Role of entrainment in the stabilisation of jet-in-hot-coflow flames

The aim of the research on the Delft jet-in-hot coflow (DJHC) burner is to gain understanding in the interplay of turbulence and chemistry in conditions as encountered in devices operating in flameless combustion mode, and to test the validity of numerical models when applied to these flameless combustion conditions. Datasets on velocities, temperatures and qualitative OH data of several Dutch natural gas flames in the DJHC burner have been obtained and are discussed in this paper. It was found that the mean velocity and turbulent stresses are not significantly affected by the chemical reactions, which is in line with the very moderate increase of mean temperatures in the flames. Even at heights where flame structures are present, peak temperatures do not always approach the adiabatic flame temperature. With both flame luminescence and OH-PLIF measurements, it is seen that chemical reactions begin to occur at a lower location when the jet velocity (and thereby the jet Reynolds number) is increased. By analysing the velocity and temperature data in the near-nozzle region, the entrainment of coflow fluid into the turbulent jet has been quantified. The increased entrainment of the higher Reynolds number jet, in combination with the positive temperature gradient in radial direction in the near field of the jet, is shown to be responsible for the decrease of the height where reactions start to occur.