Fabrice Lemoine

Simultaneous concentration and velocity measurements using combined laser-induced fluorescence and laser Doppler velocimetry: Application to turbulent transport

This paper describes the implementation of an optical technique, allowing to perform concentration and velocity measurements simultaneously and at the same point. This method is based on the coupling of laser-induced fluorescence of rhodamine B, applied to the determination of local concentration, and laser Doppler velocimetry. The method developed provides an accurate measurement of the concentration-velocity cross-correlation. The latter is a parameter linked to the eddy diffusivity tensor of a passive contaminant. This method was tested with a turbulent submerged free jet and it allowed the determination of the mean field of concentration and velocity, the concentration-velocity cross-correlation, and the local eddy diffusivity.

Investigation on temperature of evaporating droplets in linear stream using two-color laser-induced fluorescence

This article presents investigations on the temperature evolution of monodisperse, low evaporating, and interacting ethanol droplets in a linear stream. The droplets are injected over the ambient temperature, and the influence of injection parameters such as velocity, temperature, interdroplet spacing, and droplet size on the droplets cooling process is analyzed. It is shown that the typical droplet cooling process is characterized by two well-separated phases. In the first, the temperature decreases strongly, since the effect of forced convection is enhanced by the transport of air and the fuel vapor concentration in the boundary layer is far from saturation conditions. The second phase exhibits a reduction of the temperature rate of change, in connection with the decrease of the forced convection effects and enhancement of the fuel vapor concentration in the boundary layer. The effect of the injection velocity on the droplet cooling process is low, as a significant effect of the interdroplet spacing is observed. The respective influences of the injection temperature and droplet size on the heat transfer are jointly studied. It is shown that for a given injection temperature, the total energy extracted from the droplet per unit surface is insensitive to the droplet size. In this article, the mean droplet temperature is measured by two-color laser-induced fluorescence.

High-pressure Diesel spray temperature measurements using two-colour laser-induced fluorescence

This paper presents a new implementation of an optical technique which aims to measure the temporal and spatial evolution of the liquid temperature of a high injection pressure Diesel spray. Measurements are performed using the two-colour laser-induced fluorescence technique in which a temperature-sensitive fluorescent tracer is added to the fuel (ISO 4113 normafluid in the present case) and all the parameters except the temperature (e.g. tracer concentration, incident laser excitation, droplet number density) are eliminated by processing the ratio of the fluorescence intensity measured on two spectral bands. Diesel sprays, issuing from a single-hole injector, are injected at high pressures ranging from 500 to 1500 bar at a frequency of 10 Hz. The signal acquisition is then triggered on the injection cycle to enable the phase-locked monitoring of the liquid phase temperature. Temperature maps of the spray are presented and the influence of the injection pressure on the droplet temperature can be finally inferred from these results.

Simulation by solutal convection of a thermal plume in a confined stratified environment: application to displacement ventilation

Abstract This paper describes the experimental study, by solutal simulation, of a thermal plume in a confined stratified environment, a situation encountered in displacement ventilation systems. The criteria enabling similarity to be established between the thermal plume in air and the solutal plume in a hydraulic model are discussed. Density stratification is detected by a planar laser-induced fluorescence (PLIF) technique. Criteria for defining the interface height and thickness are determined After validation of these criteria in the fully developed region of the plume in a confined stratified environment, a formulation of the stratification height in the region close to the source has been established.

Experimental investigation of mass transfer in a grid-generated turbulent flow using combined optical methods

The aim of this paper is to provide some significant experimental results for the mass diffusion process from a point source in a quasi-isotropic homogeneous turbulent field generated by means of a grid. Experimental data for the mean, the fluctuating concentration fields, and the turbulent mass flux, which is the cross-correlation between concentration and velocity, are provided. Two combined optical non-intrusive methods, laser-induced fluorescence and laser Doppler velocimetry, have been used to measure simultaneously and instantaneously the concentration of the passive contaminant and the velocity of the flow. The experimental results are compared with a theoretical development, including the concept of turbulent diffusivity. An experimental determination of the turbulent diffusivity is also performed.

