【作者】 史俊瑞；

【导师】 解茂昭；

【作者基本信息】 大连理工大学 ， 动力机械及工程， 2007， 博士

【摘要】 与自由空间的燃烧相比，预混气体在多孔介质中的燃烧是一项先进并具有广阔应用前景的燃烧技术，它不仅能够显著地拓展贫富燃极限，而且在污染物排放方面具有显著的优越性。研究超绝热燃烧机理和火焰特性，包括多孔介质中的热回流效应、燃烧波传播特性、反应区域的最高燃烧温度、燃烧器的贫可燃极限和最大半周期等，将有助于发展超绝热火焰理论，设计和开发多孔介质燃烧—换热器、反应器和多种目标的新型燃烧技术。本文通过实验测量、理论分析和数值模拟研究预混气体在多孔介质中的超绝热燃烧。首先，自行设计和制作了可以研究多孔介质中燃烧波和热波传播规律的实验台，该实验台包括燃烧器(石英玻璃管)、气体供给系统和测量系统等。在燃烧波的传播过程中，对火焰的形状和发展进行了观测。在不同的工况参数下，利用热电偶系统地测量了燃烧器内的温度分布。基于这些实验结果，对超绝热燃烧、燃烧波的传播、反应区域的最高燃烧温度和火焰特性进行了分析。其次，对单向和往复流动下稀薄预混气体在多孔介质中的低速过滤燃烧进行了理论分析。(1)对稀薄预混气体多孔介质中的低速过滤燃烧进行分析。首先假设燃烧区域为无限薄的区域，建立燃烧波波速与燃烧区域最高温度的第一个关系式。然后利用层流预混火焰理论，将整个区域分为预热区域和反应区域，建立燃烧波波速与燃烧区域最高温度的另一个关系式。从而得到燃烧波波速和燃烧区域最高温度的封闭解。在宽广的工况范围内，理论解与实验取得了相同的趋势，解析地证实了超绝热燃烧是燃烧波和热波在特定条件下相互叠加的结果。(2)通过与稳态的逆流燃烧器类比，得到了往复流动下绝热惰性多孔介质内的超绝热燃烧的简化理论解。该解包括两个常微分方程，其中包括了所有的主要控制参数，因此有助于深入理解这些控制参数对燃烧器特性的影响。与数值模拟的结果相比，多孔介质固体的温度曲线可以用简化解的分段线性函数很好地估算。利用简化的理论解求得的燃烧器内温度的最大值与实验值取得了相同的趋势，但是通常比实验值大，二者的误差小于20％。(3)通过与实验和数值模拟结果的类比，对文中推导出的简化理论解进行进一步的研究。利用分段线性函数，构建出燃烧器内的温度分布曲线、贫可燃极限和最大半周期。该解适用于绝热条件下往复式惰性多孔介质燃烧器。结果表明，当流速小于0.12m／s时，理论解预测的可燃极限与实验取得了相同的趋势，增大流速可以获得较小的贫可燃极限。而流速大于0.17m／s时，增大流速对扩展贫可燃极限的影响很小。同时研究表明，小孔径的多孔介质材料更有利于扩展贫可燃极限。预测的最大半周期与流速的乘积与固体和气体热容的比值成线性关系；燃烧器的长度对最大半周期有显著的影响。燃烧器的长度越大，允许有较大的半周期。预测的贫可燃极限和推导出的最大半周期为燃烧器的设计和进一步改善提供了指导。最后，建立了单向和往复流动下的预混气体多孔介质中燃烧的数学模型，用以深入探索和理解超绝热燃烧的机理和火焰特性。(1)首先对稀薄预混气体的低速过滤燃烧波的传播特性进行了系统的数值研究，并与实验结果进行对比。通过一维数值模型研究多孔介质引起的热回流效应。结果表明，导热和辐射在热回流的过程中都起着重要的作用。研究了多孔固体的属性，例如热容和导热系数、系统的热损失、混合气当量比对燃烧波波速和反应区最高温度的影响。计算结果表明，多孔介质热容的大小是影响燃烧波传播速度的最重要的参数，但它对燃烧区域最高温度几乎没有影响。在宽广的工况范围内，数值预测的燃烧波波速和燃烧区最高温度与文献和本文中的实验结果取得了定性的一致。(2)研究多孔介质内往复流动下预混气体的超绝热燃烧。在该系统中，通过周期性换向，燃烧波稳定在一个瞬态多孔介质燃烧器中。应用的一维双温模型包括气体输运、多孔介质固体的辐射、导热和气固两相间的对流换热。通过数值计算研究了弥散效应、主要工况参数，如半周期、流速、当量比、多孔介质材料对预混气体超绝热燃烧主要特性的影响。数值模拟结果通过实验进行了验证，并取得了相同的趋势。结果表明，组分弥散效应对气体温度分布和反应热影响很小；同一工况下，不考虑气体混合物的热弥散效应，会导致过高的气体温度计算值。同时，计算结果表明小孔径的多孔介质更有利于贫可燃极限的扩展，对30ppi的多孔介质燃烧器，得到了当量比为0.092的可燃极限。(3)对往复式惰性多孔介质燃烧器进行了二维数值模拟和结构改进。在燃烧器中分别填充10ppi泡沫陶瓷或小球，研究其内部的燃烧温度和压力损失。结果表明，由相同材料制成但结构不同的多孔介质对燃烧器内的高温区的分布和压力损失有显著的影响。孔隙率较大的泡沫陶瓷适合于布置在燃烧区域，而孔隙率较小的小球适合于布置在热交换区域。根据此指导思想，提出一种改进的燃烧器，即在燃烧器的中间布置泡沫陶瓷，而在二端布置小球。对于当量比为0.1的极稀薄的甲烷与空气的混合气，得到了更为宽广的高温区域和适度的压力降。更多还原

【Abstract】 Superadiabatic combustion of premixed gases in porous media is a promising and advanced combustion technology, providing both low and high flammable limits and good performance regarding emission characteristics in comparison with open flame. Investigation of superadiabatic combustion mechanism and flame characteristics, including the heat recuperation effect, combustion wave propagation characteristics, the maximum combustion temperature in porous media, the lean flammable limit and the maximum half cycle in the porous burner and so on, benefits not only the development of the superadiabatic flame theory but also the design and development of porous media combustor-heator, reactors and new combustion technologies for various purposes. The thesis presents an intensive investigation on superadiabatic combustion in prous media by experimental measurements, theoretical analysis and numerical simulation.Firstly, the experimental facility is designed and set up to measure and characterize thermal wave and combustion wave in a packed bed, which consists of a combustor (quartz glass tube), a gas flow system, a measurement system and so on. Flame shapes and development is observed and recorded during the combustion wave propagating through the combustor. The temperature distributions in the packed bed are measured by thermocouples at various working parameters. Based on the experimental results, the superadiabatic combustion effect, combustion wave speed and the maximum combustion temperature and flame characteristics in the reaction zone are discussed.Secondly, theoretical analysis of low-velocity filtration combustion of lean mixture in porous media with uni-directional and reciprocating flow has been performed.