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

Published online by Cambridge University Press:  29 August 2013

Markus Gampert ,
Venkat Narayanaswamy ,
Philip Schaefer  and
Norbert Peters
Markus Gampert
Affiliation:
Institute for Combustion Technology, RWTH-Aachen University, Templergraben 64, Aachen, Germany
Venkat Narayanaswamy
Affiliation:
Institute for Combustion Technology, RWTH-Aachen University, Templergraben 64, Aachen, Germany
Philip Schaefer
Affiliation:
Institute for Combustion Technology, RWTH-Aachen University, Templergraben 64, Aachen, Germany
Norbert Peters*
Affiliation:
Institute for Combustion Technology, RWTH-Aachen University, Templergraben 64, Aachen, Germany
*
†Email address for correspondence: n.peters@itv.rwth-aachen.de
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Abstract

Using two-dimensional high-speed measurements of the mixture fraction $Z$ in a turbulent round jet with nozzle-based Reynolds numbers $R{e}_{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\sim R{ e}_{\lambda }^{- 1} $, where $L$ is an integral length scale and $R{e}_{\lambda } $ the local Reynolds number based on the Taylor scale $\lambda $, meaning that $\delta \sim \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.

JFM classification

Wakes/Jets: Jets Turbulent Flows: Shear layer turbulence
Type
Papers
Information
Journal of Fluid Mechanics , Volume 731 , 25 September 2013 , pp. 615 - 638
Copyright
©2013 Cambridge University Press 

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