Combustion of a single magnesium particle in water vapor

The combustion of magnesium particles in water vapor is of interest for underwater propulsion and hydrogen production. In this work, the combustion process of a single magnesium particle in water vapor is studied both experimentally and theoretically. Combustion experiments are conducted in a combustor filled with motionless water vapor. Condensation of gas-phase magnesia on the particle surface is confirmed and gas-phase combustion flame characteristics are observed. With the help of an optical filter and a neutral optical attenuator, flame structures are captured and determined. Flame temperature profiles are measured by an infrared thermometer. Combustion residue is a porous oxide shell of disordered magnesia crystal, which may impose a certain influence on the diffusivity of gas phases. A simplified one-dimensional, spherically symmetric, quasi-steady combustion model is then developed. In this model, the condensation of gas-phase magnesia on the particle surface and its influence on the combustion process are included, and the Stefan problem on the particle surface is also taken into consideration. With the combustion model, the parameters of flame temperature, flame diameter, and the burning time of the particle are solved analytically under the experimental conditions. A reasonable agreement between the experimental and modeling results is demonstrated, and several features to improve the model are identified.     

Microgravity droplet combustion: effect of tethering fiber on burning rate and flame structure

Droplets tethering on fibers has become a well established technique for conducting droplet combustion experiments in microgravity conditions. The effects of these supporting fibers are frequently assumed to be negligible and are not considered in the experimental analysis or in numerical simulations. In this work, the effect of supporting fibers on the characteristics of microgravity droplet combustion has been investigated numerically; a priori predictions have then been compared with published experimental data. The simulations were conducted using a transient one-dimensional spherosymmetric droplet combustion model, where the effect of the supporting fiber was implicitly taken into account. The model applied staggered convective flux finite volume method combined with high-order implicit time integration. Thermal radiation was evaluated using a statistical narrow band radiation model. Chemical kinetics and thermophysical properties were represented in rigorous detail. Tether fiber diameter, droplet diameter, ambient pressure and oxygen concentration were varied over a range for n-decane droplets in the simulations. The results of the simulations were compared to previously published experiments conducted in the Japan Microgravity Center (JAMIC) 10 second drop tower and the NASA Glenn Research Center (GRC) 5.2 second drop tower. The model reproduces closely nearly all aspects of tethered n-decane droplet burning phenomena, which included droplet burning history, transient and average burning rate, and flame standoff ratio. The predictions show that the presence of the tethering fiber significantly influences the observed burning rate, standoff ratio, and extinction.     

Modeling of HMX combustion

A mechanism of HMX combustion was proposed and the corresponding model was developed under the assumption that the combustion wave consists of two zones, with consideration given to the reaction of decomposition and vaporization of the initial energetic material in the condensed phase and the subsequent decomposition of its vapor in the gas phase. An analysis of the results showed that, at low pressures, the burning rate is largely determined by the exothermic decomposition of the material in the condensed phase, but at pressure above ∼20 atm, the processes in the gas phase begin to play an increasingly important role, where the limiting process is the bimolecular activation reaction with the subsequent dissociation of HMX accompanied by the secondary reactions between the products. A comparison of the calculation results with experimental data showed that the model adequately describes a number of characteristics of the combustion wave and ballistic properties, such as the burning rate and its sensitivity to pressure and initial temperature.     

Flame structure of HMX/GAP propellant at high pressure

The chemical and thermal structure of a HMX/GAP propellant flame was investigated at a pressure of 0.5 MPa using molecular beam mass spectrometry and a microthermocouple technique. The pressure dependence of the burning rate was measured in the pressure range of 0.5–2 MPa. The mass spectrometric probing technique developed for flames of energetic materials was updated to study the chemical structure of HMX/GAP flames at high pressures. Eleven species, including HMX vapor, were identified, and their concentrations were measured in a zone adjacent to the burning surface at pressures of 0.5 and 1 MPa. Temperature profiles in the propellant combustion wave were measured at pressures of 0.5 and 1 MPa. Species concentration profiles were measured at 0.5 MPa. Two main zones of chemical reactions in the flame were found. The data obtained can be used to develop and validate combustion models for HMX/GAP propellants.     

