弱旋湍流火焰合成纳米二氧化钛的机理研究

湍流效应在火焰合成纳米TiO2颗粒工业化规模放大中具有重要影响.本文基于一种内加环形旋流片的弱旋火焰开展了合成TiO2纳米颗粒的实验研究.对于内径12 mm管式燃烧器,加入旋片后火焰贫燃极限大幅度降低16%～33%,火焰高度也从9.0 cm减少到3.0 cm,焰区温度约减少20%～30%.研究表明,弱旋湍流火焰具有流速大、可燃当最比低和温度低的特点,使其相对无旋火焰可以合成粒径较小、烧结程度较低的TiO2纳米颗粒.     

滞止旋流火焰合成纳米TiO_2颗粒的实验研究

湍流效应是火焰合成纳米TiO2颗粒工业化规模放大的一个重要影响因素.本文基于一种新提出的滞止旋流火焰开展了合成TiO2纳米颗粒的实验研究.在旋流数0.26的条件下,可成功获得比表面积300㎡/g以上,粒径最小达5nm的锐钛矿型TiO2颗粒,这可归因于本文滞止旋流火焰具有贫燃稳定性、速度高反应快、温度梯度大等特点.实验还...     

NH3/H2预混旋流火焰稳定性及燃烧极限实验研究

采用叶轮型旋流燃烧器,选取氢气作为燃料添加剂,研究了掺氢比对氨气旋流火焰稳定性的影响,分析了不同旋流数、叶片数、当量比以及预混气总流量条件下,旋流火焰形态变化。测定并分析了不同参数对旋流火焰燃烧极限范围的影响。结果表明,随掺氢比的增大,火焰逐渐由“V”型转化为稳定的“M”型,燃烧反应愈发充分;高旋流数(1.27)或低叶片数(6片)相比低旋流数(0.42)或高叶片数(8片)更有利于旋流火焰的稳定和燃烧的充分进行;相比富燃,贫燃有利于形成稳定的旋流火焰;预混气总流量较大时,火焰高度较高.对于燃烧极限,掺氢比越高,极限范围越大;总流量的变化对贫燃极限影响较小,对富燃极限影响较大;高旋流数(1.27)条件下,燃烧极限范围较大。     

氨气预混旋流燃烧火焰稳定性及燃烧极限研究

采用叶轮型旋流燃烧器,研究了旋流数、叶片数以及流量等因素对氨气预混旋流燃烧火焰稳定性和燃烧极限的影响.实验结果表明,在一定当量比下,氨气预混旋流燃烧火焰会失稳发生回火或振荡抬举;随着旋流数的增大或叶片数的增加,火焰更易失稳发生回火;石英玻璃高度越高,内部流场结构越完整,火焰高度越高。氨气预混旋流火焰贫燃极限在φ=0.64～0.76之间,富燃极限在φ=1.47～1.74之间。随着总流量的增大,贫燃极限逐渐增大,富燃极限波动较大,总体燃烧极限范围变大;随着旋流数的增大、叶片数的增加或石英玻璃高度的升高,燃烧极限范围变窄。     

高旋流数管状火焰纳米颗粒合成实验研究

针对火焰气相合成燃烧器产量低的局限性,在弱旋流滞止火焰的基础上发展了高旋流数管状滞止火焰燃烧器,并选用了TTIP作为前驱物开展纳米TiO_2合成实验研究.通过在燃烧器中添加射流管装置,与弱旋流滞止火焰相比,在保持纳米颗粒的粒径及比表面积不变的情况下,燃烧器单位燃料产生颗粒量可提高10倍以上.在单一合成实验基础上,进一步开展了掺杂合成实验研究,通过独立控制轴向和切向进给,得到了掺杂均匀的二氧化钛载体钯基催化剂。     

低热值燃料的滑动弧稳燃实验研究

利用旋流实验台进行了滑动弧稳燃低热值燃料的研究,并探究了滑动弧对低热值燃料的火焰特性及熄火极限的影响规律。实验结果表明：低热值燃料燃烧随着当量比变化可以分为五种典型火焰形态。滑动弧可缩短火焰长度,增大火焰张角,使火焰驻定位置靠近角涡回流区和剪切层,从而增强燃烧稳定性。随着滑动弧的加入,角涡火焰间歇消失的不稳定过渡形态得到了极大的改善。随着空气流量的增加,滑动弧的稳燃效果逐渐减弱。滑动弧可有效拓宽低热值燃料的熄火极限。在空气流量40L/min时,稳燃极限当量比最大拓宽了0.35,熄火极限最大拓宽了0.33。提高滑动弧的输入电压对低热值燃料的熄火极限拓宽明显。     

近贫燃极限甲烷/空气火焰的层流燃烧速度研究

利用微重力条件下向外传播的球形火焰,对贫燃极限附近甲烷/空气预混火焰的层流燃烧速度进行了测量,得到当量比从0.512(本文微重力实验中测定的可燃极限)到0.601范围内的零拉伸层流燃烧速度,并与前人实验数据和使用3种化学反应动力学模型的计算结果进行了比较.本文实验结果与已有的微重力实验数据非常接近,而其他研究者在常重力...     

