【作者】 王林；

【导师】 柳朝晖；

【作者基本信息】 华中科技大学 ， 热能工程， 2013， 博士

【摘要】 无焰燃烧技术具有燃料适应范围广，燃料利用率高，通时又能较大程度减排NOx的新型燃烧技术，特别是能够很好地和具有减排CO₂能力的富氧燃烧技术结合，为无焰燃烧技术的开发利用提供了新的途径。目前针对无焰燃烧的研究大多都集中在无焰燃烧临界转变，无焰燃烧组织方式等流体动力学方面，而其燃烧的化学动力学过程却少有人涉及。本文针对无焰燃烧技术在常规空气气氛和富氧气氛下的化学动力学特性开展了系统研究。首先，基于无焰燃烧对冲扩散火焰模型，详细地对比了CH₄在空气环境下和富氧环境下的无焰燃烧特性差异。首先分析和解释了常规燃烧向无焰燃烧的转变特性，及非热解区域的消失。其次解释无焰燃烧和MILD燃烧的差异，构建了空气环境和富氧环境的燃烧分区图。同时分析对比了空气环境和富氧环境下，维持无焰燃烧的临界稀释率，预热温度和当量比的关系，表述了二种环境下维持无焰燃烧的最小点火能差异。除此之外，对比分析了常规无焰燃烧环境和富氧干湿燃烧环境下的燃料氧化路径异同，并对燃料N在两种不同环境下的转化进行详细地分析与探讨其次，本文构建了CH₄富氧燃烧对冲火焰扩散模型。详细的探讨了CO₂和H2O的化学，热力和扩散特性在富氧燃烧环境下，对火焰温度和燃烧组分影响，同时分析了这些特性对CH₄的氧化所造成的影响。结果表明CO₂的热力特性对温度的影响最大，而其热力特性和化学特性对关键性组分CO的影响都比较大。H2O的存在虽然对燃烧温度的影响不大，但是却能极大的改变CO的氧化过程，加速其氧化。同时还构建了常规空气环境，富氧干循环环境和富氧湿循环环境下的CH₄氧化路径图。同时，对CH₄无焰燃烧数值模拟预测，详细地讨论了其总包化学反应机理的优化和优选。针对无焰燃烧气氛下燃料燃烧速率变缓这一特性，优选和优化了CO和H2O的化学反应动力学速率，并考虑其逆反应过程。利用典型的同轴射流无焰燃烧实验数据，在优化燃烧湍流模型的基础上，详细地评估了不同的总包反应机理并最终优选出能够适用于无焰燃烧数值模拟预测的总包反应机理，为无焰燃烧工业炉膛的数值模拟预测提供了一定的帮助和指导。总包反应机理因其化学反应时间尺度过小而无法适用于无焰燃烧的着火延迟，熄火等特性的研究。而大型的化学反应体系又无法在无焰燃烧数值模拟过程中被利用，为此必须发展出适合其数值模拟预测的骨架机理或进一步的简化机理。在本文中，针对适用于CH₄富氧燃烧预测的97组分，778基元反应的Glarborg机理，利用先进的DRGEPSA方法，将其简化至25组分，168基元反应的骨架机理，通过对比骨架机理和详细机理在着火延迟，火焰传播速度，发热速率和组分预测等方面的异同，表明骨架机理的预测误差在5%以内。表明该骨架机理具有较高的预测精度而且可以大大缩短计算用时间。最后，针对20KW无焰燃烧实验平台，通过准稳态假设方法简化GRI3.0并详细参照对比Lu和Law发展的19步简化机理，通过对FLUENT进行自编UDF，将该简化机理嵌套至EDC湍化学模型当中，通过统计组分的净生成率优化计算策略，在保证计算精度的同时节省计算用时，同时对比分析了前面优化出的WD4机理对于全场组分的预测特性。对比模拟结果和实验结果发现，简化机理和总包机理都能够较为准确的预测燃烧过程的稳态组分，同时简化机理能够预测燃烧热释放过程并与实验观察取得了一致的结果而总包机理却不能预测无焰燃烧下的着火特性。通过对简化机理的模拟预测结果分析，无焰燃烧方式存在燃烧滞后性且燃烧为容积燃烧，而H离子的浓度在无焰燃烧环境中降低可能是其燃烧速率减慢的主要原因之一，同时发现C2链的质量流率下降而C1-C2链的交互作用加强。更多还原

