径向浓浓淡旋流煤粉燃烧器直流二次风对气固流动的影响

本文综述了旋流燃烧器直流二次风对燃烧器区单相流动、烟气温度、煤粉燃尽及ＮＯｘ生成的影响,采用三维ＰＤＡ系统对不同直流二次风率下的气固速度分布、颗粒体积流量分布进行了测量,得出了其对气固流动的影响,分析了对燃烧器性能的影响。     

预混段长度对旋流燃烧器气固两相流动特性的影响

本文利用三维PDA测量了预混段减小的斗山巴布科克旋流燃烧器气、固两相流动特性,并与设计的斗山巴布科克旋流燃烧器PDA测量结果对比,得出:预混段减小的燃烧器在x/d=0.1,0.3截面存在回流区,并将设计燃烧器回流区的位置由一二次风之间推迟至外二次风区域;预混段减后的燃烧器与设计燃烧器相比,轴向,径向,切向平均速度在二次风区域产生的峰值远离燃烧器轴线;在x/d=0.1～0.5截面,预混段减小的燃烧器与设计燃烧器相比一次风区域颗粒体积流量的峰值变大,二次风区域颗粒体积流量的峰值变小,在燃烧器出口附近回流区内的颗粒体积流量明显降低。     

对V形火焰稳定器回流区内的气体质量和回流区边界上紊流交换率的研究

本文在18米/秒、28.5米/秒、40米/秒三种来流速度下,对30°V形火焰稳定器下游的速度场、密度场进行了测量。在稳定器前方均匀气体的速度增加时,火焰稳定器回流区内气体旋涡向下游移动,回流区内回流气体的总质量减小,回流区边界上的紊流交换率增加。利用测量结果,计算了火焰稳定器回流区内气体的总质量和回流区边界上的紊流交换率,找到了它们和压力系数之间的关系,找到了无量纲停留时间和压力系数的关系。     

稠密颗粒射流倾斜撞击颗粒膜特征

采用高速摄像仪对稠密颗粒射流倾斜撞击形成的类液体颗粒膜特征进行实验研究,考察了颗粒粒径、射流速度以及射流含固率等因素对颗粒膜形态及动态特征的影响.结果表明:随着颗粒粒径增大,稠密颗粒倾斜撞击流由颗粒膜向散射模式转变;随着射流速度增加,气固不稳定增强,射流流量出现脉动,正面与侧面分别表现为颗粒膜的非轴对称振荡和表面波纹结构;颗粒膜非轴对称振荡的振荡频率和振荡幅度随射流速度的增大而增大;表面波纹速度和波长沿传播方向增大,波纹间存在叠加现象.颗粒膜出现非轴对称振荡主要是因为喷嘴出口由气固不稳定性引起的射流流量脉动,射流流量脉动频率与撞击面振荡频率基本相当.     

颗粒数方程的求解及流化床数值模拟

流化床内颗粒聚并和破碎将影响颗粒相的流动特性.本文运用基于颗粒动理学理论的欧拉一欧拉气固多相流模型,利用直接矩积分方法求解颗粒数平衡方程,建立颗粒数密度与连续性方程、动量方程之间的关系,数值模拟流化床内两种不同直径颗粒发生聚并时气固两相流动特性。计算结果表明,颗粒聚并伴随着床内颗粒直径逐渐增大,床内颗粒流化状态逐渐变为固定床状态,两种颗粒直径均增加,且小颗粒的体积分数逐渐减小、大颗粒的体积分数增加。当仅考虑聚并过程时增加流化速度将导致床内颗粒体积平均直径变大。随着颗粒密度减小,床内体积平均直径增加。     

纳米孔隙内混合气体流动特性的分子动力学模拟

纳米孔隙内气体流动的理论预测对气体微流控器件的设计和制造具有重要的理论指导作用,文章采用分子动力学方法研究了氮气、氧气和二氧化碳混合气体在平行壁纳米孔隙内的剪切流动特性和边界滑移特性.研究结果表明:随着加入二氧化碳比例的不断增加,混合气体滑移速度不断增大,并且当二氧化碳的比例低于20%时,混合气体流动速度沿孔隙宽度方向呈线性分布;而当比例达到40%后,其速度轮廓将呈现非线性趋势.当二氧化碳所占比例为20%时,随着孔隙宽度的增加,混合气体的整体边界滑移随之减小.探究了混合气体密度和气-固耦合强度对混合气体流动及边界滑移的影响机理.发现随着混合气体密度的减小,气流边界滑移增大;随着气-固界面耦合强度的增强,边界气体分子易被吸附而出现黏滑运动,气体分子在边界处的积聚现象增强,剪切应变率增大,边界滑移减小.      

