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

实际燃烧室中的非预混火焰通常是这样操作的，即火焰发生局部熄灭。例如，燃料相对于氧化剂的较大速度可能会导致火焰底部的标量梯度足够大，从而导致熄灭，而较大的雷诺数可能会导致下游位置的熄灭。局部消光可能导致从燃烧器升起的稳定火焰，可能导致反应表上的局部孔洞，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，它可以向未燃烧的燃料和空气传播(点火)，也可以向燃烧产物扩散(熄灭)。由于边缘火焰将燃料从空气中分离出来，边缘之外可能存在一个强烈的成分梯度。发展了边缘火焰在成分梯度中传播的理论，并对稳定传播的火焰进行了一些测量。智能优点该项目通过测量大量反应混合层中的速度和浓度场来解决边缘火焰传播问题。这些测量提供了对火焰边缘混合气和速度梯度的影响的完整检查。此外，还建立了退去的稳定火焰(负传播速度)，并首次与理论发展进行了比较。由于退缩和传播的边缘火焰对于湍流喷流火焰和局部熄灭的湍流火焰都很重要，因此所提出的测量方法可以为湍流火焰的这一重要特征发展一个更准确和更完整的模型。拉曼散射、瑞利散射、激光诱导荧光和粒子图像测速技术被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上火焰稳定性的理解。更广泛的影响这项研究的潜在重要性来自于它对未来工业燃烧室设计的影响，这些设计建议使用局部熄灭或升起火焰来减少污染物的产生。因此，对边缘火焰的理解和发展边缘火焰在湍流中的传播模型是至关重要的。这一发展还影响到燃烧的其他领域，包括火焰在液体和固体表面的蔓延(这可能是不必要的火灾的结果)，以及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生助学金参与进来。本科生专注于短期项目，这些项目只检查几个测试条件。除了这个研究元素，大四的本科生还将设计一个简单的燃烧器模型，用于与当地高中科学教育工作者进行示范。该燃烧室的设计将允许对火焰稳定性、燃烧模式和燃烧在工业中的应用进行讨论和模型演示。

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.