The numerical simulation of two-dimensional aluminized composite solid propellent combustion

A numerical framework is presented which examines, for the first time, the burning of two-dimensional aluminized solid propellants. Aluminized composite propellants present a difficult mathematical and numerical challenge because of complex physics and topological changes that occur at the propellant surface. For example, both mathematical models and appropriate numerical solvers must describe the regressing burning surface, aluminium particle detachment and evolution throughout the gas-phase flow field, surface tension effects, ignition and combustion of aluminium particles, phase change effects, agglomeration of aluminium particles, radiation feedback to the propellant surface, to name a few. All of these effects must be modelled in a consistent manner. A numerical framework for which these effects can be included in a rational fashion is currently being developed. This framework includes the level set method to capture the solid–gas interface as well as particle motion in the gas phase. Some preliminary calculations of the two-dimensional combustion field supported by a disc pack with embedded aluminium particles are presented.     

Numerical simulation of the 3-dimensional combustion of aluminized heterogeneous propellants

We report the first 3-dimensional simulations of aluminized propellant combustion, accounting for heat conduction in the solid, combustion in the gas-phase, and coupling of these via the irregularly moving propellant surface, one that can not be defined by a single-valued height function. The simulations are used to examine the dynamics of aluminum particles in the near-neighborhood of the surface after detachment, and to provide an estimate of the time to ignition of the particles, and their speed and height above the surface at ignition. In addition, we examine the temperature history of the particles during their rise to the surface, determine whether they melt or not, and in this way test Cohen’s well-known melting criterion. And, we discuss a simple model which provides insights into how aluminum particles floating on a binder melt layer would migrate because of surface tension effects, and calculate an average migration distance that is consistent with previous agglomeration studies.     

Direct numerical simulation of diluted combustion by evaporating droplets

Diluted combustion has been studied using DNS in a three-dimensional temporally developing reacting shear-layer with the oxidizer stream laden with evaporating droplets. The gaseous phase is described in the Eulerian frame while the discrete droplet phase is treated in the Lagrangian frame, with strong two-way coupling between the two phases through mass, momentum and energy exchange. Grid-resolution-independent results have been obtained in cases without and with droplets. A comprehensive parametric study has been conducted by varying the initial Stokes number (St₀) and mass loading ratio (MLR₀). Detailed field analysis has been conducted to examine the complex nonlinear interactions among droplet dynamics, evaporation, turbulence and combustion, and so on. Effects of evaporating droplets on averaged flow and combustion quantities have also been presented. In particular, the conditional scalar dissipation rate is found to be enhanced by evaporating droplets, which suggests that they can promote micromixing and combustion under certain conditions, in addition to their roles in combustion suppression. The transport equation for the mixture fraction variance has been analyzed, with a focus on the vaporization-related source terms. Such source terms exhibit more complex local variations in the present shear-flow non-premixed flame configuration, compared with the case in the homogeneous decaying turbulence configuration of Réveillon and Vervisch (2000).     

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.     

宽带相干反斯托克斯喇曼散射法诊断固体燃剂燃烧场

报道了采用单次脉冲非稳腔空间增强探测 相干反斯托克斯喇曼散射（USED CARS）技术诊断常压下固体燃剂瞬态燃烧场温度和氮气浓度。采用宽带USED CARS技术,在固体燃剂瞬态燃烧场获得了较高信噪比的单次激光脉冲氮气Q支CARS实验谱,用CARS理论计算软件拟合CARS实验谱,给出了固体燃剂瞬态燃烧场温度和氮气浓度在不同高度的分布,固体燃剂燃烧场温度约2 250K、氮气相对浓度16%～20%。     

Binder melt: Quantification using SEM/EDS and its effects on composite solid propellant combustion

The present study reports the development of a novel technique to quantify binder melt on the surface of the propellant. Non-aluminized AP-HTPB propellants of 86% particulate loading are used to illustrate the technique. Elemental maps of unburnt and extinguished propellant surface are obtained using EDS (Energy Dispersive Spectroscopy). Overlap between the elements is identified and the elemental maps are processed to calculate AP and binder area exposed in unburnt and extinguished samples. The AP area exposed is found to be around 72.3% and 63.3% for unburnt and extinguished samples, respectively, indicating a reduction in AP exposed area with extinguished samples. This has been attributed to the binder melt discussed in literature but never quantified. Simulations have been carried out to analyze and understand the effects of this binder melt. A random packing algorithm is used to simulate propellant packs. Also, a methodology to account for binder melt layer is introduced and is used to capture AP exposed areas. Effect of binder melt in propellants with different solid loading and varying particle size is discussed. It is shown that fine AP particles are more prone to being covered by binder melt than the coarse AP particles. A possible explanation to the behavior of plateau burning propellants observed in literature has been provided through this analysis.     

