Spark ignition of turbulent nonpremixed bluff-body flames

摘要

Spark ignition of turbulent nonpremixed bluff-body flames with radial fuel injection for different fuel and air bulk velocities and swirl conditions has been investigated in terms of stability and ignitability limits and ignition probability (P_(ign), defined as the probability of igniting the whole flame from a localized 2-mm spark). The velocity field and the mixture fraction distributions have been measured with laser Doppler velocimetry and planar laser-induced fluorescence (PLIF) of acetone, respectively. Successful and failed ignition events have been visualized by high-speed and OH PLIF imaging. The results show that the stability limits are much wider than the ignitability limits, but with swirl, they become closer. The mixture fraction distribution shows high sensitivity to any change in the fuel and air velocities, with the mixture inside the central recirculation zone (CRZ) tending to be relatively well-mixed and rich. Visualization of different spark realizations shows that successful ignition events manage to initiate a flame at the separation corner of the bluff body and that the direction of initial flame propagation depends more on the ignition location than on the flow conditions due to the convection of the flame kernel by the mean flow. With successful spark events inside the CRZ the flame expands in all directions, up to a certain point, when it starts forming the shape of the stable flame. The highest P_(ign) was close to the CRZ boundaries with low P_(ign) or no ignition inside CRZ when the mixture there was rich. Under conditions where the CRZ is not close to the rich flammability limit, P_(ign) is substantial. Downstream of the CRZ, P_(ign) is uniform but low due to the adverse velocity. The probability of just initiating a kernel is higher than P_(ign), especially, in the area above the CRZ. With swirl, the highest P_(ign) is at the burner exit, with no ignition inside the CRZ because the mixture is too lean. The data highlight the difference between P_(ign) and the probability of kernel initiation and the flammability factor F (the probability of finding a mixture within the nominal flammability limits) and emphasize that flame propagation in inhomogeneous mixtures must be captured by any model that aims to simulate ignition of nonpremixed flame burners.