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Influence of tip shroud cavities on low-pressure turbine main flowat design and off-design conditions
Get PDFA lot of studies on turbomachinery main flow optimisation have been performed in order
to reach actual efficiency level of modern gas turbines. To go further in the study of
aerodynamic losses sources, a better understanding on technological effects is required.
Tip shroud cavities in low pressure turbine is an example. Indeed, the by-pass flow causes
additional pressure losses. In addition, interactions between main flow and cavity flows,
as well as the re-entering flow, cause mixing losses and modifications of flow angle. This
paper investigates the contribution of tip shroud cavities in a low pressure turbine stage
on flow structures using (Unsteady) Reynolds Averaged Navier-Stokes simulations. The
ability of a steady simulation to predict the overall performance and flow physics of this
kind of flow is well documented in the literature but time-resolved simulations are needed
to deepen the analysis. This is an objective of this paper. Following the presentation of the
configuration under investigation, an analysis of flow structures is made in the upstream
region of the rotor, close to the shroud. After that, simulations at off-design conditions are
studied in order to evaluate this impact on the previous mechanisms
Nitrate and nitrite bacterial reduction at alkaline pH and high nitrate concentrations, comparison of acetate versus dihydrogen as electron donors
Get PDFThis study assesses bacterial denitrification at alkaline pH, up to 12, and high nitrate concentration, up to 400 mM. Two types of electron donors organic (acetate) and inorganic (dihydrogen) were compared. With both types of electron donors, nitrite reduction was the key step, likely to increase the pH and lead to nitrite accumulation. Firstly, an acclimation process was used: nitrate was progressively increased in three cultures set at pH 9, 10, or 11. This method allowed to observe for the first time nitrate reduction up to pH 10 and 100 mM nitrate with dihydrogen, or up to pH 10 and 400 mM nitrate with acetate. Nitrate reduction kinetics were faster in the presence of acetate. To investigate further the impact of the type of electron donor, a transition from acetate to dihydrogen was tested, and the pH evolution was modelled. Denitrification with dihydrogen strongly increases the pH while with acetate the pH evolution depends on the initial pH. The main difference is the production of acidifying CO2 during the acetate oxidation. Finally, the use of long duration cultures with a highly alkaline pH allowed a nitrate reduction up to pH 11.5 with acetate. However, no reduction was possible in hydrogenotrophy as it would have increased the pH further. Instead, bacteria used organic matter from inoculum to reduce nitrate at pH 11.5. Therefore, considering bacterial denitrification in a context of alkaline pH and high nitrate concentration an organic electron donor such as acetate is advantageous
Sediment reworking by the burrowing polychaete Hediste diversicolor modulated by environmental and biological factors across the temperate North Atlantic. A tribute to Gaston Desrosiers
Get PDFParticle mixing and irrigation of the seabed by benthic fauna (bioturbation) have major impacts on ecosystem functions such as remineralization of organic matter and sediment-water exchange. As a tribute to Prof. Gaston Desrosiers by the Nereis Park association, eighteen laboratories carried out a collaborative experiment to acquire a global snapshot of particle reworking by the polychaete Hediste diversicolor at 16 sites surrounding the Northern Atlantic. Organisms and soft sediments were collected during May – July at different geographical locations and, using a common laboratory protocol, particulate fluorescent tracers (‘luminophores’) were used to quantify particle transport over a 10-day period. Particle mixing was quantified using the maximum penetration depth of tracers (MPD), particle diffusive coefficients (Db), and non-local transport coefficients (r). Non-local coefficients (reflecting centimeter scale transport steps) ranged from 0.4 to 15 yr−1, and were not correlated across sites with any measured biological (biomass, biovolume) or environmental parameters (temperature, grain size, organic matter). Maximum penetration depths (MPD) averaged ~10.7 cm (6.5–14.5 cm), and were similar to the global average bioturbation depth inferred from short-lived radiochemical tracers. MPD was also not correlated with measures of size (individual biomass), but increased with grain size and decreased with temperature. Biodiffusion (Db) correlated inversely with individual biomass (size) and directly with temperature over the environmental range (Q10 ~ 1.7; 5–21 °C). The transport data were comparable in magnitude to rates reported for localized H. diversicolor populations of similar size, and confirmed some but not all correlations between sediment reworking and biological and environmental variables found in previous studies. The results imply that measures of particle reworking activities of a species from a single location can be generally extrapolated to different populations at similar conditions
Engineering structure and functionalities of chemical vapor deposited photocatalytic titanium dioxide films through different types of precursors
