithic porous burner are sought. The one-dimensional, local volume-averaged equations of energy and species conservation that assume a non-thermal equilibrium (i.e., the two-medium treatment), are converted by a coordinate transformation using a combustion wave speed and solved to obtain close-form solutions. A parametric examination varying inlet gas velocity, fuel equivalence ratio, porosity, and thermal conductivity and diffusivity of the solid phase of the porous burner proves the validity of the analytical solutions which are in an excellent agreement with the numerical benchmark. The analytical solutions depict the key features of the filtration combustion such as non-thermal equilibrium between the solid and gas phases, superadiabatic flame temperature, and internal heat recirculation between solid and gas phases of the porous burner.

Riemann solutions for counterflow combustion in light porous foam

The paper is motivated by a model for the injection of air into a porous medium that contains a solid fuel. In previous work, a system of three evolutionary partial differential equations that models combustion of light porous foam under air injection was considered. The existence and uniqueness of traveling waves were studied and the wave sequences appearing in Riemann solutions were identified. This analysis was done under assumption that the combustion wave velocity is positive. In the present work, this hypothesis was neglected and the results generalized including the case of negative combustion wave speed. The existence of such waves was proved and the uniqueness investigated for some particular cases using Melnikov’s integral. The wave sequences appearing in Riemann solutions were identified and the numerical examples using finite difference scheme were presented.

Influence of radiation absorption by microparticles on the flame velocity and combustion regimes

Thermal radiation of the hot combustion products usually does not influence noticeably the flame propagating through gaseous mixture. the situation is changed drastically in the presence even small concentration of particles, which absorb radiation, transfer the heat to the surrounding unburned gaseous mixture by means of heat conduction, so that the gas phase temperature in front of the advancing flame lags that of the particles. It is shown that radiative preheating of unreacted mixture ahead of the flame results in a modest increase of the advancing flame velocity for a highly reactive gaseous fuel, or to considerable increase of the flame velocity in the case of a slow reactive mixture. The effects of radiation preheating as stronger as smaller the normal flame velocity. The radiation heat transfer can become a dominant mechanism compared with molecular heat conduction, determining the structure and the speed of combustion wave in the case of a small enough velocity of the advancing flame. It is shown that in the case of non-uniform distribution of the particles, such that time of the radiation heating is longer so that the maximum temperature in the region of denser particles cloud ahead of the advancing flame is sufficient for ignition, the thermal radiation may trigger additional independent source of ignition. Depending on the steepness of the temperature gradient formed in the unburned mixture, either deflagration or detonation can be initiated via the Zeldovich's gradient mechanism. Ignition of different combustion regimes, depending on the radiation absorption length, is illustrated for the particle-laden hydrogen-oxygen flame.

Combustion Characteristics of Silicon‐Based Nanoenergetic Formulations with Reduced Electrostatic Discharge Sensitivity

AbstractThis paper details the synthesis and combustion characteristics of silicon‐based nanoenergetic formulations. Silicon nanostructured powder (with a wide variety of morphologies such as nanoparticles, nanowires, and nanotubes) were produced by DC plasma arc discharge route. These nanostructures were passivated with oxygen and hydrogen post‐synthesis. Their structural, morphological, and vibrational properties were investigated using X‐ray diffractometry, transmission electron microscopy (TEM), nitrogen adsorption‐desorption analysis, Fourier transform infrared (FTIR) spectrometry and Raman spectroscopy. The silicon nanostructured powder (fuel) was mixed with varying amounts of sodium perchlorate (NaClO4) nanoparticles (oxidizer) to form nanoenergetic mixtures. The NaClO4 nanoparticles with a size distribution in the range of 5–40 nm were prepared using surfactant in a mixed solvent system. The combustion characteristics, namely (i) the combustion wave speed and (ii) the pressure‐time characteristics, were measured. The observed correlation between the basic material properties and the measured combustion characteristics is presented. These silicon‐based nanoenergetic formulations exhibit reduced sensitivity to electrostatic discharge (ESD).

