Search

An increasing number of catalysis experiments are conducted at higher pressures to bridge the pressure gap. This means that we need to measure not only the sample surface but also the gas phase around the sample with spatial resolution. This thesis treats the development and applications of two optical techniques that can be used to image the surface of a model catalyst together with the surrounding gas: planar laser-induced fluorescence (PLIF) and 2D-surface optical reflectance (2D-SOR). The strengths and weaknesses of the techniques are discussed. Examples are used to show how the techniques can be applied to study active model catalysts \textit{in situ} under ambient pressure conditions. PLIF is used to image the CO and CO₂ concentrations around various Pd model catalysts during the CO oxidation reaction. 2D-SOR is used to show the surface oxidation process during the same reaction. We show how PLIF and 2D-SOR can be combined with additional techniques, primarily high-energy surface X-ray diffraction (HESXRD), thermography, mass spectrometry (MS) and polarisation modulation infrared reflection absorption spectroscopy (PMIRRAS).Finally, we show how PLIF and 2D-SOR can be used to bridge the so-called materials gap by taking advantage of the spatial resolution of the techniques to study polycrystalline samples. These samples, which exhibit a plethora of surface structures, can be seen as a step towards more industry-like samples.

[ES] La situación social y medioambiental hace que la legislación que regula la emisión de contaminantes sea cada vez más restrictiva, sobre todo en el campo de la automoción, lo que obliga a investigar alternativas al funcionamiento tradicional de estos motores. Hacer que funcionen con energías renovables, modificar la geometría o el modo de operación del motor son algunos de los campos de investigación. El objetivo de este trabajo de final de máster es estudiar y comparar la estructura de la llama difusiva de una serie de combustibles, tanto convencionales como es el diésel, como otro tipo de combustibles sintéticos (OMEX, OME1 y HVO). Este estudio se realizará en dos instalaciones distintas. La primera de ellas es una instalación de alta presión y temperatura en la cual la combustión no se ve perturbada por ninguna interacción entre las paredes (combustión libre). La segunda instalación es un motor óptico el cual se ha monocilindrado. En esta instalación se pueden simular condiciones de trabajo más realistas y la combustión se ve afectada por la dinámica del motor. En la instalación de alta presión y temperatura se realizará un estudio de la combustión generada por los combustibles, dodecano y oxymethylene dimethyl ether (OMEX) bajo las condiciones del Spray A del Engine Combustion Network (ECN). Esta instalación permite la posibilidad de trabajar a presión constante y controlar la concentración de oxígeno. Además, posee una serie de ventanas que proporcionan un gran acceso óptico para poder visualizar de forma óptima el proceso de combustión. El análisis de la estructura de la llama se realizará a través de la medida de la distribución espacial del formaldehido y de los radicales OH en ella, usando la técnica de fluorescencia inducida por un haz plano láser (PLIF). Para completar el análisis, esta información se combinará junto con las imágenes de la radiación de quimioluminiscencia del OH* que pertenece a la región UV. Las moléculas de formaldehido se excitan a, [EN] Due to social awareness and increasingly restrictive measures on the pollutant emission, especially in the automotive field, it is necessary to investigate alternatives to the traditional operation of these engines. To make them work with renewable energies, modifying the geometry or the operation mode of the engine are some of the research fields. The objective of this master's final project is the study and compare the diffusive flame structure of a series of fuels, both conventional such as diesel, and other types of synthetic fuels (OMEX, OME1 and HVO). This study will be carried out in two different facilities. First one, high pressure and high temperature facility in which the combustion is not disturbed by any interaction between the walls (free combustion). Then, single cylinder optical engine to simulate more realistic working conditions and combustion is affected by engine dynamics. On one hand, a comparison of the flame structure for two different fuels, dodecane and oxymethylene dimethyl ether (OMEX), will be performed under condition of Spray A of the Engine Combustion Network (ECN). The experiments will be carried out in a constant pressure vessel with wide optical access, at high pressure and temperature and controlled oxygen concentration. The flame structure analysis will be performed by measuring the formaldehyde molecules and OH radical distributions using planar Laser-Induced Fluorescence (PLIF) technique. To complement the analysis, this information will be combined with that obtained with high-speed imaging of OH* chemiluminescence radiation in the UV. Formaldehyde molecules are excited with the 355-nm radiation from the third harmonic of a Nd:YAG laser, whilst OH is excited with a wavelength of 281.00-nm from a dye laser (Nd:YAG laser coupled to complex optical system to operate in a specific range of wavelengths). In both cases, the beam will be transformed into a laser sheet to excite an axial flame plane and the fluorescence radiation