Get PDFThe photocatalytic properties of titanium dioxide TiO2 thin films, a seminal semiconductor material in solar radiation involving key enabling technologies depend on their structural characteristics which, in turn are monitored by the type and the conditions of the applied deposition technique. In this work, we investigate the physicochemical characteristics of two solid precursors, oxo-β-diketonate TiO(thd)2 (1) and mixed alkoxide-β-diketonate Ti(thd)2(OBu)2 (2), for the chemical vapor deposition (CVD) of TiO2 films as alternatives to the established liquid titanium(IV) isopropoxide (TTIP). We show that vaporization of 1 results in a complex mixture of intermediate species, while that of 2 occurs congruently. Direct liquid injection CVD from 1, 2, and TTIP at 500 °C results in anatase containing, rutile rich two phase films for the first two, and to pure anatase for the latter. Films deposited from 1 and 2 are composed of large grains with limited porosity, with smaller and more densely packed aggregates for the former, which also presents a higher O/Ti ratio in the O–Ti lattice. These differences account for a higher generation rate of H2 of films from 1, in the photocatalytic decomposition of water. This comparative experimental information can be useful to build new models for extended and more reliable predictions in a materials by design approach
Exploring the dew retting feasibility of hemp in very contrasting European environments: Influence on the tensile mechanical properties of fibres and composites
Get PDFRetting of fibrous plants such as flax is an essential step in the extraction of fibre bundles and their transformation into textiles and reinforcement fabrics for use in garments and composites. Dew-retting is traditionally performed from Northwest France to the Netherlands, as the climate is highly favourable for this process. Hemp is a plant that can be grown almost all over Europe with a low environmental impact. A retting step is also required to facilitate the separation of the hemp fibres before their transformation into textiles for garments or for 1D to 3D reinforcement composites, which requires thoroughly separated fibres. Dew-retting is currently used in flax production zones. The present work demonstrates that dew retting can be conducted under different climates on different soils, from dry Mediterranean environments up to the cooler regions of eastern France. If the ternary combination of moisture, temperature and solar radiation is appropriate, field retting (dew-retting) can be as short as about three weeks. In less favourable conditions, such as in dryer areas or when retting is performed late in the season after seed maturity (cooler temperatures), it lasts longer, but it can reach suitable levels. When conducted with care and with proper monitoring of the retting level, the dew-retting process does impact neither the tensile properties of elementary hemp fibres (by degrading crystalline cellulose I) nor the tensile properties of unidirectional and injected composite materials. Consequently, if extracted with a suitable process such as scutching and hackling, fibres suitable for load-bearing composites can be produced from dewretted hemp stems produced in a wide range of climates and locations, therefore not limited to the conventional “dew retting zone” of flax production areas
Experimental investigation of low Reynolds number rotor noise
Get PDFIn this paper, an experimental characterisation of low Reynolds number rotors is performed in an anechoic room.
Two commercially available two-bladed rotors as well as four three-dimensional (3D)-printed rotors with different
numbers of blades (from two to five) are tested. The latter have canonical geometry, with an NACA0012 blade
section profile, extruded in the radial direction with constant chord and constant 10° pitch. The experimental setup
and the 3D printing strategy are first validated using results from the literature for the commercially available rotors.
For all the tested rotors, four noise characteristics are analysed: the overall sound pressure level (OASPL), the amplitude
of the blade passing frequency (BPF), and the amplitude of its first harmonic and the high-frequency broadband
noise. For all the rotors, an increase in all noise characteristics is observed with the rotational speed (rpm) for all
directivity angles. Moreover, an interesting change of pattern is observed for the amplitudes of the BPF and of its
first harmonic, with, in the vicinity of the rotor plane, a minimum value for low rpm and/or high number of blades,
and a maximum value for high rpm and/or low number of blades. This change in directivity leads to a similar change
of directivity of the OASPL. For the broadband noise, a dipole-like pattern is obtained with a minimum value at
h ¼ �10°, i.e., aligned with the trailing edge and thus indicating the generation of trailing edge noise. Finally,
scaling laws that characterise the amplitude of the different noise components with respect to the rpm are proposed
Heterogeneous multicore SDRAM interference analysis
Get PDFThe purpose of this paper is to describe a set of DDR3 SDRAM
interference estimation cost functions. The arbitration system of
the SDRAM controller heavily impact the interference analysis.
