CAREER: Characterization of Propagating and Receding Flame Edges in Composition and Velocity Gradients

在实际的燃烧器中，经常使用非预混火焰，使火焰发生局部熄灭。例如，相对于氧化剂的较大的燃料速度会导致火焰底部足够大的标量梯度导致熄灭，而较大的雷诺数会导致下游位置的熄灭。局部熄灭可能导致稳定的火焰从燃烧器中升起，可能导致反应片上的局部孔，或者可能导致火焰的整体熄灭。局部熄灭的一个共同特征是火焰边缘，它可能向未燃烧的燃料和空气传播（着火），也可能向燃烧产物退去（熄灭）。由于边缘火焰将燃料从空气中分离出来，因此在边缘之外可能存在很强的成分梯度。已经发展了边缘火焰在成分梯度中的传播理论，并对稳定传播的火焰进行了一些测量。本项目通过测量众多反应混合层中的速度场和浓度场来解决边缘火焰传播问题。测量提供了一个完整的检查的影响，混合和速度梯度在火焰边缘。此外，建立了后退的稳定火焰（负传播速度），并首次与理论发展进行了比较。由于后退和传播边缘火焰对湍流升力射流火焰和具有局部消光的湍流火焰都很重要，因此所提出的测量允许为湍流火焰的这一重要特征开发更准确和完整的模型。拉曼散射、瑞利散射、激光诱导荧光和粒子图像测速技术应用于许多选定火焰的几何形状。这项工作有望提供对反应流和非反应流之间界面的火焰稳定性的理解。更广泛的影响这项研究从其对未来工业燃烧器设计的影响中得出了其潜在的重要性，这些燃烧器建议使用局部熄灭或提升火焰来减少污染物的产生。因此，了解边缘火焰和发展边缘火焰在湍流中传播的模型是至关重要的。这一发展也影响了燃烧的其他领域，包括火焰在液体和固体表面的蔓延（这可能是不必要的火的结果）和固体推进剂的火焰传播。本科生通过机械工程系的加速硕士课程和直接的本科助教奖学金参与。本科生专注于只考察少数测试条件的短期项目。除了这个研究元素，高年级本科生将设计一个简单的燃烧器模型，用于与当地高中科学教育者进行演示。这个燃烧器将被设计为允许火焰稳定性、燃烧模式和燃烧在工业中的应用的讨论和模型演示。

Non-premixed flames in practical combustors are often operated such that local extinction of the flame occurs. For example, large fuel velocities relative to the oxidizer can lead to sufficiently large scalar gradients at the flame base to cause extinction, and large Reynolds numbers can lead to extinction at downstream locations. Localized extinction can result in stable flames that are lifted from the burner, can result in local holes in the reaction sheet, or can lead to global extinction of the flame. A common feature of localized extinction is a flame edge, which may either propagate toward unburned fuel and air (ignition) or may recede toward combustion products (extinction). Because the edge flame separates fuel from air, a strong composition gradient can exist beyond the edge. Theories for the propagation of edge flames in composition gradients have been developed, and a few measurements of stable propagating flames have been made. Intellectual meritThis project addresses edge-flame propagation through measurements of velocity and concentration fields in numerous reacting mixing layers. The measurements provide a complete examination of the effects of mixture and velocity gradients at the flame edge. In addition, stable flames that recede (negative propagation velocities) are established and compared to theoretical developments for the first time. Since both receding and propagating edge flames are important to turbulent lifted jet flames and turbulent flames with localized extinction, the proposed measurements permit development of a more accurate and complete model for this important feature of turbulent flames. Raman scattering, Rayleigh scattering, laser-induced fluorescence, and particle image velocimetry are applied to the proposed geometry for many selected flames. The work is expected to provide an understanding of flame stability at interfaces between reacting and nonreacting flows. Broader impactsThis study draws its potential importance from its impact on future industrial combustor designs that propose to use local extinction or lifted flames to reduce pollutant production. Therefore, an understanding of edge flames and the development of models for edge-flame propagation in turbulent flows are critically important. This development also impacts other areas of combustion including flame spread over liquid and solid surfaces (which may be the result of unwanted fire) and flame propagation with solid propellants. Undergraduates are involved through the accelerated M.S. program in the Department of Mechanical Engineering, and through direct undergraduate assistantships. The undergraduates focus on short-duration projects that examine only a few test conditions. In addition to this research element, senior undergraduates will design a simple model combustor to be used for demonstrations with local high-school science educators. This combustor will be designed to allow for discussion and model demonstrations of flame stability, modes of combustion, and the application of combustion in industry.