Modeling the combustion of JA2 and solid propellants of similar composition

A theoretical study on combustion of JA2, RPD-380, and RPD-351, which are modified double-base propellants composed primarily of three identical nitrate ester ingredients, is presented. A one-dimensional, two-phase model was used [M.S. Miller, W.R. Anderson, in: V. Yang, T.B. Brill, W.Z. Ren (Eds.), Solid Propellant Combustion Chemistry, Combustion and Motor Interior Ballistics, Progress in Astronautics and Aeronautics, vol. 185, AIAA, Reston, VA, 2000, pp. 501–531, (a) M.S. Miller, W.R. Anderson, J. Propul. Power 20 (3) (2004) 440–454. (b) M.S. Miller, W.R. Anderson, CYCLOPS, A Breakthrough Code to Predict Solid-Propellant Burning Rates, U.S. Army Research Laboratory Technical Report, 1987 ARL-TR-2910.]. This approach has been shown to give good agreement between predicted and experimental results for several nitrate ester propellants, including JA2 [(a) M.S. Miller, W.R. Anderson, J. Propul. Power 20 (3) (2004) 440–454. (b) M.S. Miller, W.R. Anderson, CYCLOPS, A Breakthrough Code to Predict Solid-Propellant Burning Rates, U.S. Army Research Laboratory Technical Report, 1987 ARL-TR-2910.]. Extension of the model to the two RPD variants yields results in good agreement with existing experimental data. Comparisons of the response of predicted burning rates to experimental formulation changes at gun pressures, and to the initial propellant temperature are particularly encouraging. Our results show the burning rate ordering of these propellants is JA2 < RPD-380 < RPD-351 at all pressures. Chemistry which appears to account for this ordering is discussed. Also, an upgraded mechanism was used, and the reasons for some slight changes in results vs. an older one are identified. Sensitivities of the computed temperatures near the propellant surface to the various reactions’ rate coefficients are discussed; these provide insights regarding which reactions are centrally important to the computed burning rates and solutions. The spatial structure of one propellant flame – temperature and species profiles – is given; variations vs. the formulations and pressure are discussed. The fidelity of burning rate response to mixture ratio and initial propellant temperature are encouraging that the model may find application in propellant formulation science and elsewhere.     

Evaluating the efficiency of thermo-electric conversion of heat from gas combustion in a small-scale system with counterflow heat exchange

The efficiency of thermoelectric conversion of heat from gas combustion was evaluated in a small-scale system consisting of two channels with opposing gas flows and thermocouples located in the separating wall. Combustion occurred in the chamber fed with fresh mixture heated by combustion products through heat-conducting walls of the channel. In the channel walls, there were thermoelectric converters. It has been shown that in this system, the maximum conversion efficiency of heat from gas combustion may be close to the maximum efficiency of thermoelectric conversion calculated by the maximum acceptable working temperature of the hot side of the converter. This conclusion is valid in the case when the adiabatic combustion temperature of the gas mixture is below the maximum allowable operating temperature of the hot side of the thermoelectric converter. The considered system is promising for the burning of low-calorific gas mixtures and does not require additional energy for cooling the cold side of the thermoelectric converter.     

傅里叶红外光谱法研究GAP/AP混合体系的快速热裂解

用T-Jump/FTIR在线联用分析技术,研究了GAP/AP混合体系在模拟燃烧条件下快速加热高温高压的热裂解。结果表明,GAP/AP混合体系的主要热裂解气相产物的组成发生了变化,说明组分之间存在相互作用。压力对GAP/AP混合体系气相产物有明显的影响,表明混合体系组分GAP和AP之间的相互作用是通过AP分解气相产物进行的,混合体系不但存在气相之间的反应,也存在气相/凝聚相反应。而温度并没有影响AP对GAP的作用。用T-Jump/FTIR在线分析技术能够实现模拟燃烧条件下含能材料实时气体产物分析,为从微观反应的角度探索含能材料的快速高压热裂解及其组分之间的相互作用提供一条技术途径。     

Combustion of volatile condensed systems during pressure decay

The combustion and extinction of volatile condensed systems during pressure decay are studied. In contrast to the existing theories of this phenomenon, where only the thermal inertia of the condensed phase (t _c -approximation) is considered, an analysis of the time-dependent behavior of the gas phase is also included. Extinction curves, i.e., dependence between the pressure decay depth and the pressure decay rate at which the burning of the propellant ceases are calculated. The analysis is performed within the framework of the Belyaev model. A comparison of the results with calculations based on the t _c -approximation shows that, at high pressures, the thermal inertia of the gas phase is of considerable importance.     

