【作者】 刘慧；

【导师】 陈海耿； 李本文；

【作者基本信息】 东北大学 ， 热能工程， 2010， 博士

【摘要】 相对于以自由火焰为特征的传统燃烧方式来说,多孔介质中的预混燃烧是一项新型、洁净、主动有效的技术。它能够实现低热值,甚至超低热值气体的稳定燃烧,具有燃烧稳定、燃烧速率高、可燃极限宽和污染物排放低等显著优点。这项燃烧技术在提高燃烧效率、扩展可燃极限、节约燃料、改善环境及处理各类垃圾和废弃物方面具有优越性,可广泛应用于冶金、化工能源、建材、食品加工等各种领域。这是一种与传统燃烧完全不同,且新颖独特的燃烧方式。本研究是与多孔介质材料相关的燃烧、流动、传热之基础与应用研究。本研究以气体燃料在多孔介质燃烧器中的预混燃烧为主线,以多孔介质燃烧器的实际应用为最终目的,进行实验和数值模拟研究。实验方面：1.自行设计并搭建实验台,采用在液化气中加入氮气的方法,进行气体在多孔介质内的预混燃烧实验。实验过程中比较不同形状截面多孔介质内燃烧的特性,并根据燃烧情况对多孔介质燃烧器的结构进行了改善。改造过的燃烧器结构(燃烧器内径为100mm)可在燃气流量为1.122 m3·h-1,燃烧强度为1345 kW·m-2的条件下稳定燃烧。经过多次实验得出,在当前的多孔介质材料条件下,也就是大孔介质孔分布为10 PPI (Pores Per Inch,每英寸的孔数),小孔材料孔分布为50～60 PPI,实验能够在燃烧强度不高于1200 kW·m-2的工况下进行长时间稳定燃烧,此时多孔介质辐射面温度不高于1200℃。2.进行多孔介质燃烧器的工业应用模拟实验,并与类似情况下的自由火焰燃烧进行对比。对惰性多孔介质内的气体燃烧规律进行研究,其中包括多孔介质中的预混火焰的传播和稳定性,多孔介质内燃烧效率和污染物排放。为燃烧器的实际工业应用提供了操作参数参考值,并进一步指导多孔介质燃烧器的设计和实际应用,对接下来的数值模拟结果起到对比验证作用。数值模拟方面：1.建立一维层流预混燃烧模型,以高炉煤气为研究对象,通过分析温度对反应速率的敏感度系数,以及高炉煤气主要组分在基元反应中对燃烧速率的影响,得到高炉煤气燃烧的基干机理。用详细反应机理计算高炉煤气燃烧反应的时间为18秒钟,而简化反应机理的计算时间则不到1秒钟,充分显示了简化反应机理在计算上的优势。将化学反应简化机理与详细机理的计算结果进行对比,主要组分的浓度差值均不超过0.4%,验证了简化机理的可靠性。2.使用简化的化学反应机理,对比了甲烷／空气在多孔介质燃烧器与传统燃烧器的燃烧温度和组分分布,在给定的模拟条件下,多孔介质燃烧器出口温差比传统燃烧器高300K左右,燃烧过程中多孔介质中CO的含量大约是自由空间CO含量的一半。对比结果充分显示多孔介质燃烧器在燃烧温度和燃烧程度上的优势。3.建立两段式多孔介质燃烧器的二维模型,在FLUENT软件的基础上经过二次开发,用C语言自定义编程,对甲烷／空气的预混气体在多孔介质中燃烧进行了数值模拟,得到了燃烧温度和压力场的二维,并考查了当量比、多孔介质辐射衰减系数及导热系数,对燃烧温度和压力分布的影响。模拟结果可以看出：多孔介质燃烧中的压力分布不仅受到当量比、多孔介质辐射衰减系数和导热系数的影响,而且与燃烧温度及燃烧区域有着密切的相互关系；辐射衰减系数的改变使反应区域有明显的改变,同时使燃烧温度和压力分布发生相应的变化,温度峰值和压力梯度突变处互相对应。4.将速度极限范围的模拟结果与实验结果及Barra的模拟结果进行了对比,可以得出：本次研究的入口速度极限结果比实验结果偏高,比Barra的模拟结果偏低,原因在于在数值模拟中忽略了热损失,且本研究中采取的热弥散系数与Barra的不同,而稳定燃烧极限值受弥散系数影响比较大。5.进一步考查当量比、多孔介质辐射衰减系数和导热系数对速度极限的影响,模拟结果表明,当入口速度为最低极限值时,火焰稳定在界面的上游区域；当入口速度为最高极限值时,火焰稳定在界面的下游区域；当量比越大,稳定燃烧的速度范围越宽,且速度的最小和最大极限值都变大；与参考算例相比,下游区域导热系数增加5倍,上游导热系数保持不变时,稳定燃烧速度范围增加到0.47 m·s-1；上游多孔介质的辐射传热系数增加而下游保持不变,可以达到相对最大的稳定操作范围。因此在实际应用中,应综合参考多孔介质各个特性的参数值对速度极限的影响,从而得到更大的稳定操作范围。更多还原

