서강대학교

초록/요약

In the present study, large eddy simulations (LES) of a turbulent diffusion flame with a hybrid fuel of methane and hydrogen are carried out to investigate the effects of operating conditions on the flame shape and the pollutant formation. A parametric study is performed by changing the values of hydrogen concentration and the background pressure and temperature. A flamelet-progress variable approach (FPVA) is used for a tabulated chemistry and a dynamic subgrid scale (SGS) model is used as the LES model. The result shows a good agreement with a previous experiment in the statistics of flow fields and composition. Also, the results affected by each operating parameter show consistent tendency with previous studies. Then, we investigate how the effect of added hydrogen changes for different background pressure and temperature conditions including those of a direct injection internal combustion engine. It is found that both the high compression ratio and hydrogen addition can viii lead to more compact and focused reaction zone by changes in the reaction and mixing rates. This also affects the pollutant formation, and the reduction of CO and NO is observed with hydrogen addition. Turbulent statistics such as RMS flow fields, flame stability index and combustion noise are analyzed to evaluate the flame stability. A normalized Rayleigh index proves to be a useful indicator for the stability of turbulent diffusion flame, as compared to the RMS of flow variables. From the combustion noise spectra, hydrogen addition stabilizes mainly the primary vortex shedding in the standard condition but reduces the noise level in the entire frequency range as the compression ratio increases. Also, it is concluded that stabilization of unsteady fluctuation is highly correlated to the reduced flame length.