Thermomechanical modeling of regressing heterogeneous solid propellants

A numerical framework based on the generalized finite element method (GFEM) is developed to capture the coupled effects of thermomechanical deformations and thermal gradients on the regression rate of a heterogeneous solid propellant. The thermomechanical formulation is based on a multiplicative split of the deformation gradient and regression of the heterogeneous solid propellant is simulated using the level set method. A spatial mesh convergence study is performed on a non-regressing solid heterogeneous propellant system to examine the consistency of the coupled thermomechanical GFEM solver. The overall accuracy (spatial and temporal) of the coupled thermomechanical solver for regressing solid propellants is obtained from a periodic sandwich propellant configuration, where the effects of thermomechanical deformations on its regression rate is investigated. Finally, the effects of thermomechanical deformations in a regressing two-dimensional heterogeneous propellant pack are studied and time-average regression rates are reported.     

A numerical simulation of transient ignition and ignition limit of a composite solid by a localised radiant source

An unsteady three-dimensional numerical model has been formulated, coded, and solved to study ignition and flame development over a composite solid fuel sample upon heating by a localised radiant beam in a buoyant atmosphere. The model consists of an unsteady gas phase and an unsteady solid phase. The gas phase formulation consists of full Navier-Stokes equations for the conservation of mass, momentum, energy, and species. A one-step, second-order overall Arrhenius reaction is adopted. Gas radiation is included by solving the radiation transfer equation. For the solid phase formulation, the energy (heat conduction) equation is employed to solve the transient solid temperature. A first-order in-depth solid pyrolysis relation between the solid fuel density and the local solid temperature is assumed. Numerical simulations provide time-and-space resolved details of the ignition transient and flame development and the existence of two types of ignition modes: one with reaction kernel initiated on the surface and the other with ignition kernel initiated in the gas phase. Other primary outputs of the computation are the minimum ignition energy (Joule) for the solid as a function of the external heating rate (Watt). Both the critical heat input for ignition and the optimal ignition energy are identified. Other parameters that were varied over the simulations include: sample thickness, ignition heat source spatial shape factor, and gravity level.     

Transient simulation of the combustion of fuel-lean hydrogen/air mixtures in platinum-coated channels

The start-up of platinum-coated, hydrogen-fuelled planar channels with heights of 1 mm is investigated numerically using 2-D transient simulations with detailed hetero-/homogeneous chemistry, heat conduction in the solid wall and surface radiation heat transfer. Simulations encompass pressures of 1 and 5 bar and fuel-lean H₂/air equivalence ratios of 0.10 to 0.28. Catalytic ignition is inhibited by rising pressure and increasing hydrogen concentration. However, at temperatures above the catalytic ignition temperature T_ign, the dependencies of the heterogeneous reactivity reverse, showing a positive order ～1.5 with respect to hydrogen concentration and an overall positive pressure order of ～0.97. Despite the longer catalytic ignition times for the larger equivalence ratios, the times required to reach steady state are shorter at larger stoichiometries due to their enhanced catalytic reactivity at T > T_ign and the resulting higher exothermicity. Following catalytic ignition, the wall temperatures eventually attain superadiabatic values due to the diffusional imbalance of hydrogen. Homogeneous chemistry considerably moderates the superadiabatic surface temperatures at 5 bar, as the gaseous combustion zone extends parallel to the channel wall and thus shields the catalyst surface from the hydrogen-rich channel core. Furthermore, gas-phase chemistry reduces the steady-state times and substantially increases the hydrogen conversion.     

