Optimierung nicht-linearer laserbasierter Messmethoden zur Charakterisierung von reaktiven Strömungen

In this work advancements of two four-wave mixing based laser measurement techniques, the rotational coherent anti-Stokes Raman spectroscopy (RCARS) and the laser-induced grating technique (LIG), for the diagnostic of reactive flows are presented. The developed approaches aim at optimizing the diagnostic techniques regarding applicability and accuracy as compared to the so far available methods.
For the study of oxygen as a probe molecule in combustion processes using the RCARS-technique the temperature and collider dependent S-branch Raman linewidths are determined. These are required in contour-fitting algorithms for the determination of temperature, species concentration and pressure. So far, the use of the only available Q-branch linewidths and the neglection of the proper gas phase collision partners result in deviations and inaccuracies of the obtained thermodynamic conditions. Therefore, the S-branch Raman linewidths of pure oxygen, oxygen in air and a defined mixture with CO2 are determined in a temperature range from 295K to 1900K applying the time resolved RCARS-technique. To this end the influence of the triplet electronic ground state of oxygen on the temporal evolution of the signal intensity need to be considered and a novel evaluation procedure needs to be developed. Furthermore, the S-branch linewidths of CO2 in the mixture with oxygen are to be determined to allow for accurate diagnostics of oxyfuel-combustion processes using RCARS-diagnostics in the future.
In application of the LIGS-technique for the diagnostics of reactive flows and mixture formation processes, the evaluation is often based on the easily obtainable sound velocity. The sound velocity, however, exhibits multiparametric dependences on substance data and thermodynamic conditions, which are included in the same LIG signal profile, but are tedious to extract. Hence, in this work procedures are developed, which make use of additional signal contributions recorded within the same LIGS-measurement, to facilitate simultaneous determination of multiple thermodynamic parameters. These are on the one hand a direct two-color four-wave mixing (TCFWM) signal contribution in CO2 in mixture with nitrogen and on the other hand a thermal signal contribution which is generated by pump-laser absorption and subsequent heat release in ammonia-nitrogen gas mixtures. For both process a physical model is developed, which allows application of these methods for different gas mixtures by inserting the respective substance data into the model.