Development of Dual-broadband Rotational CARS for Combustion Diagnostics

Abstract: The present thesis concerns development and application of dual-broadband rotational coherent anti-Stokes Raman spectroscopy (DB-RCARS) for temperature and species concentration measurements in combustion processes. Both fundamental development of the technique, including experimental as well as modelling results, and measurements in practical combustion devices were conducted. A code for calculation of rotational CARS spectra of pure acetylene as well as mixtures of acetylene and nitrogen was developed. Using this code, temperatures and relative acetylene to nitrogen concentrations were evaluated from DB-RCARS measurements in pure acetylene and different acetylene/nitrogen mixtures. Moreover, rotational CARS spectra of dimethyl-ether (DME) have been analyzed. A powerful tool for simultaneous temperature and multiple species concentration measurements was developed by combining rotational CARS with vibrational CARS. The concept was demonstrated for measurements of temperature, oxygen, and carbon monoxide concentrations simultaneously in a premixed sooting ethene/air flame. Rotational CARS spectra of nitrogen at very high pressures (0.1-44 MPa) at room temperature were investigated. The experimental spectra were compared with calculated spectra using different Raman linewidth models. The results indicate some shortcomings in the present model, basically the density calculation and neglecting overlapping effects between adjacent spectral lines. A new method for CARS measurements in several spatially separated points simultaneously was developed. By using DB-RCARS the method was demonstrated for quantitative measurements of profiles of temperatures and oxygen concentrations. An atomic filter for rejection of stray light was developed. The filter was shown to efficiently reject stray light from the narrowband laser without affecting the shape of the rotational CARS spectrum or causing any signal losses. Within an interdisciplinary project intended to increase the fundamental knowledge about engine knock, DB-RCARS was used for the first time for thermometry in a spark-ignition engine running on a fuel consisting of a mixture of higher hydrocarbons (iso-octane and n-heptane). Cycle-resolved temperature measurements were successfully performed in the unburned gas mixture (the end-gas) ahead of the propagating flame front at pressures below 3.0 MPa and temperatures below 1000 K. In order to investigate the performance of a catalytic combustor, simultaneous temperature and relative oxygen-to-nitrogen concentrations were measured by DB-RCARS in the exhaust gas of the combustor. From a theoretical sensitivity analysis it was found that the accuracy of temperature and oxygen concentration could be improved by fitting the nonresonant susceptibilty in the evaluation process.