燃烧系统的离散Boltzmann建模与模拟研究进展

燃烧系统的诸多模拟依托于流体建模, 离散Boltzmann方法(discrete Boltzmann method, DBM) 是近年来发展起来的一种新的流体介观建模方法. 本文简要评述DBM发展的两个方向——Navier-Stokes等偏微分方程的数值逼近解法和复杂系统的微介观动理学建模. 主要介绍在燃烧系统模拟方面DBM已有的工作、新近的思路、与传统流体建模的异同以及近期的研究成果. 本文重点传递的信息为: 作为复杂系统微介观动理学建模出现的DBM在模拟过程中同时给出“流动”及其相伴随的、关系最密切的那部分“热动”非平衡效应; 它为燃烧等复杂系统中各类非平衡行为的描述、非平衡信息的提取、非平衡程度的度量提供了一种简洁、有效的方法; 它所提供的热动非平衡测量量有两类: 一类是直接比较分布函数和平衡态分布函数的动理学矩关系得到的, 一类是来自于Chapman-Enskog多尺度分析给出的热传导和黏性项. 基于第二类DBM, 可以实现(燃烧等)一大类复杂流体系统的多尺度物理建模.     

A normalised residence time transport equation for the numerical simulation of combustion with high-temperature air

This study concerns the numerical simulation of turbulent non-premixed combustion in highly preheated air streams. One of the objectives is to settle an efficient computational procedure to proceed with the numerical simulation of large-scale industrial devices. It is also expected that the availability of such a computational framework may facilitate comprehensive sensitivity analyses as well as the development of mathematical models able to represent turbulence-chemistry interactions (TCI) in such conditions. Based on the salient physical ingredients that characterise scalar mixing, propagation, and self-ignition processes, a turbulent combustion modelling framework is thus introduced and applied to the numerical simulation of well-documented laboratory flames. In the corresponding geometries, the bulk flow velocities of the reactants streams can reach rather large values, which lead the flame to lift from the burner rim. Partially premixed flame edges thus stabilise the whole flame structure and the temperature of the oxidising stream can be increased by vitiation with burned gases so as to promote the corresponding flame-stabilisation processes. For sufficiently large values of the vitiated airstream temperature, self-ignition mechanisms may be triggered thus leading to a competition between mixing, propagation, and ignition processes. In this context, the ratio of the residence time to the self-ignition delay is thought to be a relevant variable to delineate the possible influence of ignition phenomena. Therefore, a modelled transport equation for this normalised residence time is considered. The performance of the corresponding modelling proposal is analysed with special emphasis placed on its ability to reproduce ‘memory’ or ‘lagrangian’ effects related to thermal aging processes. In this respect, it is noteworthy that the present set of computations makes use of tabulated quantities associated to (i) steady laminar one-dimensional diffusion flamelets, so as to describe the composition of combustion products, (ii) steady laminar one-dimensional premixed flamelets, to describe the flame brush propagation, and (iii) temporal evolution of zero-dimensional homogeneous mixtures to account for the possible occurrence of self-ignition phenomena. In particular, the tabulated self-ignition time value is used to evaluate the increase in the normalised residence time. Finally, two modelling parameters are put into evidence and studied through a detailed sensitivity analysis.     

Towards a classification of models for the numerical simulation of premixed combustion based on a generalized regime diagram

The inner structure, and the physical behaviour of turbulent premixed flames are usually described, and classified by means of the regime diagram introduced by Borghi and Peters. Thereby properties related to both the flame and the (turbulent) flow are considered. In this work a diagram valid for all physical regimes, comprising suitable requirements for laminar simulations, direct numerical simulation (DNS), large-eddy simulation (LES), and Reynolds averaging based numerical simulation (RANS) is proposed. In particular the diagram describes essential situations within the validity limits of the “Borghi, Peters diagram” which physical phenomena are resolved by the simulation, and which have to be modelled. This information is used for systematic classification of various models by suggesting specific models that are appropriate depending on the regime and numerical resolution, and may provide guidance for numerical simulation methods and model development in turbulent premixed combustion. This might help users as a guideline in choosing appropriate models for a given device, and numerical effort available. The regime diagram suggested by Pitsch and Duchamp de Lageneste, which includes DNS and LES by explicitely accounting for the numerical related variable filterwidth, emerges here as one of the special two-dimensional cases possible. In contrast to the generalized regime diagram, their diagram does not include laminar simulations, and RANS based considerations, while transition between wrinkled and corrugated flamelets is not clearly established.     

