Dynamic Characteristics of the Turbulent Premixed Bluff Body Flames in the Confined Combustor

Abstract

The dynamic characteristics and the combustion instability phenomenon of a premixed bluff body flame were investigated in a confined duct combustor. When the lean blowoff occurred due to the decrease of the equivalence ratio from the stable flame condition, the characteristics of the lean blowoff was compared and analyzed according to acoustic excitation to consider the external disturbance. Meanwhile, self-excited combustion instability that occur when the equivalence ratio increase from the stable flame conditon and the flashback phenomenon coupled with self-excited combustion instability was investigated. The blowoff equivalence ratio increases with the Reynolds number and changes depending on the extent of the recirculation zone. Using the relation between the Damköhler number and the Reynolds number, it was confirmed that the flow velocity at the downstream tip of the bluff body and the laminar flame speed are decisive blowoff factors. Although a periodic flame hole appeared far from the blowoff only with acoustic excitation, the blowoff observed by OH radical chemiluminescence occurred using a similar process regardless of the excitation. The recirculation zone collapses and the flame becomes small when it is close to the blowoff. Then, the flame is locally extinguished downstream from the bluff body and the recirculation zone completely collapses. Eventually, the unburned gas does not ignite and the flame is extinguished. The blowoff equivalence ratio rapidly increases at specific acoustic excitation frequencies. This was investigated using proper orthogonal decomposition analysis, the two-microphone method, and phaselock particle imaging velocimetry measurement. Resonance occurs when the excitation frequency approaches the harmonic frequency of the combustor and it increases the velocity fluctuation in the combustor and the infiltration velocity of the unburned gas in the shear layer of the recirculation zone. Consequently, because the burning velocity must have a larger value corresponding to the enhanced mixture velocity for a sustained flame, the blowoff occurs at a higher equivalence ratio. In addition, the size of the recircualtion zone was changed according to the phase of the excitation frequency occuring the resonance and it was considered as the cause of attenuation for flame stability. Previous studies investigating flashback in a bluff body have found that the flame moves back and forth around the trailing edge of the bluff body

however, the phenomenon in which the flame propagates beyond the bluff body has not been sufficiently studied. Therefore, it was necessary to understand a strong flashback, which can damage the upper section of a combustor and which is vulnerable to heat due to flame propagation over the front of the bluff body. The combustion instability frequency resulting from changes in the combustion length occurred within the range of the resonance frequency of the combustor, thereby confirming that thermal-acoustic combustion instability occurred in the combustor. In order to examine the flame structure at the moment of flashback, high-speed OH-PLIF and pressure fluctuation measurement were simultaneously conducted and the phase-locked PIV technique was applied. When the strong flashback occurs, an instantaneous adverse pressure gradient is formed within a combustion instability cycle. Consequently, the generated reverse flow pushed the flame attached at the trailing edge of the bluff body to the upstream from the bluff body. The flame propagated rapidly along the side of the bluff body by the influence of the boundary layer flow and the decreased quenching distance. This propagated flame became the ignition source at the front tip of the bluff body and generated the flame surface that propagated in all directions

thus, it was found to be the primary cause of the increase in the flashback distance. The flame flashback distance also varied depending on the combustor length and the initial flow condition. An attempt was made to concentrate the measured data of flashback distance under various conditions into a single line, and turbulence intensity and combustion instability frequency were the dominant factors that impacted the flashback distance. Additionally, in order to confirm the applicability of the research results of flashback under more practical conditions, an experiment was conducted using the hydrogen to fuel ratio, the blockage ratio of the perforated plate, and the reactant temperature as variables. The addition of hydrogen increases the flame speed and flashback distance compared to the methane flame. However, when the proportion of hydrogen exceeds 50%, a flashback proceeds to the fuel injector and the condition for which a flashback distance measurement is meaningless, easily occurs. The increase in the blockage ratio of the perforated plate attenuates the turbulence intensity flow in a non-reacting condition. This property decreases the pressure fluctuation, when combustion instability occurs and also the flashback distance. A rise in the reactant temperature increases the flame stability as well as the flame speed, and a flashback does not occur at a high reactant temperature. As the data gathered with the turbulence intensity and the instability frequency only is incomplete, a modification of the parameters is necessary. The pressure fluctuation data near the orifice is used to represent the pressure fluctuation characteristics of the combustion instability instead of the data at the position of the bluff body. Additionally, the turbulent flame speed is considered to represent the laminar flame speed and the turbulence intensity in each condition. Consequently, using the modified parameters, the relationship amongst the velocity fluctuation, turbulent flame speed, mean velocity, and the combustion instability frequency, and the factors affecting the flashback are confirmed by effectively collecting the experimental results obtained for various cases, including the methane flame condition.