Influence of flowing fluid property through an elastic tube on various deformations along the tube length

The study of fluid flow characteristics in collapsible elastic tubes is useful to understand biofluid mechanics encountered in the human body. The research work presented here is aimed at thoroughly investigating the influence of both Newtonian and/or non-Newtonian fluids (low and high shear thinning) during steady flow through an elastic tube on various tube deformations, which enables understanding of the interaction between wall motion, fluid flow, and intestinal transmembrane mass transfer as a crucial contribution to a mechanistic understanding of bioaccessibility/bioavailability. It is observed that for a given steady volume flow rate, the tube is buckled from an elliptical shape to a line or area contacted two lobes as the critical external pressure is increased. The downstream transmural pressure is found to get more negative than that at the upstream as the outlet pressure decreased due to stronger tube collapse resulting in a reduced cross-sectional area. The experimental results depict that the tube cross-sectional area decreased by only about a factor of one for PEG (polyethylene glycol) and about a factor of six for both CMC (carboxymethyl cellulose) and PAA (polyacrylamide) from the undeformed one under an applied external pressure of 105 mbar. The corresponding maximum velocity increased by a factor of two during steady flow of shear-thinning fluids. The shear-thinning behavior of both CMC and PAA solutions is clearly observed at a constant flow rate of 17 ml/s as the tube cross-sectional area decreased due to an increase in compressive transmural pressure. In addition, the viscosity of PAA is drastically decreased due to its high shear-thinning behavior than that of the CMC under the same applied external pressure

A three-stage freezing model for liquid droplets with applications to food sprays

The development and validation of a three-stage freezing model for polymorphous materials, with applications to food sprays, is presented. In the first stage, the cooling of the droplet down to the freezing temperature is described as a convective heat transfer process in turbulent flow. In the second stage, when the droplet has reached the freezing temperature, the solidification process is initiated, which results in the release of latent heat. The amount of latent heat released is related to the amount of heat convected away from the droplet. The solidification process is monitored with a progress variable that is used to determine the specific heat capacity of the semisolid droplet. After completion of the crystallization process, in stage three, the cooling of the solidified particle is described again by a convective heat transfer process until the particle temperature is close to that of the gaseous environment. The freezing model has been validated with experimental data of a single cocoa butter droplet in an air flow. Subsequently, the model has been implemented into the computational fluid dynamics code KIVA-3 and has been validated with experimental data of a cocoa butter spray. In addition, the sensitivity of drop sizes with respect to variations in material and processing parameters has been investigated. © 2010 by Begell House, Inc

Investigation of the dispersing characteristics of antral contraction wave flow in a simplified model of the distal stomach

The dispersing characteristics of antral contraction wave (ACW) flow in the antrum are investigated by reproducing the flow generated by an ACW and determining its effect on liquid drops. The goal is to gain information about the flow field and mechanical stresses, which are responsible for the food disintegration. Toward this end, a model antrum prototype was constructed, consisting of a cylinder that was closed at one end to represent the antrum and closed pylorus. A moving hollow piston with a parabolic inner contour was used to model an ACW. A computational model was developed that reflects this prototype. Experiments and simulations were first performed for fluids with different rheological properties, two relative occlusions (0.60 and 0.75), and several ACW speeds (1.0-7.5 mm/s). The simulations were validated with velocity measurements, and the characteristics of the retropulsive jet were quantified at different Reynolds numbers (0.5-105.3). Experiments were then performed in which liquid drops of different viscosity were placed in a highly viscous fluid with low interfacial tension, similar to conditions in a stomach. It was found that the viscosity ratio (0.001-0.1) influences the retraction dynamics of a drop\u27s tail after stresses are relaxed. The flow and stress information from the simulations was used to analyze fluid transport in the antrum and to quantify drop breakup conditions. It was found that a drop broke up if both a critical capillary number of 0.51 was exceeded and the drop passed within a critical dimensionless distance of 0.3 to the wave apex

In-line rheometry of particulate suspensions by pulsed ultrasound velocimetry combined with pressure difference method

