Machine d'Anticythère | EPFL Graph Search

La machine d'Anticythère, appelée également mécanisme d'Anticythère, est considérée comme le premier calculateur analogique antique permettant de calculer des positions astronomiques. C'est un mécanisme de bronze comprenant des dizaines de roues dentées, solidaires et disposées sur plusieurs plans. Il est garni de nombreuses inscriptions grecques. On connaît de la machine d'Anticythère un unique exemplaire, dont les fragments ont été trouvés en 1901 dans une épave, près de l'île grecque d'Anticythère, entre Cythère et la Crète. L'épave d'Anticythère était celle d’une galère romaine, longue d'une quarantaine de mètres, qui a été datée comme antérieure à 87 La machine d'Anticythère est le plus vieux mécanisme à engrenages connu. Ses fragments sont conservés au musée national archéologique d'Athènes. Histoire Origine et datation de la machine d'Anticythère En 2010, Giovanni Pastore a daté le mécanisme entre la fin du . En 2014, deux chercheurs, l'un argentin,

Dynamic CO2 sorption on MgO-based sorbent in the presence of CO and H2O at elevated pressures

Seongmin Jin

The presence of gas species other than the target adsorbate in practical conditions causes complications in implementing a sorption-based process because of synergistic or detrimental effects on the sorption mechanism. MgO-based sorbents have attracted massive attention for the pre-combustion CO2 capture, where H2O and CO coexist in emission gas. Although H2O has been reported to greatly enhance CO2 sorption kinetics, CO2 sorption behavior and mechanism under a large quantity of H2O as in the practical condition remain unclear. Furthermore, CO2 sorption behavior in the presence of CO has not been adequately examined. Here, the dynamic CO2 sorption behavior of MgO-based sorbent was understood by coupling the breakthrough experiment under practically relevant conditions with in situ DRIFTS results. At elevated pressures, the sorption performance was considerably enhanced, whereas the influence by temperatures was insignificant. In addition, H2O enhanced CO2 sorption kinetics and initial CO2 sorption on MgO-based sorbent, whereas CO hindered them. In situ DRIFTS analyses for the mechanism study revealed that OH groups generated from H2O dissociation created highly basic sites that promoted the formation of monodentate carbonate, thereby enhancing the initial CO2 sorption. In contrast, CO hampered CO2 sorption through two mechanisms: (1) competitive sorption between CO2 generated from the water-gas shift reaction and CO2 in the feed; (2) interference of the promoted sorption by the molten nitrate salts. This study demonstrated that the practical condition of the pre-combustion CO2 capture significantly alters the CO2 sorption behavior of MgO, thereby providing insights for the ultimate implementation of MgO-based sorbents.

ELSEVIER SCIENCE SA2022

Gas phase chemistry mechanisms for air quality modeling

Martin Junier

During the last few decades, air pollution has become one of the major environmental and public health issue in every important cities over the world. Photochemical pollutants, like ozone which play a central role in today's air pollution problems, are formed in the atmosphere by reaction of two emitted precursors: Volatile Organic Carbons (VOCs) and NOx (NO + NO2). Ozone is a highly non linear process because its formation is driven by complex chain reactions. The decrease of ozone concentration produced by a reduction of its emitted precursors is therefore unpredictable, unless calculated by numerical photochemical models. The simulation of photochemical air pollution requires detailed chemical mechanisms and a lot computer resources. The number of chemical species within the chemical mechanism has to be confined to the strict minimum in order to minimise the CPU time. A solution is to lump the immense number of VOC species involved in atmospheric pollution in a convenient smaller number of mechanism species, keeping enough details to generate accurate results in reasonable calculations times. The calculation of all kinetic data of a lumped mechanism is a tremendous work unless carried out by a generation programme. CHEMATA, presented in this work, is a chemical mechanisms generation programme designed to create lumped and explicit tropospheric gas phase chemical mechanisms. Based on the widely used mechanism RACM, CHEMATA generated an extended mechanism to test the carbonyl species parameterisation of RACM and two smaller mechanisms to compare two lumping methods (the reduced mechanism and the small mechanism). The new mechanisms have been implemented in a bOx model and in the 3D eulerian air quality model TAPOM, also presented in this work. TAPOM have been run with the four chemical mechanisms on three simulations domains (Mexico City, Milan and Bogota) presenting different emissions strengths and meteorological conditions. The comparisons between the different mechanisms, in a bOx or 3D model and with or without emission reductions lead to the following conclusions: The comparison between the extended mechanism and RACM shows that the treatment of the carbonyl species in RACM does not induce notable errors in mesoscale modelling. The use of the extended mechanism should be kept for special simulations when enhanced precision in VOCs is required or for time periods longer than 2 or 3 days. The reduced mechanism is the best compromise between CPU time and accuracy. When calculating photochemical pollution, or emission reduction scenarios, this mechanism can save a lot of time. The small mechanism presents a clear tendency to produce more "VOC sensitive" results, which can lead to severe ozone overestimations. It should only be used for qualitative simulation when CPU time is a critical issue.

EPFL2004

Characterization of Grey Cast Iron-Steel Tribo-system under Starved Lubrication Conditions

Fatemeh Saeidi

Reducing friction and wear is essential for building efficient systems with low energy consumption and a long lifetime. Surface texturing is one of the methods to reduce friction and wear, especially in oil-lubricated systems. However, there is still a lack of knowledge about the fundamental aspects involved in the improvement of tribological performance, especially for the mechanical parts sliding under starved lubrication conditions. Moreover, failure in such lubricated contacts is poorly understood and there are little agreements regarding the mechanisms leading to scuffing. The present research focuses on three main objectives. The first is to characterize a cast iron-steel tribo-system under starved lubrication conditions, and to improve its tribological performance by laser surface texturing. All tribo-tests are performed using a flat-on-flat set-up with reciprocal movements. This set-up simulates specific industrial operating conditions of a semi-journal bearing used in a cutting machine. A Design of Experiments (DoE) approach is selected to investigate the effect of the different geometrical micro-texture parameters on the coefficient of friction (COF) and the lifetime of the cast iron samples. An optimum micro-texture geometry leading to a low COF and a long lifetime is determined based on a fractional factorial design. The second goal is to investigate the failure mechanisms of the selected tribo-pair. Failure of the tribo-system is characterized for both textured and un-textured surfaces. Scuffing is found to be the failure mechanism for the un-textured cast iron samples. For the textured cast iron samples, in contrast, two different failure mechanisms are observed depending on the distance between the micro-textures in direction of sliding (DMS). The textured surfaces with a DMS > 3.5 mm fail via the scuffing mechanism, whereas the surfaces with a DMS < 3 mm fail by a different mechanism named the oxidation mechanism. In contrary to the scuffing mechanism, the oxidation failure is found to be more a gradual failure. Based on the experimental observations, two hypotheses are suggested for the scuffing mechanism, and one for the oxidation mechanism. Finally, predicting the lifetime of a tribo-system is of the utmost importance to save costs. The DoE model developed for the lifetime was found to be a weak predictive model due to the sudden nature of scuffing. The accuracy of this model could only be improved by performing large statistics, which is not time and cost effective. Consequently, the last objective of this work is to employ an acoustic emission (AE) technique to detect precisely the surface state of our tribo-system. The application of wavelet packet decomposition, as an advanced signal processing method used for AE signals, is found to be promising for early detection of scuffing. The extension of this work using random forest regression shows the possibility of predicting scuffing 5 min before it occurs.

EPFL2016