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978-3-8439-2110-7, Reihe Thermodynamik
Flash-atomization and vaporization at near vacuum conditions
187 Seiten, Dissertation Universität Stuttgart (2015), Softcover, A5
The objective of this study was to provide a better understanding of liquid flash-atomization and vaporization at near vacuum condition. This understanding of these complex phenomena is relevant for the design of upper stage liquid rocket engines and so to increase the combustion efficiency. Within this thesis, flash-atomization and vaporization in a vacuum chamber was experimentally investigated. The experiments give information on the flashing spray morphology, velocity, temperature and droplet size evolution in the spray. From the gathered experimental database a superheated fuel atomization and vaporization model was developed.
For the experiments, a new test bench is designed. The morphology of a flash-atomizing jet were investigated by means of high-speed shadowgraphy. The analysis of shadowgraphs indicates that bubble nucleation is the rate-controlling process for both the transition to fully flashing and for the spray lateral spreading.
The velocity of the particles in a flashing spray was measured by means of the Laser Correlation Velocimetry. Temperature measurements are performed with the Differential Infrared Thermography. Based on the temperature and velocity experimental data, the spray is divided into two regions: flash-atomization and spray main region. In the first region, the properties of the flow are mainly controlled by nucleate boiling. In the main region, the evolution of the flow field is mainly controlled by entrainment and evaporation. The Global Rainbow Thermometry was applied to measure the droplets diameter. The measurements' results showed a homogeneous droplet size distribution in the flashing spray due to the explosive disintegration of the superheated liquid jet.
For the numerical simulations, new flash-atomization and vaporization models were developed. The droplet size measurements and the spray spreading angle correlation were used to establish and validate the new flash atomization model based on the nucleation theory. The flash-vaporization model is based on the extension of the D2-theory presented by Abramzon and Sirignano. However the model was modified to be suitable for flashing conditions. To validate both models, a 1-D simulation using the Euler-Lagrange method was performed. The results are then compared with the experimental results. The good agreement between the simulation results and the experiments shows the ability of the new models for predicting flash-atomization and vaporization processes.