Characterisation of inelastic power-law fluids using falling sphere data

Two simple methods are presented for the characterization of inelastic power law fluids from falling sphere data. The methods involve the application of shear rate or shear stress correction factors which have been derived theoretically using Slattery's solution for creeping flow about spheres. Flow curves obtained using these methods are in excellent agreement with those measured on a Weissenberg rheogoniometer for 0.83 ≤ n ≤ 1.0. The experimentally determined drag coefficients are found to be in good agreement with the predictions of Slattery's creeping flow first approximation solution. The wall correction factors of Faxen and Francis appear to be valid for inelastic non-Newtonian fluids up to a diameter ratio of at least 0.08.     

Direct numerical simulations of a freely falling sphere using fictitious domain method: Breaking of axisymmetric wake

In the present paper, numerical simulations of the wake generated by a freely falling sphere, under the action of gravity, are performed. Simulations have been carried out in the range of Reynolds numbers from 1 to 210 for understanding the formation, growth and breakup of the axisymmetric wake. The in-house code used is based on a non-Lagrange multiplier fictitious-domain method, which has been developed and validated by Veeramani et al. (2007). The onset of instability in the wake and its growth along with the dynamic behavior of a settling sphere is examined at Reynolds number (Re) of 210. It is found that at the onset of instability the sphere starts to rotate and gives rise to a lift force due to the break of the axisymmetry in the wake which in turns triggers a lateral migration of the sphere. The lift coefficient of a freely falling sphere is 1.8 times that of a fixed sphere at a given sphere density of 4000 kg m^?3 and sphere to fluid density ratio of 4. This is attributed to the Robin's force which arises due to the rotation of the sphere. At this Reynolds number (Re=210) a double threaded wake is observed, which resembles the experimental observations of Magarvey and MacLatchy (1965).     

Free convection in power-law fluids from a heated sphere

In this work, the governing field equations describing heat transfer from a heated sphere immersed in quiescent power-law fluids have been solved numerically. In particular, consideration has been given to elucidate the role of Grashof number (Gr), Prandtl number (Pr) and power-law index (n), on the value of the Nusselt number (Nu) for a sphere in the natural convection regime. Further insights are provided by presenting streamline and constant temperature contours. The results presented herein encompass the following ranges of conditions: 10≤Gr≤10⁷; 0.72≤Pr≤100 and 0.4≤n≤1.8 thereby covering both shear-thinning and shear-thickening types of fluid behaviours. Broadly, all else being equal, shear-thinning behaviour can enhance the rate of heat transfer by up to three-fold where as shear-thickening can impede it up to ～30?40% with reference to that in Newtonian fluids. The paper is concluded by presenting detailed comparisons with the scant experimental data and the other approximate treatments of this problem available in the literature.     

Laminar natural heat transfer from an isothermal sphere to non-newtonian fluids

An experimental study of natural convection heat transfer conducted with water and thirteen aqueous polymer solutions obeying the power-law model, varying in flow behavior index from 0.3 to 0.99, and consistency index between 10 and 5 × 10³, showed that the behavior of these solutions justifies the theoretical treatment of this phenomenon proposed by Acrivos⁽¹⁰⁾. The heat source was an internally electrically heated copper sphere, 5 inches in diameter, and designed to provide an isothermal surface.     

The velocity field characteristics of high-viscosity fluids falling film flow down clamped channels

The complexity of falling film flow has been studied in many industrial applications. In this work, the velocity field of high-viscosity fluids falling film flow down clamped channels was investigated numerically and experimentally. The results show that the numerical simulation results are consistent with the experimental results, and the characteristics of the velocity field are related to the fluid properties, operating conditions, and structure of the clamped channels. When the fluid viscosity is greater than or equal to 10 Pa ⋅ s, the type of velocity field changes into I shape, U shape, and V shape. While the fluid viscosity drops to 0.89 Pa ⋅ s, the viscous force cannot resist the inertial force and gravity, resulting in a cardioid velocity field. By adjusting the structure of the clamped channels and operating conditions, the tension of the liquid film can be changed, and the velocity distribution of the liquid film can be manipulated. Significantly, under the fluctuating curtain flow, the liquid film coalesces and breaks frequently, which enlarges the surface area of the liquid film and strengthens the surface renewal frequency. Hence, this form of falling film flow can be applied to process intensification of high-viscosity materials.     

