Investigation of scalar mixing in the thin reaction zones regime using a simultaneous CH-LIF/Rayleigh laser technique

Simultaneous two-dimensional CH-LIF/Rayleigh measurements were carried out in highly stretched turbulent premixed stoichiometric methane-air flames. These flames fall into the thin reaction zones regime. In this regime, the Kolmogorov scale is smaller than the preheat zone thickness of a laminar flame, but it is larger than the reaction zone thickness. Therefore, small eddies can penetrate into the preheat zone but not into the reaction zone. These small eddies widen the preheat zone by turbulent mixing.

In the present flames at Karlovitz numbers Ka of 23 and 91, thin reaction zones with relatively thick turbulent preheat zones are being observed. The thickness of the preheat zone is expected to scale with the mixing length scale l_m, which is the thickness of an eddy within the inertial range that has a turnover time equal to the flame time.

The temperature/CH images are presented at different axial locations together with line profiles through the reaction zone to illustrate the different structures. In addition probability density functions (PDFs), of temperature conditioned on CH are presented.

The present data show that the temperature in the thin reaction zones' regime at the early positions in the highly stretched flames is relatively low. This is attributed to heat loss to the burner. In addition, in highly stretched flames at the borderline of this regime, local extinction has sometimes been observed, which could be due to entrainment of small eddies into the reaction zone.