Numerical Simulations of an Innovative Water Stirring Device for Fine Sediment Release: The Case Study of the Future Trift Reservoir

Reservoir sedimentation and consequently lack of storage volume and perturbation of the operation of intakes and bottom outlet is a key challenge affecting both hydropower production as well as dam safety and flood management. In the framework of a peer-reviewed research project (Jenzer-Althaus, 2011) an innovative countermeasure, called SEDMIX, was proposed allowing to keep in suspension or re-suspend the fine particles near the power water intakes, thanks to an optimized arrangement of four water jets producing an upward whirling flow like produced by a mixer. With such a system, the suspended particles can be conveyed downstream at acceptable rates through the power waterways during the normal operation of the hydropower plant. Although experimental studies have shown the very promising efficiency of such a device in simple cases and by numerical simulations in a laboratory reservoir, SEDMIX performance has not been investigated yet in a real-life reservoir under prototype conditions. The aim of this study is therefore, to analyze the performance of a real-sized SEDMIX operating in the future Trift reservoir via numerical analyses. This study allows to validate or to improve SEDMIX optimal configuration experimentally determined. The numerical simulations are performed for different positions and heights for the SEDMIX device. The performance of SEDMIX in each position has been evaluated and tested for different jet discharges. The analysis of the numerical simulation results shows that the presence of SEDMIX does create a vortex flow pattern and sediment movement upward. The sediment volume fraction in the higher layers of the reservoir increases and consequently the evacuated volume of fin sediments increases for simulation using the SEDMIX device comparing those without the device.

High Intensity Beam Issues in the CERN Proton Synchrotron

Sandra Aumon

This PhD work is about limitations of high intensity proton beams observed in the CERN Proton Synchrotron (PS) and, in particular, about issues at injection and transition energies. With its 53 years, the CERN PS would have to operate beyond the limit of its performance to match the future requirements. Beam instabilities driven by transverse impedance and aperture restrictions are important issues for the operation and for the High-Luminosity LHC upgrade which foresees an intensity increase delivered by the injectors. The main subject of the thesis concerns the study of a fast transverse instability occurring at transition energy. The proton beams crossing this energy range are particularly sensitive to wake forces because of the slow synchrotron motion. This instability can cause a strong vertical emittance blow-up and severe losses in less than a synchrotron period. Experimental observations show that the particles at the peak density of the beam longitudinal distribution oscillate in the vertical plane due to a short range wake field and following a travelling wave of about 700 MHz. In order to perform measurements, a dedicated single bunch beam was set up with a zero chromaticity plateau around transition energy. Extensive measurements were performed of the dynamics of instability in order to compute rise times and intensity thresholds. These measurements were done for several peak densities and the results show that the longer the bunch length, the higher is the threshold in intensity. Other measurements performed with a small negative chromaticity and another working point show that the intensity threshold can be pushed at higher values—in this case, the threshold was increased by 20%. The particularity of this work is that the instability is triggered during the acceleration. At transition energy, the momentum compaction factor is zero and the exchange of particles between the head and the tail of the beam from synchrotron motion, which is a natural way to damp instabilities, vanishes for few turns. Therefore the measurements at which η the instability is triggered are fundamental to know in which longitudinal regime the instability develops. Macro particle simulations were performed in order reproduce the dynamics of the instability with the HEADTAIL code, that simulates the beam interaction with the impedance of the machine. In the case of the PS, a very detailed impedance model does not exist, therefore a very simple resonator impedance was considered. The parameters of the code were adapted in order to simulate the beam as close as possible to the experimental conditions. The simulations showed that the travelling wave is well reproduced and therefore rise times were extracted for different beam intensities and compared to the measurements. A good agreement is found for a such simple impedance model. The intensity thresholds are reproduced for zero chromaticity within 30% and it is clear that some damping mechanisms occurring in the measurements are not reproduced by the code. In particular, octupolar components of the field, non-linear coupling and transverse space charge are not taken into account. These limitations could cause some discrepancies between model and measurements. This study allows to establish a broadband impedance model which can be use to predict the dynamics of the instability. A few experiments were done also with the use a gamma transition jump. In this case, the instability appears in the adiabatic regime and the rising of the instability is following the peak density of the beam. Both the intensity threshold measurements and the simulations show that the gamma jump is by far the most efficient way to increase significantly the intensity threshold. The choice of adequate working point appears also to be a cure of the instability. This study with the support of measurements provides a rough impedance model confirmed by simulations and an understanding of the different mechanisms triggering the instability. The last part of the thesis is dedicated to proton beam losses studies at injection energy. An eventual upgrade of the high intensity beam is limited by large beam losses at injection in the PS. Losses were measured with the current beam loss monitor system of the PS that they occur while the incoming beam enters in the machine and then during several hundred turns while the beam is circulating. Optics measurements were performed to check the mismatch of the transfer line between the PSBooster and the PS. A significant horizontal dispersion mismatch was found and in particular a difference in optics between the four injection lines of the PS. Aperture bottlenecks were found at the injection septum in both transverse planes and at the maximum of the injection bump. With the help of Monte Carlo simulations, the high radiation outside the ring induced by the losses was understood. However, the mismatch does not explain the continuous losses after the beam is injected. A possible explanation is that the space charge forces contribute to make particles cross resonances and are lost while they are oscillating at high amplitudes. The transverse phase space is refilled due to the combination of the synchrotron motion and space charge. Several solutions were proposed to improve the situation. A new optics of the transfer line and at injection could make the beam size smaller. Some special quadrupoles already installed in the PS could be used to adapt the optics of the incoming beam. Increase the injection energy from 1.4 GeV to 2 GeV kinetic would cause a gain of 65% in space charge forces, and these studies are currently ongoing for the upgrade of the PS in the framework of high-intensity LHC upgrade project. High intensity beams are also perturbed by space charge forces and transverse instabilities. Coherent tune shift measurements were performed at injection and extraction in order to evaluate the transverse imaginary part of the impedance and the contribution of the space charge to betatron frequency shift with the beam intensity.

