Conditional flow field statistics of jet-in-hot-coflow flames

The conditional velocity field of the Delft jet-in-hot-coflow flames is studied by the simultaneous application of planar laser-induced fluorescence of the OH radical (OH-PLIF) and particle image velocimetry (PIV). The goal of this study is to assess the role of turbulence on chemical processes in the Delft jet-in-hot-coflow flames and similar setups, that are characterised by the presence of a turbulent jet that issues into a slower-moving and much less turbulent surroundings of combustion products. An important result is that the conditional velocity field (conditional on the presence of a strong OH signal) is very different from the non-conditional velocity field at the same location. These jet-in-hot coflow flames are thus characterised by a strong intermittency, and experience a very different flow field and flow time scales than one would expect from the local mean velocity and scalar field. An other result following from the local velocity data is that the average conditional strain perpendicular to the flame surface is neither compressive nor extensive. This is caused by to the orientation of the principle strains and that of the flame interface. One millimetre inward to the jet, from the edge of the fuel-rich side of the OH contours, the average strains do become compressive, and the pdf of normal strain becomes considerably wider. In light of these results, the closure of the mean chemical source term by introducing a single (unconditional) turbulent time scale seems to be an invalid approach. Furthermore, it demonstrates that the fluid dynamic properties of the coflow stream are decisive for the applicability of these setups to the study of turbulence-chemistry interaction in industrial combustion processes.