燃气轮机合成气双旋流非预混燃烧室的设计及实验测试

本文针对上海交通大学25 kW燃气轮机性能试验台的合成气燃烧室开展了设计研究,完成了燃烧室样机的加工与实验测试。研究过程中,首先对合成气燃烧室开展了结构设计;采用双旋流结构的燃烧器进行合成气燃烧火焰组织;采用了燃烧室头部贫燃方式(低当量比)设计以保证燃烧室低排放特性;利用双层壁冷却方式进行火焰筒壁面冷却。在燃烧室结构设计的基础上,利用数值方法系统分析了合成气双旋流非预混燃烧室工作特性,完成了合成气非预混母型燃烧室的设计优化。根据优化方案,完成了燃烧室样机的加工、安装,并进行了实验性能测试。实验结果表明实验工况该燃烧室燃烧稳定,NO_x排放小于25 mg/m~3@15%O_2。     

电场对稀燃预混钝体旋流火焰结构的影响

本文研究了开放空间内的直流电场对甲烷稀燃预混钝体旋流火焰结构的影响。施加电场后,火焰面向上游移动,火焰面之间的夹角增大。电场能够提高钝体旋流火焰中上部分的火焰面褶皱率,从而提高湍流火焰速度,但是对根部火焰面结构没有影响。电场对最大火焰面密度没有明显作用,而是通过提高火焰刷厚度提高湍流火焰速度。预混气当量比和燃烧器出口平均速度能够影响电场作用强度。当量比越低,出口平均速度越高,电场的作用越不明显。电场对钝体旋流火焰根部结构影响较弱,且在临近吹熄工况下电场流体动力学作用很弱,对钝体旋流火焰的稀燃吹熄极限没有明显影响。     

非预混旋流火焰中部熄火及重新稳燃机理研究

本文利用5 kHz同步PIV/OH-PLIF实验装置,结合大涡模拟(LES)耦合PDF燃烧模型,系统研究燃料中心喷射下非预混旋流火焰中部熄火及再次稳燃机理,对比了两种旋流器出口结构的影响.结果 表明,LES-PDF模型可以准确地捕捉到旋流流场分布及火焰形态,包括中部熄火及火焰重新稳定.相比于直燃道的结构,扩张燃道的存在引导了更宽的回流区,改善了中部熄火现象,两种结构对其下游的主火焰再次稳燃高度影响不大.中央燃气射流带来的高标量耗散率使得热损失变大,从而引起中部熄火.同时进一步促进了CH4与空气的预先混合及部分反应前置物(CH2O),为非预混旋流火焰在下游重新稳燃提供了有利条件.通过对火焰再次稳燃处OH反应项及扩散项的分析,发现反应项占据主导地位,部分预混火焰传播为再次稳燃的主要机理.     

Characterization of premixed swirling methane/air diffusion flame through filtered Rayleigh scattering

Characteristics of a premixed, swirl methane/air diffusion flame at atmospheric pressure are measured by filtered Rayleigh scattering (FRS). Three operating conditions are investigated with the equivalence ratios of the methane/air flame covering a range of 0.67—0.83. Under each condition, single-shot and averaged FRS images over a region measured 39.3×65.6 mm² at seven cross sections of the flame are collected to demonstrate the flame behavior. A gradient calculation algorithm is applied to identify reaction zone locations and structures in the instantaneous FRS measurements. Statistical analysis for the mean FRS measurements is performed by means of joint probability density functions. The experimental results indicate that thermochemical state of the swirl flame is strongly influenced by equivalence ratio, leading to varieties of flame structures and temperature distributions. The gradient of the instantaneous FRS images clearly illustrates the characteristics of the reaction zone. The results also demonstrate that FRS can provide detailed insights into the behavior of turbulent flames.     

Application of the Lagrangian CMC method in elliptic bluff body and swirl flames

The Lagrangian CMC method was implemented in the open source programme OpenFOAM and applied to turbulent nonpremixed bluff body and swirl flames. Lagrangian CMC is more efficient than Eulerian CMC with the number of Lagrangian flame groups much less than the number of computational cells for Eulerian CMC equations in general. It is based on the conditional flame structure depending on the residence time of the fuel of fixed Lagrangian identity from the nozzle. According to sensitivity study the injected fuel was divided into ten flame groups according to the injection sequence with the resulting conditional profiles between those by ISR and Eulerian CMC. Minor deviation from Eulerian CMC was attributed to the flame structure that is difficult to be characterised by the residence time only in elliptic recirculating flows of the bluff body and swirl flames. The Eulerian and Lagrangian CMC showed the same trend of deviation from measurements for conditional temperature, H₂O, OH, CO and H₂ mass fractions. The significant deviation of H₂ was due to uncertainty in the reaction chemistry, as observed in the previous works based on other reaction mechanisms for methane and methanol.     