A series of turbulent premixed methane/air jet flames are studied using simultaneous planar lase diagnostic imaging of OH/CH/temperature and CH/OH/CH2O. The Karlovitz number of the flames ranges from 25 to 1500, and the turbulence intensity ranges from 16 to 200. These flames can be classified as highly turbulent flames in the thin reactions zone (TRZ) regime and distributed reaction zone (DRZ) regime. The aims of this study are to investigate the structural change of the preheat zone and the reaction zone as the Karlovitz number and turbulent intensity increase, to study the impact of the structural change of the flame on the propagation speed of the flame, and to evaluate the turbulent burning velocity computed in different layers in the preheat zone and reaction zone. It is found that for all investigated flames the preheat zone characterized with planar laser-induced fluorescence (PLIF) of CH2O is broadened by turbulent eddies. The thickness of the preheat zone increases with the turbulent intensity and it can be on the order of the turbulent integral length at high Karlovitz numbers. The reaction zone characterized using the overlapping layer of OH and CH2O PLIF signals is not significantly broadened by turbulence eddies; however, the CH PLIF layer is found to be broadened significantly by turbulence. The turbulent burning velocity is shown to monotonically increase with turbulent intensity and Karlovitz number. The increase in turbulent burning velocity is mainly due to the enhanced turbulent heat and mass transfer in various layers of the flame, while the contribution of flame front wrinkling to the turbulent burning velocity is rather minor.

Understanding how specific atom sites on metal surfaces lower the energy barrier for chemical reactions is vital in catalysis. Studies on simplified model systems have shown that atoms arranged as steps on the surface play an important role in catalytic reactions, but a direct comparison of how the light-off temperature is affected by the atom orientation on the step has not yet been possible due to methodological constraints. Here we report in situ spatially resolved measurements of the CO production over a cylindrical-shaped Pd catalyst and show that the light-off temperature at different parts of the crystal depends on the step orientation of the two types of steps (named A and B). Our finding is supported by density functional theory calculations, revealing that the steps, in contrast to what has been previously reported in the literature, are not directly involved in the reaction onset but have the role of releasing stress.

The use of an alexandrite laser for laser-induced fluorescence (LIF) spectroscopy and imaging of molecular oxygen in thermally excited vibrational states is demonstrated. The laser radiation after the third harmonic generation was used to excite the B-X (0-7) band at 257 nm in the Schumann-Runge system of oxygen. LIF emission was detected between 270 and 380 nm, revealing distinct bands of the transitions from B(0) to highly excited vibrational states in the electronic ground state, X (v > 7). At higher spectral resolution, these bands reveal the common P- and R-branch line splitting. Eventually, the proposed LIF approach was used for single-shot imaging of the two-dimensional distribution of hot oxygen molecules in flames.

