【作者】 王高峰；

【导师】 林其钊；

【作者基本信息】 中国科学技术大学 ， 工程热物理， 2008， 博士

【摘要】 如何更环保、更安全地使用能源是当今社会发展面临的主要问题,而燃烧则是能源获取的最主要方式。激波管是进行燃烧相关机理研究的重要手段。本文建立了一套激波管实验装置,在这个平台上对甲烷,特别是低热值的超低浓度甲烷燃烧的基础性问题进行了研究,并对激波管内的流场进行了数值模拟。首先,建立了激波管的基本实验装置（包括激波管及其配气、压力测试和光谱测量系统）。同时,研制了低压大电流的电热式电控破膜装置,实现对激波产生时机的控制（控制精度+1ms）,而且对测控设备电磁干扰小,不对化学反应产生污染。还研制了基于Atmega单片机的可编程时序控制器,完成对激光器预热、激波产生和到达、反应进程检测触发、诊断激光发射和ICCD曝光时序的总体同步控制,进而搭建了基于Nd:YAG激光器的PLIF测量平台。利用该平台对激波压缩后的流场进行了丙酮示踪PLIF测量,定量分析结果与理论计算相符,既验证了电热破膜方式的可靠性,也表明了PLIF用作激波管实验的流场显示的可行性。这种高时间和空间分辨率的测试手段在激波管实验上的成功尝试,为激波与气相物质相互作用的机理研究提供了新的思路和方法。其次,对激波诱导当量比甲烷/空气混和物点火的反应区结构进行了化学发光数字成像和OH-PLIF的可视化测量。在不同强度激波的诱导下,诱导区的温度不同,直接导致了甲烷/空气混合物的空间放热率不同,从而形成了强点燃和弱点燃两种典型的反应区结构特征,其化学自发光特征、压力特征和OH-PLIF分布的测量结果相互印证,也与前人的实验和计算分析结论一致。由于在弱点燃工况下,诱导区的着火延迟期相对于激波传播时间来说较长,导致不可消除的湍流、压力波等因素造成的扰动作用更为明显,反应特征出现显著的不均匀性。弱点燃工况的不同阶段都存在OH分布或化学发光相对于周围强很多的热点:这些化学反应强烈的热点在点火初期呈核状,随后交织成斑状,促进了火焰面的形成;而后,位于未燃气体和火焰面交界处的热点,加速了火焰面的推进。随着诱导激波的增强,这种不均匀性显著减小,在强点燃工况反应区呈规则的爆轰波结构。再次,搭建了并行化Fluent计算平台,采用空间项三阶MUSCL格式,时间项二阶隐式,Roe-Flux差分裂格式,RNG k-ε模型,基于密度法的时间导数预处理法和双重时间步进格式求解二维N-S方程,模拟了激波管内激波形成、推进、反射以及激波与边界层相互作用、激波与接触面相互作用的整个非定常流过程,计算结果与前人的结果定性一致,指出了影响5区实验条件的关键因素。最后,在激波管平台上,用OH^*和CH^*光谱,测量了超低浓度甲烷的点火延迟时间,并与详细机理、经典经验公式和一些简化机理进行了对比,发现GRI-Mech 3.0详细机理完全适用于超低浓度甲烷氧化历程的研究。通过数据分析发现,表观活化能与甲烷浓度成幂律关系的规律,并得到温度1100～1900K、常压条件下甲烷的点火延迟时间经验公式。该活化能修正的方法可能同样适用于总包反应机理。此外,通过局部和总体敏感性分析,探讨了低浓度甲烷燃烧的反应特征。当甲烷浓度低时,在与氧气竞争的反应中（如H+O₂＜=＞OH+O VS.H+CH₄＜=＞CH₃+H₂）竞争能力明显下降,造成链分支反应活跃,点火延迟时间减小;同样由于甲烷浓度低,CH₃的复合反应2CH₃（+M）＜=＞C₂H₆（+M）的影响被削弱,在决定低浓度甲烷氧化过程的基元反应中,CH₃+O₂＜=＞O+CH₃O占绝对主导地位。更多还原

