Density | EPFL Graph Search

Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ρ (the lower case Greek letter rho), although the Latin letter D can also be used. Mathematically, density is defined as mass divided by volume: \rho = \frac{m}{V} where ρ is the density, m is the mass, and V is the volume. In some cases (for instance, in the United States oil and gas industry), density is loosely defined as its weight per unit volume, although this is scientifically inaccurate – this quantity is more specifically called specific weight. For a pure substance the density has the same numerical value as its mass concentration. Different materials usually have different densities, and density may be relevant to buoyancy, purity and packaging. Osmium and iridium are the densest known elements at standard conditions for temperature and pressure. To simplify comparisons of density across different systems of

Theoretical study of electronic transport in TCV ELMy H-mode

Gaël Induni

High-confinement mode (H-mode) is a promising reference scenario for ITER. But we are still facing major issues because of instabilities. They expel periodically some of the energy, which can damage the device. These instabilities are called the edge localized modes (ELM) and are not yet fully theoretically understood. The present work is a study on the profiles evolution in between ELMs and on the ELM effects. This may help to have a better understanding of the conditions before the ELM. We use the simulations as theoretical tool. For the purpose of the simulations, we build an H-mode χ e profile according to a standard L-mode one that we truncate at the edge to create a transport barrier. This gives a good agreement with the experimental data. Several scaling laws were successfully used. The first one is the energy confinement time scaling which was used for the thermal diffusivity to scale the temperature profile. A scaling between the core and pedestal energies was found recently. It was used to compute the pedestal χ e to scale the temperature pedestal, which was successful. Finally, we used a scaling for transport barriers which links the density gradient length to that of the temperature to compute the density in the pedestal. It was already found to be good in TCV electron internal transport barriers and in ASDEX Upgrade H-mode pedestals. Looking at the MHD stability parameters, it was found that for our reference case, ELMs are not likely to be triggered by the time evolution of the pressure gradient and the current density profiles in our model, as these are only varying significantly during the first millisecond after the crash, and are almost constant during the long remaining time until the next crash. Studying different cases, we investigate the behavior of the plasma when replacing the edge heating by central one to observe the influence of the heating profile, but no significant difference was found, neither in the MHD stability parameters. Further we change the particle diffusion coefficient to compare the dynamic behavior of the density. Slowing down the density dynamic behavior also slows down the pressure one, this can be seen on the MHD stability parameters. We also vary the ELM period to compare to the change due to the variation of the particle diffusivity. It was found that there may be a sort of relation between the particle diffusivity and the ELM period at least for the density, since both cases change the density recovery time with respect to the ELM period. A last case considered is doubling the radial ELM interaction range. This is done in order to observe the difference to the reference simulation that takes the density top of pedestal as ELM range, and to compare the spatial range influenced by the MHD activity and the one by the transport improvement. It was found that the MHD stability parameters in the pedestal exhibit a different behavior with the pressure gradient starting to increase very fast.