Combustion characteristics of novel hybrid nanoenergetic formulations

This paper presents the combustion characteristics of various copper oxide (CuO) nanorods/aluminum (Al) nanothermite compositions and hybrid nanoenergetic mixtures formed by combining nanothermites with either ammonium nitrate (NH 4NO 3) or secondary explosives such as RDX and CL-20 in different weight proportions. The different types of nanorods prepared in this study are referred to as CuO-VD (dried under vacuum at 25 °C for 24 h), CuO-100 (at 100 °C for 16 h) and CuO-400 (short time (1 min) calcination at 400 °C). The physical and chemical characteristics of these different kinds of CuO nanorods were determined using a variety of analytical tools such as X-ray diffractometer, transmission electron microscope (TEM), Fourier transform infrared spectrometer (FTIR), surface area analyzer and simultaneous differential scanning calorimeter (DSC)/thermogravimetric analyzer (TGA). These measured characteristics were correlated with the combustion behavior of the nanoenergetic compositions synthesized in this work. The use of different drying and calcination parameters produced the synthesis of CuO nanorods with varying amount of hydroxyl (OH) and CH n ( n = 2, 3) functional groups. The experimental observations confirm that the presence of these functional groups on the surface of CuO nanorods enabled the formation of assembled nanoenergetic composite, upon mixed with Al nanoparticles. A facile one-step synthesis of assembled composite through surface functionalization is reported and it can be extended to large-scale preparation of assembled nanoenergetic mixtures. The combustion behavior was studied by measuring both combustion wave speed and pressure–time characteristics. Pressurization rate was determined by monitoring the pressure–time characteristics during the combustion reaction initiated by a hot wire in a fully-confined geometry. Different amounts of nanothermite powder were packed in the same volume of combustion chamber by applying different packing pressures and the pressure–time characteristics were measured as a function of varying percent theoretical maximum density (% TMD). The experimental setup used in this work enabled us to study the functional behavior of initiating explosives such as NH 4NO 3 nanoparticles, RDX and CL-20 using nanothermites under fully-confined test geometry. The dent tests performed on lead witness plates support the experimental observations obtained from pressure–time and combustion wave speed measurements of hybrid mixtures.

Flame Propagation Experiments of Non-gas-Generating Nanocomposite Reactive Materials

Nanoenergetic composites are being examined for their use as energy storage and delivery materials. For this application, non-gas-generating mixtures that produce enough energy to sustain reaction propagation are ideal. A potentially promising formulation for this application is comprised of manganese (Mn) powder mixed with manganese dioxide (MnO2) because thermoequilibrium calculations predict that the Mn/MnO2 reaction is non-gas-generating and highly exothermic. The objectives of this study were to examine the flame propagation behavior of various mass percentages of Mn and MnO2 and evaluate the influence of fuel particle size. Results showed that conduction is the dominant energy-transfer mechanism for the reaction even in the loose powder configuration, regardless of the fuel particle size. Combustion wave speed (CWS) was measured as a function of the composition and Mn particle diameter. Nanometric Mn particles (referred to as nano-Mn) produced higher CWS than micrometric Mn particles (referred to as...

Modified Nanoenergetic Composites with Tunable Combustion Characteristics for Propellant Applications

AbstractThis work reports on the synthesis and tunable characteristics of nanothermite compositions based on mesoporous Fe2O3 as an oxidizer and Al nanoparticles as a fuel. The reactivity (rate of increase of pressure) and the combustion wave speed were determined to evaluate the performance of these composites for various applications. A gas generating polymer, (acrylamidomethyl) cellulose acetate butyrate (AAMCAB), was loaded in the mesopores of Fe2O3 matrix following wet‐impregnation technique. The samples prepared in this work were characterized by a number of analytical techniques such as Fourier transform infrared (FTIR) absorption spectroscopy, transmission and scanning electron microscopy (TEM, SEM), energy dispersive X‐ray analysis, X‐ray diffraction, and nitrogen adsorption–desorption isotherms. Then, mesoporous Fe2O3 powder was mixed with Al nanoparticles to prepare nanoenergetic composites. The main characteristics such as peak pressure, reactivity, combustion wave speed, and pressure sustenance were determined as a function of polymer loading. The dependence of combustion wave speed on the pressure was established following the well‐known Vieille's law. The small value of 0.408 for the pressure exponent indicates the suitability of these nanothermite compositions for propellant applications. By reducing the percentage of polymer, the characteristic properties of nanoenergetic composite can be suitably tuned for other applications.

