The geometrically similar region of the centerline concentration decay of a counter-flowing jet was investigated using planar laser induced fluorescence. The jet is investigated for jet to counterflowing velocity ratios (Ur) in the range 4<Ur<19. New length scales for this flow were studied and used to generate empirical expressions to predict the concentration decay in the established flow zone. These length scales were defined using the 5% contour of the mean concentration field of the counterflowing jet. Additional experiments for two jet to counterflow velocity ratios were used to validate these empirical expressions. The new empirical expressions correlate well with the concentration decay in the centerline of the counterflowing jet within the free jet-like region. It was found that for some cases, the centerline concentration decay of the counterflowing jet was better predicted considering decay proportional to x−6/5, even though it can also be predicted by assuming decay with x−1.

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