【Abstract】 Flameless combustion technology is one of prospective combustion technologies. It isa new type combustion strategy for improving the efficiency of energy utilization anddecreasing the NOx emission. Especially for its great advantages of combing with theoxyfuel combustion technology, this technology also helps to deal with the climate change.Recently, many studies were focused on the characteristics for critical changes formconventional combustion to flameless combustion, the organizations of flamelesscombustion and so on. Most of these topics were belonged to the studies of category offluid dynamic for flameless combustion. However, the characteristic of chemical dynamicfor flameless combustion was barely disscused. In the present work, this topic issystematically discussed under air and oxyfuel combustion condition.A special opposed flame model is set up to analysis the difference of flamelesscombustion under air and oxyfuel combustion condition. Firstly, the critical changecondition from air combustion to flameless condition is analysed, i.e. the disappearance ofprolytic region. Secondly, combustion regions are divided to explain the difference offlameless combustion under these two conditions. Meanwhile, on the premise of flamelessor MILD combustion, the relationship between the fuel dilution, preheated temperature andequiverlence ratio is studied and the difference for the mixmum heat for sustaining theflameless or MILD combustion are furtherly analysed. Also, the reaction pathways for CH₄oxidation and NH3conversion are also detailedly studied.Meanwhile, a diffusive opposed-flow flame model is set to investigate the thermal,chemical and diffusion effect of CO₂and H2O on the flame under oxyfuel combustioncondition. The flame temperature and Emission Index of CO （EICO） are chose to show theresults. It is shown that the thermal property of CO₂greatly affected the flame temperature,both thermal and chemical properties greatly affect the EICO. Although the existence ofH2O hardly affects the flame temperature, it reduces the EICO. It means that the existenceof H2O accelerates the fuel oxidation rate. Besides, the reaction pathways of fuel under air,oxyfuel with dry recirculation and oxyfuel with wet recirculation combustion condition hasalso been detailedly analysed.The global combustion mechanisms were modified and optimized to be suitable forthe numerical computation prediction for flameless combustion. For flameless combustion,the oxidation rates of fuel becomes slower than that under flammable combustion condition.Aiming at this particular point, the oxidation rates of CO and H2are modified for predicting the main species adaptively. Different global combustion mechanisms are compared withthe Co-flow experimental data to optimize the best one for prediction of flamelesscombustion.Although the global combustion mechanism is suitable for the prediction of mainspecies, it is not used for the predictions of ignition delay and extinction of flamlesscombustion because of its small chemical reaction time scale. However, the detailedmechanism is also barely used for the CFD prediction of flameless combustion because itassumes a large amount of computation time. A skeletal mechanism or reduced mechanismis appropriate for sloving both of these problems. A newly developed methodogy calledDRGEPSA is adopted to reduce the97species and778elementary reactions whichdeveloped by Glarborg et al. A skeletal mechanism which contained25species and168elementary is got by this method. Comparisions of ignition delay, flame propagationvelocity, heat release rate and prediction of main species are conducted between thedetailed mechanism and skeletal mechanism. The result shows that the skeletal mechanismwas about95%accuracy compared with the detailed mechanism and it greatly saves theCPU computational time.Then, a numerical simulation for a20Kw flameless combustion is done. A reducedmechanism for QSS analysis of GRI3.0based on19reduced mechanism which developedby Lu and Law has been coupled with EDC turbulent-combustion by User DefinedFunctions （UDF）. It shows that this strategy keeps the high numerical precision and saves alot of computational afford. Also, the WD4global mechanism is also been used for thissimulation. The result shows that the reduced mechanism and global mechanism are used topredict the steady state species well. However, the global mechanism fails to predict thecharacteristic of ignition for flameless combustion but the results for reduced mechanismare coincident with experimental investigation. It also shows that the ignition lag forflameless combustion existed and it is a volumetric combustion. The reduction of H is oneof main causes for slowing of flameless combustion. The interaction of C1-C2chain isenhanced and the mass flow for C2chain was weaked.更多还原