Development of a fractal-based LES model in PHOENICS

This study concerns the development and validation of a new turbulence model for CFD simulations. The fractal theory of Mandlebrot (1974) and the dissipation-in-a-box formulation of Shreenivasan (1984) are used to determine local dissipation rates for use in a Large-Eddy-Simulation (LES) framework. Such a model has theoretical advantages over “industry standard” two-equation models such as the k-e, (a) because it removes some of the ambiguities associated with the formulation of the e – turbulence energy dissipation equation and (b) it does not assume isotropy above the sub-grid dimension. The model is in fact simpler and numerically more stable that Reynolds stress closures and therefore more useful for engineering computations. The LES model of Ciofallo (1988) is attached to PHOENICS together with the fractal subgrid formulation given here, to create the FLES model

Acoustic resonance for contactless ultrasonic cavitation in alloy melts

Contactless ultrasound is a novel, easily implemented, technique for the Ultrasonic Treatment (UST) of liquid metals. Instead of using a vibrating sonotrode probe inside the melt, which leads to contamination, we consider a high AC frequency electromagnetic coil placed close to the metal free surface. The coil induces a rapidly changing Lorentz force, which in turn excites sound waves. To reach the necessary pressure amplitude for cavitation with the minimum electrical energy use, it was found necessary to achieve acoustic resonance in the liquid volume, by finely tuning the coil AC supply frequency. The appearance of cavitation was then detected experimentally with an externally placed ultrasonic microphone and confirmed by the reduction in grain size of the solidified metal. To predict the appearance of various resonant modes numerically, the exact dimensions of the melt volume, the holding crucible, surrounding structures and their sound properties are required. As cavitation progresses the speed of sound in the melt changes, which in practice means resonance becomes intermittent. Given the complexity of the situation, two competing numerical models are used to compute the soundfield. A high order time-domain method focusing on a particular forcing frequency and a Helmholtz frequency domain method scanning the full frequency range of the power supply. A good agreement is achieved between the two methods and experiments which means the optimal setup for the process can be predicted with some accuracy

Characterizing the cavitation development and acoustic spectrum in various liquids

A bespoke cavitometer that measures acoustic spectrum and is capable of operating in a range of temperatures (up to 750 degC) was used to study the cavitation behaviour in three transparent liquids and in molten aluminium. To relate these acoustic measurements to cavitation development, the dynamics of the cavitation bubble structures was observed in three Newtonian, optically transparent liquids with significantly different physical properties: water, ethanol, and glycerine. Each liquid was treated at 20 kHz with a piezoelectric ultrasonic transducer coupled to a titanium sonotrode with a tip diameter of 40 mm. Two different transducer power levels were deployed: 50% and 100%, with the maximum power corresponding to a peak-to-peak amplitude of 17 lm. The cavitation structures and the flow patterns were filmed with a digital camera. To investigate the effect of distance from the ultrasound source on the cavitation intensity, acoustic emissions were measured with the cavitometer at two points: below the sonotrode and near the edge of the experimental vessel. The behaviour of the three tested liquids was very different, implying that their physical parameters played a decisive role in the establishment of the cavitation regime. Non dimensional analysis revealed that water shares the closest cavitation behaviour with liquid aluminium and can therefore be used as its physical analogue in cavitation studies; this similarity was also confirmed when comparing the measured acoustic spectra of water and liquid aluminium

Heat and mass transfer in two-phase porous materials under intensive microwave heating

Computational results for the intensive microwave heating of porous materials are presented in this work. A multi-phase porous media model has been developed to predict the heating mechanism. Combined finite difference time-domain and finite volume methods were used to solve equations that describe the electromagnetic field and heat and mass transfer in porous media. The coupling between the two schemes is through a change in dielectric properties which were assumed to be dependent both on temperature and moisture content. The model was able to reflect the evolution of both temperature and moisture fields as well as energy penetration as the moisture in the porous medium evaporates. Moisture movement results from internal pressure gradients produced by the internal heating and phase change

The development and validation of computational MHD techniques for the modelling of metal production processes

Magnetic fields are used in a number of processes related to the extraction of metals, production of alloys and the shaping of metal components. Computational techniques have an increasingly important role to play in the simulation of such processes, since it is often difficult or very costly to conduct experiments in the high temperature conditions encountered and the complex interaction of fluid flow, heat transfer and magnetic fields means simple analytic models are often far removed from reality. In this paper an overview of the computational activity at the University of Greenwich is given in this area, covering the past ten years. The overview is given from the point of view of the modeller and within the space limitations imposed by the format it covers the numerical methods used, attempts at validation against experiments or analytic procedures; it highlights successes, but also some failures. A broad range of models is covered in the review (and accompanying lecture), used to simulate (a) A-C field applications: induction melting, magnetic confinement and levitation, casting and (b) D-C field applications such as: arc welding and aluminium electroloysis. Most of these processes involve phase change of the metal (melting or solidification), the presence of a dynamic free surface and turbulent flow. These issues affect accuracy and need to be address by the modeller

A new computational approach to microwave heating of two-phase porous materials

Computational results for the microwave heating of a porous material are presented in this paper. Combined finite difference time domain and finite volume methods were used to solve equations that describe the electromagnetic field and heat and mass transfer in porous media. The coupling between the two schemes is through a change in dielectric properties which were assumed to be dependent on both temperature and moisture content. The model was able to reflect the evolution of both temperature and moisture fields as well as energy penetration as the moisture in the porous medium evaporates. Moisture movement results from internal pressure gradients produced by the internal heating and phase change

The hydrocyclone classifier — A numerical approach

A user-oriented mathematical model of the hydrocyclone classifier is described. The model uses state-of-the-art numerical techniques to solve the discretised form of the Navier-Stokes equations relating to pulp velocities, and the transport equations for air and particle concentrations. Turbulence closure is affected by employing a turbulence model which takes into direct account the effects of swirl and the presence of particles. An algebraic slip approach is used to model the relative movement of particles in the cyclone and also the formation of the air-core. The calculation produces point-by-point values of the three pulp velocity components, the pressure, the mass fraction of particles and the mass fraction of air which eventually forms the air core. The model is general in its formulation and not subject to geometrical or operational constraints. It is here validated by comparison to the well known Kelsall (1952) experiment. The predicted velocity profiles agree well with experiment, and flow rate (pressure drop), flow split, cut point and efficiency curve compare well with a number of empirical formulae. The particle profiles show not only a build-up at the cyclone conical surface but also the existence of an equilibrium radius for each particle size

Laminar and turbulent natural convection in an enclosed cavity

The paper presents a computational method used to obtain solutions of the buoyancy-driven laminar and turbulent flow and heat transfer in a square cavity with differentially heated side walls. A series of Rayleigh numbers, ranging from 103 to 1016 was studied. Donor-cell differencing is used, and mesh-refinement studies have been performed for all Rayleigh numbers considered. The turbulence model used for Rayleigh numbers greater than 106 is a (k ~ ε) two-equation model of turbulence, that includes gravity ~ density gradient interactions. The results are presented in tabular and graphical form, and as correlations of the Nusselt and Rayleigh numbers. Furthermore, the results for Rayleigh numbers up to 106 are compared with the benchmark numerical solution of de Vahl Davis

Une recherche des incendies tridimensionnels dans des compartiments / An investigation of three-dimensional fires in enclosures

Développement d'un modèle mathématique pour l'étude des structures d'écoulement et de flamme des incendies transitoires ou à l'état stationnaire dans des compartiments tridimensionnels