Modelling of immiscible liquid-liquid systems by Smoothed Particle Hydrodynamics

Immiscible fluid systems are ubiquitous in industry, medicine and nature. Understanding the phase morphologies and intraphase fluid motion is often desirable in many of these situations; for example, this will aid improved design of microfluidic platforms for the production of medicinal formulations. In this paper, we detail a Smoothed Particle Hydrodynamics (SPH) approach that facilitates this understanding. The approach includes surface tension and enforces incompressibility. The approach also allows the consideration of an arbitrary number of immiscible phases of differing viscosities and densities. The nature of the phase morphologies can be arbitrary and change in time, including break-up (which is illustrated) and coalescence. The use of different fluid constitutive models, including non-Newtonian models, is also possible. The validity of the model is demonstrated by applying it to a range of model problems with known solutions, including the Young-Laplace problem, confined droplet deformation under a linear shear field, and a droplet falling under gravity through another quiescent liquid. Results are also presented to illustrate how the SPH model can be used to elucidate the behaviour of immiscible liquid systems

A new method for reconstruction of the structure of micro-packed beds of spherical particles from desktop X-ray microtomography images. Part A. Initial structure generation and porosity determination

Micro-packed beds (μPBs) are seeing increasing use in the process intensification context (e.g. micro-reactors), in separation and purification, particularly in the pharmaceutical and bio-products sectors, and in analytical chemistry. The structure of the stationary phase and of the void space it defines in such columns is of interest because it strongly influences performance. However, instrumental limitations - in particular the limited resolution of various imaging techniques relative to the particle and void space dimensions - have impeded experimental study of the structure of μPBs. We report here a new method that obviates this issue when the μPBs are composed of particles that may be approximated by monodisperse spheres. It achieves this by identifying in successive cross-sectional images of the bed, the approximate centre and diameter of the particle cross-sections, replacing them with circles, and then assembling them to form the particles by identifying correlations between the successive images. Two important novel aspects of the method proposed here are: it does not require specification of a threshold for binarizing the images, and it preserves the underlying spherical geometry of the packing. The new method is demonstrated through its application to a packing of a near-monodispersed 30.5 μm particles of high sphericity within a 200 μm square cross-section column imaged using a machine capable of 2.28 μm resolution. The porosity obtained was, within statistical uncertainty, the same as that determined via a direct method whilst use of a commonly used automatic thresholding technique yielded a result that was nearly 10% adrift, well beyond the experimental uncertainty. Extension of the method to packings of spherical particles that are less monodisperse or of different regular shapes (e.g. ellipsoids) is also discussed

Explicit numerical simulation-based study of the hydrodynamics of micro-packed beds

Knowledge of the hydrodynamic character of micro-packed beds (μPBs) is critical to understanding pumping power requirements and their performance in various applications, including those where heat and mass transfer are involved. The report here details use of smoothed particle hydrodynamics (SPH) based simulation of fluid flow on models of μPBs derived from X-ray microtomography to predict the hydrodynamic character of the beds as a function of the bed-to-particle diameter ratio over the range 5.2≤≤15.1⁄. It is shown that the permeability of the μPBs decreases in a non-linear but monotonic manner with this ratio to a plateau beyond ⁄≈10 that corresponded to the value predicted by the Ergun equation. This permeability variation was best represented by the model of Reichelt (Chem. Ing. Technik, 44, 1068, 1972) and also reasonably well-represented by that of Foumeny (Intnl. J. Heat Mass Transfer, 36, 536, 1993), both of which were developed using macroscale packed beds of varying bed-to-particle diameter ratios. Four other similarly determined correlations did not match well the permeability variation predicted by SPH. The flow field within the μPBs varied in an oscillatory manner with radial position (i.e. channelling occurred at multiple radial positions) due to a similar variation in the porosity. This suggests that use of performance models (e.g. for heat and mass transfer) derived for macroscale beds may not be suitable for μPBs. The SPH-based approach here may well form a suitable basis for predicting such behaviour, however

A new method for reconstruction of the structure of micro-packed beds of spherical particles from desktop X-ray microtomography images. Part B. Structure refinement and analysis

© 2016 Elsevier Ltd The authors have reported elsewhere (Chem. Eng. Sci., 146, 337, 2016) a new method that derives models of micro-packed beds (μPBs) of near-spherical particles from X-ray microtomography grayscale images of limited resolution compared to the characteristics dimensions of the particles and porosity. The new method is distinguished by it not requiring a grayscale threshold to partition the images into solid and void phases, and its retention of the underlying spherical geometry, two issues that are particularly problematic when more traditional approaches are used to build models of μPBs. Here it is shown that a Reverse Monte Carlo (RMC) algorithm combined with Simulated Annealing (SA) can refine the models obtained from this new method to eliminate the vast majority of particle overlaps and incorporate particle size distributions. Application of the RMC-SA to an initial model of a μPB yielded a porosity estimate that was, within experimental uncertainty, the same as its directly measured counterpart. It was further shown that the porosity of μPBs is near unity at the bed wall and oscillates in a decaying fashion normal to the wall up to a distance of around three particle diameters into the bed. This leads to the porosity decreasing with increasing bed-to-particle diameter ratio. The opposite was observed, however, for the average number of particle-particle contacts (the mean coordination number). This latter behaviour has two origins: one in which the bulk of the bed where the coordination number is maximal and constant exerts increasing influence (volumetric origin), and one in which the packing density inherently decreases with the bed-to-particle diameter ratio (packing origin)