The in-line rheometer concept based on the combination of the ultrasonic velocity profiling (UVP) technique and pressure difference (PD) measurements was utilized for investigating the influence of particle concentration and size distribution on the rheology of particulate suspensions in pipe flow under realistic industrial process conditions. Well defined model suspensions were used, consisting of 11 mu m and 90 mu m diameter polyamide particles suspended in rapeseed oil at concentrations ranging from 1 to 25 % by volume. The variation of concentration and particle size distribution had the expected effects on the shear viscositiy of the investigated uni-modal and bimodal suspensions. The in-line results showed that the investigated suspensions exhibit Sisko flow behavior and demonstrated that the UVP+PD method can be used to determine the flow behavior of complex fluids and suspensions, even at high solid concentrations, under industrial conditions in-line. The obtained in-line results were in good agreement with measurement data obtained using a conventional rotational controlled-stress rheometer. Limitations of commercially available transducer technology were identified and other possible sources of inaccuracy of the UVP+PD method were investigated. Several improvements of the UVP+PD measurement method were proposed

Emulsion processing - From single-drop deformation to design of complex processes and products

The processing of complex emulsion systems is described in four main chapters which relate to the basic mechanisms of (i) single-drop deformation and breakup in laminar and turbulent flow fields; (ii) the impact of drop interactions in concentrated multi-drop systems on deformation, breakup and networking; (iii) the design of drop dispersing apparatus; and (iv) the relationship of disperse emulsion structure and product quality characteristics. For these chapter topics milestones archived during the past decades since the 1930s are highlighted. Among the most recent developments, experimental, modeling and numerical simulation tools are demonstrated to allow for complementary approaches with high potential for deriving optimized process and product design criteria. © 2005 Elsevier Ltd. All rights reserved

Droplet deformation under simple shear investigated by experiment, numerical simulation and modeling

The deformation and break-up of droplets in complex flow fields is encountered in many engineering applications such as mixing and dispersing processes. To manipulate and control such operations, rheological, interfacial and dynamical properties of the multiphase fluid as well as their interaction have to be known. In the present work, the deformation of droplets is studied experimentally in simple shear flow and compared with numerical calculations and modeling. For this purpose, a computer-controlled parallel band apparatus equipped with a digital camera records the time evolution of the sheared droplet and thus, analyzes digitally its shape. Numerical simulations are performed to calculate the drop deformation in three-dimensional space, although only two dimensions available experimentally (plane of shear) are considered for comparison. The simulations use a boundary integral method to determine drop deformation from the mass and momentum balance equations. Furthermore, a simple phenomenological model in terms of a droplet shape tensor is proposed to describe droplet deformation in homogeneous flow. © 2004 Elsevier B.V. All rights reserved

Influence of pH on colloidal properties and surface activity of polyglycerol fatty acid ester vesicles

Certain polyglycerol esters of fatty acids (PGE) form dispersions of uni- or multilamellar vesicles in dilute aqueous solution. These self-assembled aggregates reduce the surface-activity of PGE monomers such that interfacial films may take several hours to form. This is undesirable for processes, which rely on rapid surfactant adsorption, for example foaming. In the present work, we study the effect of pH on the colloidal (size distribution, morphology, surface charge) and interfacial (adsorption kinetics) properties of a commercial, non-purified PGE. Using dynamic light scattering, zeta-potential measurements and cryo-SEM, we show that changing the pH of the dispersion media can cause agglomeration and eventually osmotic rupture of PGE vesicles. The change in dispersion state also impacts the adsorption behavior at the water surface. Direct evidence that destabilized vesicle dispersion are more surface-active is provided by comparing the dynamic surface tension of solutions of different pH. The faster adsorption kinetics at low pH correlate with a remarkably increased foaming power. We suggest that an osmotic shock induced by changes in pH causes vesicles to deform and partially open, so that their hydrocarbon core is exposed to the dispersion media. This energetically unfavorable condition promotes the hydrophobically driven adsorption of surfactant monomers at surfaces and hence stimulates the foaming ability