多入口多孔介质燃烧器的数值模拟

在多入口燃烧器内加入多孔介质,以甲烷/空气为燃料,采用非预混燃烧的数值模拟方法,探究多入口燃烧器的燃烧情况.对比多孔介质燃烧与空间自由燃烧,分析了"超焓燃烧"现象;在多孔介质燃烧基础上,探究不同当量比对燃烧温度的影响;在多孔介质燃烧和不同当量比的基础上探究污染物CO和CO_2的排放情况.结果表明:多孔介质燃烧可以实现"超焓燃烧"特性,燃烧火焰温度高于自由空间燃烧温度;当量比对燃烧温度影响很大,随着当量比的增大,燃烧器内最高燃烧温度升高,但燃烧过程存在一个最佳当量比0.6,超过该当量比后最高温度将不再变化;多入口多孔介质燃烧有助于减少CO和CO_2的生成量.     

低热值C2H4-AIR-N2预混气体在多孔介质内的燃烧特性研究

利用自行设计的多孔介质实验台,对C_2H_4-AIR-N_2预混气体在多孔介质燃烧器内的燃烧特性进行了实验研究,分析燃料当量比、预混气体流速以及N_2稀释比对预混气体的可燃极限、火焰传播方向、火焰温度分布以及污染物排放的影响。研究表明:随着稀释比的上升,预混气体的可燃极限范围缩小,火焰向上游传播的工况逐渐减少;燃烧器内最高火焰温度与当量比以及气体流速正相关,与稀释比负相关;CO的排放量随着稀释比的上升而增加,与当量比以及气体流速负相关;实验中的NO排放量小于20 mg/m~3。     

多孔介质中预混火焰猝熄及自稳定性研究

分析了多孔介质中预混火焰的猝熄效应,试验测定了一系列工况下泡沫陶瓷的猝熄直径和自稳定范围,为多孔介质燃烧器的开发设计提供了依据。通过分析发现,猝熄直径受到多个参数的影响,包括：混合气体的流速u、预混气体的层流火焰传播速度SL、燃烧室空管Re、预混气体的导温系数a、当量比φ以及多孔介质固体温度Ts。通过对多孔介质中燃烧的自稳定性试验研究,发现了多孔介质燃烧器中火焰稳定极限(吹脱极限和回火极限)与多孔介质平均孔径和气流速度及燃烧当量比的关系。     

多孔燃烧器中氨/空气燃烧特性数值研究

氨具有氢密度高、生产成本低、基础设施完善等优点,作为一种潜在的可再生替代燃料受到了广泛的关注。目前,仅有少数研究关注氨气燃烧喷嘴的研究,针对氨气稳定燃烧喷嘴的研究尤其不足。为实现氨燃料的稳定燃烧和低污染物排放,本研究提出了一种氨用多孔介质燃烧器。对氨用多孔介质燃烧器建立了二维数值模型,并对预混氨/空气在多孔介质燃烧器中的燃烧性能进行了评价,考察了不同进口速度u₀、当量比Φ和多孔介质导热系数对氨/空气火焰特性和NO排放的影响。结果表明,多孔介质燃烧器能在u₀ = 3 ~ 7 m/s和Φ = 0.9 ~ 1.2条件下稳定燃烧;随着多孔介质导热系数的增大,火焰最高温度下降且火焰位置向上游移动;减小进口速度和增大当量比能够显著降低NO的排放。     

两层多孔介质燃烧器的数值模拟

通过一维数值模拟研究了预混气体在两层多孔介质燃烧器内的燃烧特性,着重研究两层多孔介质燃烧器中的超绝热燃烧和火焰的稳定区域。结果表明,预混气体在两层多孔介质内可以发生一定程度的超绝热燃烧,贫燃极限可以扩展到0.45。两层多孔介质能够在较宽的流速范围内将火焰稳定在它的交界面上。数值预测的最小和最大火焰传播速度与实验取得了相同的趋势,其火焰传播速度至少是自由空间中的3倍。     

催化增强的两段式多孔介质甲烷富燃特性

针对甲烷的富燃燃烧,设计开发了包含不同直径大小氧化铝球的两段式多孔介质燃烧器系统,研究了甲烷在添加催化剂前后的部分氧化富燃重整制取合成气的相关特性.通过调节入口气体流速和当量比,确定了多孔介质燃烧器的稳定操作区间,研究了流速和当量比对多孔介质燃烧中温度和组分的影响.结果表明,采用Ni基催化剂担载下游氧化铝球后,入口气体流速为0.15m/s,当量比为1.6时,甲烷的重整转化率可以提升至43.5%.     

