437 research outputs found

    A Priori Modeling of Thermal Runaway Consequences in Lithium-Ion Batteries

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    Numerous experimental methods are available to predict the hazards associated with thermal runaway (TR) and subsequent catastrophic failure of lithium-ion batteries (LIB), but these methods are time-intensive and costly. The current study provides a thorough review of these experimental methods which include closed-vessel gas sampling, accelerating rate calorimetry, cone calorimetry, and Tewarson calorimeters. The strengths and weaknesses of each experimental method as applied by various researchers are critically analyzed, and several shortcomings of current approaches are identified. Key deficiencies in current approaches include lack of control of reactant gases (i.e., ambient air or similar), inadequate heating rates that are not comparable to realistic conditions, and failure to measure condensable reaction products (e.g., water or liquid electrolyte). In lure of experimental approaches, an a priori modeling approach based on chemical equilibrium analyses (CEA) is proposed herein. Standard CEA software is limited in applicability, so that several improvements are required for accurate modeling. These improvements include prediction of electrolyte solution densities; inclusion of key reactant and/or product species and their respective thermodynamic properties; and accurate representation of high-temperature oxygen release from metal oxide cathodes. The current study focuses on addressing the first of these two improvements, but additional work is still required to fully address them. Future work will encompass resolving the third improvement (i.e., metal oxide oxygen release), model validation against available experimental data, and modeling of LIB failure scenarios to inform future designs

    Hydroacoustic Modeling of a Cavitation Vortex Rope for a Francis Turbine

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    Hydraulic machines subject to off-design operation involve the presence of cavitating flow regimes in the draft tube. The cavitation vortex rope at part load conditions is described as an excitation source for the hydraulic system, and interactions between this excitation source and system eigenfrequency may result in resonance phenomena and induce a draft tube surge and electrical power swings. To accurately predict and simulate a part load resonance, proper modeling of the draft tube is critical. The presence of this cavitation vortex rope requires a numerical pipe element taking into account the complexity of the two-phase flow. Among the parameters describing the numerical model of the cavitating draft tube flow, three hydroacoustic parameters require a special attention. The first hydroacoustic parameter is called cavitation compliance. This dynamic parameter represents the variation of the cavitation volume with respect to a variation of pressure and implicitly defines the local wave speed in the draft tube. The second parameter corresponds to the bulk viscosity and is related to internal processes breaking a thermodynamic equilibrium between the cavitation volume and the surrounding liquid. The third parameter is the excitation source induced by the precessing vortex rope. The methodology to identify these hydroacoustic parameters is based on the direct link that exists between the natural frequency of the hydraulic system and the wave speed in the draft tube. First, the natural frequency is identified with the help of an external excitation system. Then, the wave speed is determined thanks to an accurate numerical model of the experimental hydraulic system. By applying this identification procedure for different values of Thoma number, it is possible to quantify the cavitation compliance and the void fraction of the cavitation vortex rope. In order to determine the energy dissipation induced by the cavitation volume, the experimental hydraulic system is excited at the natural frequency. With a Pressure-Time method, the amount of excitation energy is quantified and is injected into the numerical model. A spectral analysis of the forced harmonic response is used to identify the bulk viscosity and the pressure source induced by vortex rope precession. Thus, the identification of the hydroacoustic parameters requires the development of a new numerical draft tube model taking into account the divergent geometry and the convective terms of the momentum equation. Different numerical draft tube models are compared to determine the impact of convective and divergent geometry terms on identification of the hydroacoustic parameters. Furthermore, to predict the hydroacoustic parameters for non-studied operating conditions and to break free from the dependence upon the level setting of the Francis turbine, dimensionless numbers are proposed. They have the advantage of being independent from the selected numerical model and they define a behavior law of hydroacoustic parameters when the cavitation volume oscillates at resonance operating conditions. Finally, to investigate the stability operation of the prototype, the hydroacoustic parameters need to be transposed to the prototype conditions according to transposition laws. By assuming both Thoma similitude and Froude similitude conditions, transposition laws are developed and the hydroacoustic parameters are predicted for the prototype

    L'impact du salaire minimum sur l'automatisation de l'emploi au Canada

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    Ce mémoire réunit deux grands champs de recherche économique du marché du travail, soit la recherche sur le salaire minimum et celle sur l’automatisation de l’emploi. Ainsi, l’objectif de ce mémoire est d’évaluer l’impact du salaire minimum sur l’automatisation de l’emploi au Canada entre 1998 et 2018. L’hypothèse principale est que les augmentations du salaire minimum stimulent l’automatisation des emplois nécessitant peu de qualifications, en substituant les nouvelles technologies aux travailleurs peu qualifiés. Ce mémoire reprend la méthodologie de Lordan et Neumark (2018) afin d’explorer cette hypothèse de substitution capital-travail et de mieux comprendre comment les politiques de salaire minimum façonnent le type d’emploi détenu au Canada. Les résultats montrent qu’une augmentation de 10 % du salaire minimum réduirait de 0,3 point de pourcentage la part de l’emploi automatisable occupé par des travailleurs peu qualifiés. La seconde partie de ce mémoire montre également une probabilité plus élevée de perte d’emploi pour les travailleurs peu qualifiés occupant un emploi automatisable à la suite d’une augmentation du salaire minimum. Ensemble, ces résultats sont des éléments de preuve que le salaire minimum a un effet positif sur l’automatisation de l’emploi au Canada.This thesis brings together two major field of economic research in the labor market,that is, research on minimum wage and research on job automation. The objective of thisthesis is therefore to assess the impact of the minimum wage on the automation ofemployment in Canada between 1998 and 2018. The main hypothesis is that increases in theminimum wage stimulate the automation of jobs requiring few skills, by substituting newtechnologies for low-skilled workers. The analysis uses the methodology of Lordan andNeumark (2018) in order to explore this hypothesis of capital-labor substitution and to betterunderstand how minimum wage policies shape the type of employment held in Canada. Theresults show that a 10% increase in the minimum wage would reduce the share of automatedjobs held by low-skilled workers by 0,3 percentage points. The second part of this thesis alsoshows a higher probability of losing their job for low-skilled workers in automatable jobsfollowing an increase in the minimum wage. Together, these results are evidence that theminimum wage has a positive effect on the automation of employment in Canada

