3.2. Flame structure
The flame burns detached from the nozzle. This can be clearly seen from Fig. 5 (left) showing
the mean OH distribution superimposed to the axial and radial vector plot. Mixing studies using acetone tracer PLIF [34,35] revealed that the probability of stoichiometric conditions are high in the vicinity of the mean stagnation point and coincide with the OH-onset. Although the feed- in of fuel and air is non-premixed, the present flame burns most likely in a (partially) premixed mode. This is well known for unconfined lifted non-premixed jet flames.
The influence of rising pressure on the mean OH distribution along the centreline is shown in Fig. 5 (right). Mean stagnation point and OH-on- set move upstream with increasing pressure. In the limit of the experimental uncertainty this axial shift scales linearly with the pressure. Increased turbulence intensity is expected at larger Reynolds-numbers and combustor pressures (cf. Table 1). As the flame is burning very close to premixed conditions, the turbulent burning velocity will increase with flame surface area (compare Fig. 6) and turbulence intensity. Because the bulk velocity is constant for all three operational conditions, the lift-off height decreas- es with increasing combustor pressure.