1,690 research outputs found
Fermionic semi-annihilating dark matter
No full textAbstract: Semi-annihilation is a generic feature of dark matter theories with symmetries larger than ℤ2. We investigate two examples with multi-component dark sectors comprised of an SU(2)L singlet or triplet fermion besides a scalar singlet. These are respectively the minimal fermionic semi-annihilating model, and the minimal case for a gauge-charged fermion. We study the relevant dark matter phenomenology, including the interplay of semi-annihilation and the Sommerfeld effect. We demonstrate that semi-annihilation in the singlet model can explain the gamma ray excess from the galactic center. For the triplet model we scan the parameter space, and explore how signals and constraints are modified by semi-annihilation. We find that the entire region where the model comprises all the observed dark matter is accessible to current and planned direct and indirect searches. © 2016, The Author(s)9911Nsciescopu
A systematic effective operator analysis of semi-annihilating dark matter
No full textSemi-annihilation is a generic feature of dark matter theories stabilized by symmetries larger than a ℤ2. It contributes to thermal freeze out, but is irrelevant for direct and collider searches. This allows semi-annihilating dark matter to avoid those limits in a natural way. We use an effective operator approach to make the first model-independent study of the associated phenomenology. We enumerate all possible operators that contribute to 2 → 2 semi-annihilation up to dimension 6, plus leading terms at dimension 7. We find that when the only light states charged under the dark symmetry are dark matter, the model space is highly constrained. Only fifteen operators exist, and just two for single-component dark sectors. If there can be additional light, unstable “dark partner” states the possible phenomenology greatly increases, at the cost of additional model dependence in the dark partner decay modes. We also derive the irreducible constraints on models with single-component dark matter from cosmic ray searches and astrophysical observations. We find that for semi-annihilation to electrons and light quarks, the thermal relic cross sections can be excluded for dark matter masses up to 100 GeV. However, significant model space for semi-annihilating dark matter remains. © 2017, The Author(s)2111Nsciescopu
Influence of droplet spatial distribution on spray evaporation
No full textIn aero-engines, fuel is injected as a liquid which involves two-phase flow combustion. Consequently, different phenomena such as atomization, droplet dispersion by turbulence or spray evaporation impact combustion processes. In order to study spray combustion, an experimental test rig has been developed at ONERA to partially feature the flow conditions inside the combustion chamber of a turbo-reactor. Experimental campaigns have been conducted in non-reactive and reactive conditions to obtain an experimental database. The present paper focuses on the correlation between droplet density and the nearest-neighbour droplet distance obtained from the analysis of Mie scattering images under non-reactive conditions. Results show that the nearest-neighbour droplet distance varies linearly with the inverse square-root of mean spray density. Findings are compared with the theoretical law of Hertz-Chandrasekhar and the regular arrangement law
Long-lived, colour-triplet scalars from unnaturalness
No full textAbstract: Long-lived, colour-triplet scalars are a generic prediction of unnatural, or split, composite Higgs models where the spontaneous global-symmetry breaking scale f ≳ 10 TeV and an unbroken SU(5) symmetry is preserved. Since the triplet scalars are pseudo NambuGoldstone bosons they are split from the much heavier composite-sector resonances and are the lightest exotic, coloured states. This makes them ideal to search for at colliders. Due to discrete symmetries the triplet scalar decays via a dimension-six term and given the large suppression scale f is often metastable. We show that existing searches for collider-stable R-hadrons from Run-I at the LHC forbid a triplet scalar mass below 845 GeV, whereas with 300 fb−1 at 13 TeV triplet scalar masses up to 1.4 TeV can be discovered. For shorter lifetimes displaced-vertex searches provide a discovery reach of up to 1.8 TeV. In addition we present exclusion and discovery reaches of future hadron colliders as well as indirect limits that arise from modifications of the Higgs couplings. © 2016, The Author(s)6511Nsciescopu
CFD Analysis of Fuel Atomization, Secondary Droplet Breakup and Spray Dispersion in the Premix Duct of a LPP Combustor
No full textThe two phase flow in the premix duct of a LPP combustor is computed using a Lagrangian droplet tracking method. To reproduce the characteristic spray structure of an air-assisted pressure-swirl atomizer, a sheet spray model is de-rived from measured sheet parameters and combined with an advanced concept for modeling secondary atomization effects. The sheet spray model is used for the discretization of the initial sheet fragmentation, whereas modeling of droplet deformation and breakup during trajectory integration accounts for air-blast effects on size distribution and dispersion of the spray. Droplet drag increase due to subcritical deformation is approximated by a semi-empirical correlation. Breakup modeling addresses the three distinct mechanisms bag, multimode and shear breakup which are typically encountered in fuel-air mixing processes of combustion engines. The models include mechanism-dependent deformation and breakup times, correlations for drag increase as well as size and velocity distribution functions of droplet fragments. Results of the simulation are compared to Phase Doppler measurements of the spray providing local distributions of droplet sizes, velocities and size-dependent liquid volume flux.AWEPAerospace Engineerin
Advances in Corrosion-Resistant Thermal Spray Coatings for Renewable Energy Power Plants : Part II - Effect of Environment and Outlook
No full textHigh-temperature corrosion of critical components such as water walls and superheater tubes in biomass/waste-fired boilers is a major challenge. A dense and defect-free thermal spray coating has been shown to be promising to achieve a high electrical/thermal efficiency in power plants. The field of thermal spraying and quality of coatings have been progressively evolving; therefore, a critical assessment of our understanding of the efficacy of coatings in increasingly aggressive operating environments of the power plants can be highly educative. The effects of composition and microstructure on high-temperature corrosion behavior of the coatings were discussed in the first part of the review. The present paper that is the second part of the review covers the emerging research field of performance assessment of thermal spray coatings in harsh corrosion-prone environments and provides a comprehensive overview of the underlying high-temperature corrosion mechanisms that lead to the damage of exposed coatings. The application of contemporary analytical methods for better understanding of the behavior of corrosion-resistant coatings is also discussed. A discussion based on an exhaustive review of the literature provides an unbiased commentary on the advanced accomplishments and some outstanding issues in the field that warrant further research. An assessment of the current status of the field, the gaps in the scientific understanding, and the research needs for the expansion of thermal spray coatings for high-temperature corrosion applications is also provided. © 2019, The Author(s)
Planar droplet sizing: Application to a spray of Jet A1 kerosene
No full text[EN] Optical techniques are widely employed for their non-intrusive behavior and are applied to two-phase flow
investigations. Until now, the most commonly used technique to determine the droplet size is the Phase Doppler
Anemogranulometry, although it is time consuming for an overall injector characterization. An imaging technique
called Planar Droplet Sizing has been used to offer an alternative and provide a spatially-resolved 2D map of the
Sauter Mean Diameter (SMD). The measurement is based on the ratio between laser-induced fluorescence and
scattered light intensities which are assumed to be proportional respectively to the droplet volume and droplet
surface area. However, previous studies revealed that the dependence of fluorescence intensity on the droplet
volume can be altered by the absorption of light in the liquid. The scattered light intensity depends on the
scattering angle and intensity variations within the field of view must be avoided.
