Courbes d'écoulement dans les systèmes de mélange : vers une approche simplifiée

National audienceLe comportement rhéologique de fluides cisaillés dans des géométries de mélange est étudié à partir d'une procédure simplifiée de calcul de la vitesse de cisaillement. L'applicabilité de cette procédure est examinée dans le cas de fluides newtoniens et non newtoniens, en s'intéressant à l'influence de la géométrie de mélange. Les résultats sont favorablement comparés à ceux issues de la littérature. On montre ainsi que cette procédure permet de s'affranchir de la détermination préalable d'un rayon virtuel nécessaire à l'application de l'analogie de Couette, ainsi que du choix à priori du comportement rhéologique du fluide cisaillé

High torque vane rheometer for concrete: principle and validation from rheological measurements

International audienceA high torque vane rheometer is used to measure the yield stress of cement-based materials. It is shown that this apparatus is suitable for the evaluation of the yield stress of various concretes and mortars in the fresh state in comparison with slump tests realized with ASTM Abrams cone. Then, the rheological properties (yield stress and shear flow behaviour) of a homogeneous kaolin clay suspension are studied with the apparatus and favourably compared with other rheometers and geometries

Shear flow curve in mixing systems--A simplified approach

The original contribution is available at http://www.sciencedirect.com/science/article/pii/S0009250908004545International audienceRheological behaviour of fluids in mixing systems was studied using a simplified approach in shear rate calculation. The efficiency of this approach is investigated from Newtonian and non-Newtonian fluids, exploring also the geometry of mixing systems. Results are favourably compared with those obtained from previous published works. It is shown that this approach does not require the calculation of a virtual inner radius nor a priori assumption of the rheological behaviour of the fluid mixed

Heat transfer properties of aqueous carbon nanotubes nanofluids in coaxial heat exchanger under laminar regime

International audienceThe thermal performance of water-based multi-wall carbon nanotubes nanofluids are measured in a coaxial heat exchanger under laminar regime within the range of Reynolds numbers 500-2500. The convective heat transfer properties with constant wall temperature are evaluated for four different multi-wall carbon nanotubes based nanofluids at low concentration of 0.05% in weight (0.026% in volume). The measurements of thermal and rheological properties of the nanofluids with operating temperature were investigated experimentally. The effects of the aspect ratio of carbon nanotubes, the type of base fluid and surfactant on viscosity, thermal conductivity and laminar convective heat transfer were studied. Based on the experimental results, we reported the shear-thinning behaviour of nanofluids, the nanofluid viscosity being dependant on the base fluid type in the Newtonian region. We also showed that the enhancement of the thermal conductivity and the average convective heat transfer of nanofluids increased with the aspect ratio of nanotubes and decreased when the thermal conductivity of the base fluid increases. This enhancement attains at least 10% in comparison to base fluid even with the low content of nanotubes used

Thermophysical properties and heat transfer performance of carbon nanotubes water-based nanofluids

International audienceIn this paper, thermal conductivity and rheological properties of CNT water-based nanofluids were experimentally measured, whereas density and heat capacity were evaluated from appropriate theoretical correlations. The influence of nanoparticle content and base fluids on thermophysical properties of nanofluids was presented and discussed. Then, the thermal performance and convective heat transfer of such nanofluids were investigated in a coaxial heat exchanger working in cocurrent flow. Fixed wall temperature boundary condition and laminar regime were also considered during theexperiments. The results were presented discussing the effect of the entrance region, Reynolds number and nanofluids composition

