当量比对甲烷预混低旋流燃烧的影响

通过实验和数值模拟的方法研究了甲烷/空气预混低旋流燃烧的流场结构及当量比对甲烷低旋流燃烧的影响.结果表明,甲烷/空气预混低旋流气流在喷嘴出口处扩张,形成有利于燃烧稳定的低速区;预混火焰"悬浮"于喷嘴上方,在剪切区的内侧,火焰呈W型;富燃时,随着当量比的增加,火焰的推举高度略有增加;甲烷/空气预混低旋流流场具有自相似性,无量纲轴向速度的径向分布几乎不受当量比的影响.同时,燃烧室出口的温度随着当量比的增加而增加,并且在当量比为0.8~1.4时变化较为明显,当量比超过1.4后,增加趋势变缓.     

射流喷嘴旋流强度对燃烧室回流和燃烧特性的影响研究

随着燃气轮机参数的提高和稳定低排放运行工况的拓宽,对燃烧的要求也越来越高。柔和燃烧作为一种有潜力的燃烧技术,具有温度均匀、燃烧稳定和污染物排放低等优点,而如何在燃烧室内组织流动是实现柔和燃烧的关键。采用高速射流引射掺混的方式可以较好的满足柔和燃烧产生所需的条件。预混射流喷嘴结构和布置对流场和燃烧特性有重要影响,如何选择射流喷嘴结构值得进一步研究。本文通过实验和数值模拟相结合的方式,研究了柔和燃烧器中预混射流喷嘴的旋流强度对燃烧器流动结构和燃烧排放的影响。结果表明,旋流能增强燃料/空气的掺混,低旋流作用下能使喷嘴出口掺混不均匀度ISMD下降0. 15左右;但是喷嘴旋流对燃烧室的烟气回流有减弱的作用,使回流区向喷嘴和中轴线靠近;同时,旋流会造成温度场和火焰面不均匀分布,略微拓宽燃烧工况范围并略微增加火焰的稳定性。实验结果表明喷嘴旋流进气角从0°变化到45°时,NOx排放随旋流角的增大而增加。     

BA-34型高压锅炉旋流燃烧器空气动力特性测试与研究

介绍160t／h微正压燃油锅炉炉内及旋流燃烧器出口空气动力场测试方法和测试结果,讨论加模拟火炬气前后空气动力场的变化。测试结果与分析认为,旋流燃烧器出口前具有显著的回流区和涡流区,对燃烧火焰的稳定和锅炉稳定运行有重要作用;但加模拟火炬气后,旋流燃烧器出口前的空气动力场发生了比较明显的改变,对燃烧火焰稳定有较明显的影响。     

旋流强度对F级燃气轮机燃烧稳定性影响分析

旋流器是目前燃烧室中常用的稳定火焰的结构,主要通过气体经过旋流器后在下游形成的回流区来稳定火焰。一般采用旋流数表征旋流器的旋流强度。旋流数一方面会影响空气和燃料的掺混,另一方面会影响回流区的尺寸,从而对火焰的长度和稳定性产生影响。针对某重型燃气轮机的燃烧器,采用数值模拟的方法分析了旋流数变化对热态温度场的影响,同时采用三维有限元方法分析了旋流数对燃烧稳定性的影响。结果表明:旋流数增加,会使得火焰更加紧凑,长度缩短,同时使得燃烧室二阶周向模态稳定性恶化,不稳定的风险增加;降低旋流数会使得火焰长度增加,轴向模态稳定性恶化。该研究结果可为类似机组的相关设计研究提供参考和借鉴。     

空气引射对斯特林发动机火焰特征及燃烧温度的影响

为了研究空气引射对斯特林发动机常压燃烧特性的影响,在可视化试验台上进行了不同工况下的燃烧试验,并利用高速摄影机拍摄了燃烧室内的火焰图像,利用热电偶测量了燃烧室内的燃烧温度.结果表明,无空气引射时,燃烧火焰较长,张角较小,波动较大,高温区集中在燃烧室轴线附近;加入适量引射空气,火焰缩短,张角增大,稳定性提高,高温区外扩,位置向喷嘴靠近,燃烧温度升高;过量引射空气使火焰失稳,温度场紊乱,高温区分散,燃烧温度下降.     

