Dalton J. E. Harvie

A new volume of fluid advection algorithm: the defined donating region scheme

This paper presents a new volume of fluid (VOF) advection algorithm, termed the defined donating region (DDR) scheme. The algorithm uses a linear piecewise method of free surface reconstruction, coupled to a fully multi-dimensional method of cell boundary flux integration. The performance of the new scheme has been compared with the performance of a number of alternative schemes using translation, rotation and shear advection tests. The DDR scheme is shown to be generally more accurate than linear constant and flux limited schemes, and comparable with an alternative linear piecewise scheme. The DDR scheme conserves fluid volume rigorously without local redistribution algorithms, and generates no fluid ‘flotsam’ or other debris, making it ideal in applications where stability of the free surface interface is paramount. Copyright

A hydrodynamic and thermodynamic simulation of droplet impacts on hot surfaces, Part I: theoretical model

Abstract A model is presented to simulate the behaviour of an axisymmetric volatile liquid droplet impacting on a hot solid surface in the film boiling region. A volume of fluid (VOF) algorithm is used to model the gross deformation of the droplet. This algorithm is coupled to a separate one-dimensional algorithm used to model fluid flow within the viscous vapour layer existing between the droplet and solid surface. Heat transfer within the solid, liquid and vapour phases is solved, and a kinetic theory treatment is used to calculate conditions existing at the non-equilibrium interfaces of the vapour layer.

A hydrodynamic and thermodynamic simulation of droplet impacts on hot surfaces, Part II: validation and applications

Abstract A model was previously presented to simulate the behaviour of an axisymmetric droplet impacting on a hot solid surface in the film boiling region (D.J.E. Harvie, D.F. Fletcher, International Journal of Heat and Mass Transfer 44 (2001) 2633–2642). In this paper comparisons against experimental water and n-heptane droplet impacts are made which validate the hydrodynamic and thermodynamic predictive capabilities of the model. Specifically, it is shown that the hydrodynamic behaviour of impacting droplets is predicted accurately below a Weber number of approximately 30, while above this level, at least the initial hydrodynamical aspects of an impact can be predicted. The model is found to reproduce the thermodynamic behaviour of actual droplet impacts when no contact between the solid and liquid phases occurs.

A computational fluid dynamics study of a tall-form spray dryer

Two phase simulations have been performed of a tall-form Delaval skim milk spray dryer in order to evaluate the applicability of current computational fluid dynamics models to this type of simulation, and to examine the characteristics of the flows that exist within these complex devices. The simulations have been performed with and without milk particles included, for a variety of droplet nozzle injection velocities, and for two dryer inlet throat geometries. Limited validation of the results has been achieved by comparing the simulated product moisture contents against data measured in experiments1. It was found that the simulated product moisture contents were of the same magnitude, but generally slightly lower than those found during experiments. It was also found that the behaviour of the dryer is largely determined by the relationship between the initial momentum of the injected particles and the gas fiowfield within the dryer.

A computational fluid dynamic model of fire burning rate and extinction by water sprinkler

A computational fluid dynamic (CFD) study is combined with an experimental program to develop a model of burning rate and extinction in fires.The ultimate objective is to predict extinguishment by water droplets and thereby determine the water requirement for the extinction of actual fires. The experimental,program is conducted in a full-size fire gallery with a commercial sprinkler system installed in the roof. Water droplet size distribution, velocities and mass flux from the sprinkler are measured as inputs for the computations. Horizontal sheets of polymethylmethacrylate I metre square are uniformly ignited and burned in a draft of about I m/s. Burning rates are monitored by load cell and when the fire is well established, the sprinkler is actuated. A video record of the extinguishment together with thermocouple and heat flux measurements are used to time the event and to describe the process. The primary extinguishment mechanism is considered to be due to the cooling of the burning PMMA surface below...

A computational fluid dynamics study of wood fire extinguishment by water sprinkler

Abstract A Computational Fluid Dynamics (CFD) model is developed to predict extinguishment times of an array of wood slats by water sprinkler. The model predicts flow field, combustion of wood volatiles and radiation transfer. The gas-phase model is coupled with the wood pyrolysis model to predict a volatile release rate. A sprinkler water spray is modelled using a Lagrangian particle tracking procedure, coupled with the gas flow model by a Particle-Source-In-Cell algorithm. A simple model of instant droplet evaporation at the burning surface is employed. The experimental program includes full-scale experiments in a fire gallery with a commercial sprinkler system installed in the roof. In some tests a water restrictor is used to vary the water flow rates. Water droplet size and velocity distributions are measured to serve as inputs to the spray model. A vertical array of wood slats is ignited uniformly in a slight draft of about 0·7 m/s. A few minutes after self-sustained burning is developed, the sprinkler is activated. Thermocouple and heat flux measurements in the vicinity of the flame, as well as a video record, are used to determine flame shape and to provide data for validation of the CFD model. Burning rates are measured by load cell and by CO 2 measurements. Extinguishment happens primarily due to fuel cooling, which is indicated by long extinguishment times (two orders of magnitude longer than for plastic materials). The predictions of burning rate and flame shape are reasonably accurate. Extinguishment times are modelled for different water flow rates. The dependence on water flow rate is found to be weak because the extinguishment process is controlled by the thermal time constant of the whole wood sample.

