Travelling wave mathematical analysis and efficient numerical resolution for a one-dimensional model of solid propellant combustion

We investigate a model of solid propellant combustion involving surface pyrolysis coupled to finite activation energy gas-phase combustion. Existence and uniqueness of a travelling wave solution are established by extending dynamical system tools classically used for premixed flames, dealing with the additional difficulty arising from the surface regression and pyrolysis. An efficient shooting method allows to solve the problem in phase space without resorting to space discretisation nor fixed-point Newton iterations. The results are compared to solutions from a CFD code developed at ONERA, assessing the efficiency and potential of the method, and the impact of the modelling assumptions is evaluated through parametric studies.     

A model for crack formation during active solid pyrolysis of a char-forming solid: crack patterns; surface area generation; volatile mass efflux

A model is developed for the formation and propagation of cracks in a material sample that is heated at its top surface, pyrolyses, and then thermally degrades to form char. In this work the sample is heated uniformly over its entire top surface by a hypothetical flame (a heat source). The pyrolysis mechanism is described by a one-step overall reaction that is dependent nonlinearly on the temperature (Arrhenius form). Stresses develop in response to the thermal degradation of the material by means of a shrinkage strain caused by local mass loss during pyrolysis. When the principal stress exceeds a prescribed threshold value, the material forms a local crack. Cracks are found to generally originate at the surface in response to heating, but occasionally they form in the bulk, away from ever-changing material boundaries. The resulting cracks evolve and form patterns whose characteristics are described. Quantities examined in detail are: the crack spacing in the pyrolysis zone; the crack length evolution; the formation and nature of crack loops which are defined as individual cracks that have joined to form loops that are disconnected from the remaining material; the formation of enhanced pyrolysis area; and the impact of all of the former quantities on mass flux. It is determined that the mass flux from the sample can be greatly enhanced over its nominal (non-cracking) counterpart. The mass efflux profile qualitatively resembles those observed in Cone Calorimeter tests.     

基于CARS技术的超燃冲压发动机点火过程温度测量

相干anti-Stokes Raman散射（coherent anti-Stokes Raman scattering,CARS）技术作为一种非接触测量手段,已广泛应用于多种发动机模型燃烧室温度测量及地面试验.然而,目前的工作主要集中在稳态燃烧场温度的测量,缺乏用高分辨率的单脉冲来测量瞬变的燃烧火焰温度及组分浓度的研究.基于CARS理论,结合多参数拟合算法,开发了基于MATLAB的CARS光谱计算和拟合程序CARSCF;利用McKenna平面火焰炉在不同工况下进行了温度测量,并与DLR测量结果进行对比,结果显示开发的CARSCF具有较高的测量重复性和准确性;最后将CARS技术应用于测量超燃冲压发动机点火过程中的温度测量,获取了点火过程中的温度.结果显示,在来流Mach数为3的条件下,H₂/air点火过程中温度呈现急剧上升然后缓慢下降,而CARS信号则呈现急剧上升然后急剧下降随后又缓慢上升的趋势,并且在点火过程中最高温度为1 511 K.      

Geometrically qualified ESPI vibration analysis of an engine

This paper presents an application in the automotive industry where a combination of electronic speckle-pattern interferometry and laser doppler velocimetry were used at a critical stage in the design process of an internal combustion engine. Combined deformation and surface relief measurements were used to study the phase and amplitude of deformation of a vibrating engine. The relief data was combined with the interferometer geometry and used to geometrically correct the deformation data, in an effort to improve accuracy. The measurements allowed rapid identification and quantification of design weaknesses, particularly those causing undesirable resonances. This led to a significant reduction in the design time and lowering of costs, when compared with existing design optimisation methods.     

钙基载氧体煤化学链燃烧脱硫试验研究

在钙基载氧体燃煤化学链燃烧技术过程中由于煤气化产物与载氧体之间的副反应导致反应过程中产生大量的SO₂气体。本文选择MAC铁矿石,CaO和石灰石作为脱硫剂,在小型加压固定床上研究不同温度、压力、Ca/S比条件下SO₂气体的脱除问题。结果表明,单独采用CaSO₄载氧体时,随温度升高SO₂气体浓度逐渐增加。添加铁矿石后,SO₂排放浓度降低,主要与Fe₂O₃能够催化抑制CaSO₄分解有关。添加CaO和石灰石脱硫剂后,随温度、压力以及Ca/S比增加,两种脱硫剂的脱硫效率均增加,且在相同的实验条件下得到的CaO的脱硫效果更佳。     

Predicting particle deposition for flow over a circular cylinder in combustion environments

Particle deposition on heat exchanger tubes is a serious concern in solid fuel combustion and gasification systems, such as power plants and syngas coolers. To predict deposition rates, several detailed computational fluid dynamic (CFD) models have been developed. However, these models are computationally expensive and cannot be used for quick determination of deposition rates and/or slagging tendencies. Particle impaction efficiency correlations, while not as accurate as detailed CFD models, are easier to use and are able to estimate the impaction rate of particles on the heat exchanger tubes. Nonetheless, since deposition and slagging are not just functions of particle impaction rates, but also sticking propensity, which is related to the particle temperature at impact, the impaction efficiency correlations fail to provide sufficient information. To address this shortcoming, similar correlations for particle temperature at impact have been developed in this work, based on a non-dimensional parameter that captures the flow and boundary conditions, as well as particle properties. When used alongside the impaction efficiency correlations, the new correlations developed can provide a reasonable estimate of the deposition and slagging tendencies, at negligible computational expense.     

