Stabilisation of turbulent auto-igniting dimethyl ether jet flames issuing into a hot vitiated coflow

Experimental results focusing on understanding the role of autoignition in the stabilisation of turbulent lifted jet flames of dimethyl ether issuing into a hot vitiated coflow are explored in this paper. A stability map of the flame lift-off height as a function of coflow temperature and jet partially premixing ratios is reported, indicating a strong sensitivity of the mean and rms lift-off heights to the coflow temperature. A new experimental setup that utilises high repetition rate (10 kHz) joint OH and CH2O PLIF combined with a third camera to track out of plane motion via chemiluminescence emission is employed. This allows the temporal dynamics of the flame base and autoignition kernels to be tracked at high spatial resolution over a spatial window of 20 mm (height) × 43 mm (width). To capture the temporal statistics of the flame over the full lift-off height range, a complementary, but not simultaneous, large field of view high-speed chemiluminescence imaging experiment was employed. The use of high-speed diagnostics is shown to be invaluable to understand the temporal dynamics of the flame base region, such as identifying and tracking autoignition kernels ahead of, and merging with, the main flame base. For low coflow temperatures, autoignition kernels are found to be fundamentally important to the stabilisation of the flame: small autoignition initiated kernels rapidly grow to form large flame kernels that eventually merge with the main flame base and in doing so rapidly establish a new lift-off height significantly upstream of the original flame base. For high coflow temperatures, autoignition kernels whilst still present, are significantly smaller in mean size and play a reduced role in the overall flame stabilisation. This leads to the conclusion that partially premixed flame propagation plays an increased role in overall flame stabilisation for high coflow temperature flames.