ABSTRACT

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 merit

This 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 impacts

This 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.

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