EPFL2012

Flat-Histogram Monte Carlo as an Efficient Tool To Evaluate Adsorption Processes Involving Rigid and Deformable Molecules

Berend Smit, Matthew David Witman

Monte Carlo simulations are the foundational technique for predicting thermodynamic properties of open systems where the process of interest involves the exchange of particles. Thus, they have been used extensively to computationally evaluate the adsorption properties of nanoporous materials and are critical for the in silico identification of promising materials for a variety of gas storage and chemical separation applications. In this work we demonstrate that a well-known biasing technique, known as "flat-histogram" sampling, can be combined with temperature extrapolation of the free energy landscape to efficiently provide significantly more useful thermodynamic information than standard open ensemble MC simulations. Namely, we can accurately compute the isosteric heat of adsorption and number of particles adsorbed for various adsorbates over an extremely wide range of temperatures and pressures from a set of simulations at just one temperature. We extend this derivation of the temperature extrapolation to adsorbates with intramolecular degrees of freedom when Rosenbluth sampling is employed. Consequently, the working capacity and isosteric heat can be computed for any given combined temperature/pressure swing adsorption process for a large range of operating conditions with both rigid and deformable adsorbates. Continuous thermodynamic properties can be computed with this technique at very moderate computational cost, thereby providing a strong case for its application to the in silico identification of promising nanoporous adsorbents.

