Title

Investigation of Blowoff Mechanism and Forced Response of Bluff Body Stabilized Turbulent Premixed Flames

Date of Completion

January 2010

Keywords

Applied Mechanics|Engineering, Aerospace|Engineering, Mechanical

Degree

Ph.D.

Abstract

Important problems related to the governing mechanism of flame blowoff in bluff body stabilized turbulent premixed combustion have been investigated in this dissertation. In the first part of the dissertation fundamental aspects of unforced and forced response in premixed and partially premixed fuel stratified flames encountered in low NOx gas turbine engines and afterburners are studied. Combustion in these devices presents complexities and instabilities introduced by thereto-acoustic, entropy wave and convective interactions. In this context, generalized blowoff limits and transfer functions of premixed turbulent flames under controlled acoustic perturbations and mixture gradients have been characterized. ^ The second part of the study concerns the flame dynamics of a bluff body stabilized turbulent premixed flame as it approaches lean blowoff. Experiments were performed in a laboratory scale burner as well as in a prototypical combustor with different geometries (axisymmetric and planar two dimensional), length and velocity scales. High speed chemiluminescence imaging along with simultaneous particle imaging velocimetry and OH planar laser-induced fluorescence were utilized in both these experiments for premixed propane-air flames to determine the sequence of events leading to blowoff and provide a quantitative analysis of the experimental data. It was found that near blowoff, the flame front and shear layer vortices overlap as a result of the reduced flame speed in fuel lean mixtures, to induce high local stretch rates on the flame. The high stretch rates exceeded the extinction stretch rate values instantaneously and in the mean, resulting in local flame extinction along the shear layers. Following partial or whole shear layer extinction, fresh reactants could pass through the non-reacting shear layers to react within the recirculation zone with some or all other parts of the flame extinguished. The flame kernel within the recirculation zone might survive for a few milliseconds and could reignite the shear layers such that the entire flame can be reestablished for a short period. This extinction and reignition event could happen several times before final blowoff event which occurred when the flame kernel failed to reignite the shear layers and ultimately lead to total flame extinguishment. Strikingly similar findings in the two different experimental setups suggest the general validity of the proposed flame blowoff mechanism and its insensitivity to a particular geometry. ^ Finally, recent results from ongoing research on the mechanism of forced blowoff and an experimental study on scalar mixing in an interacting field of two successively generated counter rotating laminar line vortices at the interface of two gas streams are presented. ^