|dc.description.abstract||This research uses a self-designed turbulent wake burner to investigate a downward propagating premixed flame interacting with a turbulent wake. Since the interaction of flame and wake can produce the so-called flame stretching effects. To study the stretching effects and ST, we apply high-speed laser tomography and particle image velocimetry (PIV), use research-grade methane or propane -air mixtures, and conduct a series of experiments. Basically, there are two types of wake vortices, a compressive strain (counter-rotating) vortex pair and an extensive strain vortex pair. It is found that flame-wake interactions in compressive strain vortices are more intense than in extensive strain vortices. This is because the compressive strain vortex pair can engulf directly the flame into the clockwise-rotating vortex core. Burning starts at the vortex core and through layers and layers radially. The other half, counter-rotating vortex has slower burning rates. This asymmetry of burning is due to the facts that the axis of the vortex pair is not perpendicular to the flame front and the buoyancy effect matters.
We calculate the local flame stretching rate (including the curvature rate and the aerodynamic strain rate) along the wrinkled flame front at different times using laser tomography and PIV techniques. For the extensive strain vortex pair, values of the stretching rate increase along the wrinkled flame front from the vortex tip to the position which is close to the vortex core. This result is similar to that of Driscoll et al. (1994) using a single axisymmetric vortex pair interacting with a premixed flame. It is found that the curvature term is much more important than the strain rate term. Thus, the stretching rate may be approximated by the curvature term alone. From the probability density function of the stretching rate, we found that the stretching rate changes with time , indicating that the unsteady effect cannot be neglected.
In order to measure the effect of turbulent intensity to ST, we put the thin disturbance slices with different heights on the edge of the sliding plate. The results are then compared with previous experimental and theoretical results. It is found that the present ST measurements are much higher than our previous data using a cruciform burner which can produce a near-isotropic turbulent flow field. This may be because in the 3-D near-isotropic turbulent flow field many flame-flame collisions and annihilations are frequently observed. Therefore, the increase of the total flame area is not linearly proportional to the turbulent intensity, values of ST obtained in 3-D near-isotropic turbulence field using the cruciform burner are found to be much less than that of the present measurements for 2-D flame-wake interactions. However, the latter is not stationary and must be viewed with caution.||en_US|