Preusche, Andreas (2021)
Non-invasive thermometry and wake mixture fraction determination of evaporating droplets at elevated pressures using laser spectroscopy.
Technische Universität Darmstadt
doi: 10.26083/tuprints-00019515
Dissertation, Erstveröffentlichung, Verlagsversion

Kurzbeschreibung (Abstract)

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 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. Using density estimates from measured mixture fractions, an equation of state translation into equivalent temperatures is used to indirectly approximate the liquid phase temperature. The results show good agreement between the two methods.

The second campaign in this work aims to determine the mixture fraction in the wakes of evaporating acetone and heptane droplets in carbon dioxide atmosphere with Raman imaging. The experimental setup is adapted to accommodate this inert gas, and 2D Raman scattering characterization and measurements are carried out. A 532nm laser source is used to produce a laser sheet above the droplets, which interrogates the mixing wake during their fall. Due to the strong spectral overlap of the Raman responses, the temperature and pressure dependence of this method are characterized. The characterization shows a pressure dependence, so the method is adapted before being used to measure the wake mixture fractions. The results are compared to previous campaigns using nitrogen atmosphere in matching conditions. They show that the carbon dioxide atmosphere increases diffusion of acetone in to the vapor mixture compared to nitrogen while the heptane droplet wakes mix similarly between the two inert gases. Pressures of 2 MPa and 4 MPa, as well as combinations of atmosphere temperatures from 393 K to 533 K and injector temperatures of 393 K to 493 K are measured for a fall height of 19 mm at 2 Hz droplet detachment.

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