The flame surface density Rxy is de

The flame surface density Rxy is determined in its two-dimensional projection. In a first step, the influence of the interrogation box size Dx2 is investigated to confirm its insensitivity to this parameter. Dx is varied between 1.55, 3.4, and 5.25 mm. The flame surface density initially is evaluated in radial cuts at x = 20, 30, and 40 mm. No significant influence of the box size dimension could be observed (not shown). For the smallest box size of Dx = 1.55 mm, however, scatter increases due to the decreased probability to actually find the OH contour in the rather small box.
The flame brush thickness d is determined from histograms for the probability of OH occurrence (PDFOH). The flame brush expands in down- stream direction. At x = 30 mm, the flame brush width is approximately d = 17 mm. Following the recommendation in [40], the box size should not exceed 0.2 · d. Therefore, a box size of Dx = 3.4 mm is chosen.
In modelling premixed combustion the pro- gress variable c = (T