Spatial linear analysis of the flow in a solid rocket motor with burning walls

A theoretical investigation of the coupling between the core flow process and combustion sublayer process in a rocket chamber flow is presented. The focus of the investigation is on penetration of chamber disturbances into the sublayer and on the burn rate characteristic responses. Characteristic solutions to the local non-parallel instability problem supported by gasifying propellant in a double slab geometry are sought. In the fully decoupled limit the only characteristic solution response is a pressure response, thus comparisons between coupled and decoupled pressure responses are presented. The propellant is typified by a set of burning characteristics, and their effect on the coupling is explored. The goal of the analysis is to clarify the extent strand burning characteristics (i.e., in zero cross flow conditions) can affect burn rate perturbations leading to the phenomenon of erosive burning. The findings of this research agree with experimental studies as it regards the effect of burning parameters on erosive behavior. Change in erosive sensitivity with propellant characteristics is thus correlated with a change in the core-sublayer interaction.     

Modeling of multiphase combustion and deposit formation in a biomass-fed furnace

A comprehensive computational model for biomass combustion is presented, featuring a solid phase combustion model, a fluid dynamic model for the gas phase, and a solid particle transport and deposit formation model. The submodel developed to track particle trajectories is briefly outlined. The model is implemented on the Finite Element code XENIOS++, and a test case is considered of a furnace burning wooden biomass chips added with water and inert material; a dedicated flamelet library is worked out to model combustion. Results underline the capability of the code to predict combustion conditions and, in particular, the growth rates of deposits of different particle size over the furnace walls, as well as the most critical locations for particle deposition.     

炉内煤粉燃烧一维数学模型及其仿真

为了准确计算炉内煤粉的燃尽率,从研究煤粉粒子的燃烧机理入手,以炉膛内最复杂的燃烧器区域的煤粉燃烧过程为研究对象,通过合理简化煤粉中挥发分和焦炭的燃烧过程,建立了炉内煤粉燃烧沿高度方向上的一维宏观模型,在模型中考虑了煤粉燃烧过程中氧含量的变化,以单个煤粉颗粒燃烧的等密度模型为基础,通过多种煤粉粒径的燃烧过程反映煤粉燃烧的整体过程,推导出计算炉内煤粉燃尽率的显示公式,满足了实时仿真计算的要求。计算结果与实测数据和现有的文献相符,并对结果进行了分析。     

Interaction of a moving shock wave with a two-phase reacting medium

The flow field, that develops when a moving shock wave hits a two-phase medium of gas and particles, has a practical application to industrial accidents such as explosions at coal mine and in grain elevator and furthermore to solid propellant combustion in rocket engine. Therefore, a successful prediction of the thermo-fluid mechanical characteristics development of gas and particles is very crucial and imperative for the successful design and operation of rocket nozzles and energy conversion systems. This paper describes an interaction phenomenon when a moving shock wave hits a two-phase medium of gas and particles with/without chemical reaction. A particle-laden gas is considered to be located along a ramp so that numerical integration is accomplished from the tip of ramp for a finite period. For the numerical solution, a fully conservative unsteady implicit second order time-accurate sub-iteration method and the second order Total Variation Diminishing scheme are used with the finite volume method for gas phase. For particle phase, the Monotonic Upstream Schemes for Conservation Laws as well as the solution of the Riemann problem for the particle motion equations is also used together with the schemes above. Transient development of thermo-fluid mechanical characteristics is calculated and discussed by changing the particle mass density and particle specific heat. For the case of the reacting particle-laden gas flow, a carbon particle-laden oxygen gas is considered to be located along a ramp. The results are discussed by comparison with the cases of the pure gas and the inert particle-laden gas. Major results reveal that when the particle mass density is smaller, there is a stronger interaction between two phases so that the velocity and temperature differences between two phases more rapidly decrease. When the particle specific heat is varied, only a thermal effect is observed while the other effects are minor. The case with reacting particles yields significantly different results due to chemical reaction such that the gas density does not monotonously but rapidly decrease due to the slip line in the relaxation zone, while the pressure and temperature become higher in comparison with the non-reacting case. But the dynamic variation would be only secondary to the thermal one.     