Numerical study on the realization of compression ignition in a type of porous medium engine fueled with Isooctane

The working process of a porous medium (PM) engine, characterized as periodic contact type and fueled with liquid fuel as Isooctane, is simulated by using an improved version of KIVA-3V. A modified volume-averaged method is proposed for describing the interaction between fuel droplets and the solid phase of the PM. The improved version of KIVA-3V was validated by simulating the experiment of Zhdanok for the superadiabatic combustion of CH 4–air mixtures under filtration in a packed bed. Good agreement between experimental data and computational results for the speed of combustion wave is achieved. The influences of initial PM temperature, PM structure and valve opening timing on the realization of compression ignition in the PM engine are also verified. Initial PM temperature is the crucial factor in guaranteeing the realization of the compression ignition of the PM engine. Considering influential factors, such as the properties of the PM, the compression ratio, the equivalence ratio, and the heat transfer between gas and solid phase of the PM should obtain optimized initial PM temperature. The variation in PM structure affects the convective heat transfer between the gas and solid phase and the dispersion effect of the PM. Compression ignition all can be realized in PM engines with four kinds of PM. Compression ignition is achieved at the considered four valve opening timings. Value opening timing has influence on the average temperature of the PM engine and the working of the PM engine does not allow earlier or later valve opening timing.

Nanoenergetic Composites of CuO Nanorods, Nanowires, and Al‐Nanoparticles

AbstractThis paper reports on the synthesis of the nanoenergetic composites containing CuO nanorods and nanowires, and Al‐nanoparticles. Nanorods and nanowires were synthesized using poly(ethylene glycol) templating method and combined with Al‐nanoparticles using ultrasonic mixing and self‐assembly methods. Poly(4‐vinylpyridine) was used for the self‐assembly of Al‐nanoparticles around the nanorods. At the optimized values of equivalence ratio, sonication time, and Al‐particle size, the combustion wave speed of 1650 m s−1 was obtained for the nanorods‐based energetics. For the composite of nanowires and Al‐nanoparticles the speed was increased to 1900 m s−1. The maximum combustion wave speed of 2400 m s−1 was achieved for the self‐assembled composite, which is the highest known so far among the nanoenergetic materials. It is possible that in the self‐assembled composites, the interfacial contact between the oxidizer and fuel is higher and resistance to overall diffusional process is lower, thus enhancing the performance.

Numerical study on the compression ignition of a porous medium engine

Homogeneous and stable combustion can be realized in a porous medium (PM) engine where a chemically inert PM is mounted in the combustion chamber. To understand the mechanism of the PM engine, we simulated the working process of a PM engine fueled with natural gas (CH4) using an improved version of KIVA-3V and investigated the effects of the initial PM temperature, the PM structure as well as the fuel injection timing on the compression ignition of the engine. The improved version of KIVA-3V was verified by simulating the experiment of Zhdanok et al. for the superadiabatic combustion of CH4-air mixtures under filtration in a packed bed. The numerical results are in good agreement with experimental data for the speed of combustion wave. Computational results for the PM engine show that the initial PM temperature is the key factor in guaranteeing the onset of compression ignition of the PM engine at a given compression ratio. The PM structure affects greatly both convective heat transfer between the gas and solid phase in the PM and the dispersion effect of the PM. Pore diameter of the PM is a crucial factor in determining the realization of combustion in the PM engine. Over-late fuel injection timing (near TDC) cannot assure a compression ignition of the PM engine.

Numerical simulation and theoretical analysis of premixed low-velocity filtration combustion

This paper investigates combustion wave characteristics of lean premixtures in a porous medium burner. Heat recuperation originated by the porous medium is examined by an one-dimensional numerical model. Attention is focused on the influences of solid properties, heat loss, equivalence ratio, etc., on the combustion wave speed and the maximum combustion temperature attained in the wave. Based on the flame sheet assumption a relationship between the combustion wave speed and the maximum combustion temperature is given. Then an approach from the laminar premixed flame theory is applied and the entire flame zone is divided into a pre-heating region and a reaction region, and treated separately. In this way, the second relationship between the two parameters is deduced. Thus a closed analytical solution for the combustion wave speed and the maximum combustion temperature is obtained. Over a wide range of working conditions, the numerical predictions and theoretical results show qualitative agreements with experimental data available from the literature. The results reveal that the mechanism of superadiabatic combustion is attributed to the overlapping of the thermal wave and combustion wave under certain conditions.