In this work, three arbitration are considered, corresponding to the situations where the accessed memory address belongs to the same block address, different memory banks and different rows. The aim of these functions is to estimate the instructions interference overhead may suffer when concurrently accessing these three logical addresses in a SDRAM saturation context. To develop these interference expressions, specific measurement systems, micro-benchmarks and theory on SDRAM controllers have been used
Role of Pond Sediments for Trapping Pesticides in an Agricultural Catchment (Auradé, SW France): Distribution and Controlling Factors
Get PDFIn agricultural areas, ponds are suitable wetland environments to dissipate and reduce the occurrence of pesticides in aquatic environments. However, their impact at a catchment scale is still poorly understood. This study aims to determine how these organic contaminants were trapped in a pond located in an agricultural critical zone from SW France (Auradé catchment). The spatial distribution of pesticide concentrations and their different controlling factors were investigated in waters and sediments collected during two distinct seasons. The results highlighted (i) the link between the presence of the molecules and the agricultural practices upstream, (ii) the influence of hydrological/seasonal conditions, especially on hydrophobic molecule accumulation such as tebuconazole, (iii) the key role of clay content in sediments on the control of moderately hydrophilic pesticides (metolachlor and boscalid), but also the unexpected role of coarse particles for boscalid; and (iv) the influence of sediment depth on pesticide storage. Nevertheless, other physico-chemical parameters, such as mineralogical composition of sediment, needed to be considered to explain the pesticide patterns. This study brings a new hypothesis to be investigated in the future about pesticide behaviour in such pond environments
Engineering of Microcage Carbon Nanotube Architectures with Decoupled Multimodal Porosity and Amplified Catalytic Performance
Get PDFNew approaches for the engineering of the 3D microstructure, pore modality, and chemical functionality of hierarchically porous nanocarbon assemblies are key to develop the next generation of functional aerogel and membrane materials. Here, interfacially driven assembly of carbon nanotubes (CNT) is exploited to fabricate structurally directed aerogels with highly controlled internal architectures, composed of pseudo-monolayer, CNT microcages. CNT Pickering emulsions enable engineering at fundamentally different length scales, whereby the microporosity, mesoporosity, and macroporosity are decoupled and individually controlled through CNT type, CNT number density, and process energy, respectively. In addition, metal nanocatalysts (Cu, Pd, and Ru) are embedded within the architectures through an elegant sublimation and shock-decomposition approach; introducing the first approach that enables through-volume functionalization of intricate, pre-designed aerogels without microstructural degradation. Catalytic structure–function relationships are explored in a pharma-important amidation reaction; providing insights on how the engineered frameworks enhance catalyst activity. A sophisticated array of advanced tomographic, spectroscopic, and microscopic techniques reveal an intricate 3D assembly of CNT building-blocks and their influence on the functional properties of the enhanced nanocatalysts. These advances set a basis to modulate structure and chemistry of functional aerogel materials independently in a controlled fashion for a variety of applications, including energy conversion and storage, smart electronics, and (electro)catalysis
A local thermal non-equilibrium model for coupled heat and mass transfer with dispersion and thermal diffusion in porous media
Get PDFA theoretical model is proposed to predict Darcy-scale mass transport in porous media coupled with non-equilibrium heat transfer and taking into account the thermal diffusion process. A volume-averaging technique was used with approximations leading to a two-equation or two-temperature model for the macro-scale energy balance equations. Because of Soret effect, the concentration deviation with respect to the averaged value is a function of concentration and temperature gradients in the fluid phase, the temperature gradient in the solid phase, and the difference between the two averaged temperatures. The mapping between deviations and averages involves four closure problems for the mass transport equations: problems which were solved numerically over a two-dimensional periodic-unit cell for evaluation purposes. The results show that the effective coefficients depend strongly on the thermo-physical properties of the medium and the Péclet number. In particular, the effective-Soret coefficient in porous media changes with the Péclet number and the phases thermal conductivity ratio