Influence of fuel type on advective heat flux and extinction scalar dissipation rate in negative edge flames

In turbulent combustion, such as that encountered in a gas-turbine engine, regions of locally high strain may cause a flame hole to develop in the reaction sheet. The edges of the hole caused by this local extinction point can be characterized as forwardly propagating if they move in the direction of unburned fuel and air or negatively propagating if they move in the direction of products. The hole itself may close if the forward propagation is relatively fast, or it may grow and lead to global extinction if the negative propagation is dominant. Although the forwardly propagating flame edge has been studied extensively in the form of lifted flames, the negatively propagating flame edge has received less attention experimentally, due to its transient nature. In this work, a counterflow flame geometry was used to produce negative edge flames. This burner, which has been previously characterized, utilizes a high-velocity coflow to induce stable off-axis extinction due to an increase in scalar dissipation rate. Previously, it was found for methane flames that advection through the flame edge increased the extinction scalar dissipation rate as compared to extinction on the centerline. The flame edge scalar dissipation rate was correlated to the centerline extinction value, but since advection is a function of both the local velocity and temperature gradients, the correlation depends on details of the flame including its local velocity at the reaction sheet. Additionally, at low velocities, thermal and species diffusion through the flame edge played a larger role. The current paper describes an experimental and numerical study to extend the previous analysis to additional fuels in an effort to examine the impact of varying diffusivities on the local extinction scalar dissipation rate. The flame stability as a function of velocity and composition has been investigated for ethane and hydrogen, supplementing previous results for methane. The conditions leading to stabilization of a negative edge flame indicate that the flame edge is more robust for ethane and hydrogen as compared to methane, suggesting that the role of advection at the edge is even stronger than previously reported for methane edge flames. Results from the numerical simulations are examined to calculate this advective heat flux component.