Influence of combustion parameters on NOx production in an industrial boiler

NO_x formation during the combustion process occurs mainly through the oxidation of nitrogen in the combustion air (thermal NO_x) and through oxidation of nitrogen with the fuel (prompt NO_x). The present study aims to investigate numerically the problem of NO_x pollution using a model furnace of an industrial boiler utilizing fuel gas. The importance of this problem is mainly due to its relation to the pollutants produced by large boiler furnaces used widely in thermal industrial plants. Governing conservation equations of mass, momentum and energy, and equations representing the transport of species concentrations, turbulence, combustion and radiation modeling in addition to NO modeling equations were solved together to present temperature and NO distribution inside the radiation and convection sections of the boiler. The boiler under investigation is a 160 MW, water-tube boiler, gas fired with natural gas and having two vertically aligned burners.The simulation study provided the NO distribution in the combustion chamber and in the exhaust gas at various operating conditions of fuel to air ratio with varying either the fuel or air mass flow rate, inlet air temperature and combustion primary air swirl angle. In particular, the simulation provided more insight on the correlation between the maximum furnace temperature and furnace average temperatures and the thermal NO concentration. The results have shown that the furnace average temperature and NO concentration decrease as the excess air factor λ increases for a given air mass flow rate. When considering a fixed value of mass flow rate of fuel, the results show that increasing λ results in a maximum value of thermal NO concentration at the exit of the boiler at λ = 1.2. As the combustion air temperature increases, furnace temperature increases and the thermal NO concentration increases sharply. The results also show that NO concentration at exit of the boiler exhibits a minimum value at around swirl angle of 45°.     

Contrast between steady and time-averaged unsteady combustion simulations

Time-averaged predictions from unsteady solutions of the two-dimensional Navier–Stokes equations are contrasted with Reynolds-averaged results for a reacting flow problem in a high pressure combustor. The goal is to determine whether the two-dimensional unsteady approximation can be useful as an engineering analysis in problems for which time-averaged quantities are of primary interest. The conditions are taken from an experiment in which non-premixed gaseous oxygen and hydrogen were injected into a combustion chamber through coaxial channels. The resulting flowfield is dominated by a large recirculation zone arising from the back-step created by the injector. The results of steady and time-averaged, unsteady solutions are strikingly different. The unsteady simulation produces strong unsteady structures whose time-averaged results lead to a much wider flame zone, a different recirculation zone structure, and a substantially different wall heat flux than those obtained with a steady RANS procedure. The time-averaged calculations yield the correct combustor chamber pressure and compare considerably more favorably with heat flux measurements than do the RANS results. The two-dimensional approximation, however, overstates the unsteady vortex roll up and precludes large scale mixing across the axis of symmetry, thereby giving deficient predictions near the centerline. Overall, the present results indicate that capturing large-scale unsteady characteristics can provide more accurate predictions of recirculation dominated reacting flows and suggest that two-dimensional, time-averaged solutions represent a potentially useful engineering tool for problems of this nature while also serving as a precursor for full three-dimensional simulations.     

基于粒子系统的湍流燃烧火焰的可视化研究

引入粒子系统作为可视化建模的基本方法，建立了包含火焰粒子子模型、湍流流场子模型和燃料属性子模型的湍流燃烧火焰可视化模型．通过将温度场与湍流流场的有机结合及燃烧室内网格体单元的颜色与透明度的显示，实现了湍流燃烧火焰的三维动态传播可视化．最后，结合实例给出了火焰效果图，体现了湍流涡对火焰前锋面的影响．     

某型燃气轮机等离子强化燃烧三维仿真研究

为对某型燃气轮机燃烧室性能进行预估,以提供燃烧室优化设计和试验定型的理论依据,对某型燃气轮机燃烧室等离子强化燃烧流场进行了三维仿真。网格在曲线贴体坐标系下进行非结构化划分,湍流模型采用RNG模型,压力一速度耦合的求解采用SIMPLEC算法,得到了燃烧室流场的点火时间、火焰筒壁面温度及流场温度分布、速度分布和物质分布等情况。仿真结果表明,某型燃气轮机的设计基本满足性能要求,但存在一些问题需要在进一步的工作中改进提高。     

合成气非预混燃烧的数值模拟

针对合成气非预混火焰结构开展数值模拟和试验验证,分析天然气改烧合成气后燃烧特性的变化规律.结果表明,大涡模拟(Large Eddy Simulation,LES)在速度分布和温度分布的预测中与试验结果比较吻合,而在对燃烧过程产物(如OH自由基)预测中则与试验结果有所差异.采用数值模拟与试验测量相结合的方法,探讨和分析合成气燃烧特性的变化规律:与天然气火焰相比,合成气燃烧时高温区域更大,火焰稳定性较好;随着当量比提高,燃烧室热负荷不断增大,同时最高回流速度增大,火焰根部受到压缩,逐渐呈现出推举火焰特征.     