(1). Combustion wave characteristics of low-velocity filtration combustion in a porous medium burner are analyzed. Based on the flame sheet assumption, a relationship between the combustion wave speed and the maximum combustion temperature is given at first. Then an approach from the laminar premixed flame theory is applied and the entire flame zone is divided into a pre-heating region and a reaction region, and treated separately. In this way, the second relationship between the two parameters is deduced. Thus a closed analytical solution for the combustion wave speed and the maximum combustion temperature is obtained. Over a wide range of working conditions, theoretical predictions show qualitative agreements with experimental data available from the literature. In addition, the results reveal that the mechanism of superadiabatic combustion is attributed to the overlapping of the thermal wave and combustion wave under certain conditions.(2). Based on the analogy with the steady countercurrent reactor, a simplified theoretical solution is presented, which is applicable to adiabatic inert porous media combustors with reciprocating flow. The model consists of two ordinary differential equations that link all major controlling parameters, which allow for a good physical understand of the process. Compared to the numerical study, the temperature profile in the reactor can be approximated very well by a piecewise linear function, in terms of the simplified model. The maximum temperatures in the porous media burner predicted by the simplified model show the same trends as those of experimental results, but are generally higher, and the deviation between the experimental data and predications is less than 20%.(3). Based on the experimental and simulation resuluts a simplified theoretical solution is further developed. The temperature profiles in the burner, the lean flammable limit and maximum half cycle are predicted by a piecewise linear function, which is applied to inert porous medium combustors with reciprocating flow in the absence of heat loss to the surroundings. The predicted lean flammable limits show the same trends as experiments when the gas velocity is less than 0.12m/s, this means that the lean flammable limits are getting lower with increasing gas velocity. However, greater gas velocities have little effect on the lean flammable limits at gas velocites greater than 0.17m/s. At the same time, results show that the lean flammable limit can be extended by using porous media of smaller pore size. In addition, it is shown that the predicted maximum half cycle is proportional to the product of the gas velocity and the ratio of the specific heats between the solid and gas. The combustor length has significant influence on the maximum half cycle and the longer combustor length permits larger half cycle. The predicted lean flammable limit and maximum half cycle provide guidelines for the design of the combustor and some indications for further improving the combustor performances.Finally, mathematical models of premixed combustion in prous media with uni-directional and reciprocating flows have been developed to improve the understanding of the superadiabatic combustion mechanism and flame characteristics.