A USM turbulence-chemistry model for simulating NOx formation in turbulent combustion

A unified second-order moment (USM) turbulence-chemistry model for simulating NO_x formation in turbulent combustion is proposed. All the correlations, including the correlation of the reaction-rate coefficient fluctuation with the concentration fluctuation, are closed by the transport equations in the same form. This model abandons the series expansion approximation of the exponential term or the approximation of using a product of two single-variable PDFs instead of a joint PDF. The proposed model is used to simulate methane-air jet diffusion combustion and NO_x formation. The combustion prediction results are compared with those using the EBU-Arrhenius model and other two versions of the second-order moment model. The NO_x prediction results are compared with those using the pure presumed PDF model. Validation of predictions using the experimental data given by the Sandia National Laboratory, USA indicates that the proposed model gives better results than other models, and it is much economical than other refined models.     

Simulation of swirling coal combustion using a full two-fluid model and an AUSM turbulence-chemistry model

A full two-fluid model of reacting gas-particle flows with an algebraic unified second-order moment turbulence-chemistry model for the turbulent reaction rate of NO formation are used to simulate swirling coal combustion. The sub-models are the k-ε-k_p two-phase turbulence model, the EBU-Arrhenius volatile and CO combustion model, the six-flux radiation model, coal devolatilization model and char combustion model. The prediction results are in good agreement with the experimental results taken from references.     

Large-eddy simulation of swirling gas-particle flows using a USM two-phase SGS stress model

In large-eddy simulations (LES) of gas-particle flows most investigators use single-phase subgrid scale (SGS) stress models. The Interaction between the two-phase SGS stresses is not fully taken into account. In this paper, a unified second-order moment (USM) two-phase SGS stress model for the LES of gas-particle flows is proposed, in which the interaction between the two-phase SGS stresses and anisotropy of the two-phase SGS stresses is fully taken into account. The proposed model is used in the LES of swirling gas-particle flows, together with RANS modeling using the USM two-phase turbulence model. The instantaneous results exhibit the multiple recirculating gas flow structure similar to that of the single-phase swirling flows, but the particle flow structure shows less vortices. The two-phase time-averaged velocities and RMS fluctuation velocities predicted by both LES-USM and RANS-USM models are almost the same and are in good agreement with the experimental results. However, for two-phase RMS fluctuation velocities, the LES-USM results are somewhat better than the RANS-USM results.     

Simulation of turbulent combustion and NO formation in a swirl combustor

A presumed probability density function (PDF) model for temperature fluctuation is proposed and formulated in this paper. It incorporates a two-step reaction mechanism for propane combustion and the thermal and prompt NO formation mechanisms. The present model, together with a new algebraic Reynolds stress model (ASM), is employed to simulate the turbulent combustion and NO formation in a swirl combustor. The calculated propane, carbon monoxide, and carbon dioxide concentrations agree with the measurement. The calculated gas temperature and oxygen and NO concentrations are in general agreement with the measured data. The simulated results show that NO forms mainly in the upstream region of the combustor. The flue gas recirculation effectively abates the nitrogen oxides (NO_x) emission in the combustor.     

水蒸气对oxy-coal燃烧中NO生成的影响

为了研究H₂O对燃料N向NO转化各个阶段的影响,采用可沿程取样的沉降炉反应器,研究了一种烟煤在1273 K温度下,O₂/N₂、O₂/CO₂以及O₂/CO₂/H₂O气氛下燃烧的燃尽与NO生成的情况.并通过在停留时间为0.2、0.3、0.5、1.1 s与O₂浓度为5%、21%及30%情况下的实验来研究H₂O对NO生成的影响.实验结果表明,相对于O₂/CO₂气氛,H₂O的添加抑制了NO的生成,且该影响主要集中在燃烧初期挥发分N的氧化过程.随着O₂浓度的增加,H₂O添加对oxy-coal燃烧方式下NO生成影响加大.     