【Abstract】 Comparing with the traditional premixed, free-flame characterized combustion, the premixed combustion in porous media is a new-type, clean and effective technology. It can be used to implement steady combustion with fuel of low or extremely low calorific value. The porous media burner has advantages of stable combustion, higher flame speed, wider lean flammability limit and less pollutants emission. It also has superiorities on increasing combustion efficiency, extending the flammability limit, saving energy, protecting entironment and dealing with sorts of wastes. It can be widely used in various fields, such as metallurgy, chemical process, energy, building materials, food processing, household and so on. It is a completely different, novel and unique mode of combustion.This study is a foundational and applied research on combustion, flow, and heat transfer in porous media. The main content of this study is about premixed combustion of gas fuel in porous media burner. Experiments and numerical simulations are carried out for the finnal goal of practical application of the porous media burner.Experiments:1. An experimental platform was designed and built by our group. The experiments of premixed gas combustion in porous media are carried out by adding nitrogen in liquefied gas. In experiments, the combustion characteristics in different cross-sections of porous media are compared and, structure of porous media burner is improved according to the state of combustion. The improved burner, diameter of which is 100mm, can support the stable combustion under the gas consumption of 1.122 m3·h-1 and the firing rate of 1345kW·m-2. Due to the features of selected porous media, saying, the numbers of the big pore is 10 PPI (Pores Per Inch), and the small pore is 40～50 PPI, conclusions are made that, a long-term stable combustion can be supported by the maximum firing rate of 1200kW·m-2, and the radiating temperature of porous media under this condition is no higher than 1200℃.2. To compare with the free flame combustion under the similar conditions, an industrial experiment is also carried out. The combustion characteristics in porous media burner are investigated, including the propagation and stability of premixed flame, the combustion efficiency and the pollutant emission in porous media. The reference values of operational parameters are provided for industrial applications of the porous media burner, and the results of subsequent numerical simulation are validated by the results of experiment.Numerical simulations:1. A one-dimensional laminar flow model of the premixed combustion is built, and the industrial gas with combustible components, that is, blast furnace gas is used as the research target. The simplified reaction mechanisms of blast furnace gas are obtained by analyzing the sensitivity coefficient of temperature with respect to the reaction-rate, and the effect of primary constituents of blast furnace gas on reaction-rate. The time consuming on calculating the details kinetics of blast furnace gas is about 18 seconds, while that on calculating the simplified kinetics is less than one second. The later is absolutely superior than the former one from the aspect of calculation. Compared the results of detailed reaction mechanisms with simplified reaction mechanisms, the concentration differences of the primary constituents are less than 0.4%, and the simplified reaction mechanisms are validated.2. Using the simplified chemical reaction mechanism, the temperature and constituent profiles of combustion in porous media burner are compared with those of mathane/air combustion in traditional burner. For a given condition, the temperature difference at the outlet of porous burner is about 300K higher than that of traditional burner. During the combustion process, the concentration of CO in porous burner is about half of that in traditional burner. Superiorities of porous media burner are fully demonstrated by these comparisions.3. The two-dimensional, two-section combustion model of porous media burner is set up, the methane/air mixture combusting in porous media is simulated numerically by the software FLUENT. The user defined codes of C language are used to extend the ability of FLUENT and enable two-dimensional distributions of temperature and velocity to be obtained. Eeffects of the equivalence ratio, the extinction coefficient and the thermal conductivity of porous media on these distribustions are also investigated. The results show that the profile of pressure is not only affected by the equivalence ratio, the extinction coefficient and the thermal conductivity of porous media, but also closely related to the temperature and the combustion area. The variations of extinction coefficient lead to the great changes of the combustion area, while profiles of temperature and pressure are changed responsely. The peak value of temperature corresponds to the variation of pressure gradient.4. Comparing the simulation results of speed limit with the results of experiments and Barra’s simulation, it shows that our simulated values are higher than experimental values, and are lower than pre-published value. The reasons are:heat loss is neglected in simulation, and the expression of heat dispersion, which impacted speed limits in stable combustion greatly, is different in this research with that in literature.5. The effects of equivalence ratio, radiative extinction coefficient and thermal conductivity of the porous media on speed limit are investigated. The results show that:the stable flame stays upstream while inlet veloctiy is minimum, on the contrary, it stays in the downstream zone while inlet velocity is maximum; the stable operating range enlarges and its maximum and minimum shifts to the larger values as the equivalence ratio increases; compared with the reference case, the table operating range increases to 0.47 m·s-1 if the downstream conductivity is 5 times of that in reference case and the upstream conductivity keeps the same. In practices, radiative extinction coefficient in the upstream section is desired to be larger in order to obtain a larger stabilization range.更多还原