Comparison of fire temperature and fractional area modeled from SWIR, MIR, and TIR multispectral and SWIR hyperspectral airborne data

Spectral mixture modeling has previously been used to retrieve fire temperature and fractional area from multiband radiance data containing emitted radiance from fires. While this type of temperature modeling has potential for improving understanding of fire behavior and emissions, modeled temperature and fractional area may depend on the wavelength region used for modeling. Using airborne hyperspectral (Airborne Visible Infrared Imaging Spectrometer; AVIRIS) and multispectral (MODIS/ASTER Airborne Simulator; MASTER) data acquired simultaneously over the 2008 Indians Fire in California, we examined changes in modeled fire temperature and fractional area that occurred when input wavelength regions were varied. Temperature and fractional area modeled from multiple MASTER runs were directly compared. Incompatible spatial resolutions prevented direct comparison of the AVIRIS and MASTER model runs, so total area modeled at each temperature was used to indirectly compare temperature and fractional area retrieved from these two sensors. AVIRIS and MASTER model runs using shortwave infrared (SWIR) bands produced consistent fire temperatures and fractional areas when modeled temperatures exceeded 800 K. Temperatures and fire fractional areas were poorly correlated for temperatures below 800 K and when the SWIR bands were excluded as model inputs. The single temperature blackbody assumption commonly used in mixing model retrieval of fire temperature is potentially useful for modeling higher temperature fires, but is likely not valid for lower temperature smoldering combustion due to mixed radiance from multiple fuel elements combusting at different temperatures. SWIR data contain limited emitted radiance from combustion at lower temperatures, and are thus essential for consistent modeling of fire temperature and fractional area at higher fire temperatures.     

Fuel combustion test in constant volume combustion chamber with built-in adaptor

Combustion tests of pre-mixture of methane and air in constant volume combustion chamber(CVCC) have been carried out by means of flame propagation photo and gas pressure measurement,the effects of CVCC body temperature,intake pressure of pre-mixture of methane and air,equivalence ratio and location of the built-in adaptor have been investigated.The whole combustion chamber can be divided into two parts,i.e.the upper combustion chamber and the lower combustion chamber,by the built-in adaptor with through hol...     

逼真可控的火焰动画框架设计与实现

提出一个通用的火焰模拟框架,通过前处理、火焰模拟和后处理三个阶段生成满足用户需求且真实感强的火焰动画。针对该模拟框架,给出一种有效解决方案。根据火焰的两相流特性以及流体湍流特性,提出火焰的湍流模型,在有限计算资源条件下生成细节丰富的高真实感火焰动画。实验结果表明,所提出的火焰模拟框架切实可行,能逼真模拟基本火焰现象以及在复杂曲线、曲面和蔓延规则约束下的火焰动画。     

Electro-thermal analysis of an embedded boron diffused microheater for thruster applications

One of the important design criteria of micropropulsion systems in particular VLM is the type of microheater, its layout and placement with a view to achieve uniform heating of propellant, fast heat transfer efficiency with minimum input power. Thrust produced by microthruster not only depends on the structural geometry of the thruster and propellant flow rate, but also on the chamber temperature to produce super saturated dry stream at the exit nozzle. Detailed design of microheater in thermal and electrical domains using co-solvers available in MEMS software tools along with material’s thermal property, temperature dependence of electrical resistivity and thermal conductivity have been considered in the present work to achieve precise modeling and experimental accuracy of heater operation. The chamber temperature was analytically calculated and subsequently the required resistance and power were estimated. The boron diffused microheaters of meanderline configuration in silicon substrate has been designed and its finite element based electro-thermal modeling was employed to predict the heater characteristics. The variation of microheater temperature with time, applied voltage and along chamber length has been determined from the modeling. Subsequently the designed microheater was realized on silicon wafer by lithography and boron diffusion process and its detailed testing was evaluated. It was found that boron diffused resistor of 820 Ω can generate 405 K temperature with applied input power 2.4 W. Finally the simulated results were validated by experimental data.     

Numerical simulation of forest fire propagation based on modified two-dimensional model

A modified two-dimensional two-phase mathematical model of forest wildfires propagation is considered. The model is based on the averaging of three-dimensional equations of two-phase medium over the height of the forest fuel (FF) layer and it includes the (k?ε)-turbulence model with additional turbulence production and dissipation terms in the forest layer and the Eddy Break-up Model for the combustion rate in the gas phase. The developed model can be used to carry out numerical simulation of the forest fire-front propagation under the conditions of a heterogeneous FF distribution, the presence of obstacles to the fire propagation, and the wind effects. This model can be used for real-time computation of the fire propagation, for expert assessments of emergency situations, and for assessments of the damage caused by forest fires.     

