| dc.description.abstract | This thesis investigates experimentally effect of radiative heat losses on the stretch rate of a premixed flame interacting with a decaying turbulent wake. The decaying turbulent wake was generated by quickly withdrawing a horizontal plate in a long vertical burner with 1.5m height and with a cross-sectional area of 15×15 cm2. Before a run, the burner is evacuated and then filled up methane-air mixtures with or without diluents to 1 atm, including N2 and CO2, at a given equivalence ratio ( ). A run begins by ignition at the top of the burner where the top plate with 15×15 cm2 cross-sectional area is simultaneously opened in order to release the gas from thermal expansion and generate a downward-propagating premixed flame at 1 atm. When the flame propagates 1m downwardly, very near to the top of the horizontal plate, the horizontal plate is quickly withdrawn to create a turbulent wake for flame-wake interactions. We use high-speed digital particle imaging velocimetry to measure flame-wake interactions, so that the corresponding dilatation rate, aerodynamic strain rate, curvature, and thus the stretching rate can be obtained.
Three cases of rich methane/air premixed flames at 1.4 or 1.45 are studied, including the pure methane/air case ( 1.45; negligible radiative heat loss), the N2-dilulent case ( 1.4; small radiative heat loss), and the CO2-diluent case ( 1.4; large radiative heat loss). It is found that the negative strain rate increases the positive dilatation rate, while the positive strain rate increases the negative dilatation rate. This correlation is the same among these three cases, regardless different degrees of radiative heat loss. The effect of radiative heat loss reduces the magnitude of the strain rate, but it has little influence on the curvature. It is found that the curvature term plays a more dominate role on the stretch rate than the strain rate term. When the dilatation rate of the CH4/air/N2 case is compared with that of the CH4/air/CO2 case, it is found that increasing the degree of radiative heat loss corresponds to a decrease of the dilatation rate, indicating the dilatation rate may be used as an indication of flame burning strength, as suggested by Driscoll and his co-workers (1996).Using the wavelet analysis, the integral length scale of flame-wake interactions among these three cases are obtained. Finally, the unsteady effect of flame-wake interactions will be also discussed. | en_US |