Identification and analysis of instability in non-premixed swirling flames using LES

Large eddy simulations (LES) of turbulent non-premixed swirling flames based on the Sydney swirl burner experiments under different flame characteristics are used to uncover the underlying instability modes responsible for the centre jet precession and large scale recirculation zone. The selected flame series known as SMH flames have a fuel mixture of methane-hydrogen (50:50 by volume). The LES solves the governing equations on a structured Cartesian grid using a finite volume method, with turbulence and combustion modelling based on the localised dynamic Smagorinsky model and the steady laminar flamelet model respectively. The LES results are validated against experimental measurements and overall the LES yields good qualitative and quantitative agreement with the experimental observations. Analysis showed that the LES predicted two types of instability modes near fuel jet region and bluff body stabilised recirculation zone region. The mode I instability defined as cyclic precession of a centre jet is identified using the time periodicity of the centre jet in flames SMH1 and SMH2 and the mode II instability defined as cyclic expansion and collapse of the recirculation zone is identified using the time periodicity of the recirculation zone in flame SMH3. Finally frequency spectra obtained from the LES are found to be in good agreement with the experimentally observed precession frequencies.     

Impact of swirl and bluff-body on the transfer function of premixed flames

The frequency response of three lean methane/air flames submitted to flowrate perturbations is analyzed for flames featuring the same equivalence ratio and thermal power, but a different stabilization mechanism. The first flame is stabilized by a central bluff body without swirl, the second one by the same bluff body with the addition of swirl and the last one only by swirl without central insert. In the two last cases, the swirl level is roughly the same. These three flames feature different shapes and heat release distributions, but their Flame Transfer Function (FTF) feature about the same phase lag at low frequencies. The gain of the FTF also shows the same behavior for the flame stabilized by the central insert without swirl and the one fully aerodynamically stabilized by swirl. Shedding of vortical structures from the injector nozzle that grow and rollup the flame tip controls the FTF of these flames. The flame stabilized by the swirler-plus-bluff-body system features a peculiar response with a large drop of the FTF gain around a frequency at which large swirl number oscillations are observed. Velocity measurements in cold flow conditions reveal a strong reduction of the size of the vortical structures shed from the injector lip at this forcing condition. The flame stabilized aerodynamically only by swirl and the one stabilized by the bluff body without swirl do not exhibit any FTF gain drop at low frequencies. In the former case, large swirl number oscillations are still identified, but large vortical structures shed from the nozzle also persist at the same forcing frequency in the cold flow response. These different flame responses are found to be intimately related to the dynamics of the internal recirculation region, which response strongly differs depending upon the injector used to stabilize the flame.     

Stability characteristics and flowfields of turbulent non-premixed swirling flames

A simple, yet representative, burner geometry is used for the investigation of highly swirling turbulent unconfined, non-premixed, flames of natural gas. The burner configuration comprises a ceramic faced bluff-body with a central fuel jet. The bluff-body is surrounded by an annulus that delivers a swirling primary flow of air. The entire burner assembly is housed in a wind tunnel providing a secondary co-flowing stream of air. This hybrid bluff-body/swirl burner configuration stabilizes complex turbulent flames not unlike those found in practical combustors, yet is amenable to modelling because of its well-defined boundary conditions. Full stability characteristics including blow-off limits and comprehensive maps of flame shapes are presented for swirling flames of three different fuel mixtures: compressed natural gas (CNG), CNG–air (1:2 by volume) and CNG–H₂ (1:1 by volume).

It is found that with increased fuel flow, flame blow-off mode may change with swirl number, S_g. At low swirl, the flame remains stable at the base but blows off in the neck region further downstream. At higher swirl numbers, the flames peel off completely from the burner's base. Swirling CNG–air flames are distinct in that they only undergo base blow-off. In the low range of swirl number, increasing S_g causes limited improvement in the blow-off limits of the flames investigated and (for a few cases) can even lead to some deterioration over a small intermediate range of S_g. It is only above a certain threshold of swirl that significant improvements in blow-off limits appear. Six flames are selected for further detailed flowfield and composition measurements and these differ in the combination of swirl number, primary axial velocity through the annulus, U_s, and bulk fuel jet velocity, U_j. Only velocity field measurements are presented in this paper. A number of flow features are resolved in these flames, which resemble those already associated with non-reacting swirling flows of equivalent swirl obtained with the present burner configuration. Additionally, asymmetric flowfields inherent to some flames are revealed where the fluidic centreline of the flow (defined in the two-dimensional (U–W velocity pair) velocity field by the ?ω? = 0 tangential velocity contour), meanders strongly on either side of the geometric centreline downstream by about one bluff-body diameter. Flow structures revealed by the velocity data are correlated to flame shapes to yield a better understanding of how the velocity field influences the flames physical characteristics.     