2011

Anode dusting during the electrolytic production of aluminium

Timea Tordai

The disintegration of the carbon anodes during the electrolysis of alumina produces dust particles contaminating the electrolytic bath. This phenomenon, called dusting, is one of the most deleterious Al electrolysis failures, as it leads to a significant deterioration of the current efficiency, which causes severe economical losses for Al smelters. The mechanisms at work, related to carbon oxidations leading to the selective burning of the binder matrix that is responsible of the anode dusting, were reviewed through a detailed study of the literature. In the same manner, an overview of the "state of the art" knowledge, but also of the existing controversies in the field of raw material and anode manufacturing processes that affects the dusting, was delivered. The open questions concerning the root causes of dusting, as well as the ways to alleviate this phenomenon during the anode production, were considered. The process peculiarities of anodes made in China, as well as the atypical properties and behaviour of some anodes exported to the western world, were largely integrated into the frame of the experimental sections. Of particular interest was the fact that rather dense and highly calcined coke was used in the dry aggregate that does not contain recycled materials (baked scrap and butts). This situation results from the usage of a unique calcining process made in shaft (vertical) kilns, where the residence time exceeds those experienced in western rotary kiln/hearth type calcining plants by one order of magnitude. The effects of coke calcining and anode baking heat treatment temperature combinations were studied in a pilot scale simulating shaft and rotary kiln conditions. Representative types of green coke with different levels of volatiles and impurities (as S, V and Ca mainly) were selected for this study. The addition of Na contamination, in form of cryolite used as the main bath component was also examined in order to cover the aspects of the catalytic impact of contaminants present in the binder matrix on oxidation. As expected, the effects of baking were found to be of chief importance. However, the new element brought by this work is that not only the coke's sulphur content is important for its sensitivity to the negative impact of Na, but also its degree of calcination. Coke with a low calcining degree, measured by its real density/crystallite size, is showing a much better resistance to the catalytic effect of Na during the anode oxidation, and this especially when underbaking is a process issue. Recommendations on the optimum calcining heat treatment for western and Chinese calcining technologies are given as a conclusion of this part; this was done considering different scenarios of butts Na contaminations and baking issues in the anode production. The effects of desulphurization during calcining and especially during baking on the anode's physical, electrical and thermal properties were also addressed, so as to give a complete picture including the anode's density but also the butts' weight after usage of anodes during their cycle time in the pots. Optical light microscopy examinations of the porosity created on soft butts, related to permeable low density anodes, as well as on pilot anode cores oxidized in isobar laboratory conditions allowed laying emphasis on the prime importance of anode permeability in the dusting mechanisms. Some Chinese carbon plants are still using batch mixers with low temperatures and mixing intensity potential. As relatively low densification severity vibrators are also used, it felt adequate to study the role of porosity in the dusting propensity in an anode bench scale plant. For this purpose, a multi-factorial design of experiments was set up using two levels of pitching, mixing and forming intensities, and this at two levels of Na contamination and two baking temperatures. The levels of pitching, mixing (sigma blade versus high speed impeller mixers) and forming were selected in such a way that the difference in terms of apparent density reached a significant level (about 0.05 kg/dm3). Their individual impacts on the air and CO2 reactivity dust measured after laboratory oxidation tests were comparable to about 500 ppm Na contamination or 100 °C baking temperature differences, which is quite substantial for the anode behaviour in the pots. As the effect of the porosity differences on gas permeability exceeds one order of magnitude, it is obvious that the level of apparent density and therefore the pitching, mixing and forming conditions are of utmost importance for the anode dusting. The implications for equipment suppliers and plant designers are obvious: for the production of paste, efficient preheaters and intensive kneader mixers as well as controlled paste coolers need to be selected. Efficient forming machines working with vacuum conditions should be preferred and further developed in terms of densification severity and control efficiency. A parallel study on an ultra high intensive laboratory mixer, working with a mixture of fines and pitch only, i.e. producing after forming binder matrix electrodes, demonstrated the enormous potential left for porosity reduction of baked specimens. Substantial shrinkage of the dense green anodes during the baking step was observed on specimens produced under intensive mixing conditions. Concerning the dry aggregate recipe, bench scale trials with extreme ranges of fines content and fineness confirmed that, as long as the level of fineness is compensated by the fines content, optimum anode properties can be achieved with unchanged optimum pitch content. For a laboratory intensive impeller mixer, a range of fineness between 2500 and 5000 Blaine with a corresponding content between 42 % and 21 % provided anodes showing practically the same properties relevant for dusting. The conclusions drawn from the studies performed in the frame of this thesis allow a much better understanding on the traps of manufacturing world class quality anodes with zero dusting in a very different technological environment (as in China). In addition to this, the axis of development for western technology and equipment suppliers could be formulated based on the numerous quantitative pilot information gathered during this thesis work.

EPFL2007

Data-driven whole mouse brain modeling for multi-scale simulations

Dimitri Bruno Marie Rodarie

In this thesis, we present a data-driven iterative pipeline to generate, simulate and validate point-neuron models of the whole mouse brain. The ultimate goal is to replicate close loop experiments with a virtual body in a virtual world. This pipeline was origi-nally created by a PhD student of the Blue Brain Project and this pioneer work has yielded the first versions of the model and their simulations. My objective is to refine, extend and consolidate the previous version of the workflow and in particular to extend the repertoire of neuron types by integrating and consolidating available literature data.The pipeline has four main parts. First, we define a pair of reference atlases to create a spatial reference for every dataset that we want to integrate. Second, we create a cell atlas with estimates of the number and density of the major cell types in each brain region. Then, we build a connectivity atlas that describes the connections of each neuron of the mouse brain and their properties. Finally, we assign point-neuron parameters to each of the neuron types, and synaptic parameters to each of the connection types of our model to obtain a point-neuron network of the mouse brain. This network can be simulated to perform mouse brain experi-ments in-silico.In this thesis, we evaluate the quality of the different reference atlases, released by the Allen Institute for Brain Science and pro-vide methods to mitigate the impact of artifacts in the original images. We further extend the cell atlas with density estimates of more inhibitory neuron types. To do so, we introduce a new method to combine estimates of inhibitory neuron counts from the literature into a consistent framework. We also estimate inhibitory neuron density in regions where no literature data are avai-lable, using collected literature estimates and gene expression data from in situ hybridization image stacks. Our approach can be further extended to other cell types and provides a resource to build more detailed circuits of the mouse brain. Next, we refine the connectivity atlas, including literature findings on short-range connectivity. Additionally, we construct a database to store data collected on neuron types point-neuron parameters and synaptic parameters. With the results of the previous steps, we generate a new version of the whole mouse brain point-neuron network. We tested this new model against its previous version using three benchmark experiments. We explore the possibility to embed and connect detailed circuit reconstructions of the mouse brain to our point neuron model and co-simulate them. Finally, we constrain a 3D skeleton model of the mouse with joint constraints from the literature and attach muscle to its bones to create a musculoskeletal model of the mouse body that can be simulated in close-loop with our model to reproduce motor control experiments.

EPFL2022