Thermal and multiphase flow simulations of polytetrafluoroethylene-based grease flow in restricted geometry

This work presents a numerical model for predicting flow behaviour and temperature distribution of polytetrafluoroethylene thickened grease in a roller bearing configuration of obstructed flow. A finite-element code with a laminar flow multiphase mixture model is used to predict the flow patterns and volume fraction distribution of a thickener in a rectangular channel with two cylindrical rollers rotating at constant angular speeds ranging from 50 to 200 r/min. A heat transfer analysis has been done to study the temperature variation near the rollers. Simulations are carried out for 5%, 10% and 20% concentrations of the thickener. Rheological characterisation of the proposed grease samples is done. An experimentally developed rheological model is incorporated in the numerical model. The rheological characteristics of the samples follow the Herschel–Bulkley model. The velocity profiles obtained from the present numerical model are compared with the nearest available experimental microparticle image velocimetry profiles for lithium complex grease to find qualitative similarity. The variation of volume fraction distribution and temperature at different times and locations of the channel is predicted. The rotation of rollers affects the local distribution of a thickener as well as the rate of homogenisation with the oil. As polytetrafluoroethylene grease is able to maintain its rheology even at elevated temperatures, most of the channel portion is thermally unaffected due to heat dissipation up to 5 W.

Asymptotics of polygons in restricted geometries subject to a force

International audience We consider self-avoiding polygons in a restricted geometry, namely an infinite L × M tube in Z3. These polygons are subjected to a force f, parallel to the infinite axis of the tube. When f > 0 the force stretches the polygons, while when f < 0 the force is compressive. In this extended abstract we obtain and prove the asymptotic form of the free energy in the limit f → −∞. We conjecture that the f → −∞ asymptote is the same as the free energy of Hamiltonian polygons, which visit every vertex in a L × M × N box.

“Restricted Geometry” Effect on Phase Transitions in KDP, ADP, and CDP Nanocrystals

The dielectric properties of composite materials prepared by the embedding of ferroelectrics potassium dihydrogen phosphate (KDP), cesium dihydrophosphate (CDP), as well as antiferroelectric ammonium dihydrogen phosphate (ADP) into porous glass matrices with an average size of through pores of 7, 46, and 320 nm have been studied. It was found that an increase occurred in the phase transitions temperature (TC) for embedded particles in comparison with corresponding bulk materials. Some possible mechanisms of influence of “restricted geometry” on the Curie temperature are discussed. Estimates of TC shifting as a result of the “pressure effect” caused by elastic stresses in embedded particles as well as the result of bias electric field influence arising due to the piezoelectric effect are made. The possibility of using the tunneling Ising model to explain the experimental results is discussed.

FERROELECTRIC NANOCOMPOSITES WITH GOVERNED INTERFACE ON BASE OF MAGNETIC POROUS GLASSES

Two-phase (nonporous) magnetic alkali borosilicate glasses have been produced by induction melting. Their macroscopic properties and crystal structure have been studied and it is shown that in the silica skeleton there are the agglomerates of Fe3O4. These agglomerates are formed by monodomain nanoparticles of magnetite and demonstrate the superparamagnetic properties. After special thermal treatment (liquation process) and chemical etching the nanoporous matrices with random dendrite pore structure and magnetic properties have been produced. The channels (porous space) were filled by ferroelectric materials KH2PO4 (KDP), KH2PO4+(NH4)H2PO4 (KDP-ADP or KADP), and NaNO2 and the effect of applied magnetic fields on phase transitions in these nanocomposite have been studied. It has also been established that a restricted geometry changed essentially the phase diagram of KADP.

Теплоемкость и тепловое расширение нанокомпозитов триглицинсульфат-пористое стекло

The effect of restricted geometry on specific heat capacity and thermal expansion of the triglycine sulfate (TGS)–borosilicate glass composites have been studied first. A decrease in the entropy and temperature of the P 2_1 ↔ P 2_1/ m phase transition in the TGS component with decreasing the glass matrix pore diameter at the invariable specific heat and thermal expansion coefficient has been observed. The estimates are indicative of the minor effect of internal pressure on the TGS pressure coefficient dT _ C / dp in the composites.

Влияние ограниченной геометрии на структуру и фазовые переходы в наночастицах нитрата калия

The effect of restricted geometry and thermal prehistory of sample preparation on phase transitions in nanocomposites on base of porous glasses with average pore diameters 7 and 46 nm filled by potassium nitrate have been studied by X-rays and neutron diffraction. The nanoparticle sizes have been determined and phase diagrams (on cooling) for these nanoparticles have been described. It is shown that there is a critical nanoparticle size in the interval from 30 till 20 nm, at which in nanocomposite the ferroelectric phase is realized only regardless of preparation method.