Mechanism of combustion catalysis by ferrocene derivatives. 2. Combustion of ammonium perchlorate-Based propellants with ferrocene derivatives

Combustion of mixtures of a narrow fraction of ammonium perchlorate (AP) with hydrocarbon binders and combustion catalysts diethylferrocene and 1,1′-bis(dimethyloctyloxysilyl)ferrocene, as well as nano-sized Fe₂O₃ is studied. It is shown that the efficiency of ferrocene compounds from the viewpoint of increasing the burning rate depends on the oxidizer/fuel ratio in the propellant and on the place of the leading reaction of combustion. In composites with a high oxidizer/fuel ratio whose combustion follows the gas-phase model, the catalyst efficiency is rather low. In systems with a low oxidizer/fuel ratio where the contribution of condensed-phase reactions to the burning rate of the system is rather large, the catalyst efficiency is noticeably greater, and it is directly related to the possibility of formation of a soot skeleton during combustion. The close values of the catalytic activity of ferrocenes and Fe₂O₃ in the case of their small concentrations in such compositions testify that the main contribution to the increase in the propellant burning rate is made by Fe₂O₃ formed due to rapid oxidation of ferrocene on the AP surface and accumulated on the soot skeleton. Thermocouple measurements of propellants with a low oxidizer/fuel ratio are performed, and it is shown that the temperature of their surface is determined by plasticizer evaporation. A phenomenological model of combustion of the examined propellants is proposed.     

Combustion mechanism of nitro ester binders with nitramines

The combustion of binary compositions of nitramines (HMX, RDX, Bi-HMX, and CL-20) with two nitro ester binders, one of which is characterized by gas-phase combustion, and the other by the burning-rate controlling reaction in the condensed phase, was studied in the pressure range 2–15 MPa. It is shown that in compositions with the binder characterized by gas-phase combustion, HMX in concentrations up to 50% acts as an inert additive. Depending on the size and concentration of HMX particles, three types of combustion of the mixtures can be identified: combustion along the binder layers, combustion as a single system, and the model of combustion with a coolant. At higher nitramine concentrations, combustion control passes to nitramine, and the burning-rate controlling reaction occurs in the liquid phase of nitramine. For Bi-HMX and CL-20 nitramines, which are less stable and more fast burning than HMX and in a mixture with a binder characterized by gas-phase combustion, only two combustion models are observed: the model with fast burning additives or combustion as a single unit. In compositions of the nitramines RDX and HMX with a binder burning by the c-phase mechanism, the combustion model with fast burning additives is applicable only in a narrow range of conditions. The compositions mostly burn only as a single unit, and the addition of nitramine increases the burning rate of nitro ester by transferring heat from the overlying zone to the condensed phase.     

Effect of fuel properties on biomass combustion. Part II. Modelling approach—identification of the controlling factors

Biomass fuels come from many varieties of sources resulting in a wide range of physical and chemical properties. In this work, mathematical models of a packed bed system were employed to simulate the effects of four fuel properties on the burning characteristics in terms of burning rate, combustion stoichiometry, flue gas composition and solid-phase temperature. Numerical calculations were carried out and results were compared with measurements wherever possible. It was found that burning rate is mostly influenced by fuel size and smaller fuels result in higher combustion rate due to increased reacting surface area and enhanced gas-phase mixing in the bed; combustion stoichiometry is equally influenced by fuel LCV and size as a consequence of variation in burning rate as well as the mass ratio of combustible elements to the oxygen in the fuel; for the solid-phase temperature, material density has the strongest influence and a denser material has a higher maximum bed temperature as it results in a less fuel-rich combustion condition; while CO concentration in the flue gases is mostly affected by both fuel calorific value and size, CH₄ in the exiting flow is greatly affected by material density due to change in reaction zone thickness.     

