【作者】 艾育华；

【导师】 周怀春；

【作者基本信息】 华中科技大学 ， 热能工程， 2006， 博士

【摘要】 随着我国经济的高速发展,能源供应状况日趋紧张,燃料燃烧产生的环境污染也日趋严重。烟黑是化石燃料燃烧产生的污染物之一,是没有实现的燃料化学能;烟黑颗粒随燃烧排气进入大气环境,从而造成环境污染。烟黑颗粒表面吸附有多环芳烃PAH,具有诱变性与致癌性,从而对人体健康造成不利影响;光化烟雾、酸雨及消光效应也是烟黑对环境的不利影响。在燃气透平燃烧室中,烟黑的强吸收性对燃烧室壁面辐射传热,需增大额外的冷却空气量;烟黑颗粒可直接对涡轮叶片造成冲蚀,或沉积在叶片引起腐蚀。在火灾中,烟黑的连续辐射是火灾生长与蔓延的主要原因,是火灾救险、逃生的主要障碍。而在锅炉中,烟黑辐射是主要的传热方式,这里烟黑既是重要的,又是必须的,更是无法避免的。因此问题的关键在于掌握烟黑生长的物理化学机理,有效控制烟黑生成、表面生长及表面氧化过程。由于燃烧现象是受多种物理和化学因素控制的复杂过程,因此从发展数值燃烧科学的角度,深入研究火焰中烟黑生长机理,寻求一种数值模拟烟黑生长、火焰辐射与污染排放的实用方法,具有重要的现实意义和科学价值。本文首先详细分析了国内外有关烟黑生长与氧化研究的现状,重点阐述了烟黑模型研究的进展。烟黑测量是烟黑机理研究与烟黑生长过程控制与可视化的关键,本文详细介绍了国内外常用的烟黑测量方法及其研究现状,重点阐述了直接取样技术和发射CT技术的特点及其应用。燃烧测量科学中,燃烧火焰图像处理及可视化是一个重点,本文详细分析了国内外燃烧火焰图像处理可视化以及辐射传递逆问题求解方面的研究现状和进展。论文还介绍了化学发光分析技术及其应用方面的国内外研究进展。本文基于详细的基元反应模型和简单双方程烟黑半经验模型,利用复杂热特性及输运特性实验数据,针对轴对称、层流、同向流扩散乙烯-空气火焰,用简单双方程烟黑模型与详细气相化学反应耦合模拟烟黑的生成、生长与氧化过程。火焰中CO,CO2,H2O以及烟黑的辐射热传递用离散坐标法计算,它们的辐射特性计算采用统计窄带相关-k基带模型(SNBCK)。完成完全椭圆型控制方程的求解,给出了合理的火焰温度与烟黑容积份额分布计算结果。并与发射CT法的测量结果比较,取得比较满意的结果。针对非均匀吸收、发射、无散射轴对称含烟黑扩散火焰对象,常规双色法不再适用,本文基于烟黑辐射特性,提出并模拟研究了同时重建火焰温度与烟黑容积份额的发射CT法,模拟结果显示发射CT法适用于重建单峰或峰谷差较小的双峰温度与烟黑容积份额分布。本文所完成的是火焰温度场与烟黑容积份额分布的同时辐射成像重建,这是本文的一个重要创新。利用发射CT法对非冒烟蜡烛火焰与冒烟煤油火焰的温度与烟黑容积份额进行测量,可以准确得到非冒烟与冒烟火焰扩散火焰的分布特征。进一步将发射CT法应用于流量可控的乙烯/空气扩散火焰研究,从火焰温度与烟黑容积份额分布可以看到,温度达到最大值的径向位置比烟黑容积份额达到最大值的径向位置稍大,从而可以确定扩散火焰中烟黑颗粒形成区发生在火焰反应区的富燃侧。当燃料流量增大,烟黑浓度增大,辐射损失增大,反过来影响火焰温度,使火焰温度水平更低。火焰温度反过来影响烟黑的氧化速率,使火焰中烟黑浓度增加。这与文献中的实验结论是一致的。通过与模拟计算和热电偶的测量比较,发射CT法不但可以定性还可以定量给出温度分布与烟黑容积份额分布,而且比热电偶法测量烟黑在火焰上部的氧化区有更强的适应性。在燃烧传热数值计算中,通常假定燃烧释放的化学能全部转化为燃烧介质的内能,介质温度升高,热辐射增强,但这与实际情况不符。事实上,燃烧反应释放的总能量可分成二个部分,一部分为燃烧产物吸收成为内能;另一部分则作为辐射能直接对外发射。因此本文在辐射传递方程与能量守恒方程中引入了燃烧反应放热的自吸收份额ψ,针对一个简单二维发射吸收的矩形系统,研究了燃烧反应中考虑化学发光辐射对整个燃烧系统能量平衡的影响。从总能量平衡角度看,化学发光并不影响壁面热流分布以及非反应区的温度分布。而直接辐射能份额对反应区温度的影响十分明显,自吸收份额ψ越小,直接辐射份额越大,反应区温度越低。因此基于热辐射成像的测量方法需要作相应的修正,而首先要解决的问题就是通过相关实验来确定自吸收热量份额的大小。实验结果表明,发射CT法测得的反应区温度明显高于热电偶的测量结果,除计算与测量误差外,化学发光的影响也应给予必要的考虑。因此,考虑化学发光为我们进行燃烧诊断提供了一个新的思路。更多还原

