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Influence of hydrogen content and injection scheme on the describing function of swirled flames
Get PDFThe dynamics of V-shape swirled lean premixed methane/air flames enriched with hydrogen is examined for two injection schemes. The response of the flames submitted to harmonic flowrate modulations is compared when hy- drogen is premixed with the main methane/air flow and when it is injected pure as a pilot jet, directly at the flame base. Experiments are carried out at constant thermal power. Results obtained for the flame describing function (FDF) show that for a given hydrogen content, the premixed and pilot injection strategies lead to drastically different responses, although the shape of these flames are close. In the fully premixed strategy, the frequency bandwidth over which the flame is very responsive widens as the flame shortens due to the higher reactivity of the hydrogen enriched combustible mixture. Increasing the hydrogen content leads to an increased receptivity of the premixed flame sheet to incident flow perturbations. The opposite effect is seen for the pilot injection strategy due to a rebalancing of the heat release rate distribution along the flame brush with higher reaction rates close to the flame base compared to the reference methane/air case and the fully premixed hydrogen injection strategies. With hydrogen pilot injection, heat release rate fluctuations at the flame base interfere with those further downstream along the reaction layer, leading to an overall reduction of the FDF gain. This mechanism is evidenced with a set of experiments and confirmed by a low order model that considers a non uniform distribution of the heat release along a wrinkled flame sheet. It is also shown to persist when the forcing level is varied. These experiments indicate that the FDF of swirling V-flames can be lowered over a broad frequency range with hydrogen piloting due to a higher reactivity at the flame base
Risk knowledge modeling for offer definition in customer-supplier relationships in Engineer-To-Order situations
No full textThis work deals with the customer-supplier relationship and concerns offer definition in Engineer-To-Order situations (ETO), by adopting the supplier point of view. In such cases, when the offer definition relies simply on key design choices without a detailed design, there is a specific risk (ETO-specific risk) that customer expectations cannot be fulfilled. This kind of risk is in addition to the conventional risks (non ETO-specific risk) involved in any delivery process (machine break down, resource not available, scrapped part…). In order to minimize the supplier risk of not being able to complete the offer as accepted and contracted by the customer, a knowledge-based system can be used to assist risk engineering. Consequently, this article proposes two interrelated knowledge modeling contributions. Firstly, a risk knowledge model which, when implemented in a knowledge-based system that supports risk characterization and risk treatment by using knowledge re-use techniques, is proposed and discussed. Secondly, two knowledge typologies for risk characterizations and treatments (both for ETO and non-ETO situations) in order to support risk knowledge, identification and modeling are also proposed and discussed. These contributions are innovative and groundbreaking in terms of both academics and applications: they provide a formal model to structure risk knowledge and a first list of risks and treatments to be taken into account in ETO and non ETO situations. After an introduction that presents the research gap, our objectives and an analysis of related works, our two contributions are described in two sections with respect to ISO31000 recommendations. The first section covers risk identification and evaluation while the second deals with risk treatments
Few-layered-graphene/zirconia composites: Single-step powder synthesis, spark plasma sintering, microstructure and properties
Get PDFThe chemical vapor deposition of carbon is performed onto a commercial yttria-stabilized zirconia (3YSZ) powder bed. This produces few-layered-graphene (FLG) film uniformly covering the 3YSZ grains, without the manipulation of any pre-existing nanocarbon in the form of graphene platelets. The powders are then consolidated by spark plasma sintering, producing specimens where FLG is located along the grain boundaries of the 3YSZ matrix, which is below 0.3 μm in grain size. The samples are characterized by Raman spectroscopy, X-ray photoelectron spectroscopy, scanning and transmission electron microscopy. The pure 3YSZ exhibits higher toughness and fracture strength compared to composites, but the trend is that their toughness increases upon the increase in carbon content. Crack-deflection and crack-bridging are observed. The composites are electrically conducting with a percolation threshold between 1.48 and 1.98 vol.% of carbon, reflecting the continuous nature of the FLG film over very long distances