Extension of spread-slump formulae for yield stress evaluation

International audienceThis paper provides a new model to evaluate the yield stress of suspensions, slurries or pastes, based on the release of a finite volume of material onto a horizontal surface. Considering the height (h) and the radius (R) of the sample at the flow stoppage, two asymptotic regimes, where h higher than R or h lower than R, lead to different analytical models that allow the determination of yield stress. Experimental observations show typical sample shape at stoppage between slump and spread. Based on these observations, we have developed a new analytical model to evaluate accurately the yield stress of materials in this intermediate regime. The validity of this model was evaluated from data obtained using various Carbopol® dispersions. The yield stress measured with the proposed model was compared with the yield stress evaluated from shear flow curves obtained with roughened plate/plate geometry fitted to the Herschel-Bulkley model. Results show the relevance of the proposed model which that can be applied in the range between models used for the two asymptotic regimes

Processing the Couette viscometry data using a Bingham approximation in shear rate calculation

The original contribution is available at http://www.sciencedirect.com/science/article/pii/S037702570800013XInternational audienceThis paper presents an approach to computing the shear flowcurve from torque-rotational velocity data in a Couette rheometer. The approximation techniques in shear rate calculation are generally dictated by the radius ratio between coaxial cylinders and the rheological behaviour of fluid tested. Here, the approach consists in analysing the sheared material as a Bingham fluid and computing an average shear rate when the fluid in the cylindrical gap is partially and fully sheared. We focus in particular on the applicability of the Bingham approximation in shear rate calculation. First, the approach is assessed by examining synthetic data generated with Newtonian, non-Newtonian and yield stress materials with known properties, varying the gap radius ratio. The results, which are compared with commonly used techniques in shear rate calculation, prove the relevance of the proposed approach. Finally, its efficiency is examined by applying it to process Couette data of yield stress fluids taken from published works

Efficiency of carbon nanotubes water based nanofluids as coolants

International audienceThe thermo-physical properties of water-based nanofluids containing carbon nanotubes, stabilized by SDBS as surfactant, are experimentally studied. The effect of low nanoparticle volume fraction, ranging from 0.0055% to 0.278%, on density, thermal conductivity and viscosity of nanofluids is investigated for temperature range of 20°C to 40°C. Enhancement in density, thermal conductivity and viscosity of nanofluids with volume fraction in nanotubes is shown in comparison to base fluids and modelled from simple theoretical relationships. The influence of temperature on the thermo-physical properties of tested nanofluids is also discussed, as well as the shear rate dependence on the nanofluids viscosity. Finally, the efficiency of the tested nanofluids as cooling fluids is evaluated under laminar and turbulent flows regimes from the thermo-physical values previously determined. This may be helpful for using these nanofluids in real cooling systems

Rhéologie de pâtes dentaires par extrusion

International audienceL'objectif de ce travail est d'étudier le comportement rhéologique de différentes pâtes de soins dentaires dont le comportement est principalement plastique. Des tests sont réalisés avec une géométrie d'extrusion et par cisaillement direct (géométrie vane). Ces deux types d'essais permettent d'évaluer les seuils de mise en écoulement et de friction du matériau. Il apparaît que ces seuils sont fortement affectés par une modification des conditions cinématiques. Les résultats permettent de sélectionner les formulations les plus pertinentes pour garantir des conditions d'exploitation optimisées

Etude des performances thermiques en convection forcée des nanofluides à base de nanotubes de carbones dans un échangeur coaxial

International audienceLe présent travail s'attache à caractériser expérimentalement les propriétés thermo-physiques ainsi que les performances thermiques de quatre types de nanofluides à base de nanotubes de carbone à la fraction massique 0.01% afin d'évaluer les effets liés à leurs compositions dont le type de surfactant, le rapport d'aspect des NTC et l'ajout de l'antigel. Une section d'essais constituée d'un échangeur coaxial a été conçue et instrumentée finement avec des thermocouples au niveau de la paroi d'échange et à l'entrée/sortie du fluide, ainsi qu'avec des capteurs de pression afin de permettre une détermination précise des flux échangés au sein de cette section d'essais. Les mesures de la conductivité thermique et de la viscosité dynamique des nanofluides ont été effectuées expérimentalement. Le coefficient d'échange convectif dans l'échangeur ont été déterminés en fonction de la distance axiale