|dc.description.abstract||This thesis investigates experimentally the effect of unsteady stretch on laminar premixed flames interacting with turbulent flows. Using a turbulent wake burner and a large cruciform burner, a von Kármán turbulent wake and near-isotropic turbulent flows can be generated, respectively. We applied high-speed particle image velocimetry (PIV) and the laser tomography to quantitatively measure the corresponding strain rate, curvature, stretch rate, and dilatation rate along the interacting flame front with turbulent wake and near-isotropic turbulence.
Experiments in the turbulent wake burner were conducted to study the effect of radiative heat losses on stretching of premixed CH4 flames by using two diluting gases, CO2 (large radiative heat loss) and N2 (small radiative heat loss),respectively. Note that the laminar burning velocities for both CO2- and N2-diluted flames are kept constant with SL = 10 cm/s at a fixed equivalence ratio Φ = 0.7. Experimental results reveal that the maximum burning rate that may be indicated by the maximum dilatation rate occurs in regions of high positive curvature rates. This confirms that the reaction rate of Le＜1 flames is increased by the positive stretch, as already suggested by Law and many other researchers. The curvature team is more important than the strain rate term in the overall stretch consideration for the present lean CH4/air premixed flames with Le＜1, at least for the ratio of the mean tangential velocity of the staggered vortex pair of the wake to the laminar burning velocity, uθ/SL , up to 2. By comparing CO2- and N2-diluted flames, the wrinkled flame propagation speeds and the peak values of the dilatation rate are largely decreased by the increase of radiative heat loss.
Experiments in the cruciform burner were conducted to investigate the effect of unsteady stretch for rich (Φ = 1.45) methane/air flames interacting with near-isotropic turbulence, where the root-mean-square turbulent intensity u’’=32.3 cm/s and u’’/SL = 2.2. The experimental data suggest that the reaction of rich CH4 flames (Le＞1) is strengthened by the negative strain rate, but the flame is burned more intensely near regions of the flame front whose curvature is positive. For the unsteady stretch of rich CH4 flames, the strain rate term plays a dominate role on the stretch rate in the beginning of the flame-turbulence interaction, but during the interaction the curvature term gradually becomes a dominate term.||en_US|