Indirect searches for dark matter through Standard Model products of its annihilation generally assume a cross-section which is dominated by a term independent of velocity (
-wave annihilation). ...However, in many DM models an
-wave annihilation cross-section is absent or helicity suppressed. To reproduce the correct DM relic density in these models, the leading term in the cross section is proportional to the DM velocity squared (
-wave annihilation). Indirect detection of such
-wave DM is difficult because the average velocities of DM in galaxies today are orders of magnitude slower than the DM velocity at the time of decoupling from the primordial thermal plasma, thus suppressing the annihilation cross-section today by some five orders of magnitude relative to its value at freeze out. Thus
-wave DM is out of reach of traditional searches for DM annihilations in the Galactic halo. Near the region of influence of a central supermassive black hole, such as Sgr A*, however, DM can form a localized over-density known as a "spike". In such spikes the DM is predicted to be both concentrated in space and accelerated to higher velocities, thereby allowing the
-ray signature from its annihilation to potentially be detectable above the background. We use the
Large Area Telescope to search for the
-ray signature of
-wave annihilating DM from a spike around Sgr A* in the energy range 10 GeV-600 GeV. Such a signal would appear as a point source and would have a sharp line or box-like spectral features difficult to mimic with standard astrophysical processes, indicating a DM origin. We find no significant excess of
rays in this range, and we place upper limits on the flux in
-ray boxes originating from the Galactic Center. This result, the first of its kind, is interpreted in the context of different models of the DM density near Sgr A*.
Here we investigate the structural evolution of TX100 micelles upon loading with several linear and cyclic alkanes by DLS, PGSE-NMR, 2D NOESY NMR, viscosity measurements, and molecular dynamic ...simulations. Our results confirm that TX100 alone forms spherical, onion-like micelles made of several partially interpenetrating surfactant layers where the polyethylene glycol chains are in contact with the tetramethyl-butyl-phenyl moieties. Loading with non-penetrating oils larger than decane induces a decrease in micellar size and hydration because the alkane molecules compete with both water and tetramethyl-butyl-phenyl groups for the polyethylene glycol chains. This results in the partial peeling of the "onion" and in the dehydration of polyethylene glycol chains so that the micelles increase in number and decrease in size upon alkane loading. In contrast, small and penetrable oils (mainly cyclo-alkanes) first swell the onion-like micelles (inducing an increase in size) and only above a critical oil/surfactant ratio does the oil induce the weakening of the multilayer structure and the dehydration of polyethylene glycol chains found in long linear alkanes.
Here we investigate the structural evolution of TX100 micelles upon loading with several linear and cyclic alkanes by DLS, PGSE-NMR, 2D NOESY NMR, viscosity measurements, and molecular dynamic simulations.
► Implementation of comprehensive CFD model of air-blown coal-fired updraft gasifier. ► Unsteady simulation of Wellman–Galusha gasifier. ► Time-dependent fresh coal introduction and ash removal ...considered. ► Analysis of time-dependent and averaged final syngas composition. ► Thermo-fluid dynamic analysis of gasifier during one coal-charge cycle.
A comprehensive CFD model has been developed to simulate the gasification process within an air-blown updraft coal gasifier. Updraft fixed bed gasification processes are characterized by complex behavior, since they involve different space- and time-dependent sub-processes where coal preheating, drying, de-volatilization and char reactions take place. Simplified models, such as non-dimensional ones, useful for preliminary gross mass and energy balance, are unable to correctly simulate the overall gasification phenomena and more sophisticated approaches are required.
In particular, CFD models could be used to describe in a detailed way the complex time- and space-dependent phenomena involved in the gasification process. Considering the high volume fraction of the solid phase, close to the packing condition, the Euler–Euler approach is required to model this multiphase flow. The solid phase is considered as a continua according to the kinetic and plastic theory of granular flows.
The operation of a Wellman–Galusha gasifier is investigated, considering a non-continuous loading of coal and extraction of the ash, with the aim of characterizing the space- and time-dependent behavior of the process.