Investigation of lubricant induced pre-ignition and knocking combustion in an optical spark ignition engine

An experimental study was performed to investigate lubricant oil induced pre-ignition and knocking combustion process in a single cylinder spark ignition (SI) engine with full bore overhead optical access. Lubricant oil was deliberately injected to the exhaust area through a specially modified direct injector to trigger the stochastic pre-ignition in a premixed air and fuel mixture. Simultaneous heat release analysis and high speed combustion imaging were used to study the pre-ignition and combustion processes. Outlier detection based on robust statistical methods was validated as an effective and efficient approach to identify sporadic pre-ignition. When pre-ignition occurred, the pre-ignited flame-front exhibited much faster propagating speed than that of the normal spark-ignited flame-front in the first stage of flame development. In several cycles, pre-ignition was followed by the pre-ignited propagating flame-front and then a separate spark-ignited flame-front before they subsequently merged together. In a few other cycles, pre-ignition led to heavy knocking combustion caused either by the auto-ignition close to the flame-front or near the cylinder wall, or both. The ultimate knock intensity of such cycles was determined by the timing, size, and location of end-gas auto-ignition of the unburned gas. Furthermore, optical detection of the oil droplet entrained combustion in the cycle subsequent to the knocking combustion cycle implied that high frequency oscillation pressure waves ejected lubricant from the piston-ring crevice.     

Effect of water vapour on particulate matter emission during oxyfuel combustion of char and in situ volatiles generated from rapid pyrolysis of chromated-copper-arsenate-treated wood

This paper reports the effect of water vapour on particulate matter (PM) during the separate combustion of in situ volatiles and char generated from chromated-copper-arsenate-treated (CCAT) wood at 1300 °C. Combustion of in situ volatiles produces only PM with aerodynamic diameter?<1?µm (i.e., PM₁), dominantly PM with aerodynamic diameter?<0.1?µm (i.e., PM_0.1). Water vapour could significantly enhance the nucleation, coagulation and condensation of fine particles and reduce the capture of Na and K by the alumina reactor tube via reduced formation of alkali aluminates, leading to increases in both yield and modal diameter of PM_0.1. Water vapour could also enhance char fragmentation hence increase the yield of PM with aerodynamic diameter between 1 and 10?µm (i.e., PM_1–10) during char combustion. For trace elements, during in situ volatiles combustion, volatile elements (As, Cr, Ni, Cu and Pb) are only presented in PM₁ and water vapour alters the particle size distributions (PSDs) but has little effect on the yields of these trace elements. During char combustion, As, Cr, Cu and Ni are present in both PM₁ and PM_1–10 while the non-volatile Mn and Ti are only present in PM_1–10. Increasing water vapour content increases the yields of As, Cr, Cu, Ni, Mn and Ti in PM_1-10 due to enhanced char fragmentation. During char combustion, water vapour also originates less oxidising conditions locally for enhancing As release, promotes the generation of gaseous chromium oxyhydroxides and inhabits the production of NiCl₂ (g), leading to increased yields of As and Cr and decreased yield of Ni in PM_0.1.     

Experimental study of a confined premixed metal combustor: Metal flame stabilization dynamics and nitrogen oxides production

This work presents a study of a magnesium/air combustion process in the context of innovative zero carbon dioxide (CO₂) energy carriers for reducing global warming effects. In order to analyze more deeply the confined combustion of magnesium under fluctuating overpressure conditions (0 to 24 hPa) and the generated gaseous by-products, magnesium/air flames have been realized in a combustion chamber with a conical bluff-body as flame holder and different contraction ratios diaphragms at the exit duct. Sieved magnesium samples with two size-fractions were tested: 20–50?µm and 50–70?µm. The gaseous emissions of nitrogen oxides (NO_x) and dioxygen (O₂) were analyzed with on-line infrared, ultraviolet and paramagnetic analyzers. A flame pulsating behavior was clearly observed from light emission intensity (monitored by a photodiode) and pressure fluctuations (monitored by a pressure sensor); the frequencies obtained ranged between 3 and 10?Hz. The frequency of the pulsation exhibited strong dependence on the geometric configuration of the chamber: a contraction diaphragm divided by two the frequency level of the fluctuations in the studied range of maximum overpressure. Such fluctuations may probably be the consequence of periodic perturbations of the recirculation zone behind the bluff-body. These periodic perturbations are themselves caused by strong periodic overpressure variations due to stiff contraction downstream responding to gas velocity fluctuations. This feed-back-loop mechanism was considered in this study. NO_x emissions produced through the thermal pathway were analyzed for equivalence ratios ranging from 0.29 to 1. The representation of NO_x versus equivalence ratio exhibited a parabolic shape with a maximum for an equivalence ratio of 0.4. Moreover, NO_x emissions of this metal combustor have shown a similar order of magnitude than current internal combustion engines.