Non-invasive thermometry and wake mixture fraction determination of evaporating droplets at elevated pressures using laser spectroscopy

Andreas Preusche
2021
In order to improve the modeling and understanding of evaporation and mixing phenomena at elevated pressures, experimental data are needed for comparison and validation. In this work, the evaporation of free falling droplets in such conditions is studied experimentally. A pressure vessel with optical access is used to inject liquid acetone and heptane into pressurized nitrogen and carbon dioxide atmospheres. A temperature controlled injector then forms droplets that detach either freely or with
more » ... the help of electrostatic charge. First, laser induced fluorescence and phosphorescence (LIFP) of acetone is used to noninvasively measure the average temperature of the acetone droplets during their fall. An ultra-violet dye laser setup for emission at 320nm is build to excite the liquid acetone and the resulting fluorescence and phosphorescence emissions are recorded. Using a ratio metric approach, the total emission as well as the separated phosphorescence alone are used to characterize their temperature dependence for pressures 2, 4 and 6 MPa and temperatures of 393 K to 508 K. The signal separation is realized in the time domain using a time gated image intensifier. The results of this characterization show a significant non-linear influence of the emission. The reason for this is the saturation of the acetone phosphorescence, even for laser energy densities less than 1 mJ/cm^2. This is successfully mitigated by including an excitation energy dependence in the calibration, which is shown to be pressure independent for the three characterized pressures. Subsequently, free falling droplets of acetone in nitrogen atmosphere are measured using this technique to determine their average temperature. Atmosphere temperatures ranged from 433 K to 513 K and injection temperatures ranged from 433 K to 503 K. Two different fall heights, 12 mm and 23 mm from the injection capillary, as well as the detachment frequencies 1 Hz and 2 Hz are investigated. The temperature results are put into context with previous measurement campaigns [...]
doi:10.26083/tuprints-00019515 fatcat:owivvsh66zaz7eol523glr2z3e