Selection of working fluids for electrohydrodynamic experiments in terrestrial conditions and microgravity

International audienceElectrohydrodyamic (EHD) heat transfer enhancement and flow control methods are becoming increasingly popular in engineering science and applica- tions both in terrestrial and low gravity applications. The correct choice of the working fluid is essential for the design and performance of EHD hardware and can pose challenge because some working fluids with favorable EHD properties can be unstable or hazardous. In this paper key properties and criteria for the selection of working fluids for single-phase (liquid) as well as gas-liquid and vapor-liquid two-phase electrohydrodynamic experiments and applications are discussed. Key physical and electrical properties as well as environmental and safety issues are reviewed for the sample fluids PF-5052, FC-72, R141b, cyclohexane and pure water. Microgravity experiments impose additional demands on the selection of the working fluids. Some of these demands are addressed by contrasting bubble dimensions and shapes at detachment, estimated using a simple thermodynamic model, in terrestrial and microgravity conditions with and without electric fields. Data are obtained using a simplified analytical model and verified experimentally

UPSCALING METHODOLOGY FOR RADIATIVE TRANSFER IN POROUS MEDIA; NEW TRENDS.

International audienc

APPLICATION DE L'INTERFEROMETRIE HOLOGRAPHIQUE A L'ETUDE DE TRANSFERTS THERMIQUES COUPLES DANS UN GAZ AU SEIN D'UNE CAVITE. ESSAI DE MODELISATION

CE TRAVAIL EST CONSACRE A L'ETUDE EXPERIMENTALE DU COUPLAGE ENTRE TRANSFERTS RADIATIFS ET CONVECTION NATURELLE LAMINAIRE, AU SEIN D'UN GAZ SEMI-TRANSPARENT, A TEMPERATURE ELEVEE (JUSQU'A 700 K). IL CONCERNE UN ECOULEMENT EN CAVITE PRESENTANT UN OBSTACLE INTERNE. L'ENCEINTE ETANCHE MISE AU POINT EST DE GRANDE DIMENSION, REGULEE A HAUTE TEMPERATURE ET MUNIE DE FENETRES PERMETTANT LES VISUALISATIONS. LES TECHNIQUES OPTIQUES MISES EN UVRE POUR DETERMINER, LES CHAMPS THERMIQUES AUTOUR DE L'OBJET D'ETUDE SONT LA DEVIATION DE FAISCEAU LASER ET L'INTERFEROMETRIE HOLOGRAPHIQUE (FAISCEAU OBJET DE 15 CM DE DIAMETRE). CETTE DERNIERE EST UTILISEE DANS DES CONDITIONS OU LA DEVIATION DE FAISCEAUX PARASITE, PRISE EN COMPTE EN EXPLOITATION, EST IMPORTANTE. L'ENSEMBLE DE DONNEES EXPERIMENTALES OBTENU, CONCERNE LES TRANSFERTS EN CONVECTION NATURELLE PURE (N 2 A DIFFERENTES TEMPERATURES) COMME LES TRANSFERTS COUPLES (CO 2 A TEMPERATURE ELEVEE). LES RESULTATS EXPERIMENTAUX COMPARES AUX RESULTATS D'UN MODELE CLASSIQUE DE CONVECTION NATURELLE EN CHAMP LIBRE SUR UNE PAROI VERTICALE, PERMETTENT D'UNE PART DE VALIDER LES PROCEDURES EXPERIMENTALES ET D'AUTRE PART D'ETUDIER EXPERIMENTALEMENT L'EFFET DU RAYONNEMENT. LE TRANSFERT RADIATIF AU SEIN DU GAZ, COUPLE AU TRANSFERT CONVECTIF LAMINAIRE AFFECTE CONSIDERABLEMENT LES PROFILS DE TEMPERATURE ET AUGMENTE LE GRADIENT DE TEMPERATURE A LA PAROI. UN MODELE BIDIMENSIONNEL DE TRANSFERTS COUPLES EST APPLIQUE A UNE GEOMETRIE DE TYPE DE CELLE DE L'ETUDE EXPERIMENTALE. LE MODELE DE RAYONNEMENT DEVELOPPE REPOSE SUR UNE METHODE DE LANCER DE RAYON. LES PROPRIETES RADIATIVES DES GAZ SONT MODELISEES PAR L'APPROCHE CK. LA CONVECTION NATURELLE EST TRAITEE PAR LE MODELE DEVELOPPE PAR LAURENT FOUCHER DANS SA THESE. LES RESULTATS DU MODELE DE COUPLAGE SONT COMMENTES PAR RAPPORT A UNE SOLUTION DE CONVECTION NATURELLE. SI UNE CONFRONTATION ENTRE MODELE ET EXPERIENCE N'A PAS ETE REALISEE SOUS LES MEMES CONDITIONS GEOMETRIQUES ET THERMIQUES, DES TENDANCES SONT OBTENUES.CHATENAY MALABRY-Ecole centrale (920192301) / SudocSudocFranceF

Modeling of bubble detachment in reduced gravity under the influence of electric fields and experimental verification.

A simple model for predicting bubble volume and shape at detachment in reduced gravity under the influence of electric fields is described in the paper. The model is based on relatively simple thermodynamic arguments and relies on and combines several models described in the literature. It accounts for the level of gravity and the magnitude of the electric field. For certain conditions of bubble development the properties of the bubble source are also considered. Computations were carried out for a uniform unperturbed electric field for a range of model parameters, and the significance of model assumptions and simplifications is discussed for the particular method of bubble formation. Experiments were conducted in terrestrial conditions and reduced gravity (during parabolic flights in NASA's KC-135 aircraft) by injecting air bubbles through an orifice into the electrically insulating working fluid, PF5052. Bubble shapes visualized experimentally were compared with model predictions. Measured data and model predictions show good agreement. The results suggest that the model can provide quick engineering estimates concerning bubble formation for a range of conditions (both for formation at an orifice and boiling) and such a model reduces the need for complex and expensive numerical simulations for a certain applications

Experimental and RDFI calculated radiative properties of a mullite foam

The solid phase of a mullite foam has been experimentally characterized as a mainly scattering semi transparent medium with an isotropic phase function. In a second step, from the corresponding determined data, the Radiative Distribution Function Identification (RDFI) method of Zeghondy et al. has been applied to predict the bi-directional reflectance of a mullite foam sample. A direct experimental determination of this reflectance agrees with the model results