The research objective of this program is to investigate the time-varying dynamics of scalar mixing relevant to turbulent reacting flows. Specifically, the program targeted the development and application of a new class of high-repetition-rate laser-based diagnostics to measure the time-varying mixture fraction, temperature, and species concentration fields in turbulent non-reacting jets and non-premixed flames. The result of the program was five new high-speed imaging techniques that were previously unavailable. High-speed planar Rayleigh scattering was developed to measure mixture fraction and temperature fields in turbulent jets and flames; high-speed spontaneous Raman scattering was developed to measure the temporal development of major species concentrations and the mixture fraction in reacting flows; and high-speed CH and CH2O PLIF were developed to monitor the time-varying, two-dimensional field of combustion intermediate and radicals with varying levels of reactivity. Targeted applications include understanding the time-dependent interaction between the large-scale turbulence and the small-scale mixing and species proproduction/destruction; and the support of large-eddy simulation (LES) as a method of modeling time-varying turbulent combustion by providing new temporally-based visualization and statistical metrics for assessing model performance.

Burst-mode planar laser-induced fluorescence (PLIF) imaging of the OH radical is demonstrated in laminar and turbulent hydrogen-air diffusion flames with pulse repetition rates up to 50 kHz. Nearly 1 mJ/pulse at 313.526 nm is used to probe the OH P210 rotational transition in the (0,0) band of the A-X system. The UV radiation is generated by a high-speed-tunable, injection-seeded optical parametric oscillator pumped by a frequency-doubled megahertz-rate burst-mode Nd:YAG laser. Preliminary kilohertz-rate wavelength scanning of the temperature-broadened OH transition during PLIF imaging is also presented for the first time (to our knowledge), and possible strategies for spatiotemporally resolved planar OH spectroscopy are discussed. 2009 Optical Society of America, Pub. in Optics Letters, v34 n9, p1309-1311, 1 May 2009. Sponsored in part by NASA grant no. NNX07AC34A and National Science Foundation.

A method of treating data acquired with the planar laser-induced fluorescence technique has been developed to visualize the topology of two-dimensional concentration fields and to describe the dynamics of the coherent mixing structures identified; This method is based on a conditional binary transformation of the local concentration data, combined with a joint probability calculation. The methodology has been used to investigate the mixing in a stirred tank, at two injection port locations (in the bulk and in the impeller stream region). With bulk injection, a folding phenomenon of the coherent mixing structure was detected. Away from this port, large-scale spatially periodic motion was identified, with a characteristic time of oscillation of the order of 2 to 3 s. With injection in the impeller stream region, no spatial instabilities of the coherent structure were detected. Local oscillations of the coherent mixing structure were found both on short and long time-scales (i.e., similar to 1 and similar to 80 s)., QC 20100525

The measurement techniques for delineating fuel-air mixing and transport in gas turbine combustion, as well as examples of representative results, are provided in this overview. The summary is broken into applications for gaseous fuels and liquid fuels since many diagnostics which are specific to the phase of the fuel have been developed. Many possible methods for assessing the general mixing have been developed, but not all have been applied to practical systems either under scaled or under actual conditions. With respect to gaseous mixing processes, planar laser-induced fluorescence (PLIF) based on acetone is now starting to be successfully applied to actual systems and conditions. In spray-fired systems, the need to discriminate between phases leads to considerable complication in delineating fuel-air mixing. Methods that focus on the discrete phase have successfully provided details relative to the droplets. These include phase Doppler interferometry (PDI), which is becoming ubiquitous in application to practical devices and under practical conditions. PDI is typically being applied to quantify droplet sizes, although the volume flux, which is relevant to fuel-air mixing, in practical systems is also being reported. In addition, PLIF strategies that focus upon the behaviour of the droplets are now being developed. However, PLIF strategies that can discriminate between phases either in the fuel or with respect to the liquid fuel and combustion air are also being developed. In terms of characterizing the vector fields associated with the mixing process, laser anemometry (LA), although it is tedious to apply, has proven reliable even in the presence of droplets. Newer methods such as DPIV and FRS have seen only limited application in practical systems but appear promising. In terms of scalar fields, LIF and PLIF have also been applied successfully to these systems, and examples of the measurements of concentrations of various radical species such as OH are found th