固体火箭冲压发动机三维数值计算

利用FLUENT中的概率密度函数非预混模型对固体火箭冲压发动机补燃室内的气相湍流燃烧进行数值模拟.主要目的是解决在固体火箭冲压发动机含镁铝推进剂的补燃室中,存在上百种中间及最终产物.复杂的反应机理使采用有限反应速率模型难以模拟补燃室中复杂的湍流燃烧的问题.模拟结果有助于提高对固体火箭冲压发动机补燃室内部流场流动的了解.模拟结果表明:补燃室内发生着复杂的三维化学反应流动,存在对掺混燃烧有重要影响的头部回流和轴向涡流.补燃室内温度分布与空气与燃气的掺混、燃烧及流动状态有密切关系.提高空燃比,可增强补燃室中燃气的回流和轴向涡流强度,加大掺混力度.提高燃烧效率.     

Large Eddy Simulation of a turbulent non-premixed propane-air reacting flame in a cylindrical combustor

Large Eddy Simulation (LES) is applied to investigate the turbulent non-premixed combustion flow, including species concentrations and temperature, in a cylindrical combustor. Gaseous propane (C₃H₈) is injected through a circular nozzle which is attached at the centre of the combustor inlet. Preheated air with a temperature of 773 K is supplied through the annulus surrounding of this fuel nozzle. In LES a spatial filtering is applied to the governing equations to separate the flow field into large-scale and small-scale eddies. The large-scale eddies which carry most of the turbulent energy are resolved explicitly, while the unresolved small-scale eddies are modelled using the Smagorinsky model with C_s = 0.1 as well as dynamically calibrated C_s. The filtered values of the species mass fraction, temperature and density, which are the functions of the mixture fraction (conserved scalar), are determined by integration over a beta probability density function (β-PDF). The computational results are compared with those of the experimental investigation conducted by Nishida and Mukohara [1]. According to this experiment, the overall equivalence ratio of 0.6, which is calculated from the ratio of the air flow rate supplied to the combustion chamber to that of the stoichiometric reaction, is kept constant so that the turbulent combustion at the fuel nozzle exit starts under the fuel-rich conditions.     

基于图像特征的火焰识别与检测技术研究

提出了一种基于火焰图像特征的火灾火焰检测及识别算法。将火焰颜色、变化率作为火灾火焰检测与识别的判据,在火焰颜色模型和变化率的基础上引入火焰尖角以及圆形度对火灾火焰图像进行进一步判定。实验结果表明,该算法能有效提高火灾火焰检测与识别的准确率,排除干扰源。     

CO余热锅炉瓦斯燃烧器改造及仿真研究

中石化某分公司催化裂化装置CO余热锅炉运行过程中,瓦斯燃烧器燃气空气混合不充分,火焰温度分布不均匀,燃烧效果差.为解决此问题,分析了瓦斯燃烧器的工作特点及结构形式,对燃烧器的配风方式、气流混合原理及稳燃机理进行了研究,并对燃气系统进行了核算,提出了燃烧器改造方案,最后运用计算流体力学(CFD)软件FLUENT,对改造前后的燃烧器内的燃烧情况进行了模拟.仿真结果表明,改造后的燃烧器火焰温度分布均匀,燃烧完全.燃烧器改造方案是合理可行的.     

High-energy density miniature thermoelectric generator using catalytic combustion

This paper describes the components and system of a thermoelectric (TE) generator with a catalytic butane combustor. The combustion chamber with a size of 8 mm/spl times/8 mm/spl times/0.4 mm is etched in a 0.65-mm-thick silicon substrate, and bonded to both sides of a 0.77-mm-thick glass substrate with a thin-film ignition heater. A set of 34 couples of BiTe TE elements, each with a size of 0.65 mm/spl times/0.65 mm/spl times/2 mm, are directly bonded to both sides of the combustor. The combustor without the TE modules was tested using butane as fuel, and self-sustaining combustion and electrical ignition were achieved. Also, nearly 100% combustion efficiency and a uniform temperature distribution were confirmed by gas chromatography and infrared thermoimaging, respectively. When the TE modules were attached to the combustor, however, butane combustion was impossible. The characteristics of TE generation were measured using hydrogen as fuel. When a theoretical combustion power was 6.6 W, the maximum output power of 184 mW was obtained with a load of 5.68 /spl Omega/. The total efficiency in this experiment was 2.8% (184 mW/6.6 W).     

