Blowoff Dynamics of V-Shaped Bluff Body Stabilized,Turbulent Premixed Flames in a Practical Scale Rig

Abstract

Near blowoff dynamics and blowoff characteristics of premixed flames stabilized by a triangular flame holder in the midspan of a rectangular duct were studied using high speed imaging at 500 fps and simultaneous PIV and OH PLIF. Near blowoff dynamics manifested by the onset of asymmetric vortex shedding and local extinction in the form of flame holes were observed. Observations are presented describing the final blowoff event and its precursor: asymmetric sinuous modes of flame motion. It has been hypothesized that partial or total extinction of flame in the shear layers is the major factor that determines the evolution of the asymmetric mode and the final blowoff event. This phenomenon is further evidenced by an observation of the presence of flame kernels within the recirculation zone which under stable conditions contain only combution products. Whether a flame can survive an almost total extinction is governed by the ability of a reacting wake during these times to reignite the extinguished shear layers. I. Introduction Ground-based and aero gas turbine engine applications routinely incorporate bluff body-stabilized flame holders for primary or secondary combustion in high speed flows. The bluff body is placed in the mid flow of a high-speed duct and produces a recirculating wake structure that allows combustion to stabilize and then propagate into the free stream. In stable conditions, the recirculating wake structure steadily entrains hot combustion products from the adjacent shear layers and carries them upstream to ignite cool reactants as they are mixed in the wake shear layers [1]. The difficulty in design and implementation of bluff-body combustors is a result of the small range of fuel/oxidizer mixing conditions that yield stable combustion for given airflow. Temporal or spatial changes in airflow or fuel flow frequently produce changes in the flame structure that can cause extinction or combustion instabilities. Therefore, for a particular design, the stability of these flames needs to be carefully characterized over the intended operating envelope to optimize the coordination of the subsystems that control air and fuel flow with the purposes of maximizing combustion efficiency, minimizing the need for relights during times of critical operation, and minimizing thermo-acoustic instabilities. In order to predict blow off early in the design stage of any combustor, the fundamental phenomena of blow off needs to be captured conceptually and analytically and used to optimize the hardware design. Investigations have been conducted for close to sixty years with the objectives of understanding the underlying