Measurements of fuel mixture fraction oscillations of a turbulent jet non-premixed flame
This work describes new type of combustion instability for which the 3-way coupling between mixing, flame heat release, and acoustics is modified by local buoyancy effects. Measurements of fuel mixture fraction are made for a non-premixed jet flame in a combustion chamber to assess the dynamics of mixing under imposed acoustic oscillations (22–55 Hz). Infrared laser absorption and phase resolved acetone-planar laser induced fluorescence are used to measure the fuel mixture fraction and then the degree of fuel/air mixing is calculated by determining the unmixedness. Results show acoustic excitation causes oscillations in the degree of fuel/air mixing at the driving frequency, which results in oscillatory flame behavior. This oscillatory flame behavior couples to the buoyancy and this in turn affects the mixing. Results also show that the mixing becomes less effective when the excitation frequency is increased or when the flame is present, compared to the non-reacting case. This work describes a key coupling mechanism that occurs when buoyancy is a significant factor in the flow field.
© 2008 The Combustion Institute. Received 25 January 2008. Received in revised form 7 June 2008. Accepted 26 July 2008. Available online 26 August 2008. This work was supported in part by the California Institute of Technology and partly by the Air Force Office of Scientific Research (AFOSR) under Grant No. F49620-03-1-0384 (Dr. Mitat Birkan, Program Manager). The authors are grateful to the assistance of Carlos Pinedo for his help in the setting up of the experiments.