城市污泥和煤混燃特性的热重分析法研究

利用热重分析法对某城市污泥和某煤种及其两者混合物的着火温度、活化能及综合燃烧特性等参数进行了研究。试验结果表明,混合试样和煤相比其活化性能得到了提高,着火温度提前,综合燃烧性能下降。在混燃过程中,煤和城市污泥基本上保持了各自的挥发分析出特性,煤的燃烧表现得更为明显。     

A model for erosive burning of homogeneous propellants

We examine the combustion response of a homogeneous solid propellant to an imposed velocity field, as a model for erosive burning. The imposed velocity field has its roots in a multiscale analysis of a planar or cylindrical rocket. We show that for certain realistic choices of the parameters both positive and negative erosion can take place. The underlying mechanism for erosive burning is flame stretch, induced by the imposed velocity field.     

燃煤循环流化床模型与试验研究

利用循环流化床内气－固两相流动等基础方面的研究成果,根据本文床内气固浓－淡流动模型,建立适用不同结构参数的循环流化床燃烧模型,考虑了床内气体、固体颗粒的返混、循环过程,以及煤燃烧、ＮＯ的生成和分解、颗粒磨损等因素。在循环流化床燃烧试验台上进行实验研究,模型仿真结果和实验数据吻合良好,表明气固两相浓－淡流动模型所建立的循环流化床燃烧系统模型可以正确地模拟循环流化床的燃烧过程。     

On the burning behavior of pulverized coal chars

A model that predicts the physical changes that pulverized coal char particles undergo during combustion has been developed. In the model, a burning particle is divided into a number of concentric annular volume elements. The mass loss rate, specific surface area, and apparent density in each volume element depend upon the local particle conditions, which vary as a consequence of the adsorbed oxygen and gas-phase oxygen concentration gradients inside the particle. The model predicts the particle's burning rate, temperature, diameter, apparent density, and specific surface area as combustion proceeds, given ambient conditions and initial char properties. A six-step heterogeneous reaction mechanism is used to describe carbon reactivity to oxygen. A distributed activation energy approach is used to account for the variation in desorption energies of adsorbed O-atoms on the carbonaceous surface. Model calculations support the three burning zones established for the oxidation of pulverized coal chars. The model indicates two types of zone II behavior, however. Under weak zone II burning conditions, constant-diameter burning occurs up to 30% to 50% conversion before burning commences with reductions in both size and apparent density. Under strong zone II conditions, particles burn with reductions in both size and apparent density after an initial short period (<2% conversion) of constant-diameter burning. Model predictions reveal that early in the oxidation process, there is mass loss at constant diameter under all zone II burning conditions. Such weak and strong burning behavior cannot be predicted with the commonly used power-law model for the mode of burning employing a single value for the burning mode parameter. Model calculations also reveal how specific surface area evolves when oxidation occurs in the zone II burning regime. Based on the calculated results, a surface area submodel that accounts for the effects of pore growth and coalescence during combustion under zone I conditions was modified to permit the characterization of the variations in specific surface area that occur during char conversion under zones II conditions. The modified surface area model is applicable to all burning regimes. Calculations also indicate that the particle's effectiveness factor varies during conversion under zone II burning conditions. With the adsorption/desorption mechanism employed, a near first-order Thiele modulus-effectiveness factor relationship is obeyed over the particle's lifetime.     

Modeling char oxidation behavior under Zone II burning conditions at elevated pressures

For accurate modeling of the coal combustion process at elevated pressures, account must be made for variations in char-particle structure. As pressure is increased, particle swelling increases during the devolatilization of certain bituminous coals, yielding a variety of char-particle structures, from uniform high-density particles to thin-walled non-uniform low-density particles having large internal void volumes. Since under Zone II burning conditions the char conversion rate depends upon the accessibility of the internal surfaces, the char structure plays a key role in determining particle burnout times. In our approach to characterize the impact of char structure on particle burning rates, effectiveness factors appropriate for thin-walled cenospherical particles and thick-walled particles having a few large cavities are defined and related to the effectiveness factor for uniform high-density particles that have no large voids, only a random distribution of pores having a mean pore size in the sub-micron range. For the uniform case, the Thiele modulus approach is used to account for Zone II type burning in which internal burning is limited by the combined effects of pore diffusion and the intrinsic chemical reactivity of the carbonaceous material. In the paper, the impact of having a variety of char structures in a mix of particles burning under Zone II burning conditions is demonstrated.     