Explicit numerical simulation of microfluidic liquid flows in micro-packed bed

Microfluidic systems are of tremendous technological interest as demonstrated by their use in chemical analysis (so called ‘lab-on-a-chip’) and biochemical analysis (e.g. to detect biomarkers for disease), and in process intensification. Packed beds of micro-sized particles possibly utilized for enhancing heat and mass transfer in microfluidic devices, where the flow regime is normally laminar, as well as provide significant increases in surface area per unit volume for analytical chemistry and biochemistry, and for separation and purification. Whilst macro-scale packed beds have long been well understood, the same is not true of their microfluidic counterparts, which we term micro-packed beds or μPBs. Of particular concern is the effect that the small bed-to-particle diameter ratio has on the nature of the bed packing and the hydrodynamics of the flow within them. This lack of understanding stems in part from the challenges that are faced in experimentally assessing μPBs and the flow through to them. The study reported in this thesis addresses these concerns through a two developments. In the first body of work, a new method is proposed for the accurate reconstruction of the structure of a μPB from X-ray micro-computed tomography data for such beds. The porosity obtained from μPB was, within statistical uncertainty, the same as that determined via a direct method whilst use of a commonly used technique yielded a result that was nearly 10% adrift, well beyond the experimental uncertainty. This work particularly addresses the significant issues that arise from the limited spatial resolution of the tomography technique in this context. In the second part of the work reported here, a meshless computational fluid dynamics technique is used to study Newtonian fluid flow through μPBs, including determination of their permeability and the by-pass fraction due to wall effects, which are important in these beds. This use of a CFD allows determination of parameters that are difficult to determine experimentally because of the challenges faced in measuring the small pressure drops involved and the absence of the limited spatial and temporal resolutions of various imaging techniques. The meshless method used here also overcomes the challenges normally faced when seeking to discretise the complex three-dimensional pore space of the packed bed. The developments here open the way to studying more complex μPB configurations, and other processes within them such as non-Newtonian flows and mass and heat transfer.Thesis (Ph.D.) (Research by Publication) -- University of Adelaide, School of Chemical Engineering, 2015

Experimental and theoretical study of a micro-fluidized bed

Published online 18 June 2013Large industrial scale fluidized beds (FBs) have been widely used because of their ability to greatly enhance mixing and both heat and mass transfer. This suggests that fluidized beds may offer a means of overcoming the poor mixing and transport characteristics of microfluidic devices where low Reynolds number flows prevail. We report experimental findings on liquid fluidization in microfluidic channels of 200-400 μm in size. Excellent fluidization is observed for various particles fluidized in ethanol where surface forces between the particles and the microfluidic channel are weak. In contrast, adhesion of the particles to the walls and subsequent de-fluidization is observed when water is used as the fluidizing medium. These findings demonstrate the importance of surface forces in micro-fluidized beds. We also find that conventional theories are able to explain the impact of surface forces on fluidization and, provided the effect of the walls on the particle packing and porosity is accounted for, the fluidization behavior. © 2013 AIP Publishing LLC.V. Zivkovic, M.N. Kashani and M.J. Bigg

Particle packing structure in a rectangular micro-capillary

Packed beds of micro-sized particles may be used for enhancing heat and mass transfer in microfluidic devices where Reynolds numbers are small. By increasing the surface area per unit volume, such micro-packed beds can also be used to significantly enhance the functionality of such devices. As the effectiveness of a packed bed is dictated by its packing structure, it is of interest to understand this structure and how it is affected by system parameters such as the channel-to-particle ratio - this has received little attention for micro-packed beds. In the work reported here, X-ray micro-computed tomography and image analysis is used to determine the packing structure of micro-packed beds of glass particles for channel-to-particle ratios of 5.19, 5.80, 6.56 and 7.55 in a channel of D = 200 mum square cross-section. The bed-average porosity was found to be far higher than macroscale PBs, vary from approximately 67% to 60% as the channel-to-particle ratio increased, with the packing structure and porosity varying both across the bed width and along its length. The former clearly arises from the effect of the wall, whilst the latter arises from the sedimentation method used to create the packed bed.M. Navvab Kashani, V. Zivkovic, Z. Alwahabi and M.J. Bigg

Importance of surface forces in a micro-fluidized bed

Fluidized beds (FBs) potentially offer a means of significantly enhancing heat and mass transfer under the low Reynolds number flows that prevail in microfluidic devices. However, as surface forces start to dominate over gravity at the microscale, adhesion of particles to the walls of the micro-FB can prevent fluidization. We used the acid-base theory of van Oss, Chaudhury and Good to understand adhesion forces at the particlewall interface in a micro-FB. This approach, novel to the fluidization field, successfully predicts the adhesion problem in a micro-FB system as verified by our experimental results for liquid fluidization in a typical PDMS microfluidics channel. This demonstrates the importance of considering surface forces in the design of micro-FB, unlike for traditional large FBs where surface forces are often less of an issue.M.J. Biggs, V. Zivkovic, M.N. Kashani and Z. Alwahab

A new method for reconstruction of the structure of micro-packed beds of spherical particles from desktop X-ray microtomography images. Part A. Initial structure generation and porosity determination

Abstract not availableMoein Navvab Kashani, Vladimir Zivkovic, Hamideh Elekaei, Mark James Bigg