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In this study, we have examined the flow field and the flame surface density in premixed combustion by simultaneous particle image velocimetry (PIV) and OH-planar laser induced fluorescence (OH-PIIF) measurements. Turbulent flames are formed in a cyclone-jet combustor under weak to highly turbulence. The flame surface density is evaluated by the orthogonal OH-PLIF for vertical and horizontal imaging. In terms of the flame front detected by OH fluorescence, the flow field of turbulent flames including the strain rate has been discussed. It is found that the axial velocity becomes the maximum around the center axis, whereas the rms axial or radial velocity takes its maximum at the position far from the center axis. The larger velocity fluctuation is observed by increasing the mean exit velocity of Um. The radial position of peak OH fluorescence signal is not in the region of large rms velocity, except for the condition where the local extinction is observed. Hence, the large velocity fluctuation induces the local extinction through the high strain rate. The flame surface density is increased with an increase in rms velocity. The range of three-dimensional flame surface density (3D FSD) is from 0.2 to 0.8 mm, which corresponds similarly to that of the Bunsen flame. Moreover, based on two-dimensional (2D) and 3D FSDs, the increasing factor of 3D effects is not the same value of homogeneous turbulence. Recognizing a linear relationship between the rms axial velocity and the integrated FSD, the increase of FSD is simply caused by the flame wrinkling due to the turbulence, even at the condition where the local extinction occurs.

Flow field and flame surface density of turbulent flames by simultaneous PIV/OH-PLIF measurements