| Summary: | 博士 === 國立中央大學 === 機械工程研究所 === 92 === This thesis is related to the study of premixed turbulent combustion using a large cruciform burner that has previously established in our combustion laboratory at NCU. The cruciform burner consisted of a long vertical vessel and a horizontal vessel. The former can be used to provide a downwardly propagating premixed flame with pressure release valves via an ignition modulus on the top of the vertical vessel. The latter was equipped with a pair of counter-rotating fans and perforated plates at each end to generate intense near-isotropic turbulence between the two perforated plates. Thus, flame-turbulence interactions without the influence of ignition can be achieved. Turbulence statistics and visualizations of turbulent flame fronts were obtained using laser Doppler velocimetry and a high-speed laser tomography technique. Three focuses are respectively fractal characteristics of these wrinkled turbulent flames, effect of radiative heat losses on turbulent burning velocities, and global quenching conditions of premixed turbulent flames. Concerning effect of radiative heat loss on turbulent premixed flames, we employ several diluted CH4/air flames with different degrees of radiative heat loss, from small (N2-diluted) to large (CO2-diluted), in the cruciform burner, over a wide range of turbulent intensities (u□/SL) where SL is the laminar burning velocity.
Using both the co-dimension and the stepping-caliper methods to analyze these laser tomography wrinkled turbulent flame images, we found that the mean fractal dimension d3 increases slowly from about 2.1 when u□/SL □ 1 to only 2.18 when 5 < u□/SL < 10, nearly independent of u□/SL. This is finding is in support of a recent Bunsen-flame result found by Gülder and his co-workers (2000), but conflicting with several previous results in which the fractal dimension was found to approach a value of 2.33 when u□/SL > 3. The inner and outer cutoffs, □i and □o, respectively, are found to be nearly constant for all flames studied. □o is found to be slightly smaller than the integral length scale of unreacted turbulence and is an order of magnitude greater than □i. It is found that the present fractal data cannot predict ST/SL correctly when the available fractal closure model such as the model of Gouldin (1987) was used, indicating a limit of the fractal model. By comparing N2- and CO2-diluted CH4/air flames of the same SL, effect of radiative heat loss plays an important role on lean CH4/air flames, while it has little influence on rich CH4/air flames. For lean mixtures at the same SL and u□/SL, values of ST/SL and/or Kc for CH4/CO2/air flames (large heat loss) are found to be considerable smaller than that of CH4/N2/air flames (small heat loss), revealing that the radiative heat loss may inhibit turbulent premixed flame propagation. These results may be relevant to the development of internal combustion engines, new-generation reciprocating engines, and atmospheric explosions.
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