Low-lying dipole excitations in the stable Cd isotopes: A systematics

Low-lying dipole excitations in the medium-weight vibrational nuclei of the Cd isotopic chain were investigated by means of nuclear resonance fluorescence experiments performed at the bremsstrahlung beam of the Stuttgart Dynamitron accelerator (endpoint energy 4.1 MeV). Detailed information has been obtained on excitation energies, spins, decay widths, and transition probabilities of numerous excited states in ^{110–114,116}Cd. Additionally, the use of two Compton polarimeters enabled model-independent parity assignments for excitations in the even-even isotopes. Strongly excited J ^π = 1^? states are found in all even-even Cd nuclei at excitation energies near the sumof the energies of the first 2⁺ and 3^? states. These excitations are interpreted as the 1^? member of the quadrupole-octupole coupled quintuplet (2+?3^?). The fragmented strength observed in the odd isotopes ^111,113Cd is compared with the strength distributions in the neighboring even-even Cd isotopes.     

预混火焰拉伸和曲率效率的物理分析

湍流燃烧的基本火焰结构是拉伸的曲面涡管;拉伸流场中的管形火焰模拟了湍流燃烧的细微结构。本文对平面预混火焰、拉伸预混火焰和管形拉伸预混火焰进行了质量、能量和组分的守恒分析。通过对比这几种火焰,揭示了火焰拉伸效果是通过优先扩散改变火焰温度和熄火极限;而火焰曲率通过增强或削弱这种优先扩散效果来影响火焰温度,影响的大小和火焰厚度与火焰半径的比值呈正比。     

Chemical kinetic modeling study of the effects of oxygenated hydrocarbons on soot emissions from diesel engines

A detailed chemical kinetic modeling approach is used to examine the phenomenon of suppression of sooting in diesel engines by the addition of oxygenated hydrocarbon species to the fuel. This suppression, which has been observed experimentally for a few years, is explained kinetically as a reduction in concentrations of soot precursors present in the hot products of a fuel-rich diesel ignition zone when oxygenates are included. The kinetic model is also used to show how different oxygenates, ester structures in particular, can have different soot-suppression efficiencies due to differences in the molecular structure of the oxygenated species.     

Blue and infrared stimulated emission from alkali vapors pumped through two-photon absorption

The 5 ²D_3/2, 5 ²D_5/2, and 7 ²S_1/2 states of rubidium and the 7 ²D_3/2, 7 ²D_5/2, and 9 ²S_1/2 states of cesium were populated at low pressure by two photon excitation using a pulsed dye laser. Blue beams from the Rb 6 ²P_{3/2, 1/2}–5 ²S_1/2 and Cs 7 ²P_{3/2, 1/2}–6 ²S_1/2 transitions were observed. In addition, infrared beams were observed arising directly from the pumped ²D states, establishing a collisionless cascade mechanism. Threshold is modest at about 0.3 mJ/pulse or 2×10⁵ W/cm². Slope efficiencies increase dramatically with alkali concentration and peak at 0.4%, with considerable opportunity for improvement. This versatile system might find applications in both underwater communications and for infrared counter measures.     

Hydroxyl tagging velocimetry (HTV) in experimental air flows

 The new nonintrusive instantaneous molecular flow tagging method, hydroxyl tagging velocimetry (HTV), previously demonstrated only for high-temperature reacting flows, is now demonstrated in low-temperature (300 K) ambient air flowfields. Single-photon photodissociation of ground-state H₂O by a ∼193-nm ArF excimer laser ‘writes’ very long grid lines (>50 mm) of superequilibrium OH and H photoproducts in a room air flowfield due to the presence of ambient H₂O vapor. After displacement, the positions of the OH tag lines are revealed through fluorescence caused by A²Σ⁺ (ν^′=0)?X²Π_i (ν^′′=0) OH excitation using a pulsed frequency-doubled dye laser with an operating output wavelength of ∼308 nm. The dye ‘read’ laser accesses the strong Q₁(1) line, compensating for the relatively weak 193-nm absorption of room-temperature H₂O. The weak absorption of ground vibrational state H₂O has previously precluded the use of HTV at low temperatures, since previous HTV systems relied on a KrF excimer ‘read’ laser that could only access a weak (3?0) OH transition. The instantaneous velocity field is determined by time-of-flight analysis. HTV tag lifetime comparisons between experimental results and theoretical predictions are discussed. Multiple-line tag grids are shown displaced due to an experimental air flowfield, thus providing 2-D multipoint velocity information. Due to the instantaneous nature of the HTV tag formation, HTV is particularly suitable for, but not limited to, a variety of fast flowfield applications including nonreacting base flows for high-speed projectiles and low-temperature hypersonic external or internal flows. Received: 3 July 2001 / Revised version: 6 November 2001 / Published online: 17 January 2002     

