3,097 research outputs found
Combustion regimes in sequential combustors: Flame propagation and autoignition at elevated temperature and pressure
This numerical study investigates the combustion modes in the second stage of
a sequential combustor at atmospheric and high pressure. The sequential burner
(SB) features a mixing section with fuel injection into a hot vitiated
crossflow. Depending on the dominant combustion mode, a recirculation zone
assists flame anchoring in the combustion chamber. The flame is located
sufficiently downstream of the injector resulting in partially premixed
conditions. First, combustion regime maps are obtained from 0-D and 1-D
simulations showing the co-existence of three combustion modes: autoignition,
flame propagation and flame propagation assisted by autoignition. These regime
maps can be used to understand the combustion modes at play in turbulent
sequential combustors, as shown with 3-D large eddy simulations (LES) with
semi-detailed chemistry. In addition to the simulation of steady-state
combustion at three different operating conditions, transient simulations are
performed: (i) ignition of the combustor with autoignition as the dominant
mode, (ii) ignition that is initiated by autoignition and that is followed by a
transition to a propagation stabilized flame, and (iii) a transient change of
the inlet temperature (decrease by 150 K) resulting into a change of the
combustion regime. These results show the importance of the recirculation zone
for the ignition and the anchoring of a propagating type flame. On the
contrary, the autoignition flame stabilizes due to continuous self-ignition of
the mixture and the recirculation zone does not play an important role for the
flame anchoring
A comparison of atomistic and continuum theoretical approaches to determine electronic properties of GaN/AlN quantum dots
In this work we present a comparison of multiband k.p-models, the effective
bond-orbital approach, and an empirical tight-binding model to calculate the
electronic structure for the example of a truncated pyramidal GaN/AlN
self-assembled quantum dot with a zincblende structure. For the system under
consideration, we find a very good agreement between the results of the
microscopic models and the 8-band k.p-formalism, in contrast to a 6+2-band
k.p-model, where conduction band and valence band are assumed to be decoupled.
This indicates a surprisingly strong coupling between conduction and valence
band states for the wide band gap materials GaN and AlN. Special attention is
paid to the possible influence of the weak spin-orbit coupling on the localized
single-particle wave functions of the investigated structure
FIRBACK: II. Data Reduction and Calibration of the 170 micron ISO Deep Cosmological Survey
We present the final reduction and calibration of the FIRBACK ISOPHOT data.
FIRBACK is a deep cosmological survey performed at 170 microns. This paper
deals with the ISOPHOT C200 camera with the C160 filter. We review the whole
data reduction process and compare our final calibration with DIRBE (for the
extended emission) and IRAS (for point sources). The FIRBACK source extraction
and galaxy counts is discussed in a companion paper (Dole et al., 2001).Comment: Accepted for publication in A&A. 9 pages, includes new aa.cls. Also
available (with better quality figures) at http://wwwfirback.ias.u-psud.fr
and http://mips.as.arizona.edu/~hdole/firback (new aa.cls is here
Superconducting d-wave stripes in cuprates: Valence bond order coexisting with nodal quasiparticles
We point out that unidirectional bond-centered charge-density-wave states in
cuprates involve electronic order in both s- and d-wave channels, with
non-local Coulomb repulsion suppressing the s-wave component. The resulting
bond-charge-density wave, coexisting with superconductivity, is compatible with
recent photoemission and tunneling data and as well as neutron-scattering
measurements, once long-range order is destroyed by slow fluctuations or glassy
disorder. In particular, the real-space structure of d-wave stripes is
consistent with the scanning-tunneling-microscopy measurements on both
underdoped Bi2Sr2CaCu2O8+x and Ca2-xNaxCuO2Cl2 of Kohsaka et al. [Science 315,
1380 (2007), arXiv:cond-mat/0703309].Comment: 5 pages, 3 figs, (v2) final version to be published in PR
Ascertaining the Values of , , and of a Polarization Qubit
In the 1987 spin retrodiction puzzle of Vaidman, Aharonov, and Albert one is
challenged to ascertain the values of , , and of
a spin-1/2 particle by utilizing entanglement. We report the experimental
realization of a quantum-optical version in which the outcome of an
intermediate polarization projection is inferred by exploiting single-photon
two-qubit quantum gates. The experimental success probability is consistently
above the 90.2% threshold of the optimal one-qubit strategy, with an average
success probability of 95.6%.Comment: 4 pages, 2 figures; final version with new title and new abstract; to
appear in Physical Review Letter
Exploration of new data acquisition and background reduction techniques for the COBRA experiment
This work has contributed improvements to several key aspects of the COBRA
double-beta decay experiment.
A new data acquisition chain was developed for pulse-shape based readout
of the COBRA coplanar grid (CPG) detectors. Prototype electronics for detector
signal transmission and amplification were developed, as well as a software
package with algorithms for pulse shape analysis of CPG detector signals. The
pulse shape data readout has already resulted in significant improvements of the
detector energy resolution. Pulse shape analysis has added interaction depth information
to data output of the experiment, which has already shown itself to
be an effective tool to suppress part of the background. A wavelet-based data
compression technique has been developed to cope with the significant increase
of data volume inherent to the recording of the full detector pulse shapes.
A flexible data acquisition software framework was developed along with the
new DAQ hardware chain. It is designed for both research and development
applications and long-term physics data collection and is now in production use
at several sites in the COBRA collaboration. The framework was also designed to
scale up to the requirements of a large-scale experiment and its modular nature
ensures that future components can be integrated easily.
This work has contributed to the reduction of the radiation background level
by over an order of magnitude, achieved by the combination of new CPG detector
coatings and contacting methods with nitrogen flushing of the COBRA setup to
create a radon-free environment.
For the first time, low-background physics data was taken with a CdZnTe pixel
detector. The results show that pixel detectors present an exciting option for the
future of the experiment.
Work is currently under way to scale up the new CPG electronics to a high
channel count. The COBRA CPG setup at LNGS is scheduled to be upgraded to
a higher number of detectors in 2011, which will all be run using the new DAQ
system. A detailed study of the detector pulse shapes using simulations and collimated
scans, currently in progress, will yield the necessary basis for advanced
CPG pulse shape analysis. This will, in the future, enable new form of background
suppression like separation between single-site and multi-site events
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