氨气/甲烷贫预混旋转湍流火焰稳定性及NO生成

为了研究氨气/甲烷掺混燃气在贫预混旋转湍流状态下的火焰稳定性及NO的排放特性,设计建造了一个可视化的旋转湍流燃烧装置,开展了一系列的实验测量研究。研究表明：随着当量比增大,氨气火焰稳定燃烧的范围有所扩大,但当氨气掺混比大于0.60时火焰出现上下振荡现象,继续增加将导致火焰吹熄;NO的排放水平随当量比增加而提高;但在相同的当量比下,NO的排放随氨气掺混比的增加先升高再下降。此外,分别采用化学反应器网络（CRN）方法和一维层流预混火焰计算方法,对相应的火焰状态进行了数值计算分析,虽然计算结果与实验结果误差较大,但其预测的NO排放特性随氨气掺混比、当量比的变化趋势是一致的,对三者之间误差的来源进行了分析。     

氨气/甲烷贫预混旋转湍流火焰稳定性及NO生成

为了研究氨气/甲烷掺混燃气在贫预混旋转湍流状态下的火焰稳定性及NO的排放特性,设计建造了一个可视化的旋转湍流燃烧装置,开展了一系列的实验测量研究。研究表明：随着当量比增大,氨气火焰稳定燃烧的范围有所扩大,但当氨气掺混比大于0.60时火焰出现上下振荡现象,继续增加将导致火焰吹熄;NO的排放水平随当量比增加而提高;但在相同的当量比下,NO的排放随氨气掺混比的增加先升高再下降。此外,分别采用化学反应器网络（CRN）方法和一维层流预混火焰计算方法,对相应的火焰状态进行了数值计算分析,虽然计算结果与实验结果误差较大,但其预测的NO排放特性随氨气掺混比、当量比的变化趋势是一致的,对三者之间误差的来源进行了分析。     

燃料环缝进口的丙烷-空气旋流扩散燃烧的LES-EBU和RANS-SOM模拟的比较

用Smagorinsky-Lilly亚网格尺度湍流模型和EBU亚网格尺度湍流燃烧模型,对燃料环缝进口的丙烷-空气旋流湍流燃烧进行了大涡模拟（LES）,并且用二阶矩（SOM）统观湍流燃烧模型进行了RANS模拟,将模拟所得统计平均的热态三个方向速度,湍流度,温度,丙烷、氧和CO₂浓度分布和详细的实验数据对比.结果表明,LES-EBU大涡模拟的统计结果不如RANS-SOM模拟结果.因此,EBU亚网格尺度湍流燃烧模型有待改进.RANS-SOM模型可以进一步在预报工程中的旋流扩散燃烧中应用.     

模拟分解炉中煤焦燃烧生成NO的特性

对国内水泥行业中几种煤焦在模拟的分解炉中NO生成特性进行了研究,考察了煤种、温度、生料对焦炭氮的释放特性的影响,探讨了焦炭氮在工业分解炉中的释放机理.研究结果表明,不同煤种的NO生成特性有较大的不同;温度对NO的生成特性也有明显的影响,基本随温度的升高而加大,但有时也有下降现象;生料的加入大大加速了NO的生成速率和转化率,表明生料对NO生成有显著的催化作用.工业分解炉中产生的NO主要来源于燃料NO,焦炭氮转化为NO的概率主要有两个互相竞争的反应决定：包括N的氧化反应和NO的还原反应.     

Simulation of coal combustion by AUSM turbulence-chemistry char combustion model and a full two-fluid model

An algebraic unified second-order moment (AUSM) turbulence-chemistry model of char combustion is introduced in this paper, to calculate the effect of particle temperature fluctuation on char combustion. The AUSM model is used to simulate gas-particle flows, in coal combustion in a pulverized coal combustor, together with a full two-fluid model for reacting gas-particle flows and coal combustion, including the sub-models as the k-ε-k_p two-phase turbulence model, the EBU-Arrhenius volatile and CO combustion model, and the six-flux radiation model. A new method for calculating particle mass flow rate is also used in this model to correct particle outflow rate and mass flow rate for inside sections, which can obey the principle of mass conservation for the particle phase and can also speed up the iterating convergence of the computation procedure effectively. The simulation results indicate that, the AUSM char combustion model is more preferable to the old char combustion model, since the later totally eliminate the influence of particle temperature fluctuation on char combustion rate.     

生物质燃烧NO生成的影响因素实验研究

以稻壳、木屑、甘蔗渣和秸秆等4种典型生物质颗粒为研究对象,在自主设计的十字形管式炉内,研究了燃料种类、温度、氧量和气氛因素对NO生成的影响.实验结果表明:燃料含N量越高,NO生成总量亦越高,最高可达0.9848 mg·g-1,而此时燃料N向NO的转化率是下降的,仅为7.18％;NO生成总量随着温度升高而升高,但不同生物...     