混合气流布置方式对循环流化床气固流动特性的影响

煤层气和煤矸石CFB混烧中,由于喷入的是可燃气体,燃烧所需的空气量及配风方式需要改变,必然会对炉内的气固流动特性产生影响.本文在煤层气和煤矸石某一混烧比下,确定所需的配风量和配风方式,通过改变混合气流的布置方式,对CFB炉内气固流动特性进行数值模拟.研究结果表明合理的布置方式可使炉内的气固混合较好,流动较稳定,且混合气流布置在两层二次风之间时,炉内密相区大部分颗粒速度较高,气固相间混合充分;密相区固体颗粒浓度较其它两种布置方式高,稀相区则相反,是比较合理的布置方式.研究结果对CFB混烧优化设计具有重要指导意义.     

基于PIV图像处理法的管内低浓度液固两相流颗粒运动特性研究

为了研究管内低浓度液固两相流颗粒运动特性,提出一种基于PIV图像处理法的液固两相流颗粒速度场、涡量场及速度大小的分析方法。通过高速摄影仪获得不同工况下流场的运动图像,运用Canny算子边缘检测法分割图像提取粒子,由互相关函数获得粒子速度和方向,重建二维场。速度矢量场与颗粒运动轨迹相符,说明该方法可用于管内低浓度液固两相流动测量。研究发现:流量越大,流场越复杂,颗粒速度分布越混乱,涡量变化范围越大。流量、浓度和粒径对颗粒速度有显著影响。颗粒速度随浓度、粒径的增大而减小,而随流量增大正好相反。水平管中,颗粒粒径越小,颗粒速度增幅越大,最大约为2倍;流量越大,不同粒径组合下颗粒速度差越大,约达1.5倍。     

速度对聚四氟乙烯摩擦系数影响的分子动力学模拟

基于分子动力学方法建立了双层聚四氟乙烯(polytetrafluoroethylene,PTFE)摩擦模型,研究了不同速度下PTFE的摩擦过程.通过分析不同速度下接触区内下层PTFE分子键长、键角、分子形状的变化及接触过程中摩擦力和正压力的变化,从微观角度研究了速度对PTFE摩擦系数的影响.研究结果表明:随着速度的增加,接触区内PTFE粒子间的键长变短,键角变小,分子链沿x;方向的变形量增加.变形后的PTFE分子产生的回弹力导致上、下层PTFE分子间相互作用力增加,从而增加了摩擦力.当速度进一步增大时,接触区内下层PTFE粒子间的键长和键角多处于平衡位置,分子链沿x方向的变形量减小.这很可能是由于接触区内下层PTFE分子沿速度方向倾斜,使上、下层PTFE分子趋于平行滑动,从而降低了摩擦力.不同速度下正压力几乎保持不变.因此,当上层PTFE所受外载荷固定时,摩擦系数随着速度的增加先增大后减小,临界速度为1.2 m/s,这与实验研究结果一致.     

旋流非预混燃烧火焰流动特性的实验研究

利用粒子图像速度场测量技术(PIV)对不同工况下的旋流非预混燃烧流场进行了测量,考察不同燃空速度比下旋流火焰的流动特性.结果表明,轴向截面上径向平均速度流场以燃烧器轴线呈中心对称,轴向平均速度、轴向脉动速度和径向脉动速度沿燃烧器轴线成对称分布,且轴向平均速度和轴向脉动速度的最大值出现在轴线处.随燃空速度比的增大,轴向平均速度和脉动速度增大,随着与燃烧器表面的距离增加,流场截面上轴向平均速度和脉动速度差异不断减小.     

湿空气扩散燃烧火焰结构特性研究

利用二维粒子成像速度仪(PIV)对钝体燃烧器中的甲烷／湿空气扩散燃烧的速度场进行测量,考察其火焰的结构特性及其内部流动状况。通过对湿空气燃烧流场与普通燃烧流场的对比分析表明,湿空气燃烧情况下,两种燃烧状态的火焰(回流燃烧火焰和中心射流主导火焰)相互转换的燃空速度比(γ)值要比普通燃烧的小;湿空气燃烧使得喷嘴后的同流空气的速度降低,空气的回流作用减弱,燃料更容易冲出回流区,火焰的稳定性能变差。     

Two-dimensional gas flows under heterogeneous combustion of solid porous media

Two-dimensional unsteady gas flows in porous media with heterogeneous-combustion centers are investigated under forced filtration and free convection. With the use of numerical methods, it is shown that complex gas flows including vortex ones can arise under the combustion of solid porous media. In the case of forced filtration, the gas tends to flow around the heated portion of an object preferring to flow along cold regions. Under natural convection, the vortex gas flows, which can exist for a reasonably long time and strongly affect the oxidizer inflow into the reaction zone, arise at the initial moment of the process in the combustion zone and in its vicinities.     

