Development and Application of Rotational Coherent Anti-Stokes Raman Spectroscopy and Laser-induced Fluorescence for Combustion Diagnostics

Abstract: The thesis concerns the development and application of two laser-based methods for combustion diagnostics: dual broadband rotational coherent anti-Stokes Raman spectroscopy (CARS) and laser-induced fluorescence (LIF). In efforts to increase the usefulness of the CARS technique, the applicability of a method involving a combination of the rotational CARS technique and the closely related vibrational CARS technique was investigated. In addition an extension of the rotational CARS technique such that not only single-point but also multiple-points measurements could be performed was presented. The rotational CARS technique was also used in an experimental investigation of a dimethyl ether (DME)-air diffusion flame for determining temperatures and relative oxygen concentrations. The major combustion application of the rotational CARS technique taken up in the thesis was its use for temperature measurements in an internal combustion engine operated under knocking combustion conditions. Temperature measurements were performed at different locations within the combustion chamber and at different points in time during the engine cycle. The insights these measurements provided, both concerning the experimental application of the rotational CARS technique in engines and in the evaluation of the measured data, were summarized in a review paper. The work with the LIF technique presented in the thesis involved primarily studies of formaldehyde, CH2O. Spectroscopic investigations of CH2O using excitation at a wavelength of 355 nm were carried out in laboratory flames. Laser-induced fluorescence by use of 355 nm excitation was also employed for measurements of formaldehyde in internal combustion engines. Measurements in a spark ignition engine involving two consecutive LIF measurements being performed within a given engine cycle provided data that could be used for determining the flame propagation within the combustion chamber. The characterization of the DME diffusion flame referred to above also included measurements of formaldehyde by use of the LIF technique, particular attention being directed at obtaining a qualitatively correct profile of the formaldehyde concentration in the flame by adjusting the measured signal to take account of quenching and temperature dependence. The laser-induced fluorescence technique was also used to investigate the devolatilisation of wood particles, a process representing the first step in the combustion of solid fuel of this type. The studies carried out included measurements of absorption and of both spectral and imaging LIF. The multi-species composition of the products emitted, hydrocarbon compounds in particular, resulted for both the absorption and the spectral LIF measurements in spectral signatures without distinct features, making identification of specific species difficult. Formaldehyde, which could be identified from the measured spectra was an exception to this. The imaging LIF measurements also provided some information on the flow of the emitted volatiles in relation to the internal wood fibre structure.