The concepts of flame and combustion characteristics are important for designing combustor, combustion efficiency and stable combustion. There is a possibility of unstable combustion since the modern gas turbine engines are generally operated at lean premixed flames to reduce the emissions such as the NOx due to environmental problems. Therefore, flame structures, heat release patterns and combustion instability characteristics should be studied to predict and control the instabilities. In this paper, flame length/flame center length scaling, vortex-heat release interaction and combustion instability characteristic were experimentally investigated in a dump combustor using different hydrocarbon fuels.
First, flame lengths and flame center lengths were experimentally measured. Ethylene, ethane, propane and air, which were used for hot-firing tests were supplied from the inlet section to combustor section. Sine waves, which were generated to simulate the combustion instability were also supplied into premixed gases at the bottom of dump combustor using an acoustic driver. An ICCD camera with a bandpass filter corresponding to a CH* chemiluminescence band was used for capturing flame images. The first objective is to investigate the effects of the inlet velocity, equivalence ratio and acoustic forcing on the flame lengths and flame center lengths. The second objective is to suggest the empirical equations about the flame lengths and flame center lengths as a function of non-dimensional parameters such as the Reynolds number, Strouhal number and Damkohler number. The experimental results showed that an increase in the equivalence ratio and a decrease in the inlet velocity caused the flame lengths and flame center to become shorter. The acoustic forcing frequency played, however, a different role on the flame lengths and flame center lengths. As the Reynolds number increased or Damkoher number decreased, the flame lengths and flame center lengths increased. The Strouhal number affected the flame length and the flame center length differently.
The patterns of heat release fluctuation were studied to determine the location and timing of a secondary fuel injection. It was experimentally measured by changing the experimental conditions and phase angles using an ICCD camera. Hot spot region was observed from the leading edge of vortex and cold spot region was in the trailing edge when the Damkohler number above 4 ? 5. On the other hand, the opposite trend appears in the cases of the Damkohler number below 3.
An experimental study has been studied to understand the combustion instability characteristics. The instability frequency and power spectral density of the pressure oscillations had significantly different values under same experimental conditions. However, the combustion instability characteristics had similar values by adjusting the characteristic chemical time regardless of the fuels. It shows the importance of characteristic chemical time in predicting combustion instability. As the combustor height increased or the equivalence ratio and inlet velocity decreased, the instability frequency and maximum power spectral density decreased. CH intensities were also measured using a photomultiplier tube with a CH* chemiluminescence band under unstable combustion. Though the characteristic chemical time was similar, the maximum power spectral densities hadn’t same values. The instability frequencies of a CH intensity, however, were identical with the results of pressure oscillation.