(1). Wave propagation characteristics of low-velocity filtration combustion in a porous medium burner are systemically investigated. Heat recuperation originated by the porous medium is examined by a one-dimensional numerical model. Results show that both solid conduction and radiation play important roles in the heat recuperation process. Attention is focused on the influence of solid properties such as specific heat capacities and heat conductions, heat loss, equivalence ratio etc, on the combustion wave speed and the maximum combustion temperature attained in the wave. Results show that the heat capacity of the porous media has a significant effect on combustion wave speed. At the same time it is shown that the maximum combustion temperature is almost, independent of the heat capacity of the packed bed. Over a wide range of working conditions, the numerical predictions and theoretical results show qualitative agreements with experimental data from both this paper and the available literature.(2). Superadiabatic combustion with reciprocating flow in a porous medium has been investigated through numerical calculations. The combustion wave is confined to a transient porous burner by periodically changing the direction of the flow. The one-dimensional model takes into account gas-phase transport, radiation, interphase heat exchange, and solid conduction. Attention is focused on the formation of superadiabatic combustion, the influence of gas dispersion, equivalence ratio and the material and structure of the porous media, on the major characteristics of superadiabatic combustion in the porous body. Results are validated through comparisons with available experimental data and show the same trends as the experiments. It is indicated that species dispersion has little influence on the gas temperature and reaction heat for the lean mixture. However, the gas temperature is overpredicted at the reaction zone without the effect of mixture thermal dispersion at the same condition. In addition, it is shown that the combustible limit can be extended with smaller pore size porous media and a combustible limit with the equivalence ratio of 0.092 is achieved for 30ppi cerafoam.(3). Two-dimensional numerical investigations on the structure improvement of porous inert media bumer with reciprocating flow are presented. Attention is focused on the combustion temperature and pressure loss in the burner, which is respectively packed with 10ppi ceramic foams or alumina pellets with various sizes. Results show that material and structures of porous media have significant influence on the burner performance, and that ceramic foam with a high porosity is suitable for using in the combustion region whereas alumina pellets should be placed in the heat exchange zone. According to this principle, an improved burner design is proposed and this leads to a wider high temperature plateau and moderate pressure loss for extremely dilute CH4/air mixture with an equivalence ratio of 0.1. Numerical results are validated against experiment data.更多还原