Conversion of volatile-nitrogen and char-nitrogen to NO during combustion

The design of low emission combustion chambers using low NO_x strategies involving staged burning or stratified combustion requires a detailed understanding of the combustion processes of the fuel volatiles and char burning. In this paper some aspects of the combustion of coal-volatiles and char are considered. The extreme cases of volatile combustion, namely premixed and diffusive burning are examined in order to consider the range of NO_x reduction options available to the combustion chamber designer. A similar set of situations is examined for char burning and the release of the fuel-nitrogen to form NO.

The implications of the processes are considered in two practical applications, those of the high temperature combustion found in pulverised coal burning and in a lower temperature regime of the conditions under fluidised bed combustion. In the case of pulverised coal flame the degree of mixedness of the volatiles played a dominant part in determining the extent of NO formation whilst the role of char-nitrogen is only to form NO and NO reduction is limited because of the short residence time and low char concentrations at the end of the reaction zone. In a circulating fluidised bed combustor it was concluded that a different situation can arise. If the bed is sufficiently large enough to give a residence time of several seconds, then the NO initially formed in the fluidised bed is reduced by the carbon in the top of the bed and the riser under steady state conditions and its concentration at the exit can be estimated by equilibrium calculations.     

A second-order-moment–Monte-Carlo model for simulating swirling gas–particle flows

A second-order moment (SOM) gas-phase turbulence model, combined with a Monte-Carlo (MC) simulation of stochastic particle motion using Langevin equation to simulate the gas velocity seen by particles, is called an SOM–MC two-phase turbulence model. The SOM–MC model was applied to simulate swirling gas–particle flows with a swirl number of 0.47. The prediction results are compared with the PDPA measurement data and those predicted using the Langevin-closed unified second-order moment (LUSM) model. The comparison shows that both models give the predicted time-averaged flow field of particle phase in general agreement with those measured, and there is only slight difference between the prediction results using these two models. In the near-inlet region, the SOM-MC model gives a more reasonable distribution of particle axial velocity with reverse flows due to free of particle numerical diffusion, but it needs much longer computation time. Both models underpredict the gas and particle fluctuation velocities, compared with those measured. This is possibly caused by the particle–wall and particle–particle interaction in the near-wall region, and the effect of particles on dissipation of gas turbulence, which is not taken into account in both models.     

简化的Solomon热解模型和旋流煤粉燃烧NO生成的数值模拟

提出一种简化的Solomon热解模型, 用于模拟煤粉燃烧NO生成数值模拟中HCN的释放.用纯双流体模型、k-ε-k_p两相湍流模型、EBU-Arrhenius燃烧模型、六热流辐射模型、双方程热解模型、简化的Solomon热解模型以及NO生成湍流反应二阶矩代数模型对旋流煤粉燃烧器内两相流动、煤粉燃烧、HCN释放以及NO生成进行了数值模拟.模拟结果与文献中实验结果的对比表明,基于简化Solomon热解模型的HCN释放模型预报结果比基于双方程热解模型的HCN释放模型预报结果好.     

The effect of the mixing characters of primary and secondary air on NOx formation in a swirling pulverized coal flame

The characteristics of burning pulverized coal motion and the combustion process are investigated by numerical simulation to analyze the relations between the NO_x formation and the gas flow and burning pulverized coal moving in a swirl pulverized coal burner. The depth of particle penetration into the IRZ L_dav and the effective time of IRZ τ_eav are obtained under three swirl numbers s=0.5, 1.03 and 1.2, respectively. The results show that the minimum outlet NO_x emission appears at the maximum L_dav and τ_eav and the minimum outlet NO emission, the maximum L_dav and the maximum τ_eav occur at the same time (s=1.03). It is also found that, the diameter of pulverized coal first increases, but then decreases the particle residence time τ_int in IRZ.     