百叶窗煤粉浓缩器阻力特性研究

水平浓缩煤粉燃烧器（HBC）可以同时实现煤粉的高效燃烧,防止结渣,稳定燃烧和低NOx排放,一种基于惯性力浓度的新型煤粉浓缩器－百叶窗式煤粉浓给器是实现水平浓缩煤粉燃烧的关键,成功的采用这种百叶窗式浓缩器要研究四个问题,阻力特性,气流和压力分配特性,浓缩特性和耐磨特性,本文着重研究结构参数对浓缩器阻力损失的影响,实验结果表明,随着百叶窗浓缩栅个数和间距的增加及浓缩栅倾角的增加,百叶窗浓缩器的阻力损失减小。     

Combustion of single coal particles in a jet

Fluidized bed combustion has attracted much interest in recent years, but there is very little data on the behavior of coal particles at these new conditions. Coal of much larger diameter (1–10 mm), much lower furnace temperatures (~850 °C), and different fluid mechanical conditions exist compared to pulverized coal furnaces. This paper presents experimental data on the behavior and combustion rates of individual coal particles aerodynamically suspended in a heated jet, to stimulate flow conditions in a fluidized bed.Tests of bituminous, sub-bituminous and lignite coals from 2 to 12 mm at jet temperatures of 705 and 816 °C in air and air diluted with equal parts of nitrogen were conducted. The ignition delay time varied from 2 to 44 sec. The devolatilization time extended up to 80 sec and was dependent mainly on particle size. The total burn time was independent of coal type and temperature, and varied as the square of the size and inversally with the oxygen concentration. The total turn time varied from 25 to 740 sec independently of coal type. The square law for the char burning rate was investigated.     

稳燃腔煤粉燃烧器对不同煤种燃烧稳定性的研究

本文利用三维粒子动态分析仪测量了稳燃腔煤粉燃烧器回流区的流场,并比较了钝体不同位置的回流区特征。实践说明通过调整钝体的位置及改变稳腔煤燃烧器的某些结构尺寸,可以极大的改善不同煤种的燃烧稳定性。     

水处理固体废物用作燃煤脱硫添加剂的实用性

水处理固体废物用作燃煤脱硫添加剂的机理及实验研究表明,水处理固体废物可以有效地催化脱硫反应,减缓高温下CaSo4分解速度,使燃煤脱硫过程的最佳脱硫温度更接近煤的燃烧温度,提高燃煤脱硫效率及钙利用率;其用作燃煤脱硫添加剂进行脱硫,实施系统及流程简单,既能减少水处理固体废物排放量,减轻对水体的污染,也能有效地减少燃煤SO2的排放量。     

Numerical Studies on Combustion Characteristics of Interacting Pulverized Coal Particles at Various Oxygen Concentration

The two-dimensional laminar combustion characteristics of coal particles at various oxygen concentration levels of a surrounding gas have been numerically investigated. The numerical simulations, which use the two-step global reaction model to account for the surrounding gas effect, show the detailed interaction among the inter-spaced particles, undergoing devolatilization and subsequent char burning. Several parametric studies, which include the effects of gas temperature (1700 K), oxygen concentration, and variation in geometrical arrangement of the particles on the volatile release rate and the char burning rate, have been carried out. To address the change in the geometrical arrangement effect, multiple particles are located at various inter-spacings of 4–20 particle radii in both streamwise and spanwise directions. The results for the case of multiple particles are compared with those for the case of a single particle. The comparison indicates that the shift to the multiple particle arrangement resulted in the substantial change of the combustion characteristics and that the volatile release rate of the interacting coal particles exhibits a strong dependency on the particle spacing. The char combustion rate is controlled by the level of oxygen concentration and gas composition near particles during combustion. The char combustion rate is highly dependent on the particle spacing at all oxygen levels. The correlations of the volatile release rate and the change in total mass of particles are also found.     