The water gas shift reaction (WGSR) is an attractive process for enhancing the conversion of syngas carbon monoxide contents into carbon dioxide and hydrogen by means of steam in medium- and ...small-scale coal gasification power plants integrated with Carbon Capture and Storage (CCS) technologies.
Fixed bed gasifiers, in fact, may become an interesting solution for medium- and small-scale applications in distributed power generation, although a syngas with a low LHV (about 6–7MJ/kg), high N2 (40–45%) and low H2 content, is generated when air is used, as usual, as oxidant. Syngas LHV can be enhanced by using oxygen (or oxygen enriched air) instead of air. However, when oxygen is used an appropriate amount of diluent agent as additional steam, recirculated syngas or CO2 is fed into the gasifier to control gasification temperature. In the case of syngas recirculation, the gasification process produces a syngas characterized by a high CO2 content, suitable for a more efficient separation.
This paper concerns the experimental investigation and numerical simulation of carbon monoxide conversion and hydrogen generation from WGSR, when high CO2 content operating mixtures are considered. A baseline mixture was assumed that simulates the fuel gas produced by coal gasification in a small-scale air-blown updraft gasifier. During the experiments the fraction of N2 in the baseline mixture was gradually replaced with increasing CO2 fraction in order to investigate its effects of CO2 concentration on reaction and catalysts performances. An experimental apparatus, designed and built at the Department of Mechanical Engineering of the University of Cagliari (DIMECA), was used to investigate the main performance of WGSR, using high CO2-content operating mixtures and different catalyst types (Fe/Cr or Pt/Al for high temperature and Cu/Zn or Pt/Al for low temperature), and varying fundamental parameters, such as residence time of reactants in the catalyst bed, reaction temperature and H2O/CO ratio.
The experimental results are supported by an in-depth numerical analysis. Numerical simulations were carried out using zero-dimensional (CSTR-based equilibrium model), one-dimensional (PFR-based) and CFD two-dimensional computational models to characterize the spatial and time-dependent behavior of the process.
The efficiency of a pump for engine cooling system in automotive sector can be very low (15%-20%) during the homologation cycle which is more oriented to medium and low engine loads. Actual pump ...technology makes reference always to centrifugal pumps, which suffer in terms of efficiency when the speed changes as well as when head and flow rate delivered. In order to reduce the power absorbed by the pump, a different type is needed.
A sliding vane rotary pump (SVRP) is a serious alternative having all the characteristics to fulfil the engine cooling circuit with high efficiency and reliability. In this work, a SVRP has been designed, built and tested for an existing engine cooling circuit: its performances were compared to the traditional (centrifugal) pump which today is mounted on that engine. The benefits over the homologation cycle in terms of mechanical energy and CO2 saving have been emulated thanks to a comprehensive mathematical model.
To verify incidence rates and their temporal trend in a homogeneous, ethnically, and genetically distinct population of central Sardinia (the Nuoro province).
Intensive epidemiologic studies carried ...out in Sardinia since the 1970s have suggested that the prevalence and incidence of MS are much higher in this Mediterranean island compared with those found on mainland Italy.
The study area had a population of approximately 274, 000 people in the 1991 census. The authors adopted a complete enumerative approach by reviewing all possible sources of case collection available in the investigative area.
Based on 469 MS patients, the mean annual incidence for 1955 to 1995 was 4.18 per 100,000 (or 4.3 per 100,000 if age- and sex-adjusted to the European population). The incidence, averaging 1.95 per 100,000 during 1955 to 1959, rose progressively over time, reaching rates of 6.6 in the quinquiennium 1985 to 1989 and 6.4 per 100,000 in 1990 to 1995. On December 31, 1994, the crude prevalence, based on 415 MS patients alive in the study area, was 151.9 per 100,000 (156.6 if adjusted to the European population).
These incidence and prevalence rates are the highest to date that have been estimated for a large community in southern Europe, and they constitute some of the highest rates in the world. Based on other surveys, these results reinforce the position of Sardinia as a higher and rising prevalence area for MS compared with other Mediterranean populations. Genetic and social-historic data strengthen the hypothesis of the environmental role and genetic factors among Sardinians in determining the notable difference in MS frequency between Sardinians and other Mediterraneans.
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