The aim of this study is to make the PDS technique operational for a Jet A-1 kerosene spray. A strong absorption
of liquid kerosene appears under UV excitation at 266 nm making the technique unsuitable. Under visible
excitation at 532 nm, a fluorescent tracer (Pyrromethene 597) must be added to the kerosene to enhance the
fluorescence signal. To prevent scattered light intensity variations within the field of view, an optimal scattering
angle close to 115° is required. An image processing algorithm is proposed in order to reduce the effects of
multiple scatteringThis work is conducted in cooperation between ONERA and SAFRAN AIRCRAFT ENGINES (Safran Group). The authors gratefully acknowledge the ANRT (Association Nationale de la Recherche et de la Technologie) which supports part of the PhDDoublet, P.; Lempereur, C.; Bodoc, V.; Orain, M.; Gajan, P. (2017). Planar droplet sizing: Application to a spray of Jet A1 kerosene. En Ilass Europe. 28th european conference on Liquid Atomization and Spray Systems. Editorial Universitat Politècnica de València. 653-660. https://doi.org/10.4995/ILASS2017.2017.4698OCS65366
Experimental study of spray from wave impact
No full textOvertopping plumes from wave impact is relevant to coastal defence for overtopping analysis of sea walls, levees, and gates. Improved insight into this phenomenon will further enhance the prediction of wave overtopping and its induced hazard, e.g., erosion, saltwater ingress, and a hindrance to traffic. A series of small-scale experiments have been carried out in the WaterLab at TU Delft to characterize the droplets formed by wave impacts. Focused waves were generated by the piston-type wavemaker to control the wave breaking point on the wall, which allowed the creation of different types of wave impact. Impacts were investigated respectively: non-breaking, flip-through, and air pocket. After the wave impact, all the stages of the plume formation were filmed using a high-speed camera at a frame rate of 500fps. In this study, the spray sheet breakup and droplet formation are investigated. A simple approach to estimate the maximum spray height is proposed, which can be used for the splash type overtopping in the future.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Hydraulic Structures and Flood Ris
THREE NEW SPECIES OF THE GENUS CLOSTRIDIUM
No full textAt the last meeting of our society in Philadelphia a brief description was pre-sented of three interesting species of the genus Clostridium (Spray, 1947) that had apparently not been previously identified. To these were assigned names pre-sumed to be suitable to their morphologic and physiologic characters. Time did not permit their full description, and it is the intention to give here more detail, following the form used in Bergey's Manual of Determinative Bacteriology. In addition to the routine description certain reactions reported by the author (Spray, 1936, 1937) are included. These three species were isolated from the sources indicated below, by methods given in the previously mentioned publications, including various procedures of enrichment, with subsequent use of the anaerobic dish proposed by the author (Spray, 1930). CLOSTRIDIUM NAUSEUM N. SP. This species was isolated thrice from topsoil of the University campus. Soil samples were shaken in tubes of sterile tap water and heated for 10 minutes a
Marine spray from wave run-up to a hull as input to icing models
No full textShips at sea will encounter deck wetness events that usually are just annoying and rarely lead to damage. Discomfort is experienced by spray, as it is limiting view and hampering deck work. In freezing conditions, deck wetting will lead to icing. This study is a continuation of publications regarding the development of a marine icing model based on spray predictions, taking into account ship shape and the physics of wave run-up leading to spray jets. The modelling of jet development from wave run-up against a wall, representing a ship hull, is investigated by experiments and mathematical modelling using nonlinear wave theory. Run-up jets occur frequently for ships at sea and are responsible for most of the generated spray. Detailed measurements of the run-up jet were obtained from high-speed video registration yielding information on the droplet distribution. The measurements indicate that the thickness of the run-up jet above the wall determines the maximum droplet size, and that this size is significantly larger than generally assumed in icing models. Based on these insights, new computations are performed with the ‘SHIPICE’ marine icing model to demonstrate the applicability and accuracy of the approach.Accepted Author ManuscriptOffshore Engineerin
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