基于多孔介质燃烧的端部辐射器的实验研究

设计了基于多孔介质燃烧技术的端部辐射器,研究不同预混气体流速(功率)下当量比对燃烧器燃烧稳定性、多孔介质内部温度、辐射器表面温度及其均匀性、污染物排放、辐射效率等特性的影响.结果表明,燃烧器辐射表面的温度均匀性较好.最大相对温差小于3%:多孔介质燃烧器可实现最低当量比0.33的稳定可持续燃烧;小功率燃烧时.多孔介质内部温度及端部辐射表面温度都随当量比增大而增加,且流量越大增加程度越大,可据此提出实现更高辐射表面温度的方案.实验工况范围内.最大辐射效率达23%;NO<,x>排放体积分数低于25×10<'6>,在当量比大于0.45时,CO排放体积分数均低于10×10<'6>.     

CH4/air预混气体双层多孔介质燃烧特性数值模拟研究

基于局部热平衡假设,定义了无量纲参数——火焰宽度比,在过量空气系数为1.2条件下,研究了CH_4/air预混气体在双层多孔介质中浸没燃烧和表面燃烧的燃烧特性。结果表明:表面燃烧具有更高的烟气出口温度以及更高的火焰宽度比。浸没燃烧火焰轮廓类似抛物线,而表面燃烧的火焰宽度比则基本不变;入口速度不同时,距离着火面同一位置浸没燃烧火焰宽度基本保持不变,火焰轮廓仍类似于抛物线;而对于表面燃烧,相同位置的火焰宽度比在很小范围内(约为0.03)呈现先增大后减小的变化规律。同时研究也表明表面燃烧具有更高的NO_x排放,且随着速度的增加两种燃烧方式NO_x排放变化规律一致,均呈现先增大后减小再增加的变化规律。在速度为1.1～1.2μm/s左右时,两种燃烧方式NO_x排放大致相当,这表明在不扩大污染的条件下,可以使用表面燃烧获得更多的对外辐射以节约能源。     

关于过滤燃烧非平衡性和火焰结构特性的研究

为研究预混气体在小球填充床中的过滤燃烧特性和火焰结构,建立了简化的二维多孔介质结构模型,考虑燃烧过程中存在气固对流换热及多孔介质固体辐射换热的影响,在获取了填充床中的火焰传播速度和温度分布特点后。采用化学反应速度表征孔隙间的火焰结构特性和传播速度,并通过实验加以验证,结果表明,过滤燃烧系统的反应区域存在着明显的非平衡性。在化学反应区域,气固温差非常大。同一小球内部的温度分布也不均衡。流速和当量比对火焰面结构的影响很大。随流速的增大,火焰锋面变得弯曲,抛物线形状变得非常明显。随当量比的增大,火焰面变得平整。     

多孔介质特性参数对燃烧温度及压力影响的数值模拟研究

考查了两段式多孔介质内预混气燃烧的温度与压力分布情况。建立了甲烷／空气预混气体在多孔介质内燃烧的二维数学模型,运用FLUENT软件求解瞬态控制方程的方法计算出燃烧稳定后多孔介质内的温度、与压力分布,并考查了不同当量比、多孔介质辐射衰减系数和导热系数对温度和压力分布的影响。结果表明,甲烷／空气预混气体在多孔介质中燃烧,当量比越大温度峰值越高,压力梯度越大;小孔介质辐射衰减系数的改变对温度分布和压力分布没有明显的影响,而大孔介质辐射衰减系数对温度分布和压力分布有较大的影响;增加多孔介质的导热系数,会使固相与气相温度均有所升高,燃烧区域压力降低。     

Experimental study of propane/air premixed flame dynamics with hydrogen addition in a meso-scale quartz tube

The combustion process of propane/air premixed flame in meso-scale quartz tubes with different hydrogen additions was investigated experimentally to explain the flame-wall interaction mechanism. The ranges of different flame regimes were obtained by changing the flow rates of propane and hydrogen. The effects of hydrogen addition, inlet velocity and equivalence ratio were analyzed. The results show that the hydrogen addition broadens the operation ranges of fast flame regime and slow flame regime significantly. The flame propagation speed is in the same order of the thermal wave speed in solid wall for the slow flames. In fast flame regime, the flame propagation speed has an inverse correlation with the inlet flow velocity irrespective of the equivalence ratio. With the increase of the equivalence ratio, the maximum flame speed in fast flame regime decreases gradually, while the maximum flame speed in slow flame regime increases continually. It indicates that rich fuel condition suppresses the fast flame and promotes the slow flame. In slow flame regime, the output thermal efficiency is dominated by the inlet velocity and equivalence ratio.     

