Autoignition in Turbulent Flows: Experimental Observations and Investigation of the Effects of Turbulence and Mixture Inhomogeneities

In this work gaseous fuels were released continuously and concentrically into confined annular co-flows of turbulent hot air. Following injection the fuel and air mixed and at some length downstream of the nozzle the reactive mixture autoignited. Original phenomena are reported of autoignition spots, unsteady flame propagation and extinction or flashback. The frequency of the spots was measured, as were their acoustic and chemiluminescence characteristics. Optical measurements of the autoignition locations were made and used to estimate mean delay times from injection. As would be expected by considerations of simple chemical kinetics and the mean concentration field, higher air temperatures and lower fuel velocities resulted in autoignition closer to the injector. However, as the air velocity and hence also turbulent fluctuations were increased, autoignition shifted downstream and was delayed, while its frequency and sound intensity decreased. Such and other situations are presented that cannot be explained purely in terms of chemical arguments, i.e. homogeneous delay times, highlighting the significance of the mixing field through the mixture fraction and scalar dissipation rate.