RICERCA ITALIANA     

Research program

Multiscale modelling and development of process reactors for polymeric nanoparticle precipitation

University Co-ordinator

Università degli Studi di UDINE - ENERGETICA E MACCHINE - ()

Research Unit Leader

Alfredo Soldati

Description

The object of this project is to simulate the behavior of a dispersed light phase constituted by a swarm of nanoparticles, into a turbulent flow field in order to determine the interaction between flow field turbulent structures and particle dispersion. We plan to use Direct Numerical Simulation (DNS). We will simulate two different flow fields: The first will be homogeneous isotropic turbulence. This will be done as a benchmark test to assess our data against the literature (Fox and Yeung, 2003); The second will be the flow field generated by two impinging jets. This flow field is examined experimentally by the Unit of Politecnico di Torino. These flow fields will be used to perform Lagrangian simulations to characterize the dispersion field of nanoparticles.

The dynamics of particle dispersion will be evaluated under the hypothesis of one-way coupling -- particles do not affect the flow field. However, we are planning to consider the energy exchange due to interparticle collision. This may be due to the two following causes: i) Interception due to streamline bending: when two particle follow two different streaklines which are getting closer, they may interact due to their finite size; ii) Interception due to Brownian agitation: in this case two particles which are close enough may just interact due to thermal agitation.

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Phase I. DNS simulation of Homogeneous Isotropic Turbulence (HIT) with Lagrangian nanoparticle tracking.

In this phase, we will modify our pseudo spectral DNS code (Soldati and Banerjee, 1998) and we will compute a database necessary to obtain statistics. The database will then be used to track swarms of nanoparticles using a Lagrangian approach. DNS is a model free parameter and allows to resolve the fluid dynamics up to the smaller scales. This detailed description of the flow field is necessary to simulate accurately nanoparticle dispersion by direct integration of the equation of motion (Maxey and Riley, 1983). We will add Brownian motion to particles (Sbrizzai et al, 2005). We will test the influence of the several forces acting on nanoparticles (inertia, gravity and fluid drag, the Basset force, the lift and Magnus Force, the added mass force and the pressure gradient force, and the Brownian correction).
The relative dispersion of fluid particle pairs in isotropic turbulence will be studied. We will compare Lagrangian statistics with available Eulerian statistics. Intermittency will be examined.

The object of this phase is:
i) Obtain accurate dispersion fields using Lagrangian particle tracking in the DNS field. The database will be used to assess the influence of Brownian motion on particle dynamics and to further benchmark future simulations with interparticle collisions. (11 man-months).
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Phase II. Theoretical Framework for InterParticle collisions.

As examined previously, there is scarce literature on interparticle collision algorithms. However, since the phenomenon we want to investigate may be strongly influenced by particle-particle interactions, we will set up a collision model according to previous experiences (Chen et al, 1998 a, and Chen et al, 1998 b). In this phase, we plan to base on previous literature (Sommerfeld, 2001, Sommerfeld, 2003, Lopez and Puglisi, 2004) to derive a theoretical framework for the collision model. The model will be coded and applied to preliminary benchmark tests.

Synchronization and objects of Phase II:
This phase is preliminary to the final full Lagrangian simulation and will be performed mostly in parallel to Phase I and to the beginning of Phase III.
(9 man-months).

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Phase III. DNS simulation of the Homogeneous Isotropic Turbulent flow with Lagrangian nanoparticle tracking including collisions.

In this phase, we will set up and use the model for the Lagrangian simulations of particle tracking including collisions in the DNS calculated Homogeneous Isotropic Turbulence. Lagrangian models are based on the calculation of the trajectory of a large number of particles in a previously simulated flow field. These calculations can give the overall as well as the local evolution of the particle swarm helping to clarify the mechanisms which control particle transport and mixing.

Synchronization and objects of Phase III:
This phase must follow the collision model development performed by this Unit. Object will be to obtain accurate dispersion fields using Lagrangian particle tracking with collisions in the DNS field. The database will be used to assess the influence of collisions on particle dynamics. Micromixing results obtained for HIT will be used to develope sub-grid scale models used by the Unit of Bologna to simulate particle behavior in production reactors.
(15 man-months)


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Phase IV. DNS simulation of the turbulent flow generated by the impinging jets.

In this phase we will simulate numerically the flow field existing in the small reactor cell developed at the Politecnico di Torino. The cell is based on a vertical cylindrical container (scale of centimeters) covered with a small dome and equipped with two facing flushed holes through which the solvent with polymer and active principle are fed. The two jets impinge in correspondence of the cylinder axis and generate the turbulence required to promote mixing of the species. This flow is likely not developed and non-steady. Direct Numerical Simulation will help us to clarify some of the mechanisms producing turbulence. We will not be able to use the PseudoSpectral code to solve for this flow field since the boundary conditions are too complicated. We will use the Finite difference code, written in cylindrical coordinates, developed by Verzicco and Orlandi (1996) with which we already run several simulations in complex geometries (Cerbelli et al., 2001, Marchioli et al., 2003, Sbrizzai et al., 2004).

Synchronization and objects of Phase IV:
This phase must follow the experiments run by the Unit of Politecnico di Torino but it can start immediately and in parallel with the other phases run by this Unit. Experimental conditions will be obtained and Numerical Experiments will be performed. This phase will require large computational capabilities and will be run only once the available computational resources have been secured for the project.
(13 man-months)

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Phase V. DNS simulation of the turbulent flow generated by the impinging jets with Lagrangian nanoparticle tracking (including collisions).

We will use the database developed during phase IV and we will run Lagrangian simulation of nanoparticle tracking. We will run initially a benchmark test for non-interacting particles and we will examine the influence of collisions in a further stage. This procedure will allow us to examine independently the influence of collisions on the mixing process.

Synchronization and objects of Phase V:
This phase will come at last and must follow the collision model development performed by this Unit and the generation of the database relative to the experimental unit produced by the Unit of Politecnico di Torino. Object will be to obtain accurate dispersion fields in the real rector cell using Lagrangian particle tracking with collisions in the DNS field. The data available from this simulation will be fundamental to produce subgrid and/or closure models required to run CFD simulations with commercial codes. The data produced in this phase will be used by the Units in Torino and Palermo.
(11 man-months)