4,694 research outputs found
Special Issue on Power Converter of Electric Machines, Renewable Energy Systems, and Transportation
On a new theoretical framework for RR Lyrae stars I: the metallicity dependence
We present new nonlinear, time-dependent convective hydrodynamical models of
RR Lyrae stars computed assuming a constant helium-to-metal enrichment ratio
and a broad range in metal abundances (Z=0.0001--0.02). The stellar masses and
luminosities adopted to construct the pulsation models were fixed according to
detailed central He burning Horizontal Branch evolutionary models. The
pulsation models cover a broad range in stellar luminosity and effective
temperatures and the modal stability is investigated for both fundamental and
first overtones. We predict the topology of the instability strip as a function
of the metal content and new analytical relations for the edges of the
instability strip in the observational plane. Moreover, a new analytical
relation to constrain the pulsation mass of double pulsators as a function of
the period ratio and the metal content is provided. We derive new
Period-Radius-Metallicity relations for fundamental and first-overtone
pulsators. They agree quite well with similar empirical and theoretical
relations in the literature. From the predicted bolometric light curves,
transformed into optical (UBVRI) and near-infrared (JHK) bands, we compute the
intensity-averaged mean magnitudes along the entire pulsation cycle and, in
turn, new and homogenous metal-dependent (RIJHK) Period-Luminosity relations.
Moreover, we compute new dual and triple band optical, optical--NIR and NIR
Period-Wesenheit-Metallicity relations. Interestingly, we find that the optical
Period-W(V,B-V) is independent of the metal content and that the accuracy of
individual distances is a balance between the adopted diagnostics and the
precision of photometric and spectroscopic datasets.Comment: 51 pages, 20 figures, 9 tables, accepted for publication on Ap
Synthesis of Pyrazoles by 1,3-Dipolar Cycloaddition under Aqueous Micellar Catalysis
Ethyl diazoacetate (EDA), which is easily prepared from ethyl glycinate and NaNO2, reacts in situ with alkynes in a water micelle environment without organic solvent to form pyrazoles. The reaction is pH dependent, as in the presence of protic catalysis (H2SO4 4%, pH 3.5) a mixture of 3,5- and 4,5-disubstituted pyrazoles was obtained, while, at pH 5.5, only the 3,5-disubstituted isomer was obtained. The presence of the surfactant TPGS-750-M was crucial to secure clean crude reaction mixtures and high yields of the products. The same protocol was successfully applied to the synthesis of substituted pyrazolines. © 2022 The Author
Transcritical CO2 commercial refrigeration plant with adiabatic gas cooler and subcooling via HVAC: field tests and modelling
Subcooling methods at the exit of the gas cooler in transcritical CO2 commercial refrigeration systems have been studied in the recent years showing that overall remarkable improvements can be obtained. Another strategy that results efficient is the use of evaporative systems at the gas cooler (adiabatic cooling) as it allows to significantly reduce the refrigerant quality at the liquid receiver and to lower the heat rejection pressure. In this work, a fully instrumented CO2 transcritical booster system with parallel compression, in operation in a small size supermarket in northern Italy, made available measured data of its performance when subcooling and/or adiabatic cooling are active. The plant operates in a mild climate, where it suffers operation at transcritical conditions for most of the year. Subcooling in this plant is performed by coupling the refrigeration system with the HVAC system. Taking advantage of experimental measurements, a model in the TRNSYS environment is validated and allows the prediction of the annual plant performance when these strategies are adopted. The adiabatic cooling showed to allow a significant reduction (about 10%) in the energy use, and makes unnecessary the use of a parallel compressor. Subcooling by the HVAC gives rise to a reduced saving (2.9 %) due to the absence of a dedicated mechanical subcooler, however it is almost comparable to parallel compression. These trends are confirmed in two other hot and humid climates
Effect of dedicated mechanical subcooler size and gas cooler pressure control on transcritical CO2 booster systems
Dedicated Mechanical Subcooling (DMS) is one of the most investigated and effective strategies applied to increase the performance of CO2 commercial refrigeration systems in transcritical operation. Further performance benefits can be obtained by a reduction of the gas cooler pressure of the main cycle at transcritical conditions. In this work the most important parameters for the design and operation of such a system, i.e. the DMS cooling capacity, the subcooling degree and the gas cooler pressure, are considered and their effect on the annual energy use of the plant is estimated in warm and hot climate conditions by means of a validated model. DMS is also compared to the parallel compression scheme and subcooling performed through a water chiller dedicated to HVAC. DMS results to be the most effective solution among those investigated, and the choice of the best design and operating parameters allows further energy saving and cost reduction
FIELD TESTS, MODEL VALIDATION AND PERFORMANCE OF A CO2 COMMERCIAL REFRIGERATION PLANT INTEGRATED WITH HVAC SYSTEM
A fully instrumented CO2 trans-critical booster system with parallel compression, in operation in a small size supermarket in northern Italy, made available measured data that allowed monitoring its performance.