Direct numerical simulation of turbulence/radiation interaction in premixed combustion systems

An important fundamental issue in chemically reacting turbulent flows is turbulence/radiation interaction (TRI); TRI arises from highly nonlinear coupling between temperature and composition fluctuations. Here, a photon Monte Carlo method for the solution of the radiative transfer equation has been integrated into a turbulent combustion direct numerical simulation (DNS) code. DNS has been used to investigate TRI in a canonical configuration with systematic variations in optical thickness. The formulation allows for nongray gas properties, scattering, and general boundary treatments, although in this study, attention has been limited to gray radiation properties, no scattering, and black boundaries. Individual contributions to emission and absorption TRI have been isolated and quantified. Of particular interest are intermediate values of optical thickness where, for example, the smallest hydrodynamic and chemical scales are optically thin while the largest turbulence scales approach an optically thick behavior. In the configuration investigated, the temperature self-correlation contribution (emission) is primarily a function of the ratio of burned-gas temperature to unburned-gas temperature, and is the dominant contribution to TRI only in the optically thin limit. Even in the most optically thin case considered, the absorption coefficient–Planck function correlation and absorption coefficient–intensity correlation are not negligible. At intermediate values of optical thickness, contributions from all three correlations are significant.     

Experimental and numerical investigation of the hetero-/homogeneous combustion of lean propane/air mixtures over platinum

The pure heterogeneous and the coupled hetero-/homogeneous combustion of fuel-lean propane/air mixtures over platinum have been investigated at pressures 1 bar  p  7 bar, fuel-to-air equivalence ratios 0.23  φ  0.43, and catalytic wall temperatures 723 K  T_w  1286 K. Experiments were performed in an optically accessible catalytic channel-flow reactor and involved 1-D Raman measurements of major gas-phase species concentrations across the reactor boundary layer for the assessment of catalytic fuel conversion and planar laser induced fluorescence (LIF) of the OH radical for the determination of homogeneous ignition. Numerical predictions were carried out with a 2-D elliptic CFD code that included a one-step catalytic reaction for the total oxidation of propane on Pt, an elementary C₃ gas-phase chemical reaction mechanism, and detailed transport. A global catalytic reaction step valid over the entire pressure–temperature-equivalence ratio parameter range has been established, which revealed a p^0.75 dependence of the catalytic reactivity on pressure. The aforementioned global catalytic step was further coupled to a detailed gas-phase reaction mechanism in order to simulate homogeneous ignition characteristics in the channel-flow reactor. The predictions reproduced within 10% the measured homogeneous ignition distances at pressures p  5 bar, while at p = 7 bar the simulations overpredicted the measurements by 19%. The overall model performance suggests that the employed hetero-/homogeneous chemical reaction schemes are suitable for the design of propane-fueled catalytic microreactors.     

A three-dimensional Eulerian method for the numerical simulation of high-velocity impact problems

In the present paper, a three-dimensional （3D） Eulerian technique for the 3D numerical simulation of high-velocity impact problems is proposed. In the Eulerian framework, a complete 3D conservation element and solution element scheme for conservative hyperbolic governing equations with source terms is given. A modified ghost fluid method is proposed for the treatment of the boundary conditions. Numerical simulations of the Taylor bar problem and the ricochet phenomenon of a sphere impacting a plate target at an angle of 60~ are carried out. The numerical results are in good agreement with the corresponding experimental observations. It is proved that our computational technique is feasible for analyzing 3D high-velocity impact problems.     

The HyperCASL algorithm: A new approach to the numerical simulation of geophysical flows

We describe a major extension to the Contour-Advective Semi-Lagrangian (CASL) algorithm [D.G. Dritschel, M.H.P. Ambaum, A contour-advective semi-Lagrangian numerical algorithm for simulating fine-scale conservative dynamical fields, Quart. J. Roy. Meteorol. Soc. 123 (1997) 1097–1130; D.G. Dritschel, M.H.P. Ambaum, The diabatic contour advective semi-Lagrangian algorithm, Mon. Weather Rev. 134 (9) (2006) 2503–2514]. The extension, called ‘HyperCASL’ (HCASL), uses Lagrangian advection of material potential vorticity contours like CASL, but a Vortex-In-Cell (VIC) method for the treatment of diabatic forcing or damping. In this way, HyperCASL is fully Lagrangian regarding advection. A grid is used as in CASL to deal with ‘inversion’ (computing the velocity field from the potential vorticity field).     