外二次旋流风对旋流煤粉燃烧及NO生成的影响

对旋流煤粉火焰在两种分级进风的情况下用PIV（particle image velocimetry）测量了燃烧室内的速度分布,研究了湍流拟序结构对旋流火焰的燃烧特性及NO排放的影响。燃烧室进口附近,当外二次风率较大时,其拟序结构沿横向的扩散较早,大量小颗粒被裹入涡结构中参加反应,因此中心区域温度较高;外二次风率较小时,一次风外侧的涡结构使得更多的大颗粒在惯性离心力作用下运动到了近壁区域,因此近壁区温度更高。在本文的实验范围内,保持其他条件不变,增加外二次旋流风,有利于剪切层纵向涡结构的形成,也有利于横向涡的形成,促进了小颗粒煤粒燃烧和大颗粒煤粒挥发分析出,从而促进NO的提前生成以及燃烧中间产物对NO的还原,最终减少NO的生成。     

煤焦燃烧生成NO的特性研究

对小屯煤焦中氮的存在形态进行了X射线光电子能谱(XPS)分析。并对小屯煤焦在模拟分解炉中NO释放特性进行了实验研究,考察了气氛、温度、生料对焦炭氮释放特性的影响。研究结果表明:小屯煤焦中氮主要以吡咯五元环形式存在;氧气体积分数对NO的生成速率有明显影响;在无生料催化下,生成量基本随温度的升高而加大,但是,当生料存在时,生成量随温度的升高而减少;生料的加入大大加速了NO的生成速率和生成量,表明生料对NO生成有显著的催化作用。焦炭氮转化为NO的几率主要由2个互相竞争的反应决定:包括N的氧化反应和NO的还原反应。     

A parametric study on natural gas fueled HCCI combustion engine using a multi-zone combustion model

Homogenous Charge Combustion Ignition (HCCI) is a good method for higher efficiency and to reduce NOx and particulate matter simultaneously in comparison to conventional internal combustion engines. In HCCI engines, there is no direct control method for auto ignition time. A common way to indirectly control the ignition timing in HCCI combustion engines is varying engine’s parameters which can affect the combustion. In this work, a parametric study on natural gas HCCI combustion is conducted in order to identify the effect of inlet temperature and pressure, compression ratio, equivalence ratio and engine speed on combustion and engine performance parameters. In this paper, two kinds of parameters will be discussed. First, in-cylinder pressure diagrams and variation of start of combustion which are combustion parameters will be presented and then the second category, indicated mean effective pressure and thermal efficiency which are performance parameters will be studied. A six zone model coupled with detailed chemical kinetics code is used to simulate HCCI combustion. Both heat and mass transfer was considered in the modeling procedure. Results revealed that among the considered parameters, the equivalence ratio and inlet pressure are the most valuable parameters which can improve the combustion and performance characteristics of the HCCI engine.     

湍流非预混燃烧数值模拟的代数二阶矩模型

湍流燃烧数值模拟是研究燃烧的一种重要手段,采用的湍流燃烧模型是否恰当直接影响最终结果的准确性。在湍流燃烧中,化学反应速率不仅取决于当地的组分浓度和温度,而且与组分的湍流脉动也有密切关系。通过对湍流燃烧模型进行探讨,发现代数二阶矩模型（ASOM）能综合考虑湍流和反应动力学因素的影响,而且比其他复杂的模型简单。研究将组分混合速率对化学反应速率的影响在一个修正的代数二阶矩模型（RASOM）中进行考虑,更准确地计算出化学反应速率。为了验证模型的准确性,RASOM模型被应用到Sandia实验室测量的甲烷-空气非预混燃烧（Flame-D）的数值模拟中。模拟得到的结果与实验结果以及修正的涡破碎模型（EBU-A）和原ASOM模型的结果进行了对比,发现RASOM模型的效果较好。     

脉冲放电NO脱除过程模拟

建立了数学模型,把所考虑的NO/N₂/O₂/H₂O脉冲放电体系中的主要化学反应,归结为各物种浓度的刚性常微分方程组的初值问题来求解,计算方法选用Rosenbrock法,得到了物种浓度随停留时间的演化规律。结果表明：NO还原和氧化在其脱除过程中同时共存,当NO,N₂,O₂,H₂O和CO₂的体积浓度分别为200×10^-6,80%, 5％,6％和9％时,NO氧化所占的比例比NO还原的大很多;脉冲频率增大导致NO还原率和氧化率均增大;H₂O浓度增大导致HNO₃浓度增大,表明NO氧化所占的比例随H₂O浓度增大而增大。