Committee:

 Dr. Tim Lieuwen (Advisor)

 Dr. Jerry Seitzman

 Dr. Devesh Ranjan

Title: Ensemble-Averaged Dynamics of Harmonically Excited Turbulent Premixed Flames

Abstract:

Combustion instability is a significant problem for modern, lean, premixed combustors. This instability arises through a feedback loop between heat release and harmonic acoustic and/or hydrodynamic oscillations. Although there is a substantial body of research which examines this problem for laminar flames, virtually all practical, high power, combustion systems operate in a turbulent environment. Prior studies of turbulent flames have utilized a “quasi-laminar” approach, which utilizes the time-averaged flame position, and ensemble-averaged disturbance field, as inputs to a problem which is essentially identical to the laminar problem. Although this approach shows promise, it does not capture any dynamical effects. In this work, I analyze an ensemble-averaged turbulent burning velocity closure proposed by Shin and Lieuwen (2013), who showed that the ensemble-averaged turbulent burning velocity for a harmonically forced flame is proportional to the ensemble-averaged flame curvature. The strength of this dependence is characterized by the turbulent Markstein length, which relates ensemble-averaged flame curvature to the turbulent, ensemble-averaged burning velocity.

The research I propose expands upon the results of Shin and Lieuwen (2013) by showing that their flame speed closure and definition for the ensemble-averaged turbulent burning velocity can provide a governing equation for the ensemble-averaged flame position, and allows development of reduced-order models for flame position and heat release dynamics. Because the turbulent displacement and consumption Markstein lengths are a critical component of the flame speed closure, I propose to characterize their dependence on harmonic forcing amplitude, turbulence intensity, and turbulence length scale, through analysis of a numerically simulated turbulent flame. In addition to this analytical modeling, I propose to develop a new experimental facility designed to allow investigation of a turbulent, premixed, V-flame attached to a harmonically oscillating flame holder. This facility will be used to examine the ensemble-averaged flame position and heat release and characterize the turbulent flame speed dependence on ensemble-averaged flame curvature.