Peculiarities of aluminum particle combustion in steam

This work experimentally addresses aluminum combustion in steam, pure or mixed with diluents, for aluminum particles in size range 40∼80 µm, using an electrodynamic levitator. High-speed videos unveil an unreported and complex mechanism in steam, not observed in other oxidizers. The detached flame is quite faint and very close to the surface. Alumina smoke around the droplet rapidly condenses and coalesces into a large, single orbiting alumina satellite. It eventually collides the main aluminum droplet while generating secondary alumina droplets. A unique feature is the presence of several distinct oxide lobes on the droplet, which merge only at the end of burning and encapsulate the remaining aluminum, possibly promoting an incomplete combustion. The measured burning times in pure water vapor are longer than expected and the efficiency of steam is found to be 30% that of oxygen, lower than the usually accepted value of 60%. A general correlation on burning time, including the major oxidizers, is proposed. Direct numerical simulations are conducted and are in line with experiments, in terms of burning rate or flame stand off ratio. Combustion in steam seems mostly supported by surface reactions, giving a faint flame with low gas temperatures and high hydrogen content. It is speculated that those two specific features could help explain the peculiarity of steam.     

On combustion in a closed rectangular channel with initial vorticity

This article examines the detailed combustion process in a theoretical model with applicability to combustion in a wave rotor or wave disc engine. The model comprises a single channel into which an initial loading of methane and air is admitted and ignited after all inlet and exit ports have been closed. Combustion takes place at constant volume. However, the initial gaseous mixture in the channel is not at rest. The initial opening and closing of the ports generates significant vorticity which influences the evolution of the combustion process. Numerical evaluations are provided for the detailed flame shape for simplified (one-step) chemistry and a simulation using the detailed 235-step San Diego scheme. Quantities examined are the evolution of vorticity, pressure fluctuations, mass consumption rate, flame surface area and the influences on combustion of adiabatic and non-adiabatic channel walls. Combustion regimes are identified and compared with simpler model studies (no initial flow). Pointwise quantities are examined to describe the various stages of burning in the channel. The focus of the study is directed towards quantities that influence overall burning rate and completeness of combustion.     

Modelling Chemical-Looping assisted by Oxygen Uncoupling (CLaOU): Assessment of natural gas combustion with calcium manganite as oxygen carrier

Chemical-Looping Combustion (CLC) is a promising technology for performing CO₂ capture in combustion processes at low cost and with lower energy consumption. Fuel conversion modelling assists in optimizing and predicting the performance of the CLC process under different operating conditions. For this work, the combustion of natural gas was modelled using a CaMnO₃-type perovskite as oxygen-carrier and taking into consideration the processes of fluid dynamics and reaction kinetics involved in fuel conversion. The CLC model was validated against experimental results obtained from the 120?kW_th CLC unit at the Vienna University of Technology (TUV). Good agreement between experimental and model predictions of fuel conversion was found when the temperature, pressure drop, solids circulation rate and fuel flow were varied. Model predictions showed that oxygen transfer by means of the gas–solid reaction of the fuel with the oxygen-carrier was relevant throughout the entire fuel-reactor. However, complete combustion could be only achieved under operating conditions where the process of Chemical-Looping assisted by Oxygen Uncoupling (CLaOU) became dominant, i.e. a relevant fraction of the fuel was burnt with molecular oxygen (O₂) released by the oxygen-carrier. This phenomenon was improved by the design configuration of the 120?kW_th CLC unit at TUV, in which oxidized particles are recirculated to the upper part of the fuel-reactor. Thus, the validated model identified the conditions at which complete combustion can be achieved, demonstrating that it is a powerful tool for the simulation and optimization of the CLC process with the CaMnO₃-type material.     

Electrostatic precipitation in a small scale wood combustion furnace

Wood combustion generates a high concentration of particulate matter emission, but most of the particulates in the exhaust gas can be filtered through an electrostatic precipitator. The objective of this paper is to model the trajectory of particulates in the exhaust chimney of a small scale wood combustion furnace with an electrostatic precipitator. The precipitator consists of a central electrode subjected to a maximum high voltage of 50 kV and an outer electrode of 180 mm diameter, ground potential. The parameters including particle size, ambient temperature, pressure, gas flow rate and the applied voltage have been varied while computing the trajectories of the particles in the chimney. The trajectories of particulates have been analyzed for different sizes of a typical wood combusting stove by taking different forces into account on particulates. The critical conditions give the trajectory of particles as a function of particulate size and applied voltage together with the function of efficiency.     

