Numerical simulations of the ignition of n-heptane droplets in the transition diameter range from heterogeneous to homogeneous ignition

Spontaneous ignition of single n-heptane droplets in a constant volume filled with air is numerically simulated with the spherical symmetry. The volume is closed against mass, species, and energy transfer. The numerical model is fully transient. It continues calculation even after the droplet has completely vaporized, and therefore can predict pre-vaporized ignition. Initial pressure and initial air temperature are fixed at 3 MPa and 773 K, respectively. The droplet is initially at room temperature, and its diameter is between 1 and 100 μm. When the overall equivalence ratio is fixed to be sufficiently large, there exists no ignition limit in terms of initial droplet diameter d₀, and the ignition delay takes a minimum value at certain d₀. In such a case, transition from the heterogeneous ignition to the homogeneous ignition with decreasing d₀ is observed. When d₀ is fixed to be so small that the ignition would not occur in an infinite volume of air, the ignition delay takes a minimum value at certain , which is less than unity. Two-stage ignition behavior is investigated with this model. Ignition delay of a cool flame has the dependence on d₀ that is similar to that of ignition delay of a hot flame when is unity. When is almost zero, the ignition limit for cool flame in terms of d₀ is not identified unlike that for hot flame.     

Nonlinear ultrasonic waves in bubbly liquids with nonhomogeneous bubble distribution: Numerical experiments

This paper deals with the nonlinear propagation of ultrasonic waves in mixtures of air bubbles in water, but for which the bubble distribution is nonhomogeneous. The problem is modelled by means of a set of differential equations which describes the coupling of the acoustic field and bubbles vibration, and solved in the time domain via the use and adaptation of the SNOW-BL code. The attenuation and nonlinear effects are assumed to be due to the bubbles exclusively. The nonhomogeneity of the bubble distribution is introduced by the presence of bubble layers (or clouds) which can act as acoustic screens, and alters the behaviour of the ultrasonic waves. The effect of the spatial distribution of bubbles on the nonlinearity of the acoustic field is analyzed. Depending on the bubble density, dimension, shape, and position of the layers, its effects on the acoustic field change. Effects such as shielding and resonance of the bubbly layers are especially studied. The numerical experiments are carried out in two configurations: linear and nonlinear, i.e. for low and high excitation pressure amplitude, respectively, and the features of the phenomenon are compared. The parameters of the medium are chosen such as to reproduce air bubbly water involved in the stable cavitation process.     

Characterization of ammonia two-photon laser-induced fluorescence for gas-phase diagnostics

Two-photon laser-induced fluorescence (LIF) of ammonia (NH₃) with excitation of the C′-X transition at 304.8 nm and fluorescence detection in the 565 nm C′-A band has been investigated, targeting combustion diagnostics. The impact of laser irradiance, temperature, and pressure has been studied, and simulation of NH₃-spectra, fitted to experimental data, facilitated interpretation of the results. The LIF-signal showed quadratic dependence on laser irradiance up to 2 GW/cm². Stimulated emission, resulting in loss of excited molecules, is induced above 10 GW/cm², i.e., above irradiances attainable for LIF imaging. Maximum LIF-signal was obtained for excitation at the 304.8 nm bandhead; however, lower temperature sensitivity over the range 400–700 K can be obtained probing lines around 304.9 nm. A decrease in fluorescence signal was observed with pressure up to 5 bar absolute and attributed to collisional quenching. A detection limit of 800 ppm, at signal-to-noise ratio 1.5, was identified for single-shot LIF imaging over an area of centimeter scale, whereas for single-point measurements, the technique shows potential for sub-ppm detection. Moreover, high-quality NH₃-imaging has been achieved in laminar and turbulent premixed flames. Altogether, two-photon fluorescence provides a useful tool for imaging NH₃-detection in combustion diagnostics.     

Confinement of the field electron emission to atomic sites on ultra sharp tips

The spatially controlled field assisted etching method for sharpening metallic tips, in a field ion microscope (FIM), is used to study the evolution of the field emission when the tip apex radius is decreased below 1 nm. Unlike the conventional image formation in a field emission microscope (FEM), we demonstrate that at this scale the field emission is rather confined to atomic sites. A single atom apex fabricated at the end of such tips exhibits an outstanding brightness compared to other atomic tips. The measurements have been repeated for two double atom tips, with different atom-atom separations, and images of atomic field emission localization have also been obtained. We have found that the field emission intensity alternates between adjacent atoms when the applied voltage is gradually increased beyond a threshold value.     

Controlling spin-spin network dynamics by repeated projective measurements

We show that coupled-spin network manipulations can be made highly effective by repeated projections of the evolving quantum states onto diagonal density-matrix states (populations). As opposed to the intricately crafted pulse trains that are often used to fine-tune a complex network's evolution, the strategy hereby presented derives from the "quantum Zeno effect" and provides a highly robust route to guide the evolution by destroying all unwanted correlations (coherences). We exploit these effects by showing that a relaxationlike behavior is endowed to polarization transfers occurring within a N-spin coupled network. Experimental implementations yield coupling constant determinations for complex spin-coupling topologies, as demonstrated within the field of liquid-state nuclear magnetic resonance.     

Evidence for the ground-state resonance of 26O

Evidence for the ground state of the neutron-unbound nucleus (26)O was observed for the first time in the single proton-knockout reaction from a 82 MeV/u (27)F beam. Neutrons were measured in coincidence with (24)O fragments. (26)O was determined to be unbound by 150(-150)(+50) keV from the observation of low-energy neutrons. This result agrees with recent shell-model calculations based on microscopic two- and three-nucleon forces.     

Shadow effects in simulated ultrasound images derived from computed tomography images using a focused beam tracing model

Simulation of ultrasound images based on computed tomography (CT) data has previously been performed with different approaches. Shadow effects are normally pronounced in ultrasound images, so they should be included in the simulation. In this study, a method to capture the shadow effects has been developed, which makes the simulated ultrasound images appear more realistic. The method using a focused beam tracing model gives diffuse shadows that are similar to the ones observed in measurements on real objects. Ultrasound images of a cod (Gadus morhua) were obtained with a BK Medical 2202 ProFocus ultrasound scanner (BK Medical, Herlev, Denmark) equipped with a dedicated research interface giving access to beamformed radio frequency data. CT images were obtained with an Aquilion ONE Toshiba CT scanner (Toshiba Medical Systems Corp., Tochigi, Japan). CT data were mapped from Hounsfield units to backscatter strength, attenuation coefficients, and characteristic acoustic impedance. The focused beam tracing model was used to create maps of the transmission coefficient and scattering strength maps. Field II was then used to simulate an ultrasound image of 38.9 × 55.3 × 4.5 mm, using 10(6) point scatterers. As there is no quantitative method to assess quality of a simulated ultrasound image compared to a measured one, visual inspection was used for evaluation.