Michel Lebouché

Controlled cooling of a hot plate with a water jet

Abstract Experiments are performed, under steady-state conditions, to study the boiling heat transfer from a hot plate to a planar jet of water. Temperature control of the heating surface enables the determination of entire boiling curves and the identification of each boiling regime from forced convection to film boiling. By solving the two-dimensional inverse heat conduction problem the local heat flux and corresponding temperature can be computed up to a distance of 55 mm from the stagnation line. Measurements are carried out under atmospheric pressure in a heater temperature range from 100 to 650 °C. The jet temperature is varied to study the influence of subcooling in any regime of the boiling curve. Furthermore experiments are carried out with variable jet velocity and distance between nozzle and heater, for an immersed jet and for a free surface jet, respectively. The transition from nucleate to film boiling at the stagnation point can clearly be determined. Film boiling occurs, e.g., at about 450 °C surface temperature at a jet velocity of 0.8 m/s and a subcooling of 16 K.

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.

Energetic budget on an evaporating monodisperse droplet stream using combined optical methods: Evaluation of the convective heat transfer

Aerothermal properties in a fuel spray is a central problem in the field of the design of the combustion chambers of automotive engines, turbojets or rocket engines. Heat and mass transfer models are necessary in the predictive calculation schemes used by the motorists. Reliable experimental data must be obtained for both the validation and development of new physical models linked to heat transfer and evaporation in sprays, where aerodynamic interactions have a key role. This paper proposes an experimental study of the energetic budget of a monodisperse ethanol droplet stream, injected in the thermal boundary layer of a vertical heated plate. The droplet size reduction is measured using a light scattering technique (interferential method) in order to characterize the evaporation, as the droplet mean temperature is monitored using the two colors laser-induced fluorescence technique. The convection heat transfer coefficient and the Nusselt number are inferred from the overall energetic budget, as a function of the inter-droplet distance, characterizing the interaction regime. The results are compared to physical models combined with numerical simulations available in the literature, for moving, evaporating isolated droplets and for three droplets arrangement in linear stream.

Turbulent transport of a passive scalar in a round jet discharging into a co-flowing stream

The mass transport properties of a round turbulent jet of water discharging into a low velocity co-flowing water stream, confined in a square channel, is investigated experimentally. The measurement region is the self-similar range from x/d=70 to x/d=140. Combined laser-induced fluorescence and 2D laser Doppler velocimetry are used in order to measure simultaneously, instantaneously and in the same probe volume, the molecular concentration of a passive scalar and two components of the velocity. This technique allows the determination of moments involving correlations of both velocity and concentration fields, which are necessary to validate the second-order modelling schemes. Both transport equations of Reynolds shear stress uv and turbulent mass flux vc have been considered. In both cases, advection, production and diffusion terms have been determined experimentally. The pressure-strain correlation and the pressure scrambling term are inferred with the help of the budget of Reynolds shear stress and mass turbulent transport equations. Second order closure models are evaluated in the light of the experimental data. The turbulent Schmidt number is found to be almost constant and equal to 0.62 in the center region and decreases strongly to zero in the mixing layer of the jet. The effects of the co-flow on the turbulent mixing process are also highlighted.

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.

Nucleate and convective boiling in plate fin heat exchangers

Abstract The results of laboratory experiments with CFC114 flowing in an electrically heated, serrated-fin or perforated fin test section to measure local boiling coefficients over a wide range of vapour quality, with mass fluxes up to 45 kg/m 2 s, heat fluxes up to 3500 W/m 2 and pressure of 3 bar are reported. These low mass and heat fluxes reflect the industrial process application of these heat exchangers where small temperature differences may exist between streams. An analysis of the measured heat transfer coefficients from tests with CFC114 in both serrated fin and perforated fin geometries shows the separate effects of quality, mass flux and heat flux. Two kinds of mechanism were found: a nucleate boiling regime and a convective boiling regime. The data were predicted using an asymptotic model, the nucleate boiling component was obtained from pool boiling data and the forced convective component of the two-phase heat transfer coefficient was found to be well represented by the F and Martinelli parameters used by Chen [I&EC Process Design and Development 5(3) (1966)].

two-phase gas-liquid flow in horizontal corrugated channels

Abstract Experiments have been carried out for adiabatic concurrent gas–liquid flow of nitrogen and water through two corrugated test sections. This study examines flow patterns as the gas fluxes are increased. At low gas fluxes two main patterns have been observed: stratified flow for low liquid fluxes and bubble flow for greater ones. The mean wall shear stress of the two-phase flow has been measured with the electrochemical method (polarography); its analysis reveals the flow structure modification from monophasic to two-phase flow.

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.

Inverse Heat Conduction Applied to the Measurement of Heat Fluxes on a Rotating Cylinder: Comparison Between an Analytical and a Numerical Technique

A method using either a one-dimensional analytical or a two-dimensional numerical inverse technique is developed for measurement of local heat fluxes at the surface of a hot rotating cylinder submitted to the impingement of a subcooled water jet. The direct model calculates the temperature field inside the cylinder that is submitted to a given nonuniform and time dependent heat flux on its outer surface and to a uniform surface heat source on an inner radius. In order to validate the algorithms, simulated temperature measurements inside the cylinder are processed and used by the two inverse techniques to estimate the wall heat flux. As the problem is improperly posed, regularization methods have been introduced into the analytical and numerical inverse algorithms. The numerical results obtained using the analytical technique compare well with the results obtained using the numerical algorithm, showing a good stable estimation of the available test solutions. Furthermore, real experimental data are used for the estimation, and local boiling curves are plotted and discussed.

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.