2018

Modélisation des transformations de phase à l'état solide dans les aciers et application au traitement thermique par induction

Alain Jacot

In the first part of this work a comprehensive model of the continuous hardening of 3D-axisymmetric steel components by induction heating has been developed. In the model, the Maxwell and heat flow equations are solved using a mixed numerical formulation : the inductor and the workpiece are enmeshed with finite elements (FE) but boundary elements (BE) are used for the solution of the electromagnetic equations in the ambient air. This method allows the inductor to be moved with respect to the workpiece without any remeshing procedure. The heat flow equation is solved for the workpiece using the same FE mesh. For the thermal boundary conditions, a net radiation method has been implemented to account for grey diffuse bodies and the viewing factors of the element facets are calculated using a "shooting" technique. These calculations have been coupled to a metallurgical model describing the solid state transformations that occur during both heating and cooling. From the local thermal history, the evolution of the various phase fractions are predicted from TTT-diagrams using an additivity principle. A micro-enthalpy method has been implemented in the heat flow calculations in order to account for the latent heat released by the various transformations. At each time step, the local properties of the material, in particular its magnetic permeability, are updated according to the new temperatures and magnetic field. Special attention has been taken for the description of the boundary conditions associated with the water spraying below the inductor. The heat transfer coefficient has been deduced from the inverse modelling of temperatures measured at various locations of a test piece. This preliminary work has been complemented with measurements of the magnetic permeability of the 42CrMo4 steel from which the workpieces are made. This part of the study also includes dilatometric measurements for the verification of the additivity principle used in the simulation. The model has been applied to three cases of increasing complexity : induction stream heating of a steel cylinder without quenching, stream quenching of a steel cylinder and, finally, stream quenching of a non-cylindrical workpiece. The results of the simulation have been compared with experimental cooling curves, microstructures and hardness profiles. In the second part of this work, the phase transformations that occur in hypoeutectoid steels during heating have been investigated at the scale of the microstructure according to a microscopic approach. Several models have been developed in order to describe the various steps of the austenitisation process : (i) pearlite dissolution, (ii) transformation of ferrite into austenite and homogenization, (iii) grain growth in austenite. In a first approach, each step has been modeled separately. The dissolution of pearlite has been described using a two-dimensional finite element model with a deforming mesh and a remeshing procedure. The diffusion equation has been solved in austenite (γ) for a typical domain representative of a periodic structure of ferrite (α) and cementite (θ) lamellae. The α/γ and θ/γ interfaces are allowed to move with respect to the local equilibrium condition, including curvature effects via the Gibbs-Thompson coefficient. The model has been used to predict the concentration field and the shape of the interface at different stages of the pearlite dissolution. Maps representing the steady state dissolution rate as a function of the temperature and lamellae spacing have been obtained for small values of overheating. The appearance of a non-steady state regime at higher temperature has been discussed. The transformation of ferrite into austenite has been described using a pseudo-front tracking finite volume approach for solving the diffusion equation in a 1D, 2D or 3D domain. At the start of the computation, the volume is made of ferritic particles and austenitic zones resulting from the pearlite dissolution. The model allows to calculate the kinetics of the phase transformation as a function of the temperature and the initial microstructure. Although the comparison of the transformation kinetics with experimental results was quite satisfactory, it appeared that the calculated kinetics were slightly slower. This effect has been attributed to the other alloying elements which are contained in the Ck45 steel used for the experiments. This discrepancy has also been explained by a stereological effect which is due to the calculation in a 2D section instead of a 3D domain as demonstrated by a comparison of 2D and 3D simulations. The austenitic grain growth has been described with two models based respectively on a Monte Carlo technique and a mechanical approach. A methodology for obtaining a correspondence between the simulation time scale and real time has been presented and applied to the Ck45 steel. The value of the exponent n of the grain growth law d = Κ t1/n (where d is the mean diameter and t the time) has been determined for both models. The mechanical model (n=2) turned out to describe perfectly the case of normal grain growth as it occurs in liquid-gas systems. However the results of the Monte Carlo simulation (n=2.3) are in better agreement with the non-ideal behaviour of the Ck45 steel. The influence of the impurities and particles which are present in real materials should be taken into account in both models if a more quantitative agreement is to be obtained. Finally, a combined model coupling the various steps of the austenitisation process has been proposed. It allowed to show that the assumption consisting in dividing the process in three separate steps is valid in most cases. This combined model is a first attempt for a comprehensive modelling of the austenitisation process.