Determination of heterogeneities in a scraped surface heat exchanger using electrochemical sensors

An experimental investigation of a scaled-down model of an industrial exchanger, using an electrochemical technique, was undertaken in order to show the presence of hydrodynamic heterogeneities at low axial Reynolds number. Heterogeneities were revealed in the bowls with respect to the generalised Taylor number as the result of the perturbations added to the flow by blade rotation at both ends of the exchanger. Shear heterogeneities associated to flow visualisations were correlated to temperature heterogeneities observed in the bowls. Shear fluctuations were revealed in the scraped part describing two distinctive zones at low rotation speed caused by varying viscosity in the flow field. A complex spiral flow was observed by flow visualisation characterising a mass transfer evolution comprised between these two distinctive zones at low Taylor number.     

Surface oxidation by microwave-induced plasma of candidate composite materials for space shuttle protection

Oxidation resistance of composite materials (SiC/SiC, C/SiC and C/C) which can be used to protect shuttles is studied in oxygen microwave-induced plasmas (MIP). These plasmas contain the same energetic species (electrons, ions, radicals, excited atoms or molecules) as those produced by the shock wave resulting from re-entry into the atmosphere. The plasma is sustained in a silica tube located in a resonant cavity and microwave energy is supplied by a generator operating at 2450 MHz with variable power from 15 to 1000W. Experiments are conducted at pressures from 100 to 1000 Pa with temperature ranging from 1100 to 1300 °C. The atomic oxygen flow rate is about 6.10¹⁹ at sec^–1 cm^–2. The wafers are exposed to the plasma for 10 to 25 h for periods of 15 or 30min. Kinetic behaviour of the material is studied by gravimetry and surface characteristic modifications are analysed by BET Krypton isotherms at 77 K and electron spectroscopy for chemical analysis (ESCA). Gravimetric results, measurements of specific surface area by krypton adsorption and ESCA analysis show that the samples of SiC/SiC and C/SiC are quite resistant to the oxygen plasma even after 25 h exposure. The mass loss is small and the specific surface area (BET) increases but is always lower than 1 m² g^–1. ESCA analysis shows that the surface evolves by change of the superficial silicon carbide into silica. The C/C samples behave quite differently: without any protection they disappear in less than 5 min. With an antioxidant protective layer, this material can be oxidation resistant. The BET and ESCA measurements show that the attack leads to a sintering of the silica which gathers on the fibres, reducing the protection of the matrix.     

Overall mass transfer to the rotating inner electrode of a concentric cylindrical reactor with axial flow

Overall mass transfer coefficients between a liquid flowing axially in an annulus and the surface of the rotating inner cylinder have been determined electrochemically. The experiments concern mainly the laminar vortex regime and the results show the combined effect of rotation and axial flow. The value 300 of the axial Reynolds number separates two domains; for each domain the results are satisfactorily correlated using dimensionless numbers.     

Small angle X-ray scattering from block polymers. II. The bulk state

The basic theory of X-ray scattering is applied to various hypothetical models for the bulk state of block polymers in order to explain the angular dependent scattering. The intensity of scattering is calculated for a rnicellar model, a zone model, and a molecular dumbbell model. The molecular dumbbell model, in contrast to the micellar and zone models, assumes that a block copolymer containing domains may be treated as one giant molecule and that, therefore, the locations of the domains are not random but are governed by the statistics of the segments which interconnect them. Experimental and calculated scattering curves are compared for several different types of block polymers.     

A Novel Technique for Evaluating the Work of Adhesion

One of the basic problems which one encounters in bonding dissimilar materials such as glass and polymers is the development of internal stresses. These stresses arise primarily as a result of the difference in their thermal coefficients of expansion. Other stresses may also occur due to the processing history of the polymer, swelling of the polymer due to the absorption of gases in the environment or internal reaction products, and/or the loss of absorption of solvent from the adhesive. As a result, it is obvious that techniques designed to evaluate adhesion must evaluate the effects of these intrinsic stresses without modification by the external imposed stresses of the test. For example, Ahagon and Gent evaluate a work of detachment from the time average of the 180° peel force per unit width of the detaching layer.¹ However, Gent and Hamed have recently shown that this mode of peeling involves deformation of the detaching layer due to bending.² Similar problems are encountered in other techniques used to measure adhesion.     

Reduction of aqueous chromate by Fe(II)/Fe(III) carbonate green rust: kinetic and mechanistic studies

This work describes the heterogeneous reaction between FeII in carbonate green rust and aqueous chromate, in NaHCO3 solutions at 25 degrees C, and at pH values of 9.3-9.6. Evidence for reduction of CrVI to CrIII and concomitant solid-state oxidation of lattice FeII to FeIII was found from FeII titration and from structural analysis of the solids using FTIR, XRD, SEM, and XPS methods. Results indicate the formation of ferric oxyhydroxycarbonate and the concomitant precipitation of CrIII monolayers at the surface of the iron compound that induce passivation effects and progressive rate limitations. The number of CrIII monolayers formed at the completion of the reaction depends on [FeII]t=0, the molar concentration of FeII(solid) at t=0; on [n(o)]t=0, the molar concentration of reaction sites present at the surface of the solid phase at t=0; and on [CrVI]t=0, the molar concentration of CrVI at t=0. Kinetic data were modeled using a model based on the formation of successive CrIII monolayers, -(d[CrVI]/dt) = sigma(1)j k(i)[S] [CrVI]([n(i - 1)] - [n(i)]) with k(i)[S] (in s(-1) L mol(-1)), the rate coefficient of formation of CrIII monolayer i, and [n(i)] and [n(i - 1)], the molar concentration of CrIII precipitated in monolayer i and monolayer i - 1, respectively. Good matching curves were obtained with kinetic coefficients: k(1)[S] = 5-8 x 10(-4), k(2)[S] = 0.5-3 x 10(-5), and k(3)[S] about 1.7 x 10(-6) s(-1) m(-2) L. The CrVI removal efficiency progressively decreases along with the accumulation of CrIII monolayers at the surface of carbonate green rust particles. In the case of thick green rust particles resulting from the corrosion of iron in permeable reactive barriers, the quantity of FeII readily accessible for efficient CrVI removal should be rather low.     

Numerical Simulation of Taylor–Couette Flows with Rotating Outer Wall Using a Hybrid Spectral/Finite Element Method

Theoretical Foundations of Chemical Engineering - Taylor–Couette flow between independently rotating cylinders with a relatively small aspect ratio Γ = 2.4 has been investigated...     

Fair scheduling of bag-of-tasks applications using distributed Lagrangian optimization

Large scale distributed systems typically comprise hundreds to millions of entities (applications, users, companies, universities) that have only a partial view of resources (computers, communication links). How to fairly and efficiently share such resources between entities in a distributed way has thus become a critical question.