Browsing by Author "Robertson, G N"

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    C.A.R.S. temperature measurements and chemical kinetic modelling of autoignition in a methanol-fuelled internal combustion engine
    (1993) Lockett, R D; Robertson, G N
    The temperature inside the cylinder of a methanol-fuelled single-cylinder Ricardo E6 research engine running under knocking conditions, is measured by means of Coherent Anti-Stokes Raman Spectroscopy (CARS), and the pressure is measured with a pressure transducer. In order to obviate any errors arising from deficiencies in the spectral scaling laws which are commonly used to represent nitrogen Q-branch spectra at high pressure, a purely experimental technique is employed to derive temperatures from CARS spectra by cross-correlation with a reference library of spectra recorded in an accurately calibrated high-pressure high-temperature optical cell. The temperature and pressure profiles obtained from the engine running under knocking conditions, are then used as input data for chemical kinetic modelling of end-gas autoignition. Five published mechanisms (Grotheer et al 1992, Grotheer and Kelm 1989, Norton and Dryer 1989, Dove and Warnatz 1983, .and Esser and Warnatz 1987) are used in the autoignition study, and the results for the different mechanisms are compared. A good qualitative understanding of the mechanism underlying end-gas autoignition in the engine is obtained, although the calculated autoignition points occur slightly earlier than the observed point. A sensitivity analysis of the methanol autoignition system is undertaken, and the importance of the decomposition of hydrogen peroxide and the hydroperoxyl chemistry is demonstrated. The discrepancies between the predicted results of the different mechanisms is shown to be caused by a small number of sensitive reactions for which there are conflicting data. Finally, a linear mode analysis from the geometric qualitative theory of differential equations is performed on the non-linear chemical rate equations. The equilibrium points in the generalised phase space of the non-linear chemical system are shown to be defined in terms of three regions. The equilibrium points are unstable improper nodes in the first region (T < ll00K), unstable focii in the second region ( 1100K 1170K).
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    A semi-classical model of proton tunnelling in hydrogen-bonded compounds
    (1980) Lawrence, Michael Colin; Robertson, G N
    Two back-to-back Morse potentials are used to model the protonic potential in a hydrogen bond. A mathematical method is developed to obtain semi-classical solutions for the eigenvalues and eigenfunctions of the potential. Particular care is taken over the form of approximation used in the vicinity of the barrier top. The double Morse potential is then used as the fundamental element in the analysis of the proton dynamics in a number of hydrogen-bonded species: a) Chromous acid. A consistent, quantitative explanation is found for the νs(OH)/vs(OD) bands in the mid-infrared spectra of CrOOH/CrOOD. The 226 cm⁻¹ band in the far infrared and neutron scattering spectra of CrOOH is shown to be caused by the protonic tunnelling mode. b) Carboxylic and Dicarboxylic acids. The 140 cm⁻¹ protonic upper state splitting in dimeric formic acid proposed by Excoffon and Marechal (1972) is shown to be supported by the double Morse potential analysis. The analysis suggest's however that the two hydrogen-bonded protons in (HCOOH)₂ tunnel independently. The anomalous vs(OH)/vs(OD) intensity ratios observed in the infrared spectra of both adipic acid and dimeric formic acid could not be explained in terms of mechanical anharmonicity alone. The protonic tunnelling frequency in crystalline formic acid is shown to be about 23 cm⁻¹ , which supports the order-disorder interpretation of the observed phase transition in this crystal. c) Potassium Dihydrogen Phosphate (KDP). The crystallographic and infrared spectroscopic data for KDP and DKDP are interpreted consistently using the double Norse potential. The bare single-particle tunnelling frequency Ω is estimated to be 320 ± 10 cm⁻¹ and the overtone band at 4600 cm⁻¹ in the infrared spectrum of KDP is shown to be a :0 →:3> single particle transition. The model is capable of predicting the temperature and pressure dependence of both the protonic tunnelling frequency and the protonic inter-site separation δ. The soft-mode Raman data of Peercy (1975) is re-analysed allowing for the temperature dependence of Ω.
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    Whispering-gallery-mode dye laser emission from liquid in a capillary fiber
    (1993) Knight, Jonathan Cave; Robertson, G N; Driver, H S T
    The nature of optical whispering-gallery-mode resonances in a layered microcylinder is investigated numerically by studying the scattering characteristics and the internal electromagnetic fields of a normally-illuminated cladded dielectric fiber calculated using the boundary-value method. Computed resonant mode configurations are compared to the better-known results for homogeneous spheres and cylinders and coated spheres. It is shown that high-Q whispering-gallery-mode resonances can be supported by the curved interface between the core and cladding regions of a layered fiber if the core refractive index is sufficiently greater than that of the outer layer, and that these modes can be directly related to the so-called morphology-dependent resonances of a homogeneous cylinder of the same size and relative refractive index as the fiber core. The implications of these resonant modes for inelastic optical processes are made clear by developing a model for optical emissions from a molecule in the core of a capillary fiber. The results of the model show that the transition rates of molecules in the fiber core and near to the core/cladding interface are enhanced at frequencies corresponding to cavity resonances. It is shown experimentally that these high-Q cavity modes can be excited to above the threshold for laser emission by providing gain in the fiber core material. We have used a refractive dye-doped solvent as a gain medium and a fused-silica capillary to form the resonant cavity. Upon optical excitation of the dye by illuminating the fiber normally with the green beam from a frequency-doubled Nd:YAG laser, laser emission is emitted from the fiber core in the plane perpendicular to the fiber axis. We explain the novel spatial and spectral dependences of the laser emission in terms of the calculated frequencies and Q-values of the resonant cavity modes and the bulk properties of the cavity medium. We show that the thresholds observed in the laser system can be explained using a simplified rate-equation approach, and that this also explains some of the other observed features of the emissions. The heating of the dye solvent during a laser pulse has an observable effect on the resonance mode locations due to the temperature dependence of the refractive index. We demonstrate the use of observed laser spectra to determine the size and taper of the capillary fiber core.