Nicholas Christakis

Aggregation and caking processes of granular materials: continuum model and numerical simulation with application to sugar

Aggregation and caking of particles are common severe problems in many operations and processing of granular materials, where granulated sugar is an important example. Prevention of aggregation and caking of granular materials requires a good understanding of moisture migration and caking mechanisms. In this paper, the modeling of solid bridge formation between particles is introduced, based on moisture migration of atmospheric moisture into containers packed with granular materials through vapor evaporation and condensation. A model for the caking process is then developed, based on the growth of liquid bridges (during condensation), and their hardening and subsequent creation of solid bridges (during evaporation). The predicted caking strengths agree well with some available experimental data on granulated sugar under storage conditions.

A Computational model of coupled heat and moisture transfer with phase change in granular sugar during varying environmental conditions.

As part of a comprehensive effort to predict the development of caking in granular materials, a mathematical model is introduced to model simultaneous heat and moisture transfer with phase change in porous media when undergoing temperature oscillations/cycling. The resulting model partial differential equations were solved using finite-volume procedures in the context of the PHYSICA framework and then applied to the analysis of sugar in storage. The influence of temperature on absorption/desorption and diffusion coefficients is coupled into the transport equations. The temperature profile, the depth of penetration of the temperature oscillation into the bulk solid, and the solids moisture content distribution were first calculated, and these proved to be in good agreement with experimental data. Then, the influence of temperature oscillation on absolute humidity, moisture concentration, and moisture migration for different parameters and boundary conditions was examined. As expected, the results show that moisture near boundary regions responds faster than farther away from them with surface temperature changes. The moisture absorption and desorption in materials occurs mainly near boundary regions (where interactions with the environment are more pronounced). Small amounts of solids moisture content, driven by both temperature and vapour concentration gradients, migrate between boundary and center with oscillating temperature.

Computational Modelling of Multi-Physics and Multi-Scale Processes in Parallel

This paper provides an overview of the developing needs for simulation software technologies for the computational modelling of problems that involve combinations of interactions amongst varying physical phenomena over a variety of time and space scales. Computational modelling of such problems requires software technologies that enable the mathematical description of the interacting physical phenomena together with the solution of the resulting suites of equations in a numerically consistent and compatible manner. This functionality requires the structuring of simulation modules for specific physical phenomena so that the coupling can be effectively represented. These multi-physics and multi-scale computations are very compute intensive and the simulation software must operate effectively in parallel if it is to be used in this context. An approach to these classes of multi-disciplinary simulation in parallel is described, with some key examples of application to challenging engineering problems.

Computational model for prediction of particle degradation during dilute-phase pneumatic conveying: modeling of dilute-phase pneumatic conveying

A complete model of particle impact degradation during dilute-phase pneumatic conveying is developed, which combines a degradation model, based on the experimental determination of breakage matrices, and a physical model of solids and gas flow in the pipeline. The solids flow in a straight pipe element is represented by a model consisting of two zones: a strand-type flow zone immediately downstream of a bend, followed by a fully suspended flow region after dispersion of the strand. The breakage matrices constructed from data on 90° angle single-impact tests are shown to give a good representation of the degradation occurring in a pipe bend of 90° angle. Numerical results are presented for degradation of granulated sugar in a large scale pneumatic conveyor.

Computational model for prediction of particle degradation during dilute-phase pneumatic conveying: the use of a laboratory-scale degradation tester for the determination of degradation propensity

The overall objective of this work is to develop a computational model of particle degradation during dilute-phase pneumatic conveying. A key feature of such a model is the prediction of particle breakage due to particle-wall collisions in pipeline bends. This paper presents a method for calculating particle impact degradation propensity under a range of particle velocities and particle sizes. It is based on interpolation on impact data obtained in a new laboratory-scale degradation tester. The method is tested and validated against experimental results for degradation at 90° impact angle of a full-size distribution sample of granulated sugar. In a subsequent work, the calculation of degradation propensity is coupled with a flow model of the solids and gas phases in the pipeline.

The development of a continuum framework for friction welding processes with the aid of micro-mechanical parameterisations

Continuum modelling of complex industrial systems can be addressed with the aid of micromechanical parameterisations. The appropriate framework has been demonstrated through recent work in the area of modelling complex industrial processes which involve granular material. In this paper, the framework is set for the continuum modelling of friction welding processes with the aid of information, which is derived at the microscopic level. Sliding frictional behaviour of an unlubricated metal couple was studied experimentally and was found to be strongly influenced by operating conditions such as sliding speed and interface temperature for the titanium alloy Ti6Al4V. The different friction regimes observed experimentally are explained using an analytical contact model, which represents the moving system of two interconnected plates at the sliding interface with bonds, which continuously form and rupture during sliding. Moreover, the characterised frictional behaviour, which depends on interface temperature, is used to model numerically the non-linear thermo-mechanical process of linear friction welding of Ti6Al4V.

A hybrid numerical model for predicting segregation during core flow discharge

A continuum numerical model is presented that parameterizes the interactions between particles at the microscopic level and predicts the development of moving stagnant zone boundaries during core flow discharge of granular material. The model is then employed for the prediction of segregation of multi-component granular mixtures during discharge from core flow hoppers and its capability to accurately simulate the behavior of the granular mixture is demonstrated through comparisons with experimental data.

Analysis and modeling of heaping behavior of granular mixtures within a computational mechanics framework

Abstract This paper presents a continuum model of the flow of granular material during filling of a silo, using a viscoplastic constitutive relation based on the Drucker-Prager plasticity yield function. The performed simulations demonstrate the ability of the model to realistically represent complex features of granular flows during filling processes, such as heap formation and non-zero inclination angle of the bulk material-air interface. In addition, micro-mechanical parametrizations which account for particle size segregation are incorporated into the model. It is found that numerical predictions of segregation phenomena during filling of a binary granular mixture agree well with experimental results. Further numerical tests indicate the capability of the model to cope successfully with complex operations involving granular mixtures.

Application of simulation technologies in the analysis of granular material behaviour during transport and storage

Abstract Problems in the preservation of the quality of granular material products are complex and arise from a series of sources during transport and storage. In either designing a new plant or, more likely, analysing problems that give rise to product quality degradation in existing operations, practical measurement and simulation tools and technologies are required to support the process engineer. These technologies are required to help in both identifying the source of such problems and then designing them out. As part of a major research programme on quality in particulate manufacturing computational models have been developed for segregation in silos, degradation in pneumatic conveyors, and the development of caking during storage, which use where possible, micro-mechanical relationships to characterize the behaviour of granular materials. The objective of the work presented here is to demonstrate the use of these computational models of unit processes involved in the analysis of large-scale processes involving the handling of granular materials. This paper presents a set of simulations of a complete large-scale granular materials handling operation, involving the discharge of the materials from a silo, its transport through a dilute-phase pneumatic conveyor, and the material storage in a big bag under varying environmental temperature and humidity conditions. Conclusions are drawn on the capability of the computational models to represent key granular processes, including particle size segregation, degradation, and moisture migration caking.

An Analytical Description of the Frictional Behaviour of a Titanium Alloy

In recent years, significant effort has been put in the enhancement of our understanding of the physics and mechanics of moving objects under contact. Developed theoretical models can not fully account for the observed frictional behaviour of materials due to the lack of understanding of the interaction processes which occur at the microscopic level. In this paper, an analytical contact model will be described and its application to a titanium alloy will be presented. Conclusions will be drawn on the ability of this model to describe different friction regimes. The inclusion of additional factors which impact on frictional behaviour will be discussed, as well as the derivation of constitutive equations and their utilisation in continuum models.