垃圾焚烧电厂飞灰在固定床中烧结的数值模拟

针对某垃圾焚烧飞灰固定床烧结无害化处理炉,建立了炉内流动、传热传质和燃烧的数值模型．利用Fluent软件对设计工况下的烧结炉进行数值模拟,采用多孔介质模型来计算炉内气体流动和压强分布,采用缩核模型来描述炉内物料的燃烧反应过程,得到了炉内气流速度场、温度场、气体组分分布以及烧结物料的温度分布．结果表明：鼓风进入风口0．2m后,速度变化平缓且分布较为均匀;物料的燃烧反应主要发生在风口中心线以上0．3～0．55m的区域内,物料下行至风口中心截面时已经燃烧完全;当掺煤量为12％、物料直径为16mm、过量空气系数为1．2时,炉内物料的温度为1138～1400K,完全可以满足无害化烧结工艺的要求．     

Flame stability and emission characteristics of propane-fueled flat-flame miniature combustor for ultra-micro gas turbines

An engineering model of a propane-fueled miniature combustor was developed for ultra-micro gas turbines. The combustion chamber had a diameter of 20 mm, height of 4 mm, and volume of 1.26 cm³. The flat-flame burning method was applied for lean-premixed propane–air combustion. To create the stagnation flow field for a specific flat-flame formation, a flat plate was set over the porous plate in the combustion chamber. A burning experiment was performed to evaluate the combustion characteristics. The flame stability limit was sufficiently wide to include the design operation conditions of an equivalence ratio of 0.55 and air mass flow rate of 0.15 g/s, and the dominant factors affecting the limit were clarified as the heat loss and velocity balance between the burning velocity and the premixture flow velocity at the porous plate. CO, total hydrocarbons (THC), and NO_x emission characteristics were established based on the burned gas temperatures in the combustion chamber and the temperature distribution in the combustor. At an air mass flow rate of less than 0.10 g/s, CO and THC emissions were more than 1000 ppm due to large heat loss. As the air mass flow rate increased, the heat loss decreased, but CO emissions remained large due to the short residence time in the combustion chamber. NO_x emission depended mainly on the burned gas temperature in the combustion chamber as well as on the residence time. To reduce emissions despite the short residence time, a platinum mesh was placed after the combustion chamber, which drastically decreased the CO emissions. The combustor performance was compared with that of other miniature combustors, and the results verified that the present combustor has suitable combustion characteristics for a UMGT, although the overall combustor size and heat loss need to be reduced.     

Numerical studies on the combustion properties of char particle clusters

Particle clustering is an important phenomenon in dense particle–gas two-phase flow. One of the key problems worth studying is the reacting properties of particle clusters in coal particle combustion process in the dense particle region. In this paper, a two-dimensional mathematical model for the char cluster combustion in airflow field is established. This char cluster consists of several individual particles. The comprehensive model includes mass, momentum, and energy conservation equations for both gas and particle phases. Detailed results regarding velocity vector, mass component, and temperature distributions inside and around the cluster are obtained. The micro-scale mass and heat transfer occurred inside and around the char cluster are revealed. By contrastively studying the stable combustion of char particle clusters consisting of different particles, the combustion properties of char clusters in various particle concentrations are presented and discussed.     

Burnout of pulverized biomass particles in large scale boiler – Single particle model approach

Burning of coal and biomass particles are studied and compared by measurements in an entrained flow reactor and by modelling. The results are applied to study the burning of pulverized biomass in a large scale utility boiler originally planned for coal. A simplified single particle approach, where the particle combustion model is coupled with one-dimensional equation of motion of the particle, is applied for the calculation of the burnout in the boiler. The particle size of biomass can be much larger than that of coal to reach complete burnout due to lower density and greater reactivity. The burner location and the trajectories of the particles might be optimised to maximise the residence time and burnout.