【Abstract】 As the main access to energy, combustion is much concerned with the environment-friendly and safe use of energy which becomes more and more important in today’s social and economic development. Since shock tube is a primary apparatus to investigate the chemical kinetics of combustion, a shock tube experiment platform was designed and established and CFD simulations were carried out to better understand the unsteady flow fields in it. Further more, the fundamental researches on combustion mechanism of methane, especially ultra lean methane, were conducted.Firstly, a shock tube device as the fundamental equipment together with the gas delivery, pressure test and spectral measurement system was established. In order to control the arrival time of shock wave, we developed a low- voltage/high-current electro-heating diaphragm bursting device. It is demonstrated that this method have little influence on the chemical reaction process and EMI impact on the instruments, and the shock wave arrival time tolerance is less than±1ms. All synchronizations of instruments of Nd:YAG Laser pumped PLIF measurement were harmonized by a self-developed Atmega-MCU-based central controller. The coordinating process included laser preheating, shock wave creating and arriving, reaction process monitoring, diagnostic laser pulse firing and ICCD photography. The uniformity of incident shock wave was also validated by acetone PLIF measurement. Semi-quantitative analysis of the results agrees well with the empirical results. It gives a demonstration to the visualization of flow filed in shock tube experiments and confirms the validity of the high spatial and temporal resolution PLIF measurement technique as a new method for the mechanism research of the interactions between shock wave and gaseous matter, such as combustion, detonation and nonstationary wave.Secondly, Shock-induced ignition of stoichiometric methane/air mixture was experimentally studied by digital chemiluminescence imaging and OH-PLIF visualization technique. Shock waves of different strengths lead to different post shock temperatures of methane/air mixture, and then result in different energy release powers per unit mass. So there are two typical characteristics of reaction zones in strong and weak ignition models. The OH-PLIF results agreed well with the measurements of spontaneous luminescence and pressure profiles in this paper, and were in accordance with the former conclusion from experiments and numerical simulations. In weak ignition, because ignition delay was relatively long compared with the time scale of the shock wave propagation, the role of fluctuations caused by turbulence, pressure perturbations, etc., became more obvious. That means more time was provided to non-linear chemical reaction and the reaction zone showed distinctively irregular structure. When inducing shock wave was strengthened, the nonuniform characteristics decreased. Regular detonation wave structure could be seen in reaction zone of strong ignition. Lots of hot spots, in which the OH radical distribution or chemiluminescence is much more intensive than the surrounding area, exist in different stages of weak ignition process. At the initial stage of ignition, the hot spots are small kernels, then interact with each other to plaques and form the flame; at the subsequent flame propagating stage, the hot spots will appear at the interface between the unburned mixture and the flame front, speeding up the propagation of the flame front.Thirdly, we developed a computing platform for parallel Fluent. The third order MUSCL scheme for spatial discretization, second order implicit time integration, Roe Flux-Difference splitting scheme for convective fluxes, RNG k-εturbulence model, preconditioning and dual-time formulation were employed to solve 2D, unsteady, compressible, time-averaged N-S equations. The simulation results of unsteady fields exhibit the processes of shock forming, propagating and reflecting and the interactions between reflected shock and unsteady boundary layer and between bifurcated shock and contact surface in shock tube operation and show a good qualitative agreement with the previous research. Also the key impact factors of region 5 experimental condition are pointed out.Finally, the ignition delays of ultra lean methane （ULM） were measured by OH* and CH* spectral method in shock tube platform. The experimental results were compared with predicting results from detailed mechanism, the typical empirical expression and some reduced mechanisms and verified the applicability of GRI-Mech 3.0 mechanism. Through analyzing the results, we found the power law relationship between the effective active energy and methane fraction. An improved empirical expression for predicting ignition delay of methane oxidation at temperature of 1100～1900K is obtained. Also it provides a potential correction to overall reaction mechanism parameter. The reaction characteristics of ULM were studied by local and overall sensitivity analysis. There are several competing reactions to radical between methane and oxygen, like H+O₂<=>OH+O VS. H+CH₄<=>CH₃+H₂. Low methane concentration would certainly activate the most important branch reaction of H+O₂<=>OH+O. So the ignition delay time of ULM is decreased. Meanwhile, low methane concentration case weakens the influence of complex reaction of methyl （CH₃+O₂<=>O+CH₃O）. And the oxidation of methyl （CH₃+O₂<=>O+CH₃O） would dominate ULM combustion.更多还原