984 research outputs found
CFD Predictions of Soot & CO Emissions Generated by a Partially-Fueled 9-Element Lean-Direct Injection Combustor
A study was undertaken to investigate the CO & soot emissions generated by a partially-fueled 9- element LDI (Lean-Direct Injection) combustor configuration operating in the idle range of jet engine conditions. In order to perform the CFD analysis, several existing soot/chemistry models were implemented into the OpenNCC (Open National Combustion Code). The calculations were based on a Reynolds-Averaged Navier Stokes (RANS) simulation with standard k-epsilon turbulence model, a 62- species jet-a/air chemistry, a 2-equation soot model, & a Lagrangian spray solver. A separate transport equation was solved for all individual species involved in jet-a/air combustion. In the test LDI configuration we examined, only five of the nine injectors were fueled with the major pilot injector operating at an equivalence ratio of near one and the other four main injectors operating at an equivalence ratio near 0.55. The calculations helped to identify several reasons behind the soot & CO formation in different regions of the combustor. The predicted results were compared with the reported experimental data on soot mass concentration (SMC) & emissions index of CO (EICO). The experimental results showed that an increase in either T3 and/or F/A ratio lead to a reduction in both EICO & SMC. The predicted results were found to be in reasonable agreement. However, the predicted EICO differed substantially in one test condition associated with higher F/A ratio
Vapor growth of GeTe single crystals in micro-gravity
The positive effects of micro-gravity on crystal growth and fundamental properties of the vapor transport reaction were established by analyzing the results of GeSe and GeTe vapor transport experiments performed on board Skylab. The analysis was based on a direct comparison of GeSe and GeTe crystals and of mass transport rate data obtained on earth and in space. For this purpose, a total of six transport experiments employing different concentrations of transport agent (GeI4) and two temperature gradients were performed during the Skylab 3 and 4 missions. Extensive ground-based studies demonstrated that the crystal morphology and the mass transport rates of the above systems are affected by the transport conditions, in particular by gravity-driven convection. The results demonstrate unambiguously a considerable improvement of the space crystals in terms of surface perfection, crystalline homogeneity and defect density. The observation of greater mass transport rates than expected in micro-gravity environment is of basic scientific and technological significance. This indicates that conventional transport models are incomplete and demonstrates that crystals of improved quality can be grown at reasonable rates by this technique in space. Results are of practical importance for the modification of crystal growth techniques on earth
Nanosecond electro-optical switching with a repetition rate above 20MHz
We describe an electro-optical switch based on a commercial electro-optic
modulator (modified for high-speed operation) and a 340V pulser having a rise
time of 2.2ns (at 250V). It can produce arbitrary pulse patterns with an
average repetition rate beyond 20MHz. It uses a grounded-grid triode driven by
transmitting power transistors. We discuss variations that enable analog
operation, use the step-recovery effect in bipolar transistors, or offer other
combinations of output voltage, size, and cost.Comment: 3 pages, 3 figures. Minor change
Atom-wave diffraction between the Raman-Nath and the Bragg regime: Effective Rabi frequency, losses, and phase shifts
We present an analytic theory of the diffraction of (matter) waves by a
lattice in the "quasi-Bragg" regime, by which we mean the transition region
between the long-interaction Bragg and "channelling" regimes and the
short-interaction Raman-Nath regime. The Schroedinger equation is solved by
adiabatic expansion, using the conventional adiabatic approximation as a
starting point, and re-inserting the result into the Schroedinger equation to
yield a second order correction. Closed expressions for arbitrary pulse shapes
and diffraction orders are obtained and the losses of the population to output
states otherwise forbidden by the Bragg condition are derived. We consider the
phase shift due to couplings of the desired output to these states that depends
on the interaction strength and duration and show how these can be kept
negligible by a choice of smooth (e.g., Gaussian) envelope functions even in
situations that substantially violate the adiabaticity condition. We also give
an efficient method for calculating the effective Rabi frequency (which is
related to the eigenvalues of Mathieu functions) in the quasi-Bragg regime.Comment: Minor additions, more concise text. To appear in Phys. Rev. A. 20
pages, 10 figure
Dobrushin-Kotecky-Shlosman theorem for polygonal Markov fields in the plane
We consider the so-called length-interacting Arak-Surgailis polygonal Markov
fields with V-shaped nodes - a continuum and isometry invariant process in the
plane sharing a number of properties with the two-dimensional Ising model. For
these polygonal fields we establish a low-temperature phase separation theorem
in the spirit of the Dobrushin-Kotecky-Shlosman theory, with the corresponding
Wulff shape deteremined to be a disk due to the rotation invariant nature of
the considered model. As an important tool replacing the classical cluster
expansion techniques and very well suited for our geometric setting we use a
graphical construction built on contour birth and death process, following the
ideas of Fernandez, Ferrari and Garcia.Comment: 59 pages, new version revised according to the referee's suggestions
and now publishe
Effects of Increased Drought in Amazon Forests Under Climate Change: Separating the Roles of Canopy Responses and Soil Moisture
The Amazon forests are one of the largest ecosystem carbon pools on Earth. Although more frequent and prolonged future droughts have been predicted, the impacts have remained largely uncertain, as most land surface models (LSMs) fail to capture the vegetation drought responses. In this study, the ability of the LSM JSBACH to simulate the drought responses of leaf area index (LAI) and leaf litter production in the Amazon forests is evaluated against artificial drought experiments. Based on the evaluation, improvements are implemented, including a dependency of leaf growth on leaf carbon allocation and a better representation of drought-dependent leaf shedding. The modified JSBACH is shown to capture the drought responses at two sites and across different regions of the basin. It is then coupled with an atmospheric model to simulate the carbon and biogeophysical feedbacks of drought under future climate. We separate the drought impacts into (a) the direct effect, resulting from drier soil and stomatal closure, which does not involve a change in canopy structure, and (b) the LAI effect, resulting from leaf shedding and involving canopy response. We show that the latter accounts for 35% of reduced land carbon uptake (9 ± 10 vs. 26 ± 7 g/m2/yr; mean ± 1 sd) and 12% of surface warming (0.09 ± 0.03 vs. 0.7 ± 0.07 K) during the late 21st century. A north-south dipole of precipitation change is found, which is largely attributable to the direct effect. The results highlight the importance of incorporating drought deciduousness of tropical rainforests in LSMs to better simulate land-atmosphere interactions in the future
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