    Spatial variations of nitrogen trace gas emissions from tropical mountain forests in Nyungwe, Rwanda

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    Globally, tropical forest soils represent the second largest source of N2O and NO. However, there is still considerable uncertainty on the spatial variability and soil properties controlling N trace gas emission. Therefore, we carried out an incubation experiment with soils from 31 locations in the Nyungwe tropical mountain forest in southwestern Rwanda. All soils were incubated at three different moisture levels (50, 70 and 90 % water filled pore space (WFPS)) at 17 °C. Nitrous oxide emission varied between 4.5 and 400 μg N m−2 h−1, while NO emission varied from 6.6 to 265 μg N m−2 h−1. Mean N2O emission at different moisture levels was 46.5 ± 11.1 (50 %WFPS), 71.7 ± 11.5 (70 %WFPS) and 98.8 ± 16.4 (90 %WFPS) μg N m−2 h−1, while mean NO emission was 69.3 ± 9.3 (50 %WFPS), 47.1 ± 5.8 (70 %WFPS) and 36.1 ± 4.2 (90 %WFPS) μg N m−2 h−1. The latter suggests that climate (i.e. dry vs. wet season) controls N2O and NO emissions. Positive correlations with soil carbon and nitrogen indicate a biological control over N2O and NO production. But interestingly N2O and NO emissions also showed a positive correlation with free iron and a negative correlation with soil pH (only N2O). The latter suggest that chemo-denitrification might, at least for N2O, be an important production pathway. In conclusion improved understanding and process based modeling of N trace gas emission from tropical forests will benefit from spatially explicit trace gas emission estimates linked to basic soil property data and differentiating between biological and chemical pathways for N trace gas formation

    Spatial variations of nitrogen trace gas emissions from tropical mountain forests in Nyungwe, Rwanda [Discussion paper]

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    Globally, tropical forest soils represent the second largest source of N2O and NO. However, there is still considerable uncertainty on the spatial variability and soil properties controlling N trace gas emission. To investigate how soil properties affect N2O and NO emission, we carried out an incubation experiment with soils from 31 locations in the Nyungwe tropical mountain forest in southwestern Rwanda. All soils were incubated at three different moisture levels (50, 70 and 90% water filled pore space (WFPS)) at 17 °C. Nitrous oxide emission varied between 4.5 and 400 μg N m−2 h−1, while NO emission varied from 6.6 to 265 μg N m−2 h−1. Mean N2O emission at different moisture levels was 46.5 ± 11.1 (50% WFPS), 71.7 ± 11.5 (70% WFPS) and 98.8 ± 16.4 (90% WFPS) μg N m−2 h−1, while mean NO emission was 69.3 ± 9.3 (50% WFPS), 47.1 ± 5.8 (70% WFPS) and 36.1 ± 4.2 (90% WFPS) μg N m−2 h−1. The latter suggests that climate (i.e. dry vs. wet season) controls N2O and NO emissions. Positive correlations with soil carbon and nitrogen indicate a biological control over N2O and NO production. But interestingly N2O and NO emissions also showed a negative correlation (only N2O) with soil pH and a positive correlation with free iron. The latter suggest that chemo-denitrification might, at least for N2O, be an important production pathway. In conclusion improved understanding and process based modeling of N trace gas emission from tropical forests will not only benefit from better spatial explicit trace gas emission and basic soil property monitoring, but also by differentiating between biological and chemical pathways for N trace gas formation

    Optimizing dual energy cone beam CT protocols for preclinical imaging and radiation research

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    Objective: The aim of this work was to investigate whether quantitative dual-energy CT (DECT) imaging is feasible for small animal irradiators with an integrated cone-beam CT (CBCT) system. Methods: The optimal imaging protocols were determined by analyzing different energy combinations and dose levels. The influence of beam hardening effects and the performance of a beam hardening correction (BHC) were investigated. In addition, two systems from different manufacturers were compared in terms of errors in the extracted effective atomic numbers (Z(eff)) and relative electron densities (rho(e)) for phantom inserts with known elemental compositions and relative electron densities. Results: The optimal energy combination was determined to be 50 and 90kVp. For this combination, Z(eff) and r rho(e) can be extracted with a mean error of 0.11 and 0.010, respectively, at a dose level of 60cGy. Conclusion: Quantitative DECT imaging is feasible for small animal irradiators with an integrated CBCT system. To obtain the best results, optimizing the imaging protocols is required. Well-separated X-ray spectra and a sufficient dose level should be used to minimize the error and noise for Z(eff) and rho(e). When no BHC is applied in the image reconstruction, the size of the calibration phantom should match the size of the imaged object to limit the influence of beam hardening effects. No significant differences in Z(eff) and rho(e) errors are observed between the two systems from different manufacturers. Advances in knowledge: This is the first study that investigates quantitative DECT imaging for small animal irradiators with an integrated CBCT system
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