Armelle Cessou

Statistical flow dynamic properties conditioned on the oscillating stabilization location of turbulent lifted flame

Recent works have confirmed that the propagation of leading-edge flames is the stabilization mechanism of turbulent lifted flames. Strong interactions between dynamics, mixing, and chemical reactions control the sustaining flame. Simultaneous and conditional measurements are needed to investigate the stabilization of turbulent lifted jet flames. The present paper focuses on the influence of flow dynamics on turbulent flame stabilization by means of simultaneous particle-image velocimetry (PIV) and OH planar laser-induced fluorescence (PLIF) measurements. First, the turbulent field in the cold flow upstream of the flame base was determined. Turbulent properties of the flow at the flame base are important data for numerical simulation and modeling. It is essential to discriminate the cold flow from the hot gases since the velocity is higher in hot gases due to thermal expansion. Thus, to avoid overestimating the velocity fluctuations in the flame stabilization region, the fields of average and fluctuating velocity were measured within an analysis window. This window was attached to the instantaneous flame base, which followed it in its oscillating motion. With such an analysis window, only turbulent properties in the cold flow were sampled. Such data of the turbulent field at the flame base are useful to provide initial conditions to direct numerical simulation (DNS), comparison information to large eddy simulation (LES), and relevant validation of the Reynolds average Navier-Stokes (RANS) model, where intermittency is not accounted for. Second, the unsteady influence of the turbulent flow on the leading-edge flame was investigated. The leading-edge flame was identified on each OH PLIF image, and local dynamic properties of the flow were measured from PIV meshes placed along the normal of OH outline at the leading-edge location.

Long-cavity Nd:YAG laser used in single-shot spontaneous Raman scattering measurements

The use of a 100 ns pulse Nd:YAG laser is proposed to perform single-shot measurements by spontaneous Raman scattering. The long pulse duration is obtained thanks to a White cell placed inside the laser oscillator. This lengthens the pulse duration within the laser cavity, keeping almost the same beam quality as standard Nd:YAG lasers. The laser system is described, the advantages are compared with extra-cavity stretched lasers, and single-shot measurement of concentration in gas is demonstrated.

Assessment of a fast electro-optical shutter for 1D spontaneous Raman scattering in flames

In this study, a fast electro-optical shutter composed of a large aperture Pockels cell placed within two crossed polarizers is assessed as optical gating of full-frame back-illuminated CCD camera for single-shot 1D Raman multispecies measurements in flames. The spatial homogeneity of the active response of the Pockels cell crystal is firstly checked with a laser beam. The radial efficiency of the shutter to transmit the weak Raman light is evaluated and the limitations of the setup for 1D measurements are analyzed using an optical magnification. Tests performed in methane-air flames show that the Pockels cell shutter with hundreds of nanosecond gate duration can reduce up to 500:1 the background luminescence leading to up to 10 times of enhancement in the signal-to-noise ratios for Raman-active species.