微通道内甲烷/氢气/氧气预混合火焰传播特性

在石英玻璃微圆管内,进行甲烷/氢气/氧气预混合火焰传播的实验研究,分析了管径、掺氢比、当量比及入口流速对火焰传播状态和稳定火焰位置的影响规律.结果表明:实验观测到的微火焰主要有管外射流火焰、脉动火焰、稳定火焰与反复熄燃火焰;随着管径增加,稳定火焰出现在更高当量比情况下,火焰位置更靠近燃烧室入口;掺氢比越高,形成稳定火焰对应的当量比越高,火焰位置更接近出口;高当量比时,稳定火焰仅在低入口流速下能够获得,随着当量比降低,火焰能在较高入口流速下稳定;低流速下,稳定火焰在当量比为1.85～1.925时更接近燃烧室入口,随着流速增加,火焰位置更接近出口;反复熄燃火焰在管径增加时对应的当量比维持在1.79～1.93,在掺氢比增加时对应的当量比为1.79～2.12.     

采用速度匹配法研究本生灯火焰的稳定点位置

采用理论预测与实验研究的方法分析了本生灯火焰的稳定点位置与气流速度的关系,因为火焰稳定点的存在与否直接决定了火焰的存在与否.理论预测采用速度匹配法,通过分析本生灯出口附近冷态流场中气流速度分布与火焰传播速度分布的相对关系,并引入壁面淬熄的影响,预测出火焰稳定点的空间位置并得到了经验关系式.实验则以甲烷-空气的预混层流本生灯火焰为研究对象,确定了火焰稳定点位置与气流速度的相对关系.实验结果与Bernard Lewis 的预测不同,即随流速增大,火焰稳定点会朝向射流的下游和射流中心线方向移动.而根据得到的经验关系式预测出的火焰稳定点位置与实验结果符合良好,精度在±20%以内,表明速度匹配法能够准确预测火焰稳定点的位置.     

基于相空间重构的锅炉炉膛火焰信号分析

采用混沌和分形理论对实测锅炉炉膛火焰信号进行了定性分析和定量计算,通过相空间重构得到了稳定燃烧和不稳定燃烧两种工况下燃烧火焰时间序列的相平面图和关联维。分析计算结果表明:在稳定燃烧状态下,火焰信号的二维相平面图比较宽,关联维在6.5855～6.8415之间;在非稳定燃烧状态下,火焰信号的二维相平面图比较窄,关联维在5.8843～6.0907之间。两种工况下,火焰时间序列的相平面图和关联维有明显的差异,稳定燃烧时的关联维总比不稳定燃烧时的关联维大得多,因此,采用关联维作为火焰燃烧状态识别的特征参数。文中所提出的燃烧诊断方法为发展新型光学式火焰检测器提供了一种行之有效的解决方案。     

微尺度细管内热损失对火焰动力学的影响

针对微尺度细管内火焰传播问题,通过数值模拟研究了点火能量和壁面热损失对火焰传播模式的影响.数值模拟中观察到熄火现象和低速稳定传播的火焰,但并未观察到爆燃向爆轰转变.在以二维管道高度和初始已燃区长度构成的坐标图中,低速稳定火焰传播模式呈现半岛形,具有上、下两个熄火极限,其中熄火下极限位于由初始点火区域面容比决定的双曲线上方．对于低速稳定火焰传播模式,火焰尖端的传播速度与层流火焰速度较为接近．火焰面前方的气体几乎静止,火焰面后方有一个回流区,已燃气体向闭口端运动．     

基于功率谱的一种新型燃烧器燃烧状况识别

为有效地对新型燃烧器火焰燃烧状况进行识别,通过光电传感器获得一系列火焰光强信号,然后利用快速傅里叶变换(FFT)对火焰光强信号采样时间序列进行分析得出功率谱曲线,判断出非接触式的辐射式燃烧器燃烧状况稳定,进而得出了该种燃烧器燃烧孔开口大小及位置合理的结论.应用结果表明,燃烧火焰时域信号的采样频率至少要1 000 Hz,稳定的火焰信号功率谱低频部分波动小,不稳定的火焰信号功率谱波动相对于稳定的要大很多.     

