Srdjan Sasic

Setting Up a Numerical Model of a DAF Tank: Turbulence, Geometry, and Bubble Size

This paper discusses the modeling framework and identifies a number of parameters relevant when setting up a computational fluid dynamics simulation of a dissolved air flotation (DAF) tank. The selection of a turbulence model, the choice between performing two-dimensional (2D) or three-dimensional (3D) simulations, the effects of the design of the flow geometry and the influence of the size of the air bubbles are addressed in the paper. The two-phase flow of air and water is solved in the Eulerian-Lagrangian frame of reference. The realizable k- model with nonequilibrium wall functions is suggested as a compromise between a need to effectively resolve the flow and the cost of the simulations. There is a discussion on the conditions for which the steady-state simulations are appropriate. We demonstrate that a steady 2D model can simulate a stratified flow pattern. Our results show that 2D models require adjustments in geometry (e.g., substitution of the outlet pipes to an outlet distributed over the total width of the tank) and in the parameters governing the flow in order to account for the true 3D nature of some of the flow patterns. In addition, we show that the bubble size has a larger influence on the flow in the separation zone than in the contact zone.

Behaviour and Stability of the Two-Fluid Model for Fine-Scale Simulations of Bubbly Flow in Nuclear Reactors

Abstract In the present work, we formulate a simplistic two-fluid model for bubbly steam-water flow existing between fuel pins in nuclear fuel assemblies. Numerical simulations are performed in periodic 2D domains of varying sizes. The appearance of a non-uniform volume fraction field in the form of meso-scales is investigated and shown to be varying with the bubble loading and the domain size, as well as with the numerical algorithm employed. These findings highlight the difficulties involved in interpreting the occurrence of instabilities in two-fluid simulations of gas-liquid flows, where physical and unphysical instabilities are prone to be confounded. The results obtained in this work therefore contribute to a rigorous foundation in on-going efforts to derive a consistent meso-scale formulation of the traditional two-fluid model for multiphase flows in nuclear reactors.

Studies of grain segregation patterns on a Destoner using a CFD-DEM approach

Removal of contaminants from ‘food grade’ quality grains is of great importance in food and grain processing operations. A thorough understanding of the inherent granular segregation profiles on this processing equipment is a pivotal step in the design and development of more efficient processes. One such grain cleaning operation is the ‘density-based separation’ using a destoner. This process removes stones and other heavy material from lighter food grains using a vibrating deck and fluidizing air. In this paper we formulate a CFD-DEM framework (set up and implemented in the OpenFOAM® environment) to study granular segregation patterns on a destoner. The scheme is first validated by comparing simulations with experimental data using a gas-solid fluidized-bed test case. A good agreement between the experiments and the simulations is noted. This proposed framework is then used to characterize the combined effects of deck inclination and fluidization velocities on the separation profiles generated from a virtual destoner. We have found these profiles to be highly sensitive to changes in fluidization conditions, with the gradual development of segregation zones at velocities close to the minimum fluidization velocity of the heavier component. A deck inclination of 5 degrees and a fluidization velocity of 2.0 m/s is considered optimal while steeper slopes (inclinations of 15 degrees) and lower air velocities (0 and 1.5 m/s) are deemed unsuitable for segregation.

DNS of dispersed multiphase flows with heat transfer and rarefaction effects

We propose a method for DNS of particle motion in non-isothermal systems. The method uses a shared set of momentum and energy balance equations for the carrier- and the dispersed phases. Measures are taken to ensure that non-deformable entities (solid particles) behave like rigid bodies. Moreover, deformable entities (e.g. bubbles) as well as rarefaction effects can be accommodated. The predictions of the method agree well with the available data for isothermal solid particles motion in the presence of walls and other particles, natural convection around a stationary particle, solid particles motion accompanied with heat transfer effects and isothermal solid particles motion under rarefied conditions. The method is used to investigate the simultaneous effects of heat transfer and rarefaction on the motion of a solid catalyst particle in an enclosure, the interaction of a solid particle and a microbubble in a flotation cell and a case with more than 1000 particles.

