2 results
Scalar mixing and reaction in plane liquid shear layers
- P. S. Karasso, M. G. Mungal
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- Journal:
- Journal of Fluid Mechanics / Volume 323 / 25 September 1996
- Published online by Cambridge University Press:
- 26 April 2006, pp. 23-63
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The scalar (concentration) field of two-dimensional liquid mixing layers was investigated at post-mixing-transition conditions. The planar laser-induced fluorescence technique was used for passive scalar measurements, and for chemical product measurements. Following the approach of Koochesfahani & Dimotakis (1986), the chemical product results were used to make resolution-free estimates of mixed-fluid quantities, thus providing a check on the accuracy of the passive scalar measurements. The operating conditions were systematically varied to study the effect of various parameters (Reynolds number, speed ratio, and initial boundary-layer momentum thickness) on the structure of the layer. At conditions which are just past the mixing transition, the study essentially duplicated the results obtained by Koochesfahani & Dimotakis: the chemical-product concentration profiles at high- and low-stoichio-metric ratios (flip experiments) were symmetric and the average concentration of mixed-fluid was uniform across the layer. However, when the layer was pushed to more fully developed conditions, its scalar field evolved to an asymptotic state: the two flip chemical-product concentration profiles developed modest asymmetries, and the average mixed-fluid concentration developed a small variation across the layer, but the change was less than that observed in gases. Based on the chemical reaction data, we infer that the mixture fraction probability density function (p.d.f.) for the fully-developed liquid layer evolves from a ‘non-marching’ type to a ‘tilted’ type. Despite the observed evolution, the average mixed-fluid concentration remained fixed for all the layers past the mixing transition, while the total mixed-fluid probability (the total amount of mixed fluid normalized by the layer's width) showed only a very slight increasing tendency as the layer reached fully developed conditions. The mixture fraction p.d.f., measured by the passive scalar approach, is shown and discussed for a broad range of cases. While it overpredicts the amount of mixing, it showed a qualitatively-correct ‘non-marching’ character initially, but evolved to a qualitatively-incorrect ‘marching’ character at the asymptotic state. The reasons for the poor estimation of the p.d.f. by the passive scalar approach, at fully developed conditions, are attributed to changes in the flow and lack of resolution and suggests caution when using such measures. Furthermore, the study also showed that the Reynolds number alone is inadequate to characterize the state of the layer. A different parameter (the ‘pairing parameter’), which accounts for the initial boundary layers and scales with the number of vortex mergings, was found to better explain the evolution in the structure of the scalar field.
Mixing and reaction in curved liquid shear layers
- P. S. KARASSO, M. G. MUNGAL
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- Journal:
- Journal of Fluid Mechanics / Volume 334 / 10 March 1997
- Published online by Cambridge University Press:
- 10 March 1997, pp. 381-409
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The concentration field of mixing layers subject to stabilizing and destabilizing streamwise curvature was investigated at post-mixing-transition conditions. A set of operating conditions was implemented, identical to those at which straight layers were previously investigated in the same facility, in order to compare the effects of hydrodynamic instabilities upon scalar mixing. Quantitative imaging of planar laser-induced fluorescence was used for (i) passive scalar measurements, and (ii) chemical product measurements. Similar to the straight mixing layer, the results for the curved layers show that beyond the mixing transition the layer continues to evolve, and undergoes a small change in its scalar structure. At conditions just past the mixing transition both stable and unstable layers have average mixed-fluid compositions which are uniform across the layer, and average chemical product concentration profiles which are symmetric. At more fully developed conditions, the scalar field evolved: the average mixed-fluid concentration developed a small lateral variation, while the chemical product concentration profiles became asymmetric. Similar to the straight layer, the mixture-fraction PDF is believed to be of the tilted type for the most fully developed layer examined, with the marching PDF being a poor representation. Consistent with previous investigations, the growth rate of the unstable layer was found to be higher than that of straight or stable layers. The most important result is that the measured mixing efficiency of all the layers (curved and straight) was found to be the same: both the total mixed-fluid composition, and the volume fraction of mixed fluid were the same for all unstable, stable, and straight layers. The amount of mixed fluid (and of chemical product formed) was larger for the unstable layer, but always in a fixed proportion to the layer's thickness. The lack of increase in the mixing efficiency for the unstable layer is surprising, given that previous hydrodynamic measurements had shown enhanced turbulent transport for the unstable case. Thus, for all liquid shear layers studied, the rate of scalar mixing appears to be directly proportional to the entrainment rate (which essentially determines the layer's growth rate), and not to any hydrodynamic measures.