分层比对双旋流预混火焰结构和燃烧不稳定的影响

基于双旋流预混燃烧实验系统，采用动态压力传感器、ICCD相机等装置，研究了不同分层比对双旋流预混火焰宏观结构和燃烧不稳定的影响．研究结果表明：随着分层比增大，火焰的主释热区由主燃级下游逐渐移向火焰根部，再逐渐向预燃级下游靠近，旋流火焰张角随之增大，火焰中心界面结构有“W”、“V”和“多褶”3种典型结构；随着分层比增大，火焰由稳定转为不稳定，且主频都在60 Hz左右，振幅先增大再慢慢下降，且在分层比为1.25和1.50时，有明显的不稳定第二频率出现，其大小约等于2倍的不稳定主频．     

应用火焰探测诊断煤粉燃烧的试验研究

本文通过火焰探测对煤粉燃烧诊断技术进行了试验研究.首先,利用单色仪获取了煤粉火焰的光谱分布特性,计算了煤粉火焰的色度坐标,发现其煤粉浓度的变化.对稳定燃烧工况和非稳定燃烧工况的火焰信号进行了频谱估计.以上研究成果可用于实际燃烧设备的燃烧监测与管理系统中.图8参8     

轻油的预蒸发燃烧——轻油预蒸发燃烧中的火焰稳定

分析了预蒸发燃烧中的火焰稳定问题，介绍了燃料燃烧中关于火焰稳定的原理和方法，重点介绍了目前实现轻油预蒸发燃烧中所采用的若干稳焰措施。     

直接火焰燃料电池热应力分析及性能研究

针对直接火焰燃料电池建立了二维有限元分析模型,对由于火焰温度分布不均匀而引起的燃料电池热应力进行分析,并利用Weibull方法对其失效概率进行了计算.火焰温度的不均匀分布极大提高了燃料电池的失效概率,因此均匀分布的火焰温度对于直接火焰燃料电池的稳定运行至关重要.本文设计并构建了一个基于Hencken型平焰燃烧器的新型直接火焰燃料电池装置,以甲烷为燃料,利用Hencken型平焰燃烧器产生稳定且均匀分布的平面富燃扩散火焰.在不同燃料流速下测定了电池性能,电池最大功率密度达到400,W/m2.     

乙醇对汽油火焰碳烟生成特性影响的试验研究

设计并制作了一套新型的能够产生稳定层流火焰的液体燃烧器。将体积分数为0%、20%、50%和85%的无水乙醇混入92#汽油配制出了不同掺混比的乙醇汽油混合燃料,应用光学诊断技术在常压下对各燃料稳定火焰中的碳烟浓度分布进行测量,研究了添加乙醇对汽油火焰中碳烟生成特性的影响。研究结果表明:在保证碳元素流量为6.05g/h的情况下,随着燃料内乙醇的增加,各燃料均能生成稳定火焰,火焰高度保持在(50±1)mm;火焰中各个高度上的碳烟浓度都出现不同程度的降低;碳烟生成总量呈现直线下降规律,E20、E50、E85火焰中的碳烟总量相对于E0火焰分别降低了16.20%、37.77%和61.66%。     

Determining factor for the blowoff limit of a flame spreading in an opposed turbulent flow, in a narrow solid-fuel duct

