14th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 07-10 July, 2008 - 1 - Simultaneous measurements of Scalar and Velocity in a binary Propane-Air mixing layer Benoît Talbot 1 , Nicolas Mazellier 1 , Bruno Renou 1 , Luminita Danaila 1 , Mourad Abdelkrim Boukhalfa 1 1: CNRS UMR 6614 CORIA, Saint Etienne du Rouvray, France, talbot@coria.fr Abstract The focus of this work is on the non-reacting turbulent flow, in which we have elaborated a new, reliable experimental method to measure simultaneously the turbulent velocity field and the varying composition of propane/air mixing (Sc=1.36). The scalar is measured by time-resolved Rayleigh light scattering, whilst the velocity field is inferred using the hot-wire anemometry. The difficulty (due to the variable-density character of the flow) is that the calibration of the hot-wire anemometry technique requires local instantaneous values of the propane mass-fraction. Therefore, reliable determination of the local scalar is not only important by itself, but it is a priori crucial for the velocity measurement. We show here that replacing a precise fraction of air by neon (inert gas) into the air flow (at constant flow rate) leads to independent measurements of velocity fluctuations in a propane + air-neon mixture. Physically, the reason is that the heat transferred among the resulting mixing and the hot-wire is very nearly equal to that transferred among pure propane and the hot-wire. Therefore, the calibration curve is the same as that of pure propane. A first validation is performed in a turbulent propane (round) jet discharging into air-neon at Re D  6,900 for which the densities ratio is  1.52. We compare our results involving the scalar mixing (Rayleigh scattering) and the velocity field (hot anemometry, with the straightforward –and thus reliable- calibration method due to our particular mixture) with those already reported in the literature for different gaseous (round) jets mixing with comparable densities ratio (air-air, propane-air, CO 2 -air). The mean and RMS radial profiles of both velocity and concentration are in good agreement with those reported in literature. The comparison takes into account the real density effects. As far as the second-order statistics are concerned, both velocity and scalar spectra on the jet axis at different downstream positions exhibit an inertial range with a k -5/3 slope, consistent with the Kolmogorov-Obukhov-Corrsin theory. After this successful validation, we use our technique to investigate turbulent properties in the very-near-field of a plane propane-air mixing layer. Two laminar initial conditions flow (          υ θ θ      = = 46 and 155, where  θ is the boundary layer momentum thickness of the faster stream) are considered. Both initial boundary layers characteristics are determined accurately for both propane and air-neon channels. Velocity and scalar spectra are critically compared, with a particular attention paid to the exhibited slopes and spikes, the latter translating coherent structures present in this flow. Their understanding is important for the further reacting field. 1. Introduction In non-premixed combustion, both reactants (fuel and oxidizer) are injected separately and merge in the near wake of a splitter plate. For pure laminar parallel streams of reactants, a very small non-reacting partial premixed fuel-oxidizer region takes place before the chemical zone. The flame extremity or ‘edge-flame’ exhibits a triple structure (so called a Triple flame) and it has been demonstrated that the lift-off/blow-off is mainly controlled by both partial premixing properties of reactants before ignition and initial boundary layers characteristics [13]. Nevertheless, Kelvin- Helmholtz instability and then 3D turbulence developing in the near-field of a mixing layer generate multiscale eddies composing the turbulent flow that organize the chemical reaction zone in thin layers designated as flamelets (collection of ‘edge-flames’). Although heat release modifies the turbulent flow field, large scale turbulent structures remain globally unmodified under moderate heat release. Edge-flames thus keep their global structure [19], and consequently they are convected, distorted (stretched and folded) or even quenched by the large scale structures [24],