Abstract: Positron Emission Particle Tracking (PEPT), a technique which enables high-speed detection of a radioactively labeled particle in three-dimensional opaque systems, has been used to track a single tracer particle attached to a textile as it is rotated and tumbled amongst other textiles in a commercially available domestic washing machine, with the aim of understanding the mechanisms by which mechanical action is imparted onto wet textiles during washing. These mechanisms were first identified by comparing the wash process to other well-researched chemical engineering processes, such as the mixing of solids in rotating drums, the impact of grinding media in comminution mills and the flow through bi-porous textile media observed in resin transfer molding processes. The results show that the textiles experience the greatest mechanical action upon impact after having been projected into the air by the motion of the rotating drum, and as textile loading increases, the free-fall velocity and deceleration upon impact of the textiles decreases, leading to less mechanical action in the impact region of the drum. Shear rates of the textile were also estimated based on an Eulerian approach, yielding highest measured shear rates in the impact region. The motion of the textile is also shown to be two-dimensional, with little movement along the depth of the drum over time and a dead zone region was identified near the axis of rotation of the drum, where textiles experience less mechanical action than textiles located nearer to the rotating drum wall.
Abstract: Although fluid gels have found a large number of applications (in every day food products and cosmetics), their lubrication behaviour is still not fully understood. In this work the lubrication behaviour of agarose fluid gels has been investigated and the mechanism describing the process is proposed. A wide range of agarose concentrations was studied (1% to 4%) and it gave fluid gel particles with different levels of elasticity E, which are shown to affect both their rheological and tribological behaviour. The critical velocity required to induce entrainment of the particles was found to also depend on the particles’ elasticity E but in addition to the applied normal load W. Fluid gel systems of the same elasticity E but of different particle sizes were also investigated and an overall reduction in friction with decreasing size was obtained. The present study is of interest to all processes involving soft contact between particles and surfaces; a clear example being the understanding of astringency during mastication.
Abstract: We describe the production of fluid gels from kappa-carrageenan using well-defined geometries, flow patterns and cooling rates. Narrow size distributions are achieved by inducing spinodal decomposition under temperature quench and controlled sheared conditions. The polymer concentration effect on the mechanism of decomposition is provided, as is a mathematical description of the relationship between the increase in viscosity, due to the particle formation, and the concentration of the κ-carrageenan. For all of the studies with κ-carrageenan a mean diameter of <5 μm was obtained and the fluid gels produced showed strong and highly recoverable particle bridging. A model is presented for the kinetics of aggregation of these particles, providing information on the mechanisms involved.
Abstract: Up-pumping pitched blade turbines (and similar impellers) have recently been shown to be particularly effective for achieving a variety of mixing duties. Here, their turbulent flow characteristics are analysed by angle-resolved particle image velocimetry (PIV) for the first time and compared with their down-pumping equivalent, the usual time-averaged parameters also being determined for each. The work was conducted in 0.15 m diameter vessel (T) with a 45° impeller of diameter D (=0.45T) in water. The angle-resolvedPIV enables a number of novel features to be identified. Firstly, the two pumping directions are shown to give very different vortex structures, even though the flow numbers, Fl, are the same (=0.79). In addition, the ‘spottiness’ of the normalized kinetic energy along a radius as the trailing vortex moved away from each impeller can be identified, which is not shown from time-averaged data. Often, the most important parameter for processing is the local normalized specific energy dissipation rate, and this is estimated using three methodologies: by measurement of the components of the stress tensor directly, ; by dimensional analysis, , with measured integral length scales (ILS); and by the Smagorinsky closure method, , to model unresolved scales (with a Smagorinsky constant used in the literature on stirred vessels). Again, only the angle-resolved results show the spottiness of and also higher values than the time-averaged. Differences in the values obtained by the three methods are discussed and compared with the existing literature. Most importantly, for the first time, the power input in the PIV-interrogated region is calculated from the three methods and compared to the input based on the impeller torque. Both DA and SGS methods are shown to overestimate the true power by a factor of 5 and 2, respectively, whilst the DE method provided a significant underestimate (1/5th) due to the limitation of the resolved length scales. The SGS method shows the greatest promise and by changing the value of the Smagorinsky constant in accordance with recent recommendations, good agreement is obtained. Nevertheless, it is concluded that there is still a need for improved methods for determining the important mixing parameter, .