旋流数对燃烧不稳定性及NO_x生成的影响

采用大涡模拟方法分析了旋流数对燃气轮机燃烧室内预混燃烧不稳定性以及NO_x生成特性的影响.结果表明:增大旋流数使得流场的扩张角增大,中心回流区范围扩大,对燃烧产物的卷吸能力增强,预混段内温度升高,高温区范围扩大,有利于燃料气流的着火与稳定燃烧,火焰长度也有所缩短;旋流数为0.7时,流场中仅存在一个进动涡核,旋流数较大时,则出现2个明显的进动涡核;增大旋流数使得涡旋周期性的脱落频率增加,破碎位置向上游移动,同时由于火焰长度缩短,热释放区域相对更为集中,从而导致燃烧室内压力脉动频率及其对应的压力峰值增大;增大旋流数也使得火焰宽度增大,峰值温度有所降低,有利于控制NO_x排放体积分数.     

甲烷/空气典型混合模式在约束空间内的燃烧特性研究

燃料质量浓度分布在一定程度上影响混合气体的燃烧效率,能使燃气充分混合的同轴射流、旋片同轴、轴切结合、切向旋流等典型混合模式在航空发动机、燃气轮机及火箭发动机等先进燃烧技术应用中较为常见。因此,设计了甲烷/空气部分预混的燃烧实验装置,较为系统地实验研究了旋流数和轴向流速对混合气体在约束空间燃烧室内燃烧特性的影响。结果表明：对于有中心射流的混合结构,燃气轴向流速较低时产生黄色火焰,增大轴向流速,黄色火焰转为蓝色湍流火焰,且温度分布趋于均匀;纯切向旋流燃烧器的掺混效果较好,受燃气轴向流速的影响小,火焰结构稳定,均为蓝色火焰,温度轴/径向分布均匀且趋势一致,同当量比下燃烧产物中的污染物体积分数最小。     

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．     

一种新型旋流燃烧器内甲烷扩散燃烧特性

采用两步总包反应机理、标准亚网格应力模型与有限速率/涡耗散燃烧模型等对一种新型旋流燃烧器内甲烷-空气扩散燃烧过程进行大涡模拟.凭借旋流离心效应和涡旋效应来控制反应混合和火焰传播特性,实现了一种热流分布均匀、温度波动小的蓝色旋涡状火焰.此外,通过平面激光诱导荧光技术(OH-PLIF)对反应流的OH基分布进行定性测量.研究表明,燃烧核心区与壁面隔离,使得壁面具有自冷却的效应;切向速度沿径向分布呈现中心对称的双峰结构;高速射流经过突扩的喉部强烈吸卷周围的气流,对其起到预热作用,有利于燃料空气的混合和燃烧效率的提高.     

富氧燃烧对投运混煤锅炉低负荷稳燃性的影响

火焰筒头部结构对预混燃烧性能有重要影响,为了探讨旋流器与火焰筒扩张角相互作用关系,试验研究了扩张角为35°（渐扩）、90°（突扩）的火焰筒分别匹配旋流数为0.55,0.75旋流器对燃烧性能的影响。试验结果表明：渐扩火焰筒总压损失较突扩火焰筒减小约3.4%~4.4%,且匹配较小旋流数具有更高的总压恢复系数;突扩火焰筒较渐扩火焰筒具有更低的贫油熄火极限,且无论突扩火焰筒还是渐扩火焰筒,匹配较大旋流数旋流器后均具有更低的熄火极限;突扩型火焰筒温度场对旋流器适应性好,各旋流数下均获得较均匀温度场,出口温度分布系数为0.134 1~0.141 6;渐扩火焰筒温度场对旋流器适应性差,匹配较小旋流数旋流器后温度场均匀性更好,出口温度分布系数为0.135 7;突扩火焰筒NOx排放量更低,且匹配小旋流数旋流器更佳;渐扩火焰筒CO和碳氢化合物（UHC）排放更低,且匹配大旋流数旋流器更佳。     

