An Experimental Study on Blowoff Mechanism of Bluff-Body-Stabilized Acoustically-Forced Premixed Flame

Abstract

Flame blowoff phenomenon in combustor with bluff-body has been a topic of extensive research such as gas turbine, rocket, ramjet, and so on. The blowoff generally occurs when the chemical reaction time scale for ignition exceeds the characteristic flow time of unburned reactant. In the process of the blowoff, external acoustic excitation has a significant impact on the blowoff conditions and structural features of the flame. However, there has been almost no research on blowoff in premixed combustor when external excitation is present. In this study, blowoff was investigated in a ducted combustor with the bluff-body when acoustic excitation was forced. Natural gas mainly composed of methane was used as fuel. Lowering the equivalence ratio, lean blowoff was to occur. Blowoff equivalence ratio was measured with varying m ̇air, excitation frequency and sound pressure to analyze the blowoff. As a result, we confirmed that the blowoff equivalence ratio was increased at specific frequency. This frequency is determined as a frequency related to the frequency of shear layer, therefore flame images were taken for measurement of the frequency on the flame. To observe the flame structure, OH* chemiluminescence was used. Chemiluminescence images were analyzed by applying POD (Proper Orthogonal Decomposition) algorithm to obtain the dominant mode and the frequency of shear layer. Depending on the experimental conditions, blowoff equivalence ratio was valued very differently. Vortex frequency behind the bluff-body and resonance effect in combustor are the main factors that affect the blowoff equivalence ratios with the excitation. As a final result, we defined the blowoff mechanism with and without external excitation based on the measured equivalence ratio values and image analysis.