Insightful investigation of smoke suppression behavior and mechanism of polystyrene with ferrocene: An important role of intermediate smoke

Aiming to investigate the smoke suppression mechanism of ferrocene in model polystyrene (PS) during combustion, we exploited the vapor‐phase and condensed‐phase behaviors. Cone calorimeter testing result showed that 3 wt% ferrocene imparted PS with 56.9% reduction in total smoke production. The analysis of the char after cone calorimeter testing demonstrated that the condensed‐phase smoke suppression mechanism was weak owing to the absence of charring behavior. The vapor‐phase mechanism was focused on the analysis of the small‐molecule smoke precursors and initially formed smoke intermediates. Transmission electron microscopy of initially formed smoke intermediates of PS/3Ferrocene revealed that enormous γ‐Fe₂O₃ nanoparticles from ferrocene participated in the initial formation of smoke intermediates, which subsequently underwent notable thermal oxidation degradation with decreased smoke residue. Thermogravimetric analysis coupled with Fourier transform infrared spectroscopy results manifested that the small‐molecule smoke precursors remained almost unchanged with addition of ferrocene. Conclusively, the smoke suppression mechanism with ferrocene predominantly originated from the intensive thermal oxidation of smoke intermediates, which opened a viable route for excellent smoke suppression design.     

Droplet ignition of coal–water slurries prepared from typical coal- and oil-processing wastes

The requirements for stable ignition (and subsequent combustion) of fuel suspensions prepared from typical coal- and oil-processing wastes are studied experimentally. Attention focuses on the differences between the ignition characteristics of coal–water slurries (containing petrochemicals) obtained on the basis of filter cakes containing T, K, SS, Zh, D, and G coal. To eliminate the influence of the droplet holder (traditionally, thermocouple junctions, ceramic rods, or metal wire) on the ignition characteristics of the fuel droplet, the experiments employ a special model combustion chamber and a device for introducing a single drop of suspension. The ignition time and the minimum temperature of stable ignition of a droplet of coal–water slurries suspended in an oxidant flux are established. The influence of the following factors on the initiation of fuel combustion is determined: the oxidant temperature, the droplet size, the size of the coal dust, and the properties and concentrations of the components. The compositions of the coal–water slurries corresponding to optimal ignition (minimum inertia) are identified.     

Characteristics of HMX combustion waves at various pressures and initial temperatures

Temperature profiles and combustion-wave parameters are obtained experimentally for combustion of pressed HMX at room temperature and pressures of 1–500 atm and in the case of a change in the initial temperature of the specimens from −170 to +100°C at pressures of 1–75 (90) atm. The following combustion-zone parameters are determined: the heat effect in the c-phase, the heat transfer from the gas to the c-phase by thermal conduction and radiation, the rate of heat release in the gas near the surface, and the dimensions and temperature of the combustion zones. The authorsé previous conclusion that there is one process of decomposition and evaporation of HMX during its gasification in the condensed-phase reaction layer of the combustion wave is confirmed. Dependences of the fraction of decomposed HMX on the initial temperature of the specimens and the pressure are obtained. The differential characteristics of the combustion rate, surface temperature, and radiative heat transfer, required for the nonlinear theory of HMX combustion stability, are evaluated. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 2, pp. 59–66, March–April, 1998.     

Computational Models for Simulating Multicomponent Aerosol Evaporation in the Upper Respiratory Airways

An effective model for predicting multicomponent aerosol evaporation in the upper respiratory system that is capable of estimating the vaporization of individual components is needed for accurate dosimetry and toxicology analyses. In this study, the performance of evaporation models for multicomponent droplets over a range of volatilities is evaluated based on comparisons to available experimental results for conditions similar to aerosols in the upper respiratory tract. Models considered include a semiempirical correlation approach as well as resolved-volume computational simulations of single and multicomponent aerosol evaporations to test the effects of variable gas-phase properties, surface blowing velocity, and internal droplet temperature gradients. Of the parameters assessed, concentration-dependent gas-phase specific heat had the largest effect on evaporation and should be taken into consideration for respiratory aerosols that contain high volatility species, such as n-heptane, at significant concentrations. For heavier droplet components or conditions below body temperatures, semiempirical estimates were shown to be appropriate for respiratory aerosol conditions. In order to reduce the number of equations and properties required for complex mixtures, a resolved-volume evaporation model was used to identify a twelve-component surrogate representation of potentially toxic JP-8 fuel based on comparisons to experimentally reported droplet evaporation data. Due to the relatively slow evaporation rate of JP-8 aerosols, results indicate that a semiempirical evaporation model in conjunction with the identified surrogate mixture provide a computationally efficient method for computing droplet evaporation that can track individual toxic markers. However, semiempirical methodologies are in need of further development to effectively compute the evaporation of other higher volatility aerosols for which variable gas-phase specific heat does play a significant role.     