Effects of swirl on the stability of jet diffusion flames

An experimental study was made, using a double-swirl burner, of the stability of swirling-fuel-jet diffusion flames in swirling air streams. The fuels were hydrogen and methane. The primary variables studied were swirl intensities of the fuel jet and the air stream. It was found that the stability of flame depended on the swirl intensity of both the fuel jet and the air stream. The application of swirl to the fuel jet decreased the rim stability of the flame, but increased the blowout stability of the lifted diffusion flame. For low swirl intensity of the air stream, the effect was similar to that of the fuel jet. At higher swirl intensities of the air stream, above a critical value, the flame stability increased noticeably because of the formation of a recirculation zone near the injector exit. Even in strongly swirling air streams, the favorable effect of fuel swirl on stability of the lifted flame was evident, particularly for the methane flame.     

Mechanism analysis on the pulverized coal combustion flame stability and NOx emission in a swirl burner with deep air staging

Low NOx burner and air staged combustion are widely applied to control NOx emission in coal-fired power plants. The gas-solid two-phase flow, pulverized coal combustion and NOx emission characteristics of a single low NOx swirl burner in an existing coal-fired boiler was numerically simulated to analyze the mechanisms of flame stability and in-flame NOx reduction. And the detailed NOx formation and reduction model under fuel rich conditions was employed to optimize NOx emissions for the low NOx burner with air staged combustion of different burner stoichiometric ratios. The results show that the specially-designed swirl burner structures including the pulverized coal concentrator, flame stabilizing ring and baffle plate create an ignition region of high gas temperature, proper oxygen concentration and high pulverized coal concentration near the annular recirculation zone at the burner outlet for flame stability. At the same time, the annular recirculation zone is generated between the primary and secondary air jets to promote the rapid ignition and combustion of pulverized coal particles to consume oxygen, and then a reducing region is formed as fuel-rich environment to contribute to in-flame NO_X reduction. Moreover, the NOx concentration at the outlet of the combustion chamber is greatly reduced when the deep air staged combustion with the burner stoichiometric ratio of 0.75 is adopted, and the CO concentration at the outlet of the combustion chamber can be maintained simultaneously at a low level through the over-fired air injection of high velocity to enhance the mixing of the fresh air with the flue gas, which can provide the optimal solution for lower NOx emission in the existing coal-fired boilers.     

The influence of fuel-air swirl intensity on flame structures of syngas swirl-stabilized diffusion flame

Flame structures of a syngas swirl-stabilized diffusion flame in a model combustor were measured
using the OH-PLIF method under different fuel and air swirl intensity.The flame operated under
atmospheric pressure with air and a typical low heating-value syngas with a composition of 28.5%
CO,22.5% H2 and 49% N2 at a thermal power of 34 kW.Results indicate that increasing the air swirl
intensity with the same fuel,swirl intensity flame structures showed little difference except a small
reduction of flame length;but also,with the same air swirl intensity,fuel swirl intensity showed great
influence on flame shape,length and reaction zone distribution.Therefore,compared with air swirl
intensity,fuel swirl intensity appeared a key effect on the flame structure for the model
combustor.Instantaneous OH-PLIF images showed that three distinct typical structures with an
obvious difference of reaction zone distribution were found at low swirl intensity,while a much
compacter flame structure with a single,stable and uniform reaction zone distribution was found at
large fuel-air swirl intensity.It means that larger swirl intensity leads to efficient,stable combustion of
the syngas diffusion flame.     

