Reaction zone visualisation in swirling spray n-heptane flames

Joint PLIF measurements of CH2O and OH were performed in a swirl-stabilized n-heptane spray flame at conditions close to extinction. Simulations of laminar counterflow heptane flames at different strain rates showed that the heat release could be approximately represented by the product CH2O × OH, but also that this product cannot visualise the heat release at the lean side of stoichiometry. The simulations suggest that the outline of CH2O regions in PLIF images could be an approximate indicator of the stoichiometric mixture fraction iso-line. Due to the intense turbulence and local extinction, individual PLIF images show a very variable behaviour. They indicate rich zones, reaction sheet breaks, lift-off, and they suggest that this flame is mostly of non-premixed character. The mean heat release rate as represented here is consistent with inverse Abel-transformed OH∗ chemiluminescence imaging. The usefulness of this technique for spray flames is discussed.