Effect of catalysts on the combustion of layered systems

An experimental study is made of a “chemical arc”, a model layered system of solid components, between the end surfaces of which stationary combustion takes place. The rates of combustion of the components in ammonium perchlorate-polyether, ammonium perchlorate-polyethylene, and ammonium perchlorate-butyl rubber systems with catalytic additives (diethyl ferrocene, epoxyethyl ferrocene, and iron oxide) are measured as functions of the distance between the components. A maximum mass combustion velocity is found to exist as a result of the transition from a low velocity (kinetic) to a high velocity reaction regime. The data are in qualitative agreement with theory. The mechanism by which the catalysts affect the combustion characteristics is analyzed. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 3, pp. 57–62, May–June 1999.     

Numerical study of polyethylene burning in counterflow: Effect of pyrolysis kinetics and composition of pyrolysis products

The burning behavior of polyethylene in the counterflow of oxidizing air has been studied numerically with a coupled model describing feedback heat and mass transfer between gas‐phase flame and polymeric solid fuel. A 2‐dimensional elliptic equation in axisymmetric formulation (revealing the cylindrical shape of the polymer sample used in the experiment) has been employed to simulate heat transfer in solid fuel, and a set of 1‐dimensional hyperbolic equations has been used to determine the solid‐to‐gas conversion degree of the pyrolysis reaction. Four sets of products compositions and two modifications for the kinetic parameters of solid fuel pyrolysis reaction have been taken into account. Gas‐phase formulation is presented by set of 1‐dimensional conservation equations for multi‐component flow with detailed kinetic mechanism of combustion. The profiles of temperature and species concentrations in the flame zone have been calculated and compared with the results of experimental study of combustion of ultrahigh molecular weight polyethylene. Higher hydrocarbon composition (dodecane) has been found to show the best agreement between the temperature and species concentration profiles with the measurements, especially for the low‐level mass fractions of the by‐product components—propylene, butadiene, and benzene.     

Bicomponent droplets evaporation: Temperature measurements and modelling

Development of improved energy conversion systems, having higher efficiencies and lower emissions, is central to reducing the production of green-house gases and to meeting air quality standards for other emissions. In non-premixed combustion systems, vaporization of the droplets and mixing of the vapour with the surrounding oxidizer control the overall rate of energy release. Droplet vaporization is affected by the nature of the liquid petroleum that contains species having extremely different volatilities. A fine understanding of multi-component droplet vaporization is therefore an important issue to optimize the combustion systems. This paper presents the application of a recently developed technique to measure the temperature of bi-component droplets. Based on the three-color laser-induced fluorescence (LIF) technique, droplet temperatures can be measured regardless of the composition. The method requires adding a small amount of a fluorescent organic dye to the fuel which is composed of ethyl-alcohol and acetone. The accuracy of the measurement is about ±1.3 °C. In this study, the emphasis is placed on the evaporation of the binary mixture in a hot air plume, the temperature of which (around 650 °C) largely exceeds the boiling point of the liquid components. An extensive study of the influence of the initial composition and droplet diameter is carried out. Finally, the experimental results are compared to an evaporation model based on the discrete components approach.     