Interactive effects of metals and carbon nanotubes in a microcosm agrosystem
Get PDFAgricultural soils are exposed to multiple contaminants through the use of agrochemicals or sewage sludge, introducing metals, nanomaterials and others. Among nanomaterials, carbon nanotubes (CNTs) are known for their large surface area and adsorption capabilities, possibly modifying other element behavior. However, to date, very little is known about the impacts of such interactions in agrosystems. In this study, we aimed at understanding the transfer and toxicity of contaminants (Cd, Pb, Zn and CNTs) in microcosms including native soil bacteria, earthworms and lettuce. After a 6 week exposure, no effect of the addition of CNTs to metal contaminated soils was detected on bacterial concentration or earthworm growth. However, in lettuce, an interactive effect between CNTs and metals was highlighted: in the soil containing the highest metal concentrations the addition of 0.1 mg kg−1 CNTs led to a biomass loss (−22%) and a flavonoid concentration increase (+27%). In parallel, the addition of CNTs led to differential impacts on elemental uptake in lettuce leaves possibly related to the soil organic matter content. For earthworms, the addition of 10 mg kg−1 CNTs resulted in an increased body elemental transfer in the soil with the higher organic matter content (Pb: + 34% and Zn: + 25%)
Ageing of PEEK/Carbon Fibre composite under electronic irradiations: Influence on mechanical behaviour and charge transport
Get PDFPolymer used in satellite manufacturing are exposed to surface charging phenomenon due to electronic irradiations. This phenomenon can induce electrostatic discharges (ESD) which can lead to failures. In order to limit this phenomenon, we propose to develop PEEK composite reinforced with Short Carbon Fibres (SCF). We studied the behaviour of these composites in pristine state and after electronic irradiations compared to PEEK. In pristine state, fibres induce a faster surface electron relaxation. This behaviour shows that SCF can reduce ESD risks in space which is a first validation of the concept. After irradiations, PEEK ageing mechanisms are not modified by the presence of fibres. However, SCF stabilise both the evolution of mechanical behaviour and surface potential relaxation compared to irradiated PEEK. This stabilisation comes from two contributions: fibres stabilise the ageing of the matrix and hides the influence of this ageing on composite mechanical and electrical behaviours
The Nearest Is Not The Fastest : On The Importance Of Selecting In/Out Routing Hops Over A Satellite LEO Constellation
Get PDFThis study investigates the importance of choosing the first (respectively last) hop to access (respectively to exit) a
Low Earth Orbit (LEO) satellite constellation, which is of upmost importance for the LEO routing performance. Usually, basic routing strategies connect a ground station to its nearest satellite, and this strategy does not always lead to the optimal routing path.
We propose to select this first/last satellites within a subset of knearest satellites. After performing routing simulations over one of the next-generation satellite constellations, preliminary results show that this in/out hop selection strategy leads to a better link capacity usage and a lower data loss rate, allowing a faster TCP bulk data transfer
Task and Memory Mapping Optimization for SDRAM Interference Minimization on Heterogeneous MPSoCs
Get PDFDDR SDRAM memories are resources commonly used on multicore platforms and hence, being a main source of interference. To deal with this issue, we propose a methodology based on task/memory mapping optimization through multi-objective heuristic-based algorithms. By placing the tasks on the platform cores and the memory in the DDR SDRAM banks, we minimize the DDR SDRAM interference while considering other aspects such as the task execution parallelism and deadline margin. To evaluate the fitness of the task/memory map, the optimization algorithms make use of cost function equations. In order to compute the DDR memory interference cost, we use a fast executing self-designed cost function. The execution parallelism is computed using the workload variance cost function. The deadline margin of a task is computed considering the inter and intra core interference. The task/memory mapping outcomes are checked through tests for which the heterogeneous MPSoCs Keystone II and Sitara AM5728 are used. To assure certification, the WCET constraints of the resulting near-optimal Pareto solutions are verified through formally validated bounding frameworks