高压工况下底排推进剂燃速的反向传播神经网络模型

 为了研究底排推进剂在火炮膛内随弹丸运动时的燃烧特性,采用密闭爆发器仿真实验技术,针对底排推进剂在膛内高压工况下的燃烧特性进行实验研究,获得了两种不同装填密度下平均压力随时间变化的关系,并对压力进行了全程热散失修正。采用多次平滑、滤波数据处理技术和发射药燃速处理方法,得到了燃速与压力（8~150 MPa）之间的关系。基于实验数据特征样本,建立并训练得到了底排推进剂高压工况下的反向传播（Back Propagation）神经网络燃速模型,该模型与传统的指数模型相比,具有拟合精度高和稳定性强的特点。     

Comparison of simplified models for nitramine propellant combustion

Although 1,3,5-Trinitroperhydro-1,3,5-triazine (RDX) and Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) are very similar molecularly and their burning rates as a function of pressure are nearly identical, it is well known that they differ significantly in temperature sensitivity, especially at low pressures. To understand these differences better, three simple models were applied to HMX and RDX combustion. Both the Denison–Baum–Williams and Li–Williams–Margolis models have previously been calibrated for use with RDX. However, the RDX calibration of the Ward–Son–Brewster model was developed in the present work. All three models were compared with relevant measured data including: burning rate, flame stand-off/thickness, combustion stability, and temperature sensitivity. It was shown that all models are capable of accurately determining the burning rate of HMX and RDX as a function of pressure at the baseline initial temperature, but only two of the models are capable of capturing the variation in temperature sensitivity for both HMX and RDX, and only one model can replicate all the other measured characteristics within experimental uncertainty. Analysis using this model suggests that the surface reaction of RDX is much less exothermic than HMX and that there is a shifting between the gas phase and surface reaction dominance with pressure for HMX. This explains why the temperature sensitivity for RDX is nearly flat for low pressures while the temperature sensitivity for HMX increases significantly as the pressure decreases. Importantly, these trends are achieved without adding significant model complexity or having parameters change with pressure or initial temperature.     

Large eddy simulation of a pulverized coal jet flame ignited by a preheated gas flow

Large eddy simulation (LES) is applied to a pulverized coal jet flame ignited by a preheated gas flow. The simulation results are compared to experimental data obtained for the inlet stoichiometric ratios of 0.14, 0.22, and 0.36. An accurate and computationally inexpensive devolatilization model suitable for combustion simulation in LES is proposed and incorporated into the LES. The numerical results of gas temperature and coal burnout on the centerline show good agreement with the experimental data. Two kinds of lift-off heights are introduced to verify the combustion simulation. One is the height from the primary nozzle exit to the starting point of the growing flame region. The other is the height from the primary nozzle exit to the starting point of the continuous flame region. The calculated results of the two lift-off heights show good agreement with the experimental data. In contrast to LES, the standard k–ε model overestimates the lift-off heights because it calculates time-averaged temperature which does not contain information about local flame structure. The stoichiometric ratio in the gas phase at the starting point of the growing flame region is found to be independent of the inlet stoichiometric ratio in the range from 0.14 to 0.36.     

Numerical model of two-dimensional heterogeneous combustion in porous media under natural convection or forced filtration

A novel mathematical model and original numerical method for investigating the two-dimensional waves of heterogeneous combustion in porous media are proposed and described in detail. The mathematical model is constructed within the framework of the model of interacting interpenetrating continua and includes equations of state, continuity, momentum conservation and energy for solid and gas phases. Combustion, considered in the paper, is due to the exothermic reaction between fuel in the porous solid medium and oxidiser contained in the gas flowing through the porous object. The original numerical method is based on a combination of explicit and implicit finite-difference schemes. A distinctive feature of the proposed model is that the gas velocity at the open boundaries (inlet and outlet) of the porous object is unknown and has to be found from the solution of the problem, i.e. the flow rate of the gas regulates itself. This approach allows processes to be modelled not only under forced filtration, but also under free convection, when there is no forced gas input in porous objects, which is typical for many natural or anthropogenic disasters (burning of peatlands, coal dumps, landfills, grain elevators). Some two-dimensional time-dependent problems of heterogeneous combustion in porous objects have been solved using the proposed numerical method. It is shown that two-dimensional waves of heterogeneous combustion in porous media can propagate in two modes with different characteristics, as in the case of one-dimensional combustion, but the combustion front can move in a complex manner, and gas dynamics within the porous objects can be complicated. When natural convection takes place, self-sustaining combustion waves can go through the all parts of the object regardless of where an ignition zone was located, so the all combustible material in each part of the object is burned out, in contrast to forced filtration.     