Hydrogen addition effects in a confined swirl-stabilized methane-air flame

The effect of hydrogen addition in methane-air premixed flames has been examined from a swirl-stabilized combustor under confined conditions. The effect of hydrogen addition in methane-air flame has been examined over a range of conditions using a laboratory-scale premixed combustor operated at 5.81 kW. Different swirlers have been investigated to identify the role of swirl strength to the incoming mixture. The flame stability was examined for the effect of amount of hydrogen addition, combustion air flow rates and swirl strengths. This was carried out by comparing adiabatic flame temperatures at the lean flame limit. The combustion characteristics of hydrogen-enriched methane flames at constant heat load but different swirl strengths have been examined using particle image velocimetry (PIV), micro-thermocouples and OH chemiluminescence diagnostics that provided information on velocity, thermal field, and combustion generated OH species concentration in the flame, respectively. Gas analyzer was used to obtain NO_x and CO concentration at the combustor exit. The results show that the lean stability limit is extended by hydrogen addition. The stability limit can reduce at higher swirl intensity to the fuel-air mixture operating at lower adiabatic flame temperatures. The addition of hydrogen increases the NO_x emission; however, this effect can be reduced by increasing either the excess air or swirl intensity. The emissions of NO_x and CO from the premixed flame were also compared with a diffusion flame type combustor. The NO_x emissions of hydrogen-enriched methane premixed flame were found to be lower than the corresponding diffusion flame under same operating conditions for the fuel-lean case.     

An Investigation on the Effect of Air Swirler Vane Angle on Liquid Fuel Combustion Characteristics

This is an experimental and numerical study on the effect of air swirl vane angle on combustion characteristics of liquid fuel burners. The swirl vane angle varied in a range from 0° to 75° and the values of the dependent variables were determined. The flame temperature was measured by an S‐type thermocouple and a Testo 350 XL gas analyzer was used to determine the NO and CO pollutant concentrations. Also, sprint CFD code along with suitable models was used in analytical modeling. The results indicate that there is an optimum angle for the swirl vane (approximately 45° for the case study). At the optimum angle, the average temperature of the flame increases as much as 12.5% and 28.5% in comparison with small and large angles, respectively. Therefore, combustion efficiency reaches its maximum level and CO emission is at an extremely low level. The results also demonstrate that large swirl angles decreases NO emission.     

Flame characteristics of hydrogen-enriched methane–air premixed swirling flames

The effect of hydrogen addition in methane–air premixed flames has been examined from a swirl-stabilized combustor under unconfined flame conditions. Different swirlers have been examined to investigate the effect of swirl intensity on enriching methane–air flame with hydrogen in a laboratory-scale premixed combustor operated at 5.81 kW. The hydrogen-enriched methane fuel and air were mixed in a pre-mixer and introduced into the burner having swirlers of different swirl vane angles that provided different swirl strengths. The combustion characteristics of hydrogen-enriched methane–air flames at fixed thermal load but different swirl strengths were examined using particle image velocimetry (PIV), OH chemiluminescence, gas analyzers, and micro-thermocouple diagnostics to provide information on flow field, combustion generated OH radical and gas species concentration, and temperature distribution, respectively. The results show that higher combustibility of hydrogen assists to promote faster chemical reaction, raises temperature in the reaction zone and reduces the recirculation flow in the reaction zone. The upstream of flame region is more dependent on the swirl strength than the effect of hydrogen addition to methane fuel. At lower swirl strength condition the NO concentration in the reaction zone reduces with increase in hydrogen content in the fuel mixture. Higher combustibility of hydrogen accelerates the flow to reduce the residence time of hot product gases in the high temperature reaction zone. At higher swirl strength the NO concentration increases with increase in hydrogen content in the fuel mixture. The effect of dynamic expansion of the gases with hydrogen addition appears to be more dominant to reduce the recirculation of relatively cooler gases into the reaction zone. NO concentration also increases with decrease in the swirl strength.     