Probabilistic gain, phase and disk margins with application to AOCS validation
Get PDFMonte-Carlo simulations play a key role in most Verification and Validation (V&V) processes. It is however time-consuming and may fail to detect rare worst-case configurations. In such cases, when applicable, µ-analysis offers a nice alternative but does not provide any quantification of the probability of occurrence of the identified worst-cases. A control system can then be invalidated on the basis of unlikely events. Probabilistic µ-analysis was introduced in this context 20 years ago to bridge the gap between the two techniques, but until recently no practical tools were available. This paper summarizes recent advances on this topic. A practical algorithm for probabilistic gain, phase and disk margins analysis is first proposed, and then applied to a satellite high pointing system involving uncertain flexible modes
Phosphosilicate Multimode Optical Fiber for Sensing and Diagnostics at Inertial Confinement Fusion Facilities
Get PDFWe characterized the radiation response in the visible domain of a new multimode graded-index (GI) phosphosilicate optical fiber (GIMMF), exposed to the harsh environment (pulses of 14-MeV neutrons, X-rays, and γ -rays) associated with laser experiments at the OMEGA facility. The growth of permanent radiation-induced attenuation (RIA) was measured in situ after a series of laser shots involving a large production of 14-MeV neutrons (yields > 10^14 n per shot). RIA linearly increases with accumulated neutron fluence without recovery between shots. The obtained results allow a precise evaluation of this GIMMF vulnerability when implemented as part of laser or plasma diagnostics. Our work also reveals the potential of this class of optical fiber to serve as a radiation monitor in the radiation-rich mixed environments of megajoule class laser facilities and to provide a very fast and online estimation of the accumulated deposited dose at various locations of their experimental halls. In our experimental test configuration at OMEGA, 14-MeV neutrons are estimated to contribute to about 55% of the total deposited dose on the fibers, and the other optical losses are related to X-ray and γ -ray contributions. Those measurements could be, for example, benchmarked to the radiation maps obtained by Monte Carlo simulation tools, potentially facilitating the evaluation of the aging of diagnostics, components, and systems as well as their maintenance operations
Influence of rans turbulent inlet set-up on the swirled hot streak redistribution in a simplified nozzle guide vane passage: comparisons with large-eddy simulations
No full textA high-pressure turbine is known to be a complex component
of a gas turbine from a thermo-mechanical point of view. In
modern lean-burn combustion chambers, this complexity is enhanced
with the presence of hot streaks together with swirl components
coming from the combustion chamber. These radial and
azimuthal velocities and temperature distortions have a strong
impact either on the aerodynamics inside the high-pressure turbine
or on the aero-thermal behaviour of the vanes and blades. It
is thus clear that swirled hot streaks must be taken into account
in the early development of a high-pressure turbine by using numerical
simulations. Regarding the level of turbulence, that can
reach extreme values (25%-30% at the turbine inlet), its impact
on the transport of hot streaks is not fully understood or is still
under study, either experimentally or numerically.
From a practical point of view, in classical Reynolds-Averaged
Navier-Stokes (RANS) methods, where all the turbulence is modelled,
imposing the right turbulence at the inlet boundary condition
is not an easy task. In this paper, the redistribution of a
swirled hot streak in a bended duct is studied. This work is focused
on turbulence modelling. High-fidelity Large-Eddy Simulation
(LES) results are used as reference data to validate different
RANS set-ups to predict the hot streak redistribution in terms of migration and diffusion.
Results show that in a RANS approach, imposing the turbulent
quantities from a LES causes an immediate destruction of the
swirl components and a too high total temperature diffusion. It
is found in this study that the turbulent length scale, expressed
in terms of mT =m, plays a major role in the aerodynamic and
aero-thermal behaviour of the flow. The optimal range for the
value of mT =m found is different from what is encountered in the
literature on a high-pressure turbine configuration, which could
be attributed to the anisotropy of the turbulence. By imposing
consistent quantities at the inlet, both the trajectory of the swirl
jet and the total temperature distribution can be captured