低Reynolds数空气来流正庚烷液滴蒸发过程熵产特性

车用内燃机、航空发动机燃料的主要成分为正庚烷液体。液体燃烧属于扩散型燃烧,燃料先雾化成油雾,然后蒸发,燃油蒸气在气态扩散火焰中燃烧。油雾蒸发是液体燃烧中的重要过程,是决定液体燃烧的燃烧速率的关键步骤。建立了一个二维准稳态的液滴蒸发数值模型,分析了液滴蒸发的熵产,推导出了三维笛卡尔坐标系下的传质熵产公式。基于Fluent软件进行了数值模拟,结果显示低Reynolds数空气来流中的液滴蒸发过程的熵产主要由导热熵产组成,且液滴蒸发越容易,相对总熵产越小,从热力学角度来看其液滴蒸发过程更优。     

Optimal compositions of metal-free energetic compositions with variation in the oxidizer concentration and the ratio of nitro and difluoroamine groups

A thermodynamic analysis of the energy characteristics and combustion products of model mixtures based on polyethylene, tetranitromethane, and tetra(difluoroamino) methane is performed. Optimal ratios of the components are determined, and the maximum specific impulse was found for combustion of compositions in which the gram-atomic concentrations of hydrogen and fluorine are equal and, simultaneously, those of carbon and oxygen are equal. It is studied how the composition of the combustion products changes with a change in the ratio of the fuel components and how this is reflected in the value of the specific impulse.     

麦渣与制浆废液共混制备成型颗粒燃料及其燃烧特性分析

为研究麦渣与制浆废液共混制备的成型颗粒燃料的燃烧特性,通过热重分析法对其燃烧热力学及燃烧动力学进行了研究。结果表明：制浆废液的添加使颗粒燃料出现固定碳的二次燃烧阶段,有利于降低成型颗粒燃料的挥发分、固定碳燃烧阶段的点火温度及最大燃烧速率温度,对颗粒燃料的燃烧有正向协同作用;制备的颗粒燃料的一阶动力学模型拟合曲线的相关系数在0.95以上,颗粒燃料在挥发分燃烧和固定碳燃烧阶段的活化能和指前因子均随制浆废液的添加而降低。当废液固形物质量分数为53%时制备的成型颗粒燃料,其挥发分燃烧阶段和固定碳燃烧阶段的活化能为72.85和83.52 kJ/mol,指前因子为2.82×10⁶和3.73×10⁵ min^-1。制浆废液的添加使颗粒燃料更易燃烧,且燃烧过程稳定不易爆燃。     

Performance and exhaust emission characteristics of a spark ignition engine using ethanol and ethanol-reformed gas

Since ethanol is a renewable source of energy and has lower carbon dioxide (CO₂) emissions than gasoline, ethanol produced from biomass is expected to be used more frequently as an alternative fuel. It is recognized that for spark ignition (SI) engines, ethanol has the advantages of high octane and high combustion speed and the disadvantage of ignition difficulties at low temperatures. An additional disadvantage is that ethanol may cause extra wear and corrosion of electric fuel pumps. On-board hydrogen production out of ethanol is an alternative plan.Ethanol has been used in Brazil as a passenger vehicle fuel since 1979, and more than six million vehicles on US highways are flexible fuel vehicles (FFVs). These vehicles can operate on E85 - a blend of 85% ethanol and 15% gasoline.This paper investigates the influence of ethanol fuel on SI engine performance, thermal efficiency and emissions. The combustion characteristics of hydrogen enriched gaseous fuel made from ethanol are also examined.Ethanol has excellent anti-knock qualities due to its high octane number and a high latent heat of evaporation, which makes the temperature of the intake manifold lower. In addition to the effect of latent heat of evaporation, the difference in combustion products compared with gasoline further decreases combustion temperature, thereby reducing cooling heat loss. Reductions in CO₂, nitrogen oxide (NOx), and total hydrocarbons (THC) combustion products for ethanol vs. gasoline are described.     

