Experiments on the sooting limits of aerodynamically-strained diffusion flames

An experimental study has been performed with axisymmetric counterflow diffusion flames to investigate the influence of aerodynamic straining on the relevant sooting limits of the lower alkanes. The limits are defined by the critical strain rate at which either soot luminosity, soot particle light-scattering, or fluorescence is negligible compared to the appropriate background signal. The critical strain rates are found to be greatest for the sooting limit based on the fluorescence signal, with those based on luminosity and light-scattering being similar. The fluorescence signal, if attributed to polycyclic aromatic precursors, yields a limit that can be interpreted as the extinction of soot precursors and is suggested to be a possible limit for identifying a completely nonsooting flame condition. The separate effects of flame temperature and fuel concentration on the critical strain rates for soot extinction have also been studied. The results are indicative of how temperature and concentration influence the soot particle inception process and they show that both are potentially important parameters. The critical strain rates display an Arrhenius temperature dependence and this dependence is similar for all alkanes considered.