Stabilisation of swirling dual-fuel flames

C7H16-CH4-air flames stabilised in a bluff body swirl burner have been examined with flame photographs, OH* chemiluminescence, and simultaneous 5 kHz OH-PLIF and Mie scattering with a focus on local an global extinction characteristics. The aim of this study is to investigate flame structure when more than one fuel is present and provide both insight and data for dual-fuel modellers. Flame imaging shows that the presence of an additional fuel affects the stabilisation characteristics of one fuel, whether it be liquid or premixed gaseous. With the addition of more CH4 in the oxidiser channel, dual-fuel flames with C7H16 spray became more premixed in appearance, evidenced by flame photographs, mean OH* chemiluminescence images, and instantaneous and mean OH-PLIF images. Addition of CH4 to such systems also forces the flame to stabilise on the outside of the swirled channel, similar to premixed CH4-air flames far from blow-off. However, the flame branch in the region of the shear layer directly above the bluff body edge moves further from the base of the burner with the addition of CH4, suggesting that typical spray flame behaviour is lost even with a small addition of CH4 to the system. This observation is supported by global extinction curves, which show that C7H16-CH4-air flames appear to behave more similarly to premixed flames than spray flames, but remain of fundamental interest due to their unique stabilisation behaviour and relative insensitivity to bulk velocity changes compared to spray-only flames at similar equivalence ratios.