预热段低温链反应对预混式超焓燃烧稳燃机制的影响

In premixed excess-enthalpy combustion, the fresh fuel-air mixture is extensively preheated before entering the flame zone. For a fuel with high oxidation reactivity at low-temperatures, the composition of the mixture would be modified by the low-temperature chain reactions (chain-branching and chain-termination reactions) in the preheating zone, thus affecting the combustion kinetics in the flame. This project aims to investigate the effect of low-temperature chain reactions on the mechanisms of stabilization of premixed heat recirculation excess-enthalpy combustion via both experimentation and numerical simulation. A counter-current heat recirculation excess-enthalpy combustor system will be built, and dimethyl ether and ethanol, which are isomers, will be tested in the experimentation. Given that the low-temperature oxidation reactivity of dimethyl ether is quite pronounced while that of ethanol is relatively weak, the effect of low-temperature chain reactions on the combustion stability will be readily clarified by comparing experimental observations of the combustion of these two fuels. A numerical model considering both low- and high-temperature oxidation kinetics as well as heat transfer between gas and solid wall will be constructed. Numerical computations will be performed accordingly under selected near-extinction conditions. Kinetic analyses will be carried out to elucidate the effect of the low-temperature chain reaction products such as formaldehyde and hydroperoxides on the flame stabilization mechanisms when they are ejected into the flame zone. The outcomes of this research project will advance the excess-enthalpy combustion regarding the interaction between heat-recirculation and low- and high- oxidation, and would provide guidance for the design, optimization and safe operation of excess-enthalpy combustors for utilization of various fuels on a scientific basis.

在预混式超焓燃烧中，燃料和空气在进入火焰区之前就得到了充分预热。如果燃料低温氧化特性显著，低温链反应（链支化与链终止反应）的发生会导致未燃气的组成发生改变，进而影响火焰区的燃烧动力学。本项目将选取互为同分异构体的二甲醚和乙醇为燃料，开展超焓燃烧实验。由于两种燃料高温燃烧发热量接近而低温氧化特性差异显著，对比实验结果将直观的揭示预热段低温链反应对超焓燃烧稳定性的影响规律。本项目还将构建包括低、高温氧化反应动力学机理及气固间换热的数值模型，针对濒临熄灭工况实施动力学解析，旨在阐明关键的低温链反应产物如甲醛、氢过氧化物等对超焓燃烧稳燃机制的影响。本项目的研究成果有望在回热循环与燃料低、高温氧化特性的相互作用机制方面完善超焓燃烧理论，为不同燃料工况下的超焓燃烧器设计、优化和安全运行提供科学指导。

在预混式超焓燃烧中，燃料和空气在进入火焰区之前就得到了充分预热。预热段的低温链反应会导致未燃气的组成发生改变，进而影响火焰区的燃烧特性。本项目旨在揭示预热段低温链反应对超焓燃烧机制的影响规律。..超焓燃烧的方式多种多样，相对于项目伊始采用的U型对向流动换热燃烧器，多孔介质燃烧器结构更为简单，更有利于揭示低温氧化和回热循环的耦合机制、完成项目的既定研究目标。为此，我们搭建了多孔介质燃烧实验平台，以陶瓷、石英砂等颗粒作为多孔介质体，采用二甲醚-空气预混气作为研究对象，测定了多孔介质中的火焰温度、火焰传播速度等关键参数，构建了考虑物质、动量、能量输运特性和低温、高温氧化机理的数值模型，揭示了多孔介质超焓燃烧的火焰结构。研究结果表明，预热区存在一个显著的低温释热峰，促进了燃料的消耗和转化。研究发现预热段的低温链反应可以显著提高火焰传播速度。此外，回热循环和低温链反应的耦合降低了火焰温度峰值，有利于降低热力型氮氧化物排放。..超焓燃烧现象不仅存在于预混气在多孔介质中的燃烧，也存在于可燃固体颗粒的多孔介质燃烧。我们研究了毫米级活性炭颗粒堆放的多孔介质中的燃烧和火焰传播过程。研究表明，活性炭颗粒丰富的多孔结构显著强化了低温非均相表面反应，表面反应放热速率不高却有效维持了一氧化碳火焰在亚毫米狭缝中的传播，突破了火焰熄火间距的限制。为了从本质上理解火焰传播机理，研究了微小尺度通道中火焰对固壁的“预热机制”。研究表明，前人通过预热长度评估预热效果的理念有一定的局限性，影响预热效果的关键参数应是预热面积。分析表明，减小预热面积有利于提高预热强度从而加快火焰传播。..本项目的执行完善了预混式超焓燃烧理论，研究成果可为超焓燃烧器的设计、优化和运行提供科学指导。本项目将超焓燃烧的范畴拓展到固体燃烧领域，对于复杂非均相燃烧及固体表面火焰传播现象的理解有一定参考价值。