Combustion in the future: The importance of chemistry

Combustion involves chemical reactions that are often highly exothermic. Combustion systems utilize the energy of chemical compounds released during this reactive process for transportation, to generate electric power, or to provide heat for various applications. Chemistry and combustion are interlinked in several ways. The outcome of a combustion process in terms of its energy and material balance, regarding the delivery of useful work as well as the generation of harmful emissions, depends sensitively on the molecular nature of the respective fuel. The design of efficient, low-emission combustion processes in compliance with air quality and climate goals suggests a closer inspection of the molecular properties and reactions of conventional, bio-derived, and synthetic fuels. Information about flammability, reaction intensity, and potentially hazardous combustion by-products is important also for safety considerations. Moreover, some of the compounds that serve as fuels can assume important roles in chemical energy storage and conversion. Combustion processes can furthermore be used to synthesize materials with attractive properties.A systematic understanding of the combustion behavior thus demands chemical knowledge. Desirable information includes properties of the thermodynamic states before and after the combustion reactions and relevant details about the dynamic processes that occur during the reactive transformations from the fuel and oxidizer to the products under the given boundary conditions. Combustion systems can be described, tailored, and improved by taking chemical knowledge into account. Combining theory, experiment, model development, simulation, and a systematic analysis of uncertainties enables qualitative or even quantitative predictions for many combustion situations of practical relevance.This article can highlight only a few of the numerous investigations on chemical processes for combustion and combustion-related science and applications, with a main focus on gas-phase reaction systems. It attempts to provide a snapshot of recent progress and a guide to exciting opportunities that drive such research beyond fossil combustion.     

Laminar and unstable burning velocities and Markstein lengths of hydrogen–air mixtures at engine-like conditions

Hydrogen offers an attractive alternative to conventional fuels for use in spark ignition engines. It can be burned over a very wide range of equivalence ratios and with considerable exhaust gas recirculation. These help to minimise pumping losses through throttleless operation and oxides of nitrogen (NO_x) production through reduced temperature. Full understanding of hydrogen-fuelled engine operation requires data on the laminar burning rate of hydrogen–air residuals under a wide range of conditions. However, such data are sparse. The present work addresses this need for experimental data. Spherically expanding H₂–air flames were measured at a range of temperatures, pressures, and equivalence ratios and with varying concentrations of residuals of combustion. Unstretched burning velocities, u_l, and Markstein lengths, L_b, were determined from stable flames. At the higher pressures, hydrodynamic and diffusional-thermal instabilities caused the flames to be cellular from inception and prohibited the derivation of values of u_l and L_b. The effect of pressure on the burning rate was demonstrated to have opposing trends when comparing stoichiometric and lean mixtures. The present measurements were compared with those available in the literature, and discrepancies were attributed to neglect, in some works, the effects of stretch and instabilities. From the present measurements, the effects of pressure, temperature, and residual gas concentration on burning velocity are quantified for use in a first step towards a general correlation.     

Chemical-looping combustion: Status and research needs

Chemical-Looping Combustion (CLC) has emerged in recent years as a very promising combustion technology for power plants and industrial applications with inherent CO₂ capture, which circumvent the energy penalty imposed on other competing technologies. The process is based on the use of a metal oxide to transport the oxygen needed for combustion in order to prevent direct contact between the fuel and air. CLC is performed in two interconnected reactors, and the CO₂ separation inherent to the process practically eliminates the energy penalty associated with gas separation. The CLC process was initially developed for gaseous fuels, and its application was subsequently extended to solid fuels. The process has been demonstrated in units of different size, from bench scale to MW-scale pilot plants, burning natural gas, syngas, coal and biomass, and using ores and synthetic materials as oxygen-carriers.An overview of the status of the process, starting with the fundamentals and considering the main experimental results and characteristics of process performance, is made both for gaseous and solid fuels. Process modelling of the system for solid and gaseous fuels is also analysed. The main research needs and challenges both for gaseous and solid fuel are highlighted.