Some experimental investigations on the concentration variance and its dissipation rate in a grid generated turbulent flow

Abstract This paper is devoted to an experimental investigation of concentration variance diffusion in a grid generated turbulent flow. Combined laser-induced fluorescence applied to concentration measurement and two-dimensional laser-Doppler velocimetry are implemented in order to measure simultaneously and instantaneously the molecular concentration of the passive tracer and two components of the carrier flow velocity. The different terms of the scalar variance transport equation can be measured directly in order to deduce the scalar fluctuations dissipation rate. It is shown that the approximation scalar variance advection ≃ dissipation is valid, similarly to the decay of turbulent kinetic energy in the wake of a grid. The simultaneous determination of both scalar variance and kinetic energy dissipation rates permit an experimental determination of the scalar to velocity time scale ratio. Finally, an analysis of the self similarity of the fluctuating concentration field is also provided.

Drop spreading at the impact in the Leidenfrost boiling

Although the Leidenfrost effect has been extensively studied in the past, one challenge for the modeling of this phenomenon remains, namely, how to determine the effect induced by the presence of a vapor film on the frictions exerted on the drop. To address this issue, experiments are carried out on liquids with very different viscosities including water, ethanol, and several mixtures of water and glycerol. The deformation of droplets of a few hundred micrometers, impinging a perfectly smooth solid surface heated above the Leidenfrost temperature, is observed by shadowgraphy using a high-speed camera. Experimental results are compared to a theoretical model which is based on an inviscid asymptotic solution for the flow inside the lamella. This model also considers a lamella thickness which does not depend on the viscosity, the surface tension, and thus on the Reynolds and Weber numbers. This description of the lamella is valid if Weber and Reynolds numbers are high enough. Mass and momentum balances applied to the rim bounding the spreading lamella yield an equation for the rim motion which is then solved numerically. This equation accounts for the momentum transferred to the rim by the liquid coming from the lamella, the capillary forces, and the viscous stress at the separation between the lamella and the rim. The comparison between the model and the experiments suggests that the liquid at the bottom edge of the lamella is dragged by the vapor film given that the vapor velocity in the vapor film is significantly larger than that of the liquid. This process significantly increases the drop spreading for the low viscosity liquids. An analysis of the viscous boundary layer which develops at the bottom edge of the lamella is found to confirm this scenario.

EXPERIMENTAL AND THEORETICAL INVESTIGATION OF THE HEATING OF COMBUSTING DROPLETS IN A LINEAR STREAM

ABSTRACT Combusting ethanol droplets in linear stream are investigated by two colors laser induced fluorescence. The technique enables us to follow the transient mean temperature evolution of the combusting droplets. A database, including a wide range of aerothermal injection parameters such as injection velocity, droplet diameter and inter-droplet distance (or non-dimensional distance parameter) is provided. A calculation based on the heat diffusion equation resolution, with a constant temperature fixed at the measured equilibrium temperature has been implemented. Both heat conduction and heat advection by the droplet internal motions have been taken into account, the inside droplet velocity field being modeled by the Hill vortex solution. The influence of the different injection parameters are discussed and the results are compared with the models usually available in the literature. The measurement relative to the droplets’ transient heating enable to determine the Hill vortex intensity related to the maximum velocity at the droplet.

Mass transfer properties in a grid generated turbulent flow: some experimental investigations about the concept of turbulent diffusivity

Abstract This paper is devoted to the analysis of the mass transfer properties in a grid generated turbulent flow. The closure of the transport equation of the turbulent mass flux has been experimentally investigated. Two combined optical non-intrusive methods, the laser-induced fluorescence and the laser Doppler velocimetry have been used in order to measure simultaneously and instantaneously the concentration of a passive contaminant and the velocity of the flow. It has been demonstrated that the approximation production = pressure scrambling is valid over the major part of the flowfield. A model for the pressure scrambling term has been analysed in order to define a turbulent diffusivity concept.

Mesure directe du taux de dissipation de l'énergie cinétique de turbulence par vélocimétrie laser bi-composante : validation dans une turbulence de grille

Resume Une methode non intrusive permettant la determination directe du taux de dissipation de lenergie cinetique de turbulence est proposee, puis testee dans une configuration de turbulence de grille. Les conditions dhomogeneite et disotropie facilitent la validation de cette technique, en vue dune application aux ecoulements complexes tridimensionnels.