Experimental study on premixed combustion of spherically propagating methanol-air-nitrogen flames

The outward propagation and development of surface instability of the spark-ignited spherical premixed flames for methanol-air-nitrogen mixtures were experimentally studied by using a constant volume combustion chamber and a high-speed schlieren photography system. The laminar burning velocities, the mass burning fluxes, and the Markstein lengths were obtained at different equivalence ratios, dilution ratios, initial temperatures, and pressures. The laminar burning velocities and the mass burning fluxes give a similar curve versus the equivalence ratios. They increase with the increase of initial temperature and decrease with the increase of dilution ratio. The laminar burning velocity decreases with elevating the initial pressure, while the mass burning flux increases with the increase of the initial pressure. Markstein length decreases slightly with the increase of initial temperature for the rich mixtures. High initial pressure corresponds to low Markstein length. Markstein length increases with the increase of dilution ratio, which is more obvious when the mixture becomes leaner. Equivalence ratio has a slight impact on the development of the diffusive-thermal cellular structure at elevated initial pressures. The initial pressure has a significant influence on the occurrence of the flame front cellular structure. At the elevated pressures, the cracks on the flame surface branch and develop into the cell structure. These cells are bounded by cracks emitting a bright light, which may indicate soot formation. For very lean mixture combustion, the buoyancy effect and cooling effect from the spark electrodes have a significant impact on the flame propagation. The hydrodynamic instability, inhibited with the increase of initial temperature around the stoichiometric equivalence ratio, is enhanced with the increase of initial pressure and suppressed by mixture dilution.     

Flame dynamics of hydrogen/air mixture in a wavy micro-channel

The focus of this work is the numerical study of stable and pulsatory flame burst in an undulating geometry, using premixed hydrogen and air (with an equivalence ratio of φ = 1.0). This work extends other works in the literature by considering a linear temperature profile along the wall. This allows an analysis of the flow dynamics without forcing the location of the flame (as is the case with hyperbolic temperature profiles). The interaction between the flow dynamics and the combustion reaction is then analysed, leading to a better understanding of the physics in more general flows.Simulations were performed in OpenFoam using very detailed chemical reactions and different molecular diffusivities for each species. The results obtained show that at low inlet velocity (4 m/s) the flame became stable, and, at higher inlet velocities, the flame showed pulsatory burst dynamics. The interaction between the fluid dynamics and the combustion response proved to be important, especially because of the vortices that are formed due to the nonlinear geometry of the burner. As the inlet velocity increases, the heat release rate transmitted through the vortices decreases and a delay in ignition occurs, as evidenced by a decrease in the pulsatory burst frequency and an increase in the maximum value of the heat release rate (although not sufficient to increase the maximum temperature amplitude).In addition, we also carried out an analyses of the axial velocity and of the H₂ and OH mass fractions of the flame dynamics.     

Fuel‐lean VOCs combustion in a porous burner stacked with alumina balls: A case for ethylene combustion

A porous burner stacked in turn with 3‐ and 9‐mm alumina pellets was established to perform C₂H₄ combustion experiments by acquiring the flammable limits, temperature variation characteristics, combustion wave velocity, pollutant emissions, and treatment efficiency. The burner operated well at equivalence ratios within 0.3 to 0.7. Larger alumina pellets widened the burner's lower flammable limit. As the flame propagated downstream, the higher premixed gas flow velocity and larger alumina pellets, the higher combustion wave velocity, whereas the circumstances were opposite as the flame spread upstream. The combustion temperature increased with the equivalence ratio and premixed gas flow velocity. In response to the effect of the alumina pellet dimension, 3‐mm alumina pellets corresponded to higher combustion temperatures, lower CO emissions, and higher treatment efficiency than those less than 9‐mm conditions.     