Numerical Simulations of Gas Flow Patterns Within a Tall-Form Spray Dryer

Numerical simulations of the air flow patterns within a small scale tall-form countercurrent spray dryer have been performed. The simulations were performed using CFX 4.3, a finite volume based, computational fluid dynamics package. This study represents the first application of the Very Large Eddy Simulation (VLES) approach to the simulation of spray dryers. They have been performed in order to gain a more detailed understanding of the flow patterns and their stability in this design of dryer, which is commonly used in countercurrent drying applications, such as the drying of detergents. Limited validation of the simulations was achieved through comparison against qualitative experimental flow pattern information. It was found that by altering the angle of the inlet air streams into the dryer, the nature of the flow within the dryer could be significantly altered. In the majority of the cases simulated, large transients developed in the flow, the nature of these transients being critically dependent on the inlet conditions. The existence of such transients would be detrimental to actual spray dryer performance, however the flow patterns can be stabilised by introducing a large amount of swirl into the chamber.

Microfluidic circuit analysis I: Ion current relationships for thin slits and pipes

Existing microfluidic circuit theories consider conservation of volume and conservation of total charge at each channel intersection (node) that exists within a circuit. However, in a strict sense conservation of number (or charge) for each ion species that is present should also be applied. To be able to perform such a conservation the currents due to the movement of each ion species (electrokinetic ion currents) that occur within each channel need to be known. Hence, we here present analytical and numerical methods for calculating these ion currents (and fluid flowrates) in Newtonian binary electrolyte solutions flowing within two-dimensional thin slits and pipes. Analytical results are derived in the limits of low potential, high potential, and thin double layers. We show that irrespective of double layer overlap, the Boltzmann distribution is valid provided that a local geometric mean is used for the reference ion concentration. While the real significance of the work lies in its application to multi-channel microfluidic circuit theory (see the accompanying paper of Biscombe et al. [1]), the present results show that even in single channels, ion current behaviour can be surprisingly complex.

A multiphase electrokinetic flow model for electrolytes with liquid/liquid interfaces

A numerical model for electrokinetic flow of multiphase systems with deformable interfaces is presented, based on a combined level set-volume of fluid technique. A new feature is a multiphase formulation of the Nernst-Planck transport equation for advection, diffusion and conduction of individual charge carrier species that ensures their conservation in each fluid phase. The numerical model is validated against the analytical results of Zholkovskij et al. (2002) [1], and results for the problem of two drops coalescing in the presence of mobile charge carriers are presented. The time taken for two drops containing ions to coalesce decreases with increasing ion concentration.

Stationary chemical gradients for concentration gradient-based separation and focusing in nanofluidic channels.

Previous work has demonstrated the simultaneous concentration and separation of proteins via a stable ion concentration gradient established within a nanochannel (Inglis Angew. Chem., Int. Ed. 2001, 50, 7546-7550). To gain a better understanding of how this novel technique works, we here examine experimentally and numerically how the underlying electric potential controlled ion concentration gradients can be formed and controlled. Four nanochannel geometries are considered. Measured fluorescence profiles, a direct indicator of ion concentrations within the Tris-fluorescein buffer solution, closely match depth-averaged fluorescence profiles calculated from the simulations. The simulations include multiple reacting species within the fluid bulk and surface wall charge regulation whereby the deprotonation of silica-bound silanol groups is governed by the local pH. The three-dimensional system is simulated in two dimensions by averaging the governing equations across the (varying) nanochannel width, allowing accurate numerical results to be generated for the computationally challenging high aspect ratio nanochannel geometries. An electrokinetic circuit analysis is incorporated to directly relate the potential drop across the (simulated) nanochannel to that applied across the experimental chip device (which includes serially connected microchannels). The merit of the thick double layer, potential-controlled concentration gradient as a particle focusing and separation tool is discussed, linking this work to the previously presented protein trapping experiments. We explain why stable traps are formed when the flow is in the opposite direction to the concentration gradient, allowing particle separation near the low concentration end of the nanochannel. We predict that tapered, rather than straight nanochannels are better at separating particles of different electrophoretic mobilities.