Conditional statistics of the turbulent/non-turbulent interface in a jet flow

Using two-dimensional high-speed measurements of the mixture fraction Z in a turbulent round jet with nozzle-based Reynolds numbers Re-0 between 3000 and 18 440, we investigate the scalar turbulent/non-turbulent (T/NT) interface of the flow. The mixture fraction steeply changes from Z = 0 to a final value which is typically larger than 0.1. Since combustion occurs in the vicinity of the stoichiometric mixture fraction, which is around Z = 0:06 for typical fuel/air mixtures, it is expected to take place largely within the turbulent/non-turbulent interface. Therefore, deep understanding of this part of the flow is essential for an accurate modelling of turbulent non-premixed combustion. To this end, we use a composite model developed by Effelsberg & Peters (Combust, Flame, vol. 50, 1983, pp. 351-360) for the probability density function (p.d.f.) P(Z) which takes into account the different contributions from the fully turbulent as well as the turbulent/non-turbulent interface part of the flow. A very good agreement between the measurements and the model is observed over a wide range of axial and radial locations as well as at varying intermittency factor gamma and shear. Furthermore, we observe a constant mean mixture fraction value in the fully turbulent region. The p.d.f. of this region is thus of non-marching character, which is attributed physically to the meandering nature of the fully turbulent core of the jet flow. Finally, the location and in particular the scaling of the thickness delta of the scalar turbulent/non-turbulent interface are investigated. We provide the first experimental results for the thickness of the interface over the above-mentioned Reynolds number range and observe delta/L similar to Re-lambda(-1), where L is an integral length scale and Re-lambda the local Reynolds number based on the Taylor scale lambda, meaning that delta similar to lambda. This result also supports the assumption often made in modelling of the stoichiometric scalar dissipation rate chi(st) being a Reynolds-number-independent quantity.