Context
Many industrial sectors, such as pharmaceuticals or civil and military
aeronautics, require the use of raw materials made from mixtures of constituents
that are as homogeneous as possible to ensure the efficiency of the raw
material. The most common industrial mixers are intrusive because they use
rotating blades or a grid and require relatively long process times. To reduce
mixing times, a new, non-intrusive technology using acoustic resonance (RAM) has
been developed, based on vertical oscillation of the container containing the
components. This induces both macroscopic convective movements and vortex micro-
movements. The latter are generated throughout the volume to be mixed, unlike
intrusive technologies where they are only present in areas close to the
rotating blades, which improves mixing and enables a homogenous mixture to be
obtained very quickly. As part of an ANR ASTRID project (SINRAM), a liquid
(binder) and particles (fillers) are mixed in a tank partially filled with air
at low pressure. The tank is set in oscillation, which generates complex
physical phenomena: instabilities at the interface between the air and the
binder, vortex micro-movements, collisions of solid particles with each other,
variable rheology, viscous dissipation, and so on.
Objectives
The aim of this 2-year post-doctorate is to propose a 3D numerical simulation
code for the RAM mixer, which will be used to understand the physical phenomena,
to design highly homogeneous mixtures and to optimise the sizing of the mixer.
More specifically, it will take into account: i) the effects of air
compressibility, ii) instabilities at the air/binder interface (without
particles and then in monodisperse solution), iii) the anisothermal conditions
of the flow, and iv) the degassing kinetics taking into account the transport of
air trapped in the binder and its transfer to the interface (but without
interaction with the particles). All of this work will be carried out using the
free and open source Notus code (https://notus-cfd.org). More specifically, the
proposed work will focus on 2 aspects:
1) Compressible/incompressible two-phase solver
The simulation of the process must be able to take into account the non-
negligible effects of compressibility in air and the incompressibility of the
binder. Initially, we propose to continue the work developed in the laboratory
[1] on a class of pressure correction type methods [2,3], based on the pressure
evolution equation [4] and leading to the solution of a Poisson equation similar
to that which would be solved for an incompressible flow, but including terms
and specific coefficients relative to the compressibility of the flow. The
proposed formulation will be compatible with the Volume-of-Fluid two-phase
approach, and the interface between the binder and the air will be reconstructed
linearly in the mesh using Weymouth's VOF-PLIC method [5]. Surface tensions will
be taken into account by the Continuum Surface Force model and curvature by the
height function method [6]. A non-Newtonian rheology of the binder could be
considered, as well as heat transfer in the flow. In a second phase, the Philips
model [7] and Krieger's law [8], which have already been implemented, will also
be used to take account of the particles in the binder. Together, these methods
will make it possible to simulate the tank containing the binder in vibration
for different air pressures, and thus to reproduce the instabilities of the
interface and the temperature increase of the mixture.
2) Outgassing and transfer at interfaces
In order to take account of the outgassing phenomenon in numerical simulations,
it is necessary to develop suitable physical models that deals with the mechano-
chemical coupling of air transport in the binder and its transfer to the
binder/air interface, while respecting the concentration jump at the interface
in accordance with thermodynamic equilibrium. At this stage, we will consider
air as a dissolved entity in order to solve the transport and transfer at the
interfaces using a single macroscopic advection/diffusion equation [9,10]. The
1-fluid model described in the previous paragraph will therefore be extended to
take account of an air concentration. The equivalent fluid will be reconstructed
as a function of the volume fraction of the fluid in the mesh and the air
concentration. We propose to use a methodology developed in the laboratory and
based on a species diffusion potential that has been used to simulate
dissolution/precipitation mechanisms [11]. This diffusion potential, which is
continuous across the interface, would include in this project, in addition to
the classical concentration-dependent term, terms relating to gravity and
pressure.
Candidate profile
The post-doctoral student we are looking for should have strong skills in
physical and numerical modelling of two-phase flows. He/she should have
experience both in the use of parallel CFD codes (MPI) and in the implementation
of numerical methods (FORTRAN2008).
Application
Please send a CV, a covering letter detailing your interest in the subject, PhD.
thesis reports (manuscript and defence), and contact details of referees to the
contacts below.
Contacts
Dr. Stéphane Glockner - I2M glockner@bordeaux-inp.fr
Dr. Sylvie Bordère – I2M sylvie.bordere@u-bordeaux.fr
Bibliography
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