Prediction of soot and thermal radiation in a model gas turbine combustor burning kerosene fuel spray at different swirl levels

Combustion of kerosene fuel spray has been numerically simulated in a laboratory scale combustor geometry to predict soot and the effects of thermal radiation at different swirl levels of primary air flow. The two-phase motion in the combustor is simulated using an Eulerian–Lagragian formulation considering the stochastic separated flow model. The Favre-averaged governing equations are solved for the gas phase with the turbulent quantities simulated by realisable k–? model. The injection of the fuel is considered through a pressure swirl atomiser and the combustion is simulated by a laminar flamelet model with detailed kinetics of kerosene combustion. Soot formation in the flame is predicted using an empirical model with the model parameters adjusted for kerosene fuel. Contributions of gas phase and soot towards thermal radiation have been considered to predict the incident heat flux on the combustor wall and fuel injector. Swirl in the primary flow significantly influences the flow and flame structures in the combustor. The stronger recirculation at high swirl draws more air into the flame region, reduces the flame length and peak flame temperature and also brings the soot laden zone closer to the inlet plane. As a result, the radiative heat flux on the peripheral wall decreases at high swirl and also shifts closer to the inlet plane. However, increased swirl increases the combustor wall temperature due to radial spreading of the flame. The high incident radiative heat flux and the high surface temperature make the fuel injector a critical item in the combustor. The injector peak temperature increases with the increase in swirl flow mainly because the flame is located closer to the inlet plane. On the other hand, a more uniform temperature distribution in the exhaust gas can be attained at the combustor exit at high swirl condition.     

Impact of co- and counter-swirl on flow recirculation and liftoff of non-premixed oxy-flames above coaxial injectors

Controlling the flame shape and its liftoff height is one of the main issues for oxy-flames to limit heat transfer to the solid components of the injector. An extensive experimental study is carried out to analyze the effects of co- and counter-swirl on the flow and flame patterns of non-premixed oxy-flames stabilized above a coaxial injector when both the inner fuel and the annular oxidizer streams are swirled. A swirl level greater than 0.6 in the annular oxidizer stream is shown to yield compact oxy-flames with a strong central recirculation zone that are attached to the rim of central fuel tube in absence of inner swirl. It is shown that counter-swirl in the fuel tube weakens this recirculation zone leading to more elongated flames, while co-swirl enhances it with more compact flames. These results obtained for high annular swirl levels contrast with previous observations made on gas turbine injectors operated at lower annular swirl levels in which central recirculation of the flow is mainly achieved with counter-rotating swirlers. Imparting a high inner swirl to the central fuel stream leads to lifted flames due to the partial blockage of the flow at the injector outlet by the central recirculation zone that causes high strain rates in the wake of the injector rim. This partial flow blockage is more influenced by the level of the inner swirl than its rotation direction. A global swirl number is then introduced to analyze the structure of the flow far from the burner outlet where swirl dissipation takes place when the jets mix. A model is derived for the global swirl number which well reproduces the evolution of the mass flow rate of recirculating gases measured in non-reacting conditions and the flame liftoff height when the inner and outer swirl levels and the momentum flux ratio between the two streams are varied.     

Flow characterization and dilution effects of N_2 and CO_2 on premixed CH_4/air flames in a swirl-stabilized combustor

Numerically-aided experimental studies are conducted on a swirl-stabilized combustor to investigate the dilution effects on flame stability, flame structure, and pollutant emissions of premixed CH4/air flames. Our goal is to provide a systematic assessment on combustion characteristics in diluted regimes for its application to environmentally-friendly approaches such as biogas combustion and exhanst-gas recirculation technology. Two main diluting species, N2 and CO2, are tested at various dilution rates. The results obtained by means of optical diagnostics show that five main flame regimes can be observed for Nz-diluted flames by changing excess air and dilution rate. CO2-diluted flames follow the same pattern evolution except that all the domains are shifted to lower excess air. Both N2 and CO2 dilution affect the lean blow- out （LBO） limits negatively. This behavior can be counter-balanced by reactant preheating which is able to broaden the flammability domain of the diluted flames. Flame reactivity is degraded by increasing dilution rate. Meanwhile, flames are thickened in the presence of both diluting species. NOx emissions are significantly reduced with dilution and proved to be relevant to flame stability diagrams： slight augmentation in NOx emission profiles is related to transitional flame states where instability occurs. Although dilution results in increase in CO emissions at certain levels, optimal dilution rates can still be proposed to achieve an ideal compromise.