Development and validation of an n-dodecane skeletal mechanism for spray combustion applications

n-Dodecane is a promising surrogate fuel for diesel engine study because its physicochemical properties are similar to those of the practical diesel fuels. In the present study, a skeletal mechanism for n-dodecane with 105 species and 420 reactions was developed for spray combustion simulations. The reduction starts from the most recent detailed mechanism for n-alkanes consisting of 2755 species and 11,173 reactions developed by the Lawrence Livermore National Laboratory. An algorithm combining direct relation graph with expert knowledge (DRGX) and sensitivity analysis was employed for the present skeletal reduction. The skeletal mechanism was first extensively validated in 0-D and 1-D combustion systems, including auto-ignition, jet stirred reactor (JSR), laminar premixed flame and counter flow diffusion flame. Then it was coupled with well-established spray models and further validated in 3-D turbulent spray combustion simulations under engine-like conditions. These simulations were compared with the recent experiments with n-dodecane as a surrogate for diesel fuels. It can be seen that combustion characteristics such as ignition delay and flame lift-off length were well captured by the skeletal mechanism, particularly under conditions with high ambient temperatures. Simulations also captured the transient flame development phenomenon fairly well. The results further show that ignition delay may not be the only factor controlling the stabilisation of the present flames since a good match in ignition delay does not necessarily result in improved flame lift-off length prediction.     

A detailed chemical kinetic reaction mechanism for oxidation of four small alkyl esters in laminar premixed flames

A detailed chemical kinetic reaction mechanism has been developed for a group of four small alkyl ester fuels, consisting of methyl formate, methyl acetate, ethyl formate, and ethyl acetate. This mechanism is validated by comparisons between computed results and recently measured intermediate species mole fractions in fuel-rich, low-pressure, premixed laminar flames. The model development employs a principle of similarity of functional groups in constraining the H atom abstraction and unimolecular decomposition reactions for each of these fuels. As a result, the reaction mechanism and formalism for mechanism development are suitable for extension to larger oxygenated hydrocarbon fuels, together with an improved kinetic understanding of the structure and chemical kinetics of alkyl ester fuels that can be extended to biodiesel fuels. Variations in concentrations of intermediate species levels in these flames are traced to differences in the molecular structure of the fuel molecules.     

Partitioning edge-coloured complete symmetric digraphs into monochromatic complete subgraphs

Let ${\overrightarrow{K}}_{\mathbb{N}}$ be the complete symmetric digraph on the positive integers. Answering a question of DeBiasio and McKenney, we construct a 2-colouring of the edges of ${\overrightarrow{K}}_{\mathbb{N}}$ in which every monochromatic path has density 0.However, if we restrict the length of monochromatic paths in one colour, then no example as above can exist: We show that every $(r+1)$-edge-coloured complete symmetric digraph (of arbitrary infinite cardinality) containing no directed paths of edge-length ${?}_i$ for any colour $i\le r$ can be covered by ${\prod }_{i\in \left[r\right]}{?}_i$ pairwise disjoint monochromatic complete symmetric digraphs in colour $r+1$.Furthermore, we present a stability version for the countable case of the latter result: We prove that the edge-colouring is uniquely determined on a large subgraph, as soon as the upper density of monochromatic paths in colour $r+1$ is bounded by ${\prod }_{i\in \left[r\right]}\frac{1}{{?}_i}$.     

Measurement and simulation of partially-premixed cellular tubular flames

Experimental and numerical simulation results are reported of partially-premixed cellular tubular flames. Parametric measurements across stretch rate and equivalence ratio are taken by chemiluminescent imaging and are presented for the first time. Select hybrid cases with both cellular and non-cellular flame structures are examined with laser-induced spontaneous Raman scattering. Results are spatially resolved in two dimensions and radial interpolations of reaction and extinction zones are compared to numerical simulations using multicomponent transport and detailed chemical kinetics. Experimental cell structures and extinction zones are well predicted by numerical simulation, with discrepancies of temperature and H₂O and temperature primarily observed in locations with moderate and high mole fractions of CO₂. A novel cellular structure, denoted as a “split-cell” flame, is reported for the first time with both chemiluminescent imaging and Raman scattering. Results indicate that partially-premixed flames are valuable as experimental and numerical benchmarks to advance fundamental combustion research.