进气参数对LPP燃烧室振荡燃烧特性和火焰结构的影响

为研究贫预混预蒸发(LPP)燃烧室振荡燃烧规律和LPP火焰结构,利用动态压力传感器测量了LPP燃烧室内不同进气参数下时域及频域上的压力脉动;利用激光诱导荧光(PLIF)测量系统研究了不同进气参数下的LPP火焰结构变化规律。结果表明:随着燃烧室入口流速的增加,激励出的振荡燃烧的当量比区域会减小;在一定的入口流速下,所激励的振荡燃烧主频会随着当量比的增加而增加;随着燃烧室入口空气温度的提高,激励出振荡燃烧的区域会减小,激励出的振荡燃烧的强度会下降,但振荡燃烧的主频均会增加;稳定燃烧时,LPP火焰为V型火焰;振荡燃烧则会将LPP火焰转化为平整型火焰。     

异丁醇/辛酸甲酯混合燃料预混层流火焰速度的研究

针对生物柴油与醇类混合燃料燃烧机理研究的需求,采用高速纹影光学诊断方法和定容燃烧弹系统试验研究了异丁醇/辛酸甲酯混合燃料的预混层流燃烧特性。测量了不同当量比和初始压力条件下的不同配比混合燃料—空气预混合气的层流燃烧火焰速度,火焰拉伸率以及马克斯坦长度。分析了燃烧初始条件及异丁醇掺混比例对混合燃料的无拉伸层流燃烧速度及火焰不稳定性的影响规律。结果表明:异丁醇/辛酸甲酯混合燃料的拉伸层流火焰传播速度和层流火焰燃烧速度随着当量比的增加先增加后减少,随着初始压力的增加而减小;马克斯坦长度随着当量比和初始压力的增加而减小;异丁醇掺混比例的增加加快了层流火焰燃烧速度,但使得火焰的不稳定性倾向增加。     

Experimental study of premixed syngas/air flame deflagration in a closed duct

The propagation behaviour of a deflagration premixed syngas/air flame over a wide range of equivalence ratios is investigated experimentally in a closed rectangular duct using a high-speed camera and pressure transducer. The syngas hydrogen volume fraction, φ, ranges from 0.1 to 0.9. The flame propagation parameters such as flame structure, propagation time, velocity and overpressure are obtained from the experiment. The effects of the equivalence ratio and hydrogen fraction on flame propagation behaviour are examined. The results indicate that the hydrogen fraction in a syngas mixture greatly influences the flame propagation behaviour. When φ, the hydrogen fraction, is ≥0.5, the prominently distorted tulip flame can be formed in all equivalence ratios, and the minimum propagation time can be obtained at an equivalence ratio of 2.0. When φ < 0.5, the tulip flame distortion only occurs in a hydrogen fraction of φ = 0.3 with an equivalence ratio of 1.5 and above. The minimum flame propagation time can be acquired at an equivalence ratio of 1.5. The distortion occurs when the maximum flame propagation velocity is larger than 31.27 m s^?1. The observable oscillation and stepped rise in the overpressure trajectory indicate that the pressure wave plays an important role in the syngas/air deflagration. The initial tulip distortion time and the plane flame formation time share the same tendency in all equivalence ratios, and the time interval between them is nearly constant, 4.03 ms. This parameter is important for exploring the quantitative theory or models of distorted tulip flames.     

Flame–vortex interaction driven combustion dynamics in a backward-facing step combustor

The combustion dynamics of propane–hydrogen mixtures are investigated in an atmospheric pressure, lean, premixed backward-facing step combustor. We systematically vary the equivalence ratio, inlet temperature and fuel composition to determine the stability map of the combustor. Simultaneous pressure, velocity, heat release rate and equivalence ratio measurements and high-speed video from the experiments are used to identify and characterize several distinct operating modes. When fuel is injected far upstream from the step, the equivalence ratio entering the flame is temporally and spatially uniform, and the combustion dynamics are governed only by flame–vortex interactions. Four distinct dynamic regimes are observed depending on the operating parameters. At high but lean equivalence ratios, the flame is unstable and oscillates strongly as it is wrapped around the large unsteady wake vortex. At intermediate equivalence ratios, weakly oscillating quasi-stable flames are observed. Near the lean blowout limit, long stable flames extending from the corner of the step are formed. At atmospheric inlet temperature, the unstable mode resonates at the 1/4 wavemode of the combustor. As the inlet temperature is increased, the 5/4 wavemode of the combustor is excited at high but lean equivalence ratios, forming the high-frequency unstable flames. Higher hydrogen concentration in the fuel and higher inlet temperatures reduce the equivalence ratios at which the transitions between regimes are observed. We plot combustion dynamics maps or the response curves, that is the overall sound pressure level as a function of the equivalence ratio, for different operating conditions. We demonstrate that numerical results of strained premixed flames can be used to collapse the response curves describing the transitions among the dynamic modes onto a function of the heat release rate parameter alone, rather than a function dependent on the equivalence ratio, inlet temperature and fuel composition separately. We formulate a theory for predicting the critical values of the heat release parameter at which quasi-stable to unstable and unstable to high-frequency unstable modes take place.