Flame characterisation of gas-assisted pulverised coal combustion using FPV-LES

A multiphase flamelet/progress variable (FPV) model for the large eddy simulation (LES) of gas-assisted pulverised coal combustion (PCC) is developed. The target of the simulation is the Darmstadt turbulent gas-assisted swirling solid fuel combustion chamber. The coal particles are treated as Lagrangian point particles, the position, momentum and energy of which are tracked. The gas phase is described by the low-Mach Navier-Stokes equations alongside the Eulerian transport equations of the governing variables for the FPV model. The set of chemical states of the PCC flame is pre-tabulated in a six-dimensional flamelet table and determined by the mixing of the primary fuel stream, volatiles and char off-gases with the oxidising air, the progress of chemical reactions, the interphase heat transfer, as well as sub-grid scale variations. A presumed $\beta$-PDF approach for the total mixture fraction is applied to capture sub-grid scale effects. The discrete ordinate method (DOM) with the weighted sum of grey gases model (WSGGM) is employed to model radiation. The FPV-LES results are validated against the experimental evidence and a good agreement of the predicted mean and RMS velocities, as well as the mean gas temperature between experiments and simulations is obtained. The contributions of the pilot, volatile and char off-gas fuel streams to the coal flame are analysed. It is found that most regions of the furnace are dominated by either pilot or volatile combustion, while char conversion only occurs in the far downstream and outer furnace regions. The pilot gas dominates the near-wall region inside the quarl, whereas the volatile gas mainly released from small particles dominates a first volatile combustion zone in the interior of the internal recirculation zone. Larger particles heat up more slowly and release their volatile content further downstream, leading to a secondary volatile combustion zone.     

低功率电弧放电辅助甲烷燃烧的实验研究

利用低功率电弧放电辅助甲烷燃烧,研究了在不同甲烷/空气比例的情况下,等离子体对甲烷燃烧的影响。采用发射光谱仪进行光谱诊断,比较有/无等离子体辅助甲烷燃烧火焰发射光谱的异同,讨论了等离子体辅助燃烧可能发生的过程和机理。比较有/无等离子体辅助甲烷燃烧火焰温度的变化。利用气相色谱和烟气分析仪对甲烷燃烧产物中的CH₄、CO 和O₂ 进行分析。实验结果表明,加等离子体后,火焰的温度升高,尾气中的可燃性成分减少,氧气含量降低,在很大程度上提高了甲烷的燃烧效率;甲烷/空气的比例越低,燃烧效率的提高越明显;甲烷的富燃燃烧极限从16%调高到21%。     

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Different ratios of methane/air in the low power arc discharges assisted methane combustion were studied at atmospheric pressure. Plasma emission spectroscopy was recorded by an optical emission spectrometer. The possible physical and chemical reaction process and mechanisms were discussed. With a plasma in the on or off state, the physical appearances of the methane combustion flame were observed and compared, the temperature of the flame in the main combustion zone was investigated. The gas compositions of CH₄, CO and O₂ were analyzed by the gas chromatography and flue gas analyzer. The results showed that when plasma was in the on state, the temperature of flame increased and the combustion efficiency enhanced in stable combustion due to decrease of CH₄, CO and O₂; the combustion enhancement was more obvious at low ratio of methane/air; the rich combustion limits in terms of the ratio of methane/air was extended from 16% to 21%.     

Study of vortex core precession in combustion chambers

The article presents the results of experimental investigation of swirling flow of lean propane/air flame in a model combustion chamber at atmospheric pressure. To study the unsteady turbulent flow, the particle image velocimetry technique was used. It was concluded that dynamics of high swirl flows with and without combustion was determined by a global helical mode, complying with a precessing double-spiral coherent vortex structure. The studied low swirl flame had similar size and stability characteristics, but amplitude of the coherent helical structure substantially oscillated in time. The oscillations were associated with intermittently appearing central recirculation zone that was absent in the nonreacting flow. It is expected that the low swirl flow without the permanent central recirculation zone should be more sensitive to an external active control. In particular, this result may be useful for suppression of thermoacoustic resonance in combustion chambers.     