旋流预混燃烧室燃烧特性及排放特性的数值模拟研究

为了综合考察燃气轮机燃烧室在高稳定性、低排放以及燃料适应性等方面的新要求,基于旋流预混燃烧技术,通过三维数值模拟方法开展了甲烷/空气、丙烷/空气预混燃烧特性及排放特性研究。结果表明：在一定的预混气进气质量流量条件下,当量比增大易引发回火,燃烧温度更高,同时NO_x排放指数增大,增加预混气质量流量,可在一定程度上提高回/熄火极限;当量比固定,增加预混气进气质量流量可避免潜在的回火现象,且NO_x排放指数线性降低;旋流器的旋流数增大能形成强旋流,稳定火焰,降低NO_x排放指数,但过大的旋流强度会引发回火现象;相比于甲烷/空气预混燃烧,丙烷/空气预混燃烧温度偏高,NO_x排放指数较大,但回熄火边界更宽,对应更广阔的稳定燃烧区间。     

Stability characteristics of non-premixed turbulent jet flames of hydrogen and syngas blends with coaxial air

The stability characteristics of attached hydrogen (H₂) and syngas (H₂/CO) turbulent jet flames with coaxial air were studied experimentally. The flame stability was investigated by varying the fuel and air stream velocities. Effects of the coaxial nozzle diameter, fuel nozzle lip thickness and syngas fuel composition are addressed in detail. The detachment stability limit of the syngas single jet flame was found to decrease with increasing amount of carbon monoxide in the fuel. For jet flames with coaxial air, the critical coaxial air velocity leading to flame detachment first increases with increasing fuel jet velocity and subsequently decreases. This non-monotonic trend appears for all syngas composition herein investigated (50/50 → 100/0% H₂/CO). OH^∗ chemiluminescence imaging was performed to qualitatively identify the mechanisms responsible for the flame detachment. For all fuel compositions, local extinction close to the burner rim is observed at lower fuel velocities (ascending stability limit), while local flame extinction downstream of the burner rim is observed at higher fuel velocities (descending stability limit). Extrema of the non-monotonic trends appear to be identical when the nozzle fuel velocity is normalized by the critical fuel velocity obtained for the single jet cases.     

Combustion in swirling flows: A review

Swirling flows have been commonly used for a number of years for the stabilization of high-intensity combustion processes. In general these swirling flows are poorly understood because of their compelexity. This paper describes the recent progress in understanding and using these swirling flows. The main effects of swirl are to improve flame stability as a result of the formation of toroidal recirculation zones and to reduce combustion lengths by producing high rates of entrainment of the ambient fluid and fast mixing, particularly near to the boundaries of recirculation zones. Two main types of swirl combustor can be identified as follows:The Swirl Burner. Here swirling flow exhausts into a furnace or cavity combustion occurs in and just outside the burner exit.The Cyclone Combustion Chamber. Here air is injected tangentially into a large, usually, cylindrical chamber and exhausts through a centrally located exit hole in one end. Combustion mostly occurs inside the cyclone chamber.Initially the isothermal performance of swirl combustors is considered, and it is demonstrated that, contrary to many previous assumptions, the flow is often not axisymmetric but three-dimensional time-dependent. Under most normal nonpremixed combustion conditions, the swirling flow returns to axisymmetry, although there is still a residual presence of the three-dimensionality, particularly on the boundary of the reverse flow zone. Swirl increases considerably the stability limits of most flames; in fact with certain swirl burners, the blow-off limits are virtually infinite. Cyclone combustion chambers have large internal reverse flow zones which provide very long residence times for the fuel/air mixture. They are typically used for the combustion of difficult materials such as poor quality coal or vegetable refuse. In contrast to the swirl burner which usually has one central toroidal, recirculation zone, the cyclone combustor often has up to three concentric toroidal recirculation zones. Sufficient information is also available to indicate that stratified or staged fuel or air entry may be used to minimize noise, hydrocarbon, and NO_x emissions from swirl combustors.