Lounes Tadrist

... par Alain BRICARD Ingénieur du Conservatoire National des Arts et Métiers Ingénieur de Recherche au Centre d'Études Nucléaires de Grenoble et Lounès TADRIST Docteur ès Sciences Physiques Directeur de Recherches au Centre National de la Recherche Scientifique ...

Boiling is a two-phase heat transfer process where large heat fluxes can be transferred with small driving temperature differences. The high performance of boiling makes the process very interesting for heat transfer applications and it is widely used in industry for example in power plants, refrigeration systems, and electronics cooling. Nevertheless, due to the large number of involved phenomena and their often highly dynamic nature a fundamental understanding and closed theoretical description is not yet accomplished. The design of systems incorporating the process is generally based on empirical correlations, which are commonly accompanied by large uncertainties and, thus, has to be verified by expensive test campaigns. Hence, strong efforts are currently made to develop applicable numerical tools for a reliable prediction of the boiling heat transfer performance and limits. In order to support and validate this development and, in particular as a precondition, to enhance the basic knowledge about boiling the comprehensive multi-scale experiment RUBI (Reference mUlti-scale Boiling Investigation) for the Fluid Science Laboratory on board the ISS is currently in preparation. The scientific objectives and requirements of RUBI have been defined by the members of the ESA topical team "Boiling and Multiphase Flow" and addresses fundamental aspects of boiling phenomena. The main objectives are the measurement of wall temperature and heat flux distribution underneath vapour bubbles with high spatial and tem-poral resolution by means of IR thermography accompanied by the synchronized high-speed observation of the bubble shapes. Furthermore, the fluid temperature in the vicinity and inside of the bubbles will be measured by a micro sensor array. Additional stimuli are the generation of an electric field above the heating surface and a shear flow created by a forced convection loop. The objective of these stimuli is to impose forces on the bubbles and investigate the resulting bubble behaviour such as bubble sliding on and detaching from the surface. The experiments benefits from the absence of vapour buoyancy and natural convection in the high quality and long-term microgravity of the ISS. Effects and phenomena like thermocapillary convection that are hardly observable in normal gravity conditions can be investigated. Clearly predefined conditions particularly of the thermal layer at the heating surface can be established without disturbances by natural convection. Vapour buoyancy as the main detaching force in normal gravity is missing. Hence, it is possible to study stationary, attached bubbles and alternative detaching forces. With RUBI a long history of boiling experiments is perpetuated that used microgravity as a tool for a deeper understanding of the fundamental phenomena. Several precursor experiments closely related to the RUBI project have already been conducted on parabolic flights. The subject of the paper is to provide an overview on the RUBI project, its scientific objectives and the corresponding experimental principle. The current design of the experiment container that is under development at ASTRIUM Space Transportation in Friedrichshafen will be introduced. Furthermore, results from the precursor experiments are presented. The industrial activities of the RUBI project are funded and the science team is supported by ESA.

Boiling in microchannels is widely considered as one of the front runners in process intensification heat removal. Flow boiling heat transfer in microchannel geometry and the associated flow instabilities are not well understood, further research is necessary into the flow instabilities adverse ...

This paper concerns an experimental study on thermocapillary convection at the interface between an air bubble and a silicone oil with a low Prandtl number. The silicone oil layer is heated from above and cooled from below in order to obtain a vertical temperature gradient. When the temperature gradient is increased and a critical value is exceeded, oscillations appear in

This work concerns particle image velocimetry (PIV) measurements with a high-speed camera in a circulating fluidized bed (CFB). Because of the nature of the gas-particle flow in the riser of the CFB a dense wall layer of particles is observed when the superficial gas velocity is increased beyond a certain limit. This dense wall layer reduced the quality of the measurements. A mask is developed which improves the quality of the PIV measurements. Furthermore, it increases the range of superficial gas velocities for which the measurements can be obtained in the CFB system. The improved PIV measurement is verified via laser Doppler anemometry measurements. The results are of interest for measurements of dense wall-layer flows.

In the present (second) part of the paper a linear instability analysis of a viscous capillary jet flowing into an inviscid immiscible fluid is performed. The analysis is based on the full 3D-Navier-Stokes equations written in respect of the moving trihedron of the jet axis. A dispersion equation of the sinuous disturbances is derived accounting for the effect of the

A CFD model is developed in order to complete the experimental study of a new hybrid jet impingement / microchannel cooling scheme by enlarging the spectrum of the parametric study. The most influential parameters of the heat exchange are identified and the capacity of the new design to achieve temperature profiles of the cooled object adapted to the specific needs

In this paper we present the preliminary results on the de-pinning phenomenon of wetting volatile drops. Wetting properties are analysed by measuring the drop base and the dynamic apparent contact angle. Three cases are investigated: reduced pressure environment, controlled substrate temperature and surfactants solutions. The experimental results are compared to the existing theories to interpret the de-pinning phenomenon. The Young

We study flow boiling in a rectangular minichannel of 0.5 × 4 mm2 cross-section (hydraulic diameter: 889 μm). A two-phase flow pressure drop analysis by ranging several mass flow rates for a chosen heat flux provided to the minichannel has been performed. Two kinds of upstream conditions have been investigated to show up the confinement influence on boiling. Steady and unsteady thermo-hydraulic behaviors are reported. A stability criterion is found depending on the two controlled parameters (heat flux and mass-flow rate). The upstream condition is modified by adding a compliant buffer tank and the procedure is repeated. Unsteady flows are observed with a different intensity and with a different range of operating conditions when the flow is mainly vapor. When no compliance source is connected, the unsteady behaviors are observed in the same operating conditions. Nevertheless the characteristics of the fluctuations differ notably in amplitude and frequency. In the steady state conditions a pressure loss model is proposed based on a homogeneous assumption. A total pressure loss steady state modeling of the boiling flow is realized using a homogeneous model for the two-phase flow zone to check the steady average minichannel pressure loss. A good agreement is found with the experimental and model results.

We develop a mathematical model of evaporation of a thin liquid film into air under the action of disjoining pressure. The rate of evaporation is determined from the numerical solution of a coupled system of equations describing heat conduction in the liquid and diffusion of vapor through air. Local vapor concentration near the film surface is assumed equal to its equilibrium value at the local temperature. Evolution of the film is studied for two commonly used disjoining pressure models. Conditions are formulated at which disjoining pressure suppresses evaporation. The model is applied to investigation of the effect of evaporation on contact lines on heated surfaces.