The entire refrigeration system of the supermarket, i.e. the Commercial Refrigerating Unit, the refrigerated display cabinets and cold rooms, have been modelled in TRNSYS with in-house types for all components. The comparison with the field data available from monitoring the plant allowed to thoroughly calibrate the model, spanning the operation conditions of a whole year. The validated model is a reliable and powerful tool, usable to predict the plant performance in several conditions and to evaluate the feasibility and effectiveness of the integration with DHW/HVAC systems. Measurements and simulations show that, at mild climate conditions, it is possible and effective to fully provide HVAC and refrigeration duties with a single all-in-one unit properly designed and operated
Sersic galaxy with Sersic halo models of early-type galaxies: a tool for N-body simulations
We present spherical, non-rotating, isotropic models of early-type galaxies
with stellar and dark-matter components both described by deprojected Sersic
density profiles, and prove that they represent physically admissible stable
systems. Using empirical correlations and recent results of N-body simulations,
all the free parameters of the models are expressed as functions of one single
quantity: the total (B-band) luminosity of the stellar component.
We analyze how to perform discrete N-body realizations of Sersic models. To
this end, an optimal smoothing length is derived, defined as the softening
parameter minimizing the error on the gravitational potential for the
deprojected Sersic model. It is shown to depend on the Sersic index and on
the number of particles of the N-body realization.
A software code allowing the computations of the relevant quantities of one-
and two-component Sersic models is provided. Both the code and the results of
the present work are primarily intended as tools to perform N-body simulations
of early-type galaxies, where the structural non-homology of these systems
(i.e. the variation of the shape parameter along the galaxy sequence) might be
taken into account.Comment: 14 pages, 9 figures, 1 table, accepted for publication in PAS
Coupling a distributed grid based hydrological model and MM5 meteorological model for flooding alert mapping
International audienceThe increased number of extreme rainfall events seems to be one of the common feature of climate change signal all over the world (Easterlin et al., 2000; Meehl et al., 2000). In the last few years a large number of floods caused by extreme meteorological events has been observed over the river basins of Mediterranean area and they mainly affected small basins (few hundreds until few thousands of square kilometres of drainage area) . A strategic goal of applied meteorology is now to try to predict with high spatial resolution the segments of drainage network where floods may occur. A possible way to reach this aim is the coupling of meteorological mesoscale model with high resolution hydrological model. In this work few case studies of observed floods in the Italian Mediterranean area will be presented. It is shown how a distributed hydrological model, using the precipitation fields predicted by MM5 meteorological model, is able to highlight the area where the major floods may occur
The Carina Project. VI. The helium burning variable stars
We present new optical (BVI) time-series data for the evolved variable stars
in the Carina dwarf spheroidal galaxy. The quality of the data and the
observing strategy allowed us to identify 14 new variable stars. Eight out of
the 14 are RR Lyrae (RRL) stars, four are Anomalous Cepheids (ACs) and two are
geometrical variables. Comparison of the period distribution for the entire
sample of RRLs with similar distributions in nearby dSphs and in the Large
Magellanic Cloud indicates that the old stellar populations in these systems
share similar properties. This finding is also supported by the RRL
distribution in the Bailey diagram. On the other hand, the period distribution
and the Bailey diagram of ACs display significant differences among the above
stellar systems. This evidence suggests that the properties of intermediate-age
stellar populations might be affected both by environmental effects and
structural parameters. We use the BV Period--Wesenheit (PW) relation of RRLs
together with evolutionary prescriptions and find a true distance modulus of
20.09+/-0.07(intrinsic)+/-0.1(statistical) mag that agrees quite well with
similar estimates available in the literature. We identified four peculiar
variables. Taking into account their position in the Bailey diagram and in the
BV PW relation, two of them (V14, V149) appear to be candidate ACs, while two
(V158, V182) might be peculiar RRLs. In particular, the variable V158 has a
period and a V-band amplitude very similar to the low-mass RRL
---RRLR-02792---recently identified by Pietrzynski at al. (2012) in the
Galactic bulge.Comment: Accepted for publication in ApJ, publication date September 20, 2013.
31 pages, 7 figure, 3 tables. Figs. 3 and 4 corrected, references added,
minor correction
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