四角切向燃烧煤粉锅炉炉膛内空气动力场的数值研究

1前言四角切向燃烧煤粉锅炉是目前应用较为广泛的一种炉型，在实际运行中有一些较为突出的优点。为深入研究煤粉在炉内的燃烧特性，炉内空气动力场的研究至关重要，本文数值模拟了四角切向燃烧煤粉锅炉炉内冷、热态情况下的流场，对其时均、湍流特性进行了分析，对气流的分布特性进行了研究．2研究对象本文的研究对象是锦州电厂二期工程的HG－670／14ty9型煤粉锅炉，燃烧器为多层四角切向布置，每角有4个一次风喷口，6个二次风喷口，1个三次风喷口。锅炉各部分尺寸及燃烧器的布置见图1、图2。热态流场的模拟对象是实际锅炉。模拟工况基本按…     

Flow and mixing fields of turbulent bluff-body jets and flames

The mean structure of turbulent bluff-body jets and flames is presented. Measurements of the flow and mixing fields are compared with predictions made using standard turbulence models. It is found that two vortices exist in the recirculation zone; an outer vortex close to the air coflow and an inner vortex between the outer vortex and the jet. The inner vortex is found to shift downstream with increasing jet momentum flux relative to the coflow momentum flux and gradually loses its circulation pattern. The momentum flux ratio of the jet to the coflow in isothermal flows is found to be the only scaling parameter for the flow field structure. Three mixing layers are identified in the recirculation zone. Numerical simulations using the standard k-? and Reynolds stress turbulence models underpredict the length of the recirculation zone. A simple modification to the C₁ constant in the dissipation transport equation fixes this deficiency and gives better predictions of the flow and mixing fields. The mixed-is-burnt combustion model is found to be adequate for simulating the temperature and mixing field in the recirculation zone of the bluff-body flames.     

Effect of the incident shock wave interacting with transversal jet flow on the mixing and combustion

The interaction between an incident shock wave and a transverse jet flow for mixing and combustion in a supersonic airstream was investigated experimentally and numerically. NO planar laser induced fluorescence (NO-PLIF) and particle imaging velocimetry (PIV) for non-reactive flows and three-dimensional reactive/non-reactive numerical simulations were conducted to examine the effect of the incident shock wave on the three-dimensional flow structure and mixing mechanism between the airstream and the injected gas downstream of the injection slot. Results of NO-PLIF measurement and numerical simulation showed that, in the case without the incident shock wave, injected gas is seldom seen in the recirculation zone just downstream of the injection slot, while the injected gas with higher concentration is almost uniformly distributed in the recirculation zone when the incident shock wave is introduced downstream of the injection slot. Moreover, it was shown by the numerical simulations that the profiles of the local equivalence ratio is in the combustible range due to the enhanced entrainment of the airstream when the incident shock wave is introduced downstream of the injection slot. A large-scale recirculation in the direction parallel to the wall is generated by the three-dimensional flow effects, which enhances the mixing and extends the residence time in the recirculation zone in the case with incident shock wave downstream of the injection slot, the recirculation flow being confirmed successfully by PIV measurements as well. The results of three-dimensional reactive numerical simulations were in good agreement with the experimental flame-holding characteristics at a lower total temperature, which showed that flame-holding can be attained only when the incident shock wave was introduced downstream of the injection slot, confirming that the formation of three-dimensional and large-scale recirculation flow downstream of the injection slot enlarges the recirculation zone and enhances the mixing to produce the conditions for robust flame-holding.     

Ignition delay of fatty acid methyl ester fuel droplets: Microgravity experiments and detailed numerical modeling

Recent optical engine studies have linked increases in NO_x emissions from fatty acid methyl ester combustion to differences in the premixed autoignition zone of the diesel fuel jet. In this study, ignition of single, isolated liquid droplets in quiescent, high temperature air was considered as a means of gaining insight into the transient, partially premixed ignition conditions that exist in the autoignition zone of a fatty acid methyl ester fuel jet. Normal gravity and microgravity (10⁻⁴ m/s²) droplet ignition delay experiments were conducted by use of a variety of neat methyl esters and commercial soy methyl ester. Droplet ignition experiments were chosen because spherically symmetric droplet combustion represents the simplest two-phase, time-dependent chemically reacting flow system permitting a numerical solution with complex physical submodels. To create spherically symmetric conditions for direct comparison with a detailed numerical model, experiments were conducted in microgravity by use of a 1.1 s drop tower. In the experiments, droplets were grown and deployed onto 14 μm silicon carbide fibers and injected into a tube furnace containing atmospheric pressure air at temperatures up to 1300 K. The ignition event was characterized by measurement of UV emission from hydroxyl radical (OH*) chemiluminescence. The experimental results were compared against predictions from a time-dependent, spherically symmetric droplet combustion simulation with detailed gas phase chemical kinetics, spectrally resolved radiative heat transfer and multi-component transport. By use of a skeletal chemical kinetic mechanism (125 species, 713 reactions), the computed ignition delay period for methyl decanoate (C₁₁H₂₂O₂) showed excellent agreement with experimental results at furnace temperatures greater than 1200 K.