Imaging nanostructured fluids using cryo-TEM

Cryogenic transmission electron microscopy (cryo-TEM) has gained increasing popularity among researchers in the field of amphiphilic self-assembly as an experimental method of choice because of its unique ability to visualize nanostructures in complex fluids. Due to many recent technical improvements in the instrumentation, cryo-TEM experiments are now common. However, some limitations and possible artifacts are still a problem, and awareness of them remains a prerequisite for reliable operations and interpretation. The goal of this paper is to provide a review of the basic methods and procedures by which the cryo-TEM experiments are typically practiced. As examples, this paper also presents recent results derived from our cryo-TEM studies of micellization of block copolymer blends which clearly illustrate the unique usefulness of the technique for exploring the unexpected, complex, and/or heterogeneous nanostructures in amphiphilic solutions. This paper is dedicated to Professor Hyun-Ku Rhee on the occasion of his retirement from Seoul National University.     

Cold Simulation of momentum transfer of a metal droplet falling through slag systems possessing net-work structured fluids

Most of the high temperature smelting/refining processes involve falling of metal droplets through the slag systems. The slag systems comprising oxides viz., Al₂O₃, SiO₂, CaO, FeO, MgO, MnO etc., are essentially produced from the removal of ash and gangue from coke and/or ore during the metal extraction. At high temperatures these oxides form a network structured fluids. Momentum transfer phenomena of a falling sphere through these slag systems significantly influence the kinetics of the refining reactions occurring in the high temperature metallurgical reactors. It governs the metal quality including their productivity. The paper discusses cold simulation of the momentum transfer of a metal droplet falling through the slag systems possessing network structure.     

Characterisation of clay sintering process using impedance spectroscopy

Impedance measurements were made in situ while a clay compact was being fired at different temperatures. The measured impedance spectra consist of a high frequency (HF) semicircle arc and a low frequency (LF) tail. By employing an equivalent circuit of the clay compact to simulate the impedance spectra, we have obtained values for electrical properties and the parameters of constant phase elements (CPEs) corresponding to both bulk specimen and electrode effects. Both Arrhenius plot of specimen conductance and the dielectric loss curve demonstrate a phase transition occurring in the clay at a temperature between 900 and 950 °C. Such a phase transition was believed to be the formation of a liquid phase, which was also confirmed by using XRD technique and dilatometer analysis. The variation of specimen conductance as a function of time, which was obtained from isothermal impedance measurements, was found to be in agreement with the shrinkage curve. An equation relating the electrical conductance to the density of the specimen has been established, which has been verified by examining the densification of clay during sintering. Therefore, it is promising to use impedance measurements for examining the sintering of a wide range of ceramics in situ.     

Characterisation of coatings performance using electrochemical noise analysis

Electrochemical noise measurements (the study of the spontaneous fluctuations in electrode voltage and current which occur with respect to time) have been used to study the degradation of coated metal substrates. The technique has been found capable of distinguishing between paints in terms of both the extent and the mechanisms of corrosion occurring below the paint film and should prove to be an effective means of generating comparative performance data rapidly. Nominally identical coupled pairs of electrodes immersed in 0.6 M NaCl were studied at intervals over 2300 h. Two computer-controlled digital voltmeters simultaneously monitored each coupled electrode pair potential and also the current flow between the electrode pair, via a low noise zero resistance ammeter. Simple statistics were used to analyse the data. Standard deviations of voltage noise (σV) and current noise (σi) values were found to be indicative of the quality of corrosion protection. Good coatings gave relatively large σV values, whereas poor coatings gave relatively large σi values. An estimate of the extent of delamination can be obtained from the total charge passed.     

