44,262 research outputs found
The effect of oxygen starvation on ignition phenomena in a reactive solid containing a hot-spot
In this paper, we explore the effect of oxygen supply on the conditions necessary to sustain a self-propagating front from a spherical source of heat embedded in a much larger volume of solid. The ignition characteristics for a spherical hot-spot are investigated, where the reaction is limited by oxygen, that is, reactant + oxygen ? product. It is found that over a wide range of realistic oxygen supply levels, constant heating of the solid by the hot-spot results in a self-propagating combustion front above a certain critical hot-spot power; this is clearly an important issue for industries in which hazard prevention is important. The ignition event leading to the formation of this combustion wave involves an extremely sensitive balance between the heat generated by the chemical reaction and the depletion of the reactant. As a result, for small hot-spot radii and infinite oxygen supply, not only is there a critical power above which a self-sustained combustion front is initiated there also exists a power beyond which no front is formed, before a second higher critical power is found. The plot of critical power against hot-spot radius thus takes on a Z-shape appearance. The corresponding shape for the oxygen-limited reaction is qualitatively the same when the ratio of solid thermal diffusion to oxygen mass diffusion (N) is small and we establish critical conditions for the initiation of a self-sustained combustion front in that case. As N gets larger, while still below unity, we show that the Z-shape flattens out. At still larger values of N, the supercritical behaviour becomes increasingly difficult to define and is supplanted by burning that depends more uniformly on power. In other words, the transition from slow burning to complete combustion seen at small values of N for some critical power disappears. Even higher values of N lead to less solid burning at fixed values of power
Cataract How Important Is Age of Intervention?
Purpose: To study effect of age of intervention on visual outcome following treatment of pediatric patients with cataract.
Setting: Tertiary eye care centre in Dahod at the trijunction of Gujarat, Madhya Pradesh, and Rajasthan states in central western India.
Participants: 705 eyes of 1047 patients
Methods: This is a prospective cohort study. We studied a consecutive series of pediatric patients with congenital, developing, or COMPLICATED
cataracts who underwent surgery between January, 1999 and April, 2012 at our center. Patient demographics, cataract type, presenting symptoms, surgical intervention, postoperative visual acuity, and follow-up refractive changes were recorded.
Primary Outcome measures: vision.
Results: In total, 1305 eyes of 1047 children were included: unilateral cataracts were present in 786 (60.2%) eyes. There were 600 (46.7%) traumatic and 705 (53.3%) non-traumatic cases. Ages at surgery ranged from 1 to 215 months. Eyes were grouped by the age of surgical intervention performed: Group 1,</= 5 years including 177 (25.1%) eyes, and Group 2, >5 years, including 528 (74.9%) eyes either by anterior or pars plana route ± IOL placement. The mean follow-up time was 117 days. Ultimately, 128 (18.2%) Group 1 and 213 (30.2%)
Group 2 patients achieved a visual acuity better than 20/80 (P < 0.001). Age at intervention was significantly related (all P < 0.001) to visual outcome.
Conclusions: Age of intervention affects visual outcome significantly (p<0.001)
Molecular clouds and clumps in the Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey
The Boston University-Five College Radio Astronomy Observatory (BU-FCRAO)
Galactic Ring Survey (GRS) of 13 CO (1-0) emission covers Galactic longitudes
18 deg < l < 55.7 deg and Galactic latitudes |b| <= 1 deg. Using the SEQUOIA
array on the FCRAO 14m telescope, the GRS fully sampled the 13 CO Galactic
emission (46 arcsec angular resolution on a 22 arcsec grid) and achieved a
spectral resolution of 0.21 km/s. Because the GRS uses 13 CO, an optically thin
tracer, rather than 12 CO, an optically thick tracer, the GRS allows a much
better determination of column density and also a cleaner separation of
velocity components along a line of sight. With this homogeneous, fully-sampled
survey of 13 CO, emission, we have identified 829 molecular clouds and 6124
clumps throughout the inner Galaxy using the CLUMPFIND algorithm. Here we
present details of the catalog and a preliminary analysis of the properties of
the molecular clouds and their clumps. Moreover, we compare clouds inside and
outside of the 5 kpc ring and find that clouds within the ring typically have
warmer temperatures, higher column densities, larger areas, and more clumps
compared to clouds located outside the ring. This is expected if these clouds
are actively forming stars. This catalog provides a useful tool for the study
of molecular clouds and their embedded young stellar objects.Comment: 29 pages. ApJ in pres
Large-amplitude chirped coherent phonons in tellurium mediated by ultrafast photoexcited carrier diffusion
We report femtosecond time-resolved reflectivity measurements of coherent
phonons in tellurium performed over a wide range of temperatures (3K to 296K)
and pump laser intensities. A totally symmetric A coherent phonon at 3.6
THz responsible for the oscillations in the reflectivity data is observed to be
strongly positively chirped (i.e, phonon time period decreases at longer
pump-probe delay times) with increasing photoexcited carrier density, more so
at lower temperatures. We show for the first time that the temperature
dependence of the coherent phonon frequency is anomalous (i.e, increasing with
increasing temperature) at high photoexcited carrier density due to
electron-phonon interaction. At the highest photoexcited carrier density of
1.4 10cm and the sample temperature of 3K, the
lattice displacement of the coherent phonon mode is estimated to be as high as
0.24 \AA. Numerical simulations based on coupled effects of optical
absorption and carrier diffusion reveal that the diffusion of carriers
dominates the non-oscillatory electronic part of the time-resolved
reflectivity. Finally, using the pump-probe experiments at low carrier density
of 6 10 cm, we separate the phonon anharmonicity to
obtain the electron-phonon coupling contribution to the phonon frequency and
linewidth.Comment: 22 pages, 6 figures, submitted to PR
Scaling and Formulary cross sections for ion-atom impact ionization
The values of ion-atom ionization cross sections are frequently needed for
many applications that utilize the propagation of fast ions through matter.
When experimental data and theoretical calculations are not available,
approximate formulas are frequently used. This paper briefly summarizes the
most important theoretical results and approaches to cross section calculations
in order to place the discussion in historical perspective and offer a concise
introduction to the topic. Based on experimental data and theoretical
predictions, a new fit for ionization cross sections is proposed. The range of
validity and accuracy of several frequently used approximations (classical
trajectory, the Born approximation, and so forth) are discussed using, as
examples, the ionization cross sections of hydrogen and helium atoms by various
fully stripped ions.Comment: 46 pages, 8 figure
Direct measure of the exciton formation in quantum wells from time resolved interband luminescence
We present the results of a detailed time resolved luminescence study carried
out on a very high quality InGaAs quantum well sample where the contributions
at the energy of the exciton and at the band edge can be clearly separated. We
perform this experiment with a spectral resolution and a sensitivity of the
set-up allowing to keep the observation of these two separate contributions
over a broad range of times and densities. This allows us to directly evidence
the exciton formation time, which depends on the density as expected from
theory. We also evidence the dominant contribution of a minority of excitons to
the luminescence signal, and the absence of thermodynamical equilibrium at low
densities
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