This study clarified the blowoff mechanism for a flame spreading in an opposed turbulent flow in narrow solid fuel ducts. To clarify this mechanism, two experiments were conducted. The first experiment was to investigate the influence of ambient pressure and fuel duct size on the blowoff limit. The results indicated that the flow velocity at the point when blowoff occurred, Vg,t, increased with ambient pressure. This tendency could not be confirmed by a well-known expression for the Damköhler number, which is defined as the ratio of the characteristic flow time to the characteristic chemical time. Subsequently, to clarify the determining factor for the blowoff, the second experiment, which observed the flow field near the flame leading edge, was conducted. The results show that the flow separation in front of the flame leading edge, which provided sufficient residence time of oxidizer and gaseous fuel, is necessary for the flame to spread in an opposed oxidizer flow. From the results, it is found that the oxidizer friction velocity, u_∗, which is an indicator of the turbulent momentum transfer, is the determining factor for the flame blowoff limit. When the friction velocity is larger than a critical value, flame blowoff occurs in the fuel duct, due to the absence of flow separation.     

Electric fields effect on liftoff and blowoff of nonpremixed laminar jet flames in a coflow

The stabilization characteristics of liftoff and blowoff in nonpremixed laminar jet flames in a coflow have been investigated experimentally for propane fuel by applying AC and DC electric fields to the fuel nozzle with a single-electrode configuration. The liftoff and blowoff velocities have been measured by varying the applied voltage and frequency of AC and the voltage and the polarity of DC. The result showed that the AC electric fields extended the stabilization regime of nozzle-attached flame in terms of jet velocity. As the applied AC voltage increased, the nozzle-attached flame was maintained even over the blowout velocity without having electric fields. In such a case, a blowoff occurred directly without experiencing a lifted flame. While for the DC cases, the influence on liftoff was minimal. There existed three different regimes depending on the applied AC voltage. In the low voltage regime, the nozzle-detachment velocity of either liftoff or blowoff increased linearly with the applied voltage, while nonlinearly with the AC frequency. In the intermediate voltage regime, the detachment velocity decreased with the applied voltage and reasonably independent of the AC frequency. At the high voltage regime, the detachment was significantly influenced by the generation of discharges.     

Analysis of Cold Flowfield of Multi—Annular Opposed Jets

The technique of the use of multi-annular opposed jets as different from using swirl and bluff body createsan excellent recirculation zone with desired size in a large space.The size of recirculation,the magnitude ofreverse velocity and turbulence intensity are much greater than those formed by bluff body.Factors affectingthe flowfield include the velocity ratio of the opposed jets to the primary air J,the diameter and constructionof the opposed jet ring,secondary air velocity and configuration,and confined or unconfined flow condition andso on.This method is a promising way for flame stabilization in combustion technology.     

湍流射流火焰抬举高度的实验研究

湍流射流燃烧作为工业燃烧室中普遍存在的燃烧方式,研究湍流射流火焰不仅能促进实际燃烧室的设计改造,更能增强对湍流燃烧理论的理解。在轴对称伴流射流燃烧器实验平台上,研究了湍流自由射流火焰抬举高度随射流速度的变化及氮气稀释和伴流速度对火焰抬举高度的影响。实验结果表明湍流自由射流燃烧火焰抬举高度随射流速度呈线性增长;随氮气稀释摩尔分数的增加其抬举高度的线性斜率增大,射流火焰吹出喷嘴的雷诺数降低,火焰更易发生抬举;同时,氮气稀释摩尔分数的增加也导致射流火焰发生吹熄时雷诺数减小,射流火焰在射流速度完全进入湍流之前发生吹熄;伴流速度小于0.3 m/s时对火焰抬举高度的影响不大,当伴流速度大于0.3 m/s时抬举高度随伴流速度的增加呈线性增长,当射流速度大于20 m/s时,伴流速度的影响降低;对比伴流与稀释对抬举高度的影响可知射流速度大于30 m/s时对伴流的敏感性大于稀释,而在射流速度小于30 m/s时对稀释更敏感。     

Blowoff characteristics of bluff-body stabilized conical premixed flames with upstream spatial mixture gradients and velocity oscillations