Characterisation of three different impellers using the LDV-technique

A characterisation of three commonly used impellers was made in this study by measuring local mean velocities and the fluctuations of these velocities with the LDV technique. The data was used to estimate volumetric flow, velocity fluctuations and turbulent intensity in the impeller region of the tank. The impellers investigated were a high flow impeller, a pitched blade turbine and a Rushton turbine. The cylindrical vessel used was made of Perspex, had a dished bottom (DIN 28013), was equipped with four baffles and had an inner diameter of 0.45 m. It was found that the bulk velocities could be scaled with the tip-speed of the impeller (ND). The flow rate at constant impeller speed increased in the order high flow impeller — Rushton turbine — pitched blade turbine. The corresponding order for the turbulence fluctuation is: high flow impeller — pitched blade turbine — Rushton turbine. The velocity profile of the flow out from the high flow impeller was furthermore, not as smooth as could be expected.     

Characterization of copolymers fractionated using supercritical fluids

In this study supercritical fluids were used to process several different types of oligomers. Supercritical fluid processing was found to be quite effective at removing low vapor pressure monomers from polymers and for fractionation of polymers according to molecular weight. This resulted in high purity materials with low M?_w/M?_n ratios. The polymer fractions obtained were analyzed by size exclusion chromatography, differential scanning calorimetry, and melt rheometry. Their physical and chemical properties are described.     

Deformation and fracture characterization of inelastic composite materials using potentials

An approach using strain energy-like potentials to characterize deformation and fracture of inelastic, nonlinear composite materials is described. The inelasticity may be due to various causes, including microcracking, microslipping, and rate processes responsible for fading memory (viscoelasticity). The concept of work potentials is introduced first, and then arguments are given for their existence for inelastic materials. Emphasis in the paper is on elastic composite materials with changing or constant states of distributed damage. Experimental results on polymeric composites are subsequently presented to illustrate this approach to deformation and fracture characterization. Finally, extension to viscoelastic behavior is discussed.     

A numerical study of the accelerating motion of a dense rigid sphere in non-newtonian power law fluids

The equations of motion of an accelerating sphere falling through non-Newtonian fluids with power law index n in the range 0.2 ≤ n ≤ 1.8 were integrated numerically using the assumption that the drag on the sphere was a function of both power law index and terminal Reynolds number, Re_t For 10^?2 ≤ Re_t ≤ 10³ both dimensionless time and distance travelled by the sphere under transient conditions showed a much stronger dependence on the flow behaviour index, n, for shear-thinning than for shear-thickening fluids. The form of this dependence is investigated here. Furthermore, results in four typical shear-thinning fluids suggested a strong correlation between the distance and time travelled by the sphere under transient conditions and the value of the fluid consistency index. The analysis reported herein is, however, restricted to dense spheres falling in less dense fluids, when additional effects arising from the Basset forces can be neelected.     

A note on wall effect on the terminal falling velocity of a sphere in quiescent Newtonian media in cylindrical tubes

The settling velocity of a sphere in a quiescent fluid is significantly reduced by the presence of confining boundaries. The severity of this phenomenon (wall effect) is strongly dependent on the geometrical and kinematic parameters. In this work, the simplest case of a sphere settling on the axis of a long cylindrical tube is considered. The functional dependence of the wall factor on the sphere-to-tube diameter ratio (λ) and the sphere Reynolds number has been examined. The wall factor is a function of λ only both at very low and at very high values of the sphere Reynolds number (Re). On the other hand, it depends on both λ and Re in the intermediate zone. The values of the Reynolds number marking the transitions from the viscous to intermediate and from the transitional to fully inertial flow regimes have been identified as functions of λ. Using these criteria to delineate experimental data, the relative performance of some of the available expressions for the estimation of wall factor has been evaluated against a large body of experimental results.     

Characterisation of thermoplastic matrix composites using variable frequency microwave

Abstract

In most industrial microwave processing operations, the frequency of the microwave energy launched into the waveguide or cavity containing the sample is fixed. This brings with it inherent heating uniformity problems. This paper describes a new technique for microwave processing, known as variable frequency microwave (VFM) processing, which alleviates the problems brought about by fixed frequency microwave processing. In VFM processing, microwave energy over a range of frequencies is transmitted into the cavity in a short time, e.g. 20 μs. It is therefore necessary to determine the best frequency range for processing a material. The best range frequency for microwave processing of five different thermoplastic matrix composites using the VFM facilities has been determined. The optimum frequency band for microwave processing of these five materials was in the range 8–12 GHz. This data enables bonding of the materials using microwave energy under the most favourable conditions.