1,831 research outputs found
Internal coating of air-cooled gas turbine blades
Four modified aluminide coatings were developed for IN-792 + Hf alloy using a powder pack method applicable to internal surfaces of air-cooled blades. The coating compositions are Ni-19Al-1Cb, Ni-19Al-3Cb, Ni-17Al-20Cr, and Ni-12Al-20Cr. Cyclic burner rig hot corrosion (900 C) and oxidation (1050 C) tests indicated that Ni-Al-Cb coatings provided better overall resistance than Ni-Al-Cr coatings. Tensile properties of Ni-19Al-1Cb and Ni-12Al-20Cr coated test bars were fully retained at room temperature and 649 C. Stress rupture results exhibited wide scatter around uncoated IN-792 baseline, especially at high stress levels. High cycle fatigue lives of Ni-19Al-1Cb and Ni-12Al-20Cr coated bars (as well as RT-22B coated IN-792) suffered approximately 30 percent decrease at 649 C. Since all test bars were fully heat treated after coating, the effects of coating/processing on IN-792 alloy were not recoverable. Internally coated Ni-19Al-1Cb, Ni-19Al-3Cb, and Ni-12Al-20Cr blades were included in 500-hour endurance engine test and the results were similar to those obtained in burner rig oxidation testing
Fiducial Stellar Population Sequences for the u'g'r'i'z' System
We describe an extensive observational project that has obtained high-quality
and homogeneous photometry for a number of different Galactic star clusters
(including M 92, M 13, M 3, M 71, and NGC 6791) spanning a wide range in
metallicity (-2.3<[Fe/H]<+0.4), as observed in the u'g'r'i'z' passbands with
the MegaCam wide-field imager on the Canada-France-Hawaii Telescope. By
employing these purest of stellar populations, fiducial sequences have been
defined from color-magnitude diagrams that extend from the tip of the red-giant
branch down to approximately 4 magnitudes below the turnoff: these sequences
have been accurately calibrated to the standard u'g'r'i'z' system via a set of
secondary photometric standards located within these same clusters.
Consequently, they can serve as a valuable set of empirical fiducials for the
interpretation of stellar populations data in the u'g'r'i'z' system.Comment: 16 pages, 7 tables, 13 figures; accepted for publication in A
Internally coated air-cooled gas turbine blading
Ten candidate modified nickel-aluminide coatings were developed using the slip pack process. These coatings contain additives such as silicon, chromium and columbium in a nickel-aluminum coating matrix with directionally solidified MAR-M200 + Hf as the substrate alloy. Following a series of screening tests which included strain tolerance, dynamic oxidation and hot corrosion testing, the Ni-19A1-1Cb (nominal composition) coating was selected for application to the internal passages of four first-stage turbine blades. Process development results indicate that a dry pack process is suitable for internal coating application resulting in 18 percent or less reduction in air flow. Coating uniformity, based on coated air-cooled blades, was within + or - 20 percent. Test results show that the presence of additives (silicon, chromium or columbium) appeared to improve significantly the ductility of the NiA1 matrix. However, the environmental resistance of these modified nickel-aluminides were generally inferior to the simple aluminides
The origin of the LMC stellar bar: clues from the SFH of the bar and inner disk
We discuss the origin of the LMC stellar bar by comparing the star formation
histories (SFH) obtained from deep color-magnitude diagrams (CMDs) in the bar
and in a number of fields in different directions within the inner disk. The
CMDs, reaching the oldest main sequence turnoffs in these very crowded fields,
have been obtained with VIMOS on the VLT in service mode, under very good
seeing conditions. We show that the SFHs of all fields share the same patterns,
with consistent variations of the star formation rate as a function of time in
all of them. We therefore conclude that no specific event of star formation can
be identified with the formation of the LMC bar, which instead likely formed
from a redistribution of disk material that occurred when the LMC disk became
bar unstable, and shared a common SFH with the inner disk thereafter. The
strong similarity between the SFH of the center and edge of the bar rules out
significant spatial variations of the SFH across the bar, which are predicted
by scenarios of classic bar formation through buckling mechanisms.Comment: MNRAS Letters, accepte
Luminosity and mass function of galactic open clusters: I. NGC 4815
We present deep V and I photometry for the open cluster NGC 4815 and four
surrounding Galactic fields down to a limiting magnitude . These data
are used to study cluster spatial extension by means of star counts, and to
derive the luminosity (LF) and mass function (MF). The radius turns out to be
at V=19.0 level, whereas the mass amounts at down to V=20.8. From the color-magnitude diagram, we obtain the LFs
in the V and I bands, using both the standard histogram and an adaptive kernel.