This experimental study concerns determination of blowoff equivalence ratios for lean premixed conical flames for different mixture approach velocities ranging from 5 to 16 m/s in the presence of spatial mixture gradients and upstream velocity modulation. Conical flames were anchored on a disk-shaped bluff body that was attached to a central rod in the burner nozzle. A combustible propane-air mixture flowed through a converging axisymmetric nozzle with a concentric insert, allowing radial mixture variation by tailoring the composition in the inner and outer parts of the nozzle. The radial mixture profiles were characterized near the location of the flame holder by laser Rayleigh light scattering. Additionally, a loudspeaker at the nozzle base allowed introduction of periodic velocity oscillations with an amplitude of 9% of the mean flow velocity up to a frequency of 350 Hz. The flame blowoff equivalence ratio was experimentally determined by continuously lowering the fuel flow rates and determining the flame detachment point from the flame holder. Flame detachment was detected by a rapid reduction of CH* emission from the flame base imaged by a photomultiplier detector. It was found that the flame blowoff is preceded by progressive narrowing of the flame cone for the case of higher inner jet equivalence ratios. In this case, the fuel-lean outer flow cannot sustain combustion, and clearly this is not a good way of operating a combustor. Nevertheless, the overall blowoff equivalence ratio is reduced by inner stream fuel enrichment. A possible explanation for this behavior is given based on the radial extent of the variable-equivalence-ratio mixture burning near the flame stabilization region. Fuel enrichment in the outer flow was found to have no effect on blowoff as compared to the case of uniform mixture. The results were similar for the whole range of mean flow velocities and upstream excitation frequencies.     

Effect of electric fields on the stabilization of premixed laminar bunsen flames at low AC frequency: Bi-ionic wind effect

The stabilization characteristics of laminar premixed bunsen flames have been investigated experimentally by applying AC electric fields at low frequency below 60 Hz together with DC in the single electrode configuration. The blowoff velocity has been measured for varying AC voltage and frequency. A transition frequency between low and high frequency regimes has been identified near 40–50 Hz, where AC electric fields have minimal effect on flame stabilization. In the low frequency regime, the blowoff velocity decreased linearly with AC voltage such that the flames became less stable. This was consistent with the DC result, implying the influence of the ionic wind effect. The variation of blowoff velocity with AC frequency showed a non-monotonic behavior in that the velocity decreased and then increased, exhibiting minimum blowoff velocity near 6–8 Hz. Based on the molecular kinetic theory, the developing degree of ionic wind was derived. By considering the ionic wind effects arising from both positive and negative ions in a flame zone, the bi-ionic wind effect successfully explained the non-monotonic behavior of blowoff velocity with AC frequency in the low frequency regime.     

Flame stabilization in a lifted non-premixed turbulent hydrogen jet with coaxial air

To understand hydrogen jet liftoff height, the stabilization mechanism of turbulent lifted jet flames under non-premixed conditions was studied. The objectives were to determine flame stability mechanisms, to analyze flame structure, and to characterize the lifted jet at the flame stabilization point. Hydrogen flow velocity varied from 100 to 300 m/s. Coaxial air velocity was regulated from 12 to 20 m/s. Simultaneous velocity field and reaction zone measurements used, PIV/OH PLIF techniques with Nd:YAG lasers and CCD/ICCD cameras. Liftoff height decreased with increased fuel velocity. The flame stabilized in a lower velocity region next to the faster fuel jet due to the mixing effects of the coaxial air flow. The non-premixed turbulent lifted hydrogen jet flames had two types of flame structure for both thin and thick flame base. Lifted flame stabilization was related to local principal strain rate and turbulent intensity, assuming that combustion occurs where local flow velocity and turbulent flame propagation velocity are balanced.     