【Abstract】 With the rapid development of economy of China, the situation of energy supply is becoming tighter and tighter, and the environmental pollution produced by burned fuel is becoming more and more serious. Soot is one of pollutants produced by burned fossil fuel, and it represents unrealized chemical energy of fuel. The emission of soot by combustion processes is a source of atmospheric pollution and, due to its association with mutagenic and carcinogenic polycyclic aromatic hydrocarbons (PAH), has been affecting human health. Soot particles also affect the environment in many other ways including their contribution to the formation of photochemical smog and atmospheric acids. Soot particle presented in the atmosphere scatter and absorb solar radiation that can impede atmospheric visibility. In internal combustion engines and gas turbines, the deposition of soot has deleterious consequences for the maintenance and efficiency of the device, so the designer has many good reasons to avoid soot formation. This objective also applied in the case of fires, whose mechanism of propagation often involved radiant transfer from hot soot particles. On the other hand, this same ability to radiate is obviously desirable in a furnace. Therefore, it’s critical to understand the physical and chemical mechanisms about soot formation, and make it possible to control the processes of soot nucleation, surface growth, and oxidation.Development of practical ways to numerically simulate flame environments, as a step toward developing computational combustion, requires significantly improved understanding of soot processes in flame. Clearly, improved understanding of soot formation in flames is needed to achieve effective methods of predicting flame radiation and pollutant emissions, as well as for developing practical methods of computational combustion.In this dissertation, the present situation of investigations about soot formation and oxidation, soot measurement, and visualization of physical properties in flame by flame image processing at home and abroad was analyzed in detail, and emphasis was put on the development of soot model and emission CT technique. The development of experimental use of chemiluminescence was also described. A numerical study of soot formation and oxidation in axisymmetric laminar coflow diffusion ethylene-air flame was conducted using detailed gas-phase chemistry and complex thermal and transport properties. A simple two-equation soot model was employed to predict soot formation, growth, and oxidation with interactions between the soot chemistry and the gas-phase chemistry taken into account. Radiation heat transfer by both soot and radiating gases was calculated using the discrete-ordinates method coupled with a statistical narrow-band correlated-k based band model. The governing equations in fully elliptic form were solved. Reasonable flame temperature and soot volume fraction distributions were given here.For visualizing non-uniform absorbing, emitting, non-scattering, axisymmetric sooting flames, conventional two-color emission methods are no longer suitable, so an emission CT method for the simultaneous estimation of temperature and soot volume fraction distributions is studied. The simulation results indicate that the emission CT method is suited for the reconstruction of flame structures with single peak or double peaks with small difference between the peak and valley. For a double-peaked flame structure with larger peak and valley difference, reasonable result can be obtained just when the mean square deviations of measurement data are small, for example, not more than 0.01. The simultaneous estimation of temperature and soot volume fraction distributions is one important innovation in this dissertation.The emission CT method is used to estimate temperatures and soot volume fractions simultaneously in a candle flame, a kerosene flame, and several ethylene flames from the knowledge of the monochromatic radiation intensities measured by a CCD in this dissertation. The results indicate that the greater soot concentration lies inside the higher flame temperature in both types of flame, both inside the flame front and outside the flame axis. In addition, the fuel flow rate of the kerosene flame is greater than that of the candle flame which increases the amount of soot in the flame, thus increasing radiation losses. This, in turn, causes a lower flame temperature. This is observed by other researchers.In computational combustion, it is generally assumed that all the energy released is transformed into the internal energy of the combustion medium. So the temperature of the medium increases, and then the thermal radiation emitted from it increases too. But it is opposed to the practical situation. Therefore, it was assumed in this paper that the total energy released in a combustion reaction is divided into two parts, one part is a self-absorbed heat, and the other is a directly-emitted heat. The former is absorbed immediately by the products, becomes the internal energy and then increases the temperature of the products as treated in the traditional way. The latter is emitted directly as radiation into the combustion domain and should be included in the radiation transfer equation (RTE) as a part of radiation source. For a simple, 2-D, gray, emitting-absorbing, rectangular system, the numerical study showed that the temperatures in reaction zones depended on the fraction of the directly-emitted energy, and the smaller the gas absorption coefficient was, the more strong the dependence appeared. Because the effect of the fraction of the directly-emitted heat on the temperature distribution in the reacting zones for gas combustion is significant, the measurement based on thermal radiation imaging need corresponding correction, the first problem to be solved is to determine the fraction of the directly-emitted heat. Experimental results show that the temperatures in reacting zones achieved by emission CT are greater than those measured by thermocouple. Besides computational and measuremental errors, the effect of chemiluminescence should be considered in experiment. Thus, a new clue is provided by consideration of chemiluminescence for combustion diagnosis.更多还原