After correction for incompleteness and field star contamination, the LFs have
been transformed into the present day mass functions (PDMF). The PDMFs from the
V and I photometry can be represented as a power-law with a slope and (the \cite{salp55} MF in this notation has
a slope ) respectively, in the mass range . Below this mass, the MF cannot be considered as
representative of the cluster IMF, as it is the result of the combined effect
of strong irregularities in the stellar background, probable internal dynamical
evolution of the cluster and/or interaction of the cluster with the dense
Galactic field. Unresolved binaries and mass segregation can only flatten the
apparent derived IMF, so we expect that the real IMF must be steeper than the
quoted slope by an unknown amount.Comment: 12 pages, 10 eps figures, in press in Astronomy and Astrophysic
The Star Formation Region NGC 6530: distance, ages and Initial Mass Function
We present astrometry and photometry, down to , of the very
young open cluster NGC6530, obtained from observations taken with the Wide
Field Imager camera at the MPG/ESO 2.2 m Telescope. Both the vs. and
the vs. color-magnitude diagrams (CMD) show the upper main sequence
dominated by very bright cluster stars, while, due to the high obscuration of
the giant molecular cloud surrounding the cluster, the blue envelopes of the
diagrams at are limited to the main sequence stars at the
distance of NGC6530. This particular structure of the NGC6530 CMD allows us to
conclude that its distance is about pc, significantly lower
than the previous determination of d=1800 pc. We have positionally matched our
optical catalog with the list of X-ray sources found in a Chandra-ACIS
observation, finding a total of 828 common stars, 90% of which are pre-main
sequence stars in NGC6530. Using evolutionary tracks of Siess et al. (2000)},
mass and age values are inferred for these stars. The median age of the cluster
is about 2.3 Myr; in the mass range (0.6--4.0), the Initial Mass
Function (IMF) shows a power law index , consistent with both
the Salpeter index (1.35), and with the index derived for other young clusters
; towards smaller masses the IMF shows a peak and then it starts to decrease.Comment: 32 pages, 13 ps figures, in press in Astronomy and Astrophysic
Spatially resolved LMC star formation history: I. Outside in evolution of the outer LMC disk
We study the evolution of three fields in the outer LMC disk Rgc=3.5-6.2 Kpc.
Their star formation history indicates a stellar populations gradient such that
younger stellar populations are more centrally concentrated. We identify two
main star forming epochs, separated by a period of lower activity between ~7
and ~4 Gyr ago. Their relative importance varies from a similar amount of stars
formed in the two epochs in the innermost field, to only 40% of the stars
formed in the more recent epoch in the outermost field. The young star forming
epoch continues to the present time in the innermost field, but lasted only
till ~0.8 and 1.3 Gyr ago at Rgc=5.5 degrees and 7.1 degrees, respectively.
This gradient is correlated with the measured HI column density and implies an
outside-in quenching of the star formation, possibly related to a variation of
the size of the HI disk. This could either result from gas depletion due to
star formation or ram-pressure stripping, or from to the compression of the gas
disk as ram-pressure from the Milky Way halo acted on the LMC interstellar
medium. The latter two situations may have occurred when the LMC first
approached the Milky Way.Comment: 15 pages, 13 figures, 4 tables. MNRAS, in pres
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