The evolution of autoignition kernels in turbulent flames of dimethyl ether

Simultaneous planar laser-induced fluorescence (PLIF) imaging of CH2O and OH was performed at a repetition rate of 10.kHz, jointly with chemiluminescence to explore autoigniting dimethyl ether (DME) flames in a hot vitiated coflow burner. The focus of the study is the imaging of the flame stabilization region and the temporal evolution of ignition kernels upstream of the flame base. Results detail the evolution of kernels throughout their formation, growth and final merging with the flame base. The ignition events were explored for a range of different fuel premixing and dilution ratios over two coflow temperatures which result in different lift-off heights. Images of CH2O and OH over the entire flame length show that not only is the lift-off height much higher at low coflow temperatures, but that the fluctuations are more intense and the region of kernel formation is larger both radially and axially. In these autoignition stabilized flames, increased premixing leads to the lift-off height and location of the maximum kernel formation rate being further downstream. Transient 1-D simulations of hot coflow products opposed against jet fuel mixtures identify that the overlap of CH2O and OH PLIF signals are a reliable marker of heat release in autoignition kernels. Measurements indicate that for the high coflow temperature cases, on average, the heat release of individual kernels is low, despite the high total kernel formation rate. This can be correlated to the slow growth rate and elongated aspect ratio of the kernels. For low coflow temperature cases, kernels are growing faster and have high heat release rates with near unity aspect ratios.