基于FGM模型的同轴射流非预混火焰大涡模拟

采用直接数值模拟方法对二甲醚(Dimethyl Ether,DME)射流推举燃烧进行了研究（DNS）,分析了DME射流推举火焰结构、燃烧模式和推举稳定机理。数值模拟工况条件为：燃料由狭缝射出,初始温度500 K,射流速度138 m/s;伴流空气的初始温度1 000 K,流速3 m/s,压力为0506 6 MPa。研究表明：DME射流推举火焰与传统的边火焰有很大不同,在射流核心区内存在1条低温放热分支以及紧随其后的中温着火分支,并且推举稳定点位于贫燃侧;DME湍流射流推举火焰包含冷焰反应区(Cool Flame Zone,CFZ)、中温反应区(Intermediate Temperature Zone,ITZ)、富燃高温区(High Temperature Rich Burn Zone,HTR)以及贫燃高温区(High Temperature Lean Burn Zone,HTL)4种模式;在CFZ与ITZ区内湍流混合占主导,并且湍流混合会抑制低温放热;在HTR与HTL区内放热速率占主导地位,但是湍流会显著增强超贫燃区间内的高温放热速率;大部分热量在HTL和HTR区产生,而CFZ和ITZ区对总体产热的贡献微乎其微,但是所产生的中低温组分加快了高温着火过程;射流推举稳定性由贫燃侧的高温自着火反应机制所控制。     

二甲醚湍流射流推举火焰的燃烧机理研究

对长、宽、高为650 mm×400 mm×12 mm的半闭口狭窄矩形通道（海伦-肖装置）内的甲烷/空气层流预混火焰传播过程进行了实验研究,探究当量比φ在0.6~1.2范围内、火焰传播角度ω在垂直向下-90°至垂直向上90°区间对火焰前锋轮廓发展及非标准层流火焰速度的影响。结果表明：火焰在通道内的传播分为热膨胀、准稳态传播和端壁效应3个阶段,每个阶段具有各自不同的前锋轮廓特征。由于瑞利-泰勒不稳定性机制的作用,所有当量比工况下向上传播的火焰均在准稳态传播阶段中呈现出明显的锋面褶皱与胞状结构;对向下传播的火焰而言,其在贫燃工况（φ为0.6,0.8）下的胞状不稳定性得到了有效抑制,而在当量比φ=1.0及富燃工况（φ=1.2）下,该稳定性效应并不显著。火焰瞬时速度与标准层流速度的比值Ui/UL,在φ=0.6的极贫燃工况与其他当量比工况下展现出明显不同的发展特性,极贫燃工况火焰向上传播时（ω=90°）,Ui/UL随着传播过程的进行一直增大,直到火焰触碰壁面末端熄灭,整个过程Ui/UL与火焰传播方向呈正相关关系;而对于其他当量比工况,Ui/UL在传播过程中均先升高后下降,火焰触碰壁面末端熄灭前其值趋于稳定,其平均速度与标准层流速度的比值Ua/UL在水平传播（ω=0°）时达到最大值。     

Boundary-velocity gradient and premixed flame blowoff in U-bend tubes with secondary flow

The effect of a non-uniform boundary-velocity gradient along the rim of a circular nozzle burner on flame stabilization, including partial liftoff and blowoff, has been investigated experimentally by using U-bend tubes as nozzles for both laminar methane/air and propane/air premixed flames. Secondary flow, induced by the imbalance between pressure force and centrifugal force inside the U-bend tube, generated non-uniform and non-axi-symmetric flow. The intensity of the secondary flow was controlled by varying the flow rate and the radius of curvature of the U-bend tubes. Unique features of flames were visualized with direct photography and planar laser-induced fluorescence for OH-radicals. As the flow-rate increases, the flame lifted off partially from the nozzle rim, and the nozzle attached region decreased with increasing flow rate. Finally, blowoff occurred. Stability of flames was mapped as functions of equivalence ratio, nozzle-exit velocity, and the radius of curvature. The flames in the U-bend tubes had larger velocities at blowoff compared to the case with a straight-tube burner. Flow-field measurement using a laser Doppler velocimeter showed that local boundary-velocity gradients at the critical conditions of partial liftoff and blowoff in the U-bend tubes agreed well with those in the straight-tube burner. Also, such conditions can be described with overall flow characteristics in U-bend tubes, represented by the Dean number.