Leading point behavior during the ignition of an annular combustor with liquid n-heptane injectors

Experimental and numerical investigations of ignition in combustors with multiple burners have recently emerged and have provided new insights on the last phase of ignition in gas turbine-like annular geometries where the flame propagates from burner to burner. Previous comparisons between calculations and experiments of light-round in a laboratory scale annular combustion chamber have demonstrated the ability of large-eddy simulation to predict such processes for perfectly premixed conditions and, more recently, for n-heptane spray injection. The present analysis focuses on two additional operating points with liquid n-heptane sprays and the turbulent flame propagation in the two-phase mixture is examined through the behavior of its leading points. The validation of the light-round process is characterized in terms of ignition delays. The detailed analysis of the propagation through the definition of a leading point enables to highlight some key phenomena responsible for the flame behavior, such as the influence of the liquid droplet spray and its vaporization in the chamber. Calculations indicate that the volumetric expansion due to the chemical reaction at the flame induces a strong azimuthal flow in the fresh stream at a distance of several sectors ahead of the flame, which modifies conditions in this region. This creates heterogeneities in the gas composition and wakes on the downstream side of the swirling jets formed by the injectors, with notable effects on the motion of the leading point and on the absolute flame velocity.     

Transform-Limited Pulses Are Not Optimal for Resonant Multiphoton Transitions

Maximizing nonlinear light-matter interactions is a primary motive for compressing laser pulses to achieve ultrashort transform limited pulses. Here we show how, by appropriately shaping the pulses, resonant multiphoton transitions can be enhanced significantly beyond the level achieved by maximizing the pulse's peak intensity. We demonstrate the counterintuitive nature of this effect with an experiment in a resonant two-photon absorption, in which, by selectively removing certain spectral bands, the peak intensity of the pulse is reduced by a factor of 40, yet the absorption rate is doubled. Furthermore, by suitably designing the spectral phase of the pulse, we increase the absorption rate by a factor of 7.     

Dynamics of spray and swirling flame under acoustic oscillations : A joint experimental and LES investigation

The dynamics of spray swirling flames is investigated by combining experiments on a single sector generic combustor and large eddy simulations of the same configuration. Measurements and calculations correspond to a self-sustained limit cycle operation where combustion coupled by an axial quarter wave acoustic mode induces large amplitude oscillations of pressure in the system. A detailed analysis of the mechanisms controlling the process is carried out first by comparing the measured and calculated spray and flame dynamics. Considering in a second stage that the spray and flame are compact with respect to the acoustic wavelength the analysis can be simplified by defining state variables that are obtained by taking averages over the combustor cross section and representing the behavior of these average quantities as a function of the axial coordinate and time. This reveals a first region in which essentially convective processes prevail. The convective heat release rate then couples further downstream with the pressure field giving rise to positive Rayleigh source terms which feed energy in the axial acoustic mode. In the convective region, the swirl number features oscillations around its mean value with an impact on the flow aerodynamics and flame radial displacement. Fluctuations in the fuel flow rate are initiated at the injector exhaust and likewise convected downstream. The total mass flow rate that exhibits strong convective disturbances is dominated further downstream by the acoustic motion. This information provides new insights on the convective-acoustic coupling that controls the heat release rate disturbances and reveals the time delays governing the combustion oscillation process.     

Determination of the inhomogeneous structure of a medium from its plane wave reflection response,part II: A numerical approximation

This paper is concerned with a standard one dimensional inverse scattering problem: given the reflection response of an unknown inhomogeneous medium for plane waves under normal or oblique incidence, determine its sound speed and density structures. The problem is solved by means of a simple numerical technique which involves only fast Fourier transform operations and numerical integration of ordinary differential equations. Three cases are specifically considered: (a) sound speed is unknown, density is known; (b) sound speed is known, density is unknown; (c) sound speed and density are to be determined simultaneously. Numerical simulations performed on reflection coefficients computed in Part I for a limited band of frequencies lead to accurate reconstructions of the original structures of various media.     

-algebras of proper foliations

We study the -algebras of proper foliations. In case of finite depth they are described by a tower of exact sequences. We conclude with remarks about foliations almost without holonomy and AF-embeddability.

     

Emission and laser-induced fluorescence imaging methods in experimental combustion

Imaging methods provide new insights into many fundamental combustion processes. Many imaging techniques have been devised in recent years and applied to a range of experiments. One particularly useful method is to seed the flow with oil particles and illuminate the domain of interest with a planar sheet of laser light. The droplets evaporate and vanish when they pass through the flame. The light scattered by the particles may be imaged for example with a CCD camera or with high-speed cinematography to show the structure and dynamics of the flame front. This technique, sometimes called laser tomography, is based on Mie scattering. It provides essentially qualitative information on the geometry and motion of the flame front. Another valuable method relies on spontaneous emission imaging. In this method the light emitted by certain radicals produced by the chemical reaction is detected by a camera and delivered to a computer for further processing. In some circumstances it is possible to deduce from this measurement the spatial distribution of heat release in the reactive flow. More quantitative data may be gathered with planar laser-induced fluorescence (PLIF) imaging. The reactive flow is illuminated with a planar laser sheet delivered by a tunable laser. The laser light excites the fluorescence of a species that is present in the flow, which is then detected with an intensified CCD camera. The data obtained in this way can be processed to obtain spatial measurements of the species concentration. The basic principles, equipment requirements, and experimental aspects of these three imaging techniques are reviewed. Practical applications to turbulent flames are emphasized. It is shown that emission imaging applied to turbulent ducted flames yields interesting information for modeling. A second example of application is the ignition sequence of a multiple-injector combustor, of importance to modern cryogenic rocket engines. Emission and PLIF imaging have been used to obtain data on the development of the initial flame kernel and on its propagation from the first injector to the next. The images gathered in this experiment yield a unique view on the flame patterns that lead to the final stabilization of the reactive fronts. While current imaging methods are essentially qualitative, it is possible to deduce quantitative results from the data, and some of the present limitations may be overcome with more refined measurement procedures. These issues are analyzed, and future developments in this area are evaluated.     

Direct fourier synthesis of waves in layered media and the method of stationary phase

This note describes a method of Direct Fourier Synthesis (DFS) for the calculation of wavefields in layered media. This technique may be used to obtain numerical solutions of many problems which have been previously treated by approximation methods. The DFS is here used to analyze radiation of a line source in the presence of a plane interface separating two half-spaces of different sound speeds. Approximate solutions of this problem obtained by a classical application of the method of stationary phase are compared with DFS results and it is found that their qualitative behaviour and precision is not satisfactory. However it is shown that an initial change of co-ordinate system origin allows a notable improvement of results obtained by the method of stationary phase.