A comprehensive Lagrangian flame–kernel model to predict ignition in SI engines

A Lagrangian model to predict the first stages of the combustion process in SI engines, when the size of flame kernel is small compared with the mesh size, and flame development is influenced by heat transfer from the spark, local flow, turbulence and air/fuel mixture distribution is presented. The spark channel is initially represented by a set of Lagrangian particles that are convected by the mean flow. Flame kernels are launched locally for all the particles satisfying an ignition criterion based on the local Karlovitz number. For each of them, equations of energy and mass are solved accounting for electrical power transferred from the electrical circuit, local turbulence and flame speed. The proposed model has been validated with experimental data provided by Herweg et al.; a computational mesh reproducing the geometrical details of the optical, pre-chamber SI engine was built, including the electrodes. Initially, cold-flow simulations were carried out to verify the validity of the computed flow-field and turbulent distribution at ignition time. Then, the combustion process was simulated accounting for the effects of different engine speeds, air/fuel ratio and spark-plug position. Encouraging results were achieved for a wide range of operating conditions.     

RBF neural network inferential sensor for process emission monitoring

Inferential sensing, or soft sensing, gained popularity in recent years as an alternative to continuous emission monitoring systems because of its simplicity, reliability, and cost effectiveness as compared to analogous hardware sensors. In this paper we address the problem of NO_x emission using a model of furnace of an industrial boiler, and propose a neural network structure for high performance prediction of NO_x as well as O₂. The studied boiler is 160 MW, gas fired with natural gas, water-tube boiler, having two vertically aligned burners. The boiler model is a 3D problem that involves turbulence, combustion, radiation in addition to NO_x modeling. The 3D computational fluid dynamic model is developed using Fluent simulation package. The model provides calculations of the 3D temperature distribution as well as the rate of formation of the NO_x pollutant, enabling a better understanding on how and where NO_x are produced. The boiler was simulated under various operating conditions. The generated data is then used for initial development and assessment of neural network soft sensors for emission prediction based on the conventional process variable measurements. The performance of the proposed soft sensor is then evaluated using actual data from an industrial boiler. The developed soft sensor achieves comparable accuracy to the continuous emission monitor analyzer, however, with substantial reduction in the cost of equipment and maintenance.     

Large-Eddy Simulation of double-row compound-angle film cooling: Setup and validation

Film cooling is an important technique allowing to increase the thermal efficiency of gas turbines. By blowing cool air through an array of small holes in the turbine blades a thin fluid film is set up shielding the blades from the hot gas arriving from the combustion chamber.This work presents a Large-Eddy Simulation of a particular film-cooling configuration known to provide a high level of effectiveness. It incorporates spanwise rows of holes which, by pairwise combination, generate a so-called anti-kidney-vortex. The simulation setup employs the Navier–Stokes code NSMB for compressible flow including the Approximate Deconvolution Model for subgrid turbulence modeling, the Synthetic-Eddy Method for turbulent inflow generation and reflection-reducing outflow boundary conditions. The setup is first validated by simulations of standard flat-plate turbulent boundary-layer flow without blowing. Results of the film-cooling simulation are then compared with related experimental data. They show reasonable agreement of the cooling effectiveness and temperature distribution, thus confirming the validity of the simulation approach which will be used in future studies of film cooling.     

石灰窑温度智能控制系统设计

提出了一种石灰窑燃烧室温度智能控制系统的设计方案。该系统以下燃烧室温度为控制目标,以煤气流量为调节对象,通过模糊神经网络动态调整煤气流量设定值,采用粒子群优化算法对模糊神经网络的参数进行寻优,并采用最小二乘支持向量机逆控制系统对煤气流量进行跟踪控制,实现了对石灰窑下燃烧室温度的精确控制。仿真结果表明,该系统响应速度快,稳定性好,具有很好的抗干扰性和鲁棒性。     

Flowfield initialization and approach to stationary conditions in unsteady combustion simulations

The initial transient leading to stationary conditions in unsteady combustion simulations is investigated by considering flow establishment in model combustors. Quiescent initial conditions with the chamber initially filled with an inert, hot gas are used to provide physically realistic starting conditions and robust, reliable combustion initiation. Transient processes are visualized by using a distinct initial fluid in the combustor whose concentration is tracked as it is expelled. The duration of the transient is shown to be dependent on the characteristic turn-over time for recirculation zones and the time for the chamber pressure to reach steady conditions. Substantial changes in the initial condition did not materially affect the length of the transient. Different combustor geometries changed the ratio of pressure equilibration time and species replenishment time, but did not have a major effect on the overall duration of the transient. Representative comparisons of the time-averaged, stationary results with experiment are presented to document the computations.