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 σx\sigma_x, σy\sigma_y, and σz\sigma_z of a Polarization Qubit

In the 1987 spin retrodiction puzzle of Vaidman, Aharonov, and Albert one is challenged to ascertain the values of $\sigma_x$, $\sigma_y$, and $\sigma_z$ 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