State Estimation of the Performance of Gravity Tables Using Multispectral Image Analysis

Gravity tables are important machinery that separate dense (healthy) grains from lighter (low yielding varieties) aiding in improving the overall quality of seed and grain processing. This paper aims at evaluating the operating states of such tables, which is a critical criterion required for the design and automation of the next generation of gravity separators. We present a method capable of detecting differences in grain densities, that as an elementary step forms the basis for a related optimization of gravity tables. The method is based on a multispectral imaging technology, capable of capturing differences in the surface chemistry of the kernels. The relevant micro-properties of the grains are estimated using a Canonical Discriminant Analysis (CDA) that segments the captured grains into individual kernels and we show that for wheat, our method correlates well with control measurements (\(R^2 = 0.93\)).

Particle-level simulations of flocculation in a fiber suspension flowing through a diffuser

We investigate flocculation in dilute suspensions of rigid, straight fibers in a decelerating flow field of a diffuser. We carry out numerical studies using a particle-level simulation technique that takes into account the fiber inertia and the non-creeping fiber-flow interactions. The fluid flow is governed by the Reynolds-averaged Navier-Stokes equations with the standard k-omega eddy-viscosity turbulence model. A one-way coupling between the fibers and the flow is considered with a stochastic model for the fiber dispersion due to turbulence. The fibers interact through short-range attractive forces that cause them to aggregate into flocs when fiber-fiber collisions occur. We show that ballistic deflection of fibers greatly increases the flocculation in the diffuser. The inlet fiber kinematics and the fiber inertia are the main parameters that affect fiber flocculation in the pre-diffuser region.

A numerical framework for bubble transport in a subcooled fluid flow

In this paper we present a framework for the simulation of dispersed bubbly two-phase flows, with the specific aim of describing vapor–liquid systems with condensation. We formulate and implement a framework that consists of a population balance equation (PBE) for the bubble size distribution and an Eulerian–Eulerian two-fluid solver. The PBE is discretized using the Direct Quadrature Method of Moments (DQMOM) in which we include the condensation of the bubbles as an internal phase space convection. We investigate the robustness of the DQMOM formulation and the numerical issues arising from the rapid shrinkage of the vapor bubbles. In contrast to a PBE method based on the multiple-size-group (MUSIG) method, the DQMOM formulation allows us to compute a distribution with dynamic bubble sizes. Such a property is advantageous to capture the wide range of bubble sizes associated with the condensation process. Furthermore, we compare the computational performance of the DQMOM-based framework with the MUSIG method. The results demonstrate that DQMOM is able to retrieve the bubble size distribution with a good numerical precision in only a small fraction of the computational time required by MUSIG. For the two-fluid solver, we examine the implementation of the mass, momentum and enthalpy conservation equations in relation to the coupling to the PBE. In particular, we propose a formulation of the pressure and liquid continuity equations, that was shown to correctly preserve mass when computing the vapor fraction with DQMOM. In addition, the conservation of enthalpy was also proven. Therefore a consistent overall framework that couples the PBE and two-fluid solvers is achieved.

Numerical investigation of fiber flocculation in the air flow of an asymmetric diffuser

A particle-level rigid fiber model is used to study flocculation in an asymmetric planar diffuser with a turbulent Newtonian fluid flow, resembling one stage in dry-forming process of pulp mats. The fibers are modeled as chains of rigid cylindrical segments. The equations of motion incorporate hydrodynamic forces and torques from the interaction with the fluid, and the fiber inertia is taken into account. The flow is governed by the Reynolds-averaged Navier Stokes equations with the standard k-omega turbulence model. A one-way coupling between the fibers and the flow is considered. A stochastic model is employed for the flow fluctuations to capture the fiber dispersion. The fibers are assumed to interact through short-range attractive forces, causing them to interlock as the fiber-fiber contacts occur during the flow. It is found that the formation of fiber flocs is driven by both the turbulenceinduced dispersion and the gradient of the averaged flow field

Numerical simulations of the interaction between a settling particle and a rising microbubble

In the current work the hydrodynamic interaction between a settling particle and a rising microbubble is investigated using numerical simulations. The simulations are performed in a multiphase direct numerical simulation (DNS) framework, indicating that all relevant spatial and temporal scales are resolved. It is shown that the method predicts that particle-bubble attachment is possible when the initial horizontal distance between their centers is small and that the particle will pass the bubble without attaching when this initial distance is large. Furthermore, it is shown that the probability of a successful attachment is lower if the bubble Eotvos and Morton numbers are significantly larger than unity.