Modeling and analysis of HVOF thermal spray process accounting for powder size distribution

This work focuses on the high velocity oxygen–fuel (HVOF) thermal spray processing of coatings and presents a fundamental model for the process which explicitly accounts for the effect of powder size distribution. The model describes the evolution of the gas thermal and velocity fields, as well as the motion and temperature of agglomerate particles of different sizes. In addition to providing useful insight into the in-flight behavior of particles with different sizes, the model is used to make a control-relevant parametric analysis of the HVOF thermal spray process. This analysis allows us to systematically characterize the influence of controllable process variables such as combustion chamber pressure, oxygen/fuel ratio, as well as the effect of powder size distribution, on the values of particle velocity and temperature at the point of impact on substrate. Specifically, the study shows that particle velocity is primarily influenced by the combustion chamber pressure, and particle temperature is strongly dependent on the fuel/oxygen ratio. Furthermore, it shows that the particle velocity and temperature at the point of impact depend strongly on particle size. These findings are consistent with available experimental observations and set the basis for the formulation of the control problem for the HVOF process.     

Regimes of the Combustion of Organic Coal–Water Fuels

The results of an analysis of the combustion behavior of the drops of organic coal–water fuels (OCWFs) prepared based on the flotation products (cakes) of the enrichment the coal of five grades (longflame, gas, coking, low-caking, and lean coals) and two petroleum products (petroleum residue and spent turbine oil) are presented. The experiments were performed under the conditions of the stationary fastening of an OCWF drop on the junction of a quick-response thermocouple in a flow of heated (from 500 to 1000°C) air. The following three combustion regimes were revealed for all of the test OCWF compositions: stepwise regime with slow heating (smoldering), intense gas generation regime with the boiling of liquid fuel components (boiling), and regime with a distinct tear-off zone of gas flow (torch). It was shown that the occurrence of the above combustion behaviors substantially depends on the characteristics of the OCWF components: the ash content and the yield of volatile substances of coal cakes and the boiling points and the ignition and combustion temperatures of the petroleum products used. Based on the results of the experiments, the ranges of air temperature changes characteristic of each of the three combustion regimes of fuel suspensions were established.     

Modeling effect of the biodiesel mixing ratio on combustion and emission characteristics using a reduced mechanism of methyl butanoate

This paper describes a modeling study on the effects of the mixing ratio of biodiesel derived from rice on combustion and emission characteristics. In order to model ignition and combustion processes, a combined mechanism of methyl butanoate and n-heptane was used after modifying some reaction constants. The oxygen content of rice oil was maintained at the measured value (11 vol.%) by assuming that one mole of rice oil was composed of one mole of methyl butanoate and two moles of n-heptane. The fuel property library of the KIVA code was expanded to include physical properties such as density and surface tension of rice oil (i.e., biodiesel). In addition, a discrete multi-component fuel evaporation model was employed to model the fuel injection, atomization, and evaporation.Calculated pressure histories and carbon monoxide (CO) and hydrocarbon (HC) emissions of the present study agreed reasonably with experiments at a 100 MPa injection pressure and a 10° BTDC injection timing for D100, BD20, and BD40. However some discrepancies were observed in predicting nitrogen oxide (NO_x) emissions. The effect of the start of injection timing on fuel consumption and CO emissions were also studied.     

Investigation of the mechanistic basis for ferrocene activity during the combustion of vinyl polymers

The influence of ferrocene on the flammability and smoke generation behavior and thermal characteristics of poly(vinyl chloride) (PVC) and poly(vinyl alcohol) (PVC) was examined in order to understand the basis of ferrocene's activity as a flame-retardant and smoke-suppressant additive. Ferrocene increased char formation in PVC by 20–60% while increasing the limiting oxygen index (O.I.) by 15–19%. Little char was found either with or without ferrocene in PVA, which had little improvement in O.I. Visible smoke was significantly decreased by ferrocene in PVA and in PVC of low to moderate molecular weight, but was unchanged in high molecular weight PVC. O.I. of PVC appeared to vary with molecular weight of the polymer. Thermogravimetric analyses and thermal degradation experiments showed that ferrocene promotes early weight loss and crosslinking in PVC. This effect is coincidental with the formation of ferricenium cation, which was identified by its visible spectrum and which may be a possible catalyst. Smoke inhibition of PVA by ferrocene occurs predominantly through gas-phase processes, while in PVC evidence and analogy for activity in both the gas phase and condensed phase are found.