Numerical simulation and validation of SI-CAI hybrid combustion in a CAI/HCCI gasoline engine

SI-CAI hybrid combustion, also known as spark-assisted compression ignition (SACI), is a promising concept to extend the operating range of CAI (Controlled Auto-Ignition) and achieve the smooth transition between spark ignition (SI) and CAI in the gasoline engine. In this study, a SI-CAI hybrid combustion model (HCM) has been constructed on the basis of the 3-Zones Extended Coherent Flame Model (ECFM3Z). An ignition model is included to initiate the ECFM3Z calculation and induce the flame propagation. In order to precisely depict the subsequent auto-ignition process of the unburned fuel and air mixture independently after the initiation of flame propagation, the tabulated chemistry concept is adopted to describe the auto-ignition chemistry. The methodology for extracting tabulated parameters from the chemical kinetics calculations is developed so that both cool flame reactions and main auto-ignition combustion can be well captured under a wider range of thermodynamic conditions. The SI-CAI hybrid combustion model (HCM) is then applied in the three-dimensional computational fluid dynamics (3-D CFD) engine simulation. The simulation results are compared with the experimental data obtained from a single cylinder VVA engine. The detailed analysis of the simulations demonstrates that the SI-CAI hybrid combustion process is characterised with the early flame propagation and subsequent multi-site auto-ignition around the main flame front, which is consistent with the optical results reported by other researchers. Besides, the systematic study of the in-cylinder condition reveals the influence mechanism of the early flame propagation on the subsequent auto-ignition.     

燃尽风率对新型W火焰锅炉主燃区气固流动特性的影响

针对采用偏心旋流二次风燃烧技术的300 MWe旋流燃烧器W火焰炉,借助1/10冷模试验台,通过三维激光颗粒动态分析仪测量研究了不同燃尽风率下其主燃区内气固流动特性。随着燃尽风率减小,拱下回流区内回流速度不断增加,且回流区尺寸不断增大。随着燃尽风率由25%减小到10%,在分级分区域,颗粒的最大竖直速度由2 m/s增大到4 m/s.燃尽风率由20.3%减小到10%,拱下回流区内气固两相竖直脉动速度明显增大,气固两相湍流强度将不断增大.在乏气和分级风区域,燃尽风率15%下最大颗粒体积流率是燃尽风率20.3%的2至2.7倍,拱上气流下冲深度明显增加.随着燃尽风率减小,下冲颗粒开始折转向上的位置被推迟,下炉膛空间利用率将不断增加.     

Numerical simulation of the transient multiphase field during plasma deposition manufacturing composite materials

A solid/liquid/gas unified model has been developed to investigate the gradient composition formation during the plasma deposition manufacturing (PDM) composite materials process. In this model, an enthalpy porosity model was applied to deal with the melting and solidification of the deposited layer, and a level-set approach was introduced to track the evolution of the free surface of the molten pool and the deposited layer. Moreover, complicated physical phenomena occurring at the liquid/gas interface, including forced convection heat loss, heat emission and plasma heat source, have been incorporated into the governing equations by source terms. In this study, the numerical experiment of nickel base alloy powder deposited on the medium steel substrate by PDM technique was implemented based on the staggered grid and SIMPLEC algorithm. Concentration gradient distribution of the solute material at the composite material interface, fluid flow and temperature distribution in the molten pool and the deposited layer have been investigated in detail. Supported by the National Natural Science Foundation of China (Grant No. 50474053) and the High Technology Research and Development Program of China (Grant No. 2007AA04Z142)     

氢原子传输及负氢离子产生全三维数值模拟研究

理论分析了负氢离子源中中性粒子传输特性及引出电极表面产生负氢离子(H^-)的物理过程, 研究了引出孔传输率对氢原子传输的影响,深入剖析了氢原子与不同属性导体壁碰撞以及碰撞后反射的物理情景.基于CHIPIC软件平台,成功研制了全三维 Particle-in-cell with Monte Carlo Collision 氢原子传输及负氢离子产生物理过程的模拟算法,并采用JAEA 10A负氢离子源进行模拟验证.模拟达到稳态后,氢原子平均能量约为0.57 eV, 且H原子呈现+Y漂移,当非均匀氢原子束轰击引出壁时,导致产生的负氢离子空间分布不均匀. 这些模拟结果都与文献符合,验证了算法的可靠性.