Date of Award
Doctor of Philosophy
Theodore J Heindel
Understanding the near nozzle region of a spray is integral to spray optimization and control efforts because this region is where liquid break up and spray formation occurs, setting the conditions under which the spray dynamics evolve. However, the near-nozzle region of sprays is not yet well characterized because the high optical density complicates measurements. Recent innovations in X-ray technology enable high-quality measurements and visualization techniques that are capable of capturing the high-speed structures. To improve characterization and expand upon current analysis methods, the studies presented in this thesis use synchrotron white-beam X-ray radiography, synchrotron focused-beam X-ray radiography, tube source X-ray radiography, and back-illuminated imaging on the spray from a coaxial nozzle.
In one of the following studies, a comparative analysis of near-nozzle experimental techniques is presented that shows how penetrating X-rays provide additional information over the established method of shadowgraphs. A technique to estimate the intact length of sprays from X-ray radiographs was presented and validated. Additionally, a reasonable estimate of spray angle from shadowgraphs was consistent with X-ray radiographic techniques, showing that the information obtained from shadowgraphs was sufficient for this calculation.
An initial study using focused-beam synchrotron X-ray measurements with a cross sectional area of 5 x 6 μm to measure the attenuation in the beam (used to calculate the effective path length of liquid) at an effective rate of 270 kHz for 10 seconds was also completed. Various statistical measures were applied to the X ray focused beam measurements including average, standard deviation, skewness, and kurtosis, to quantify the spray from a canonical coaxial airblast nozzle. Results showed that the average effective path length was useful in determining the intact length and spray angle. The capabilities of statistical measures in determining important spray characteristics was also discussed.
A study using image based feedback control to optimize the spray half angle (θ), obtained from shadowgraphs, with the assumption that the largest θ is desired, was also completed. While keeping the total air flow rate constant, varying ratios of swirled air to straight air (SR), determined by the image based feedback controller, were introduced into the air portion of a coaxial airblast nozzle. A golden section search converged on the SR that provided the largest θ and was validated by the distribution of θ versus SR. The ratio that produced a spray with the greatest angle of θ = 25.8 ± 2° was found at SR = 0.66 ± 0.03 for a spray with a momentum ratio of 6. The successful design and implementation of the image based feedback controller was intended to provide a foundation for developing real time active feedback controllers for sprays.
One study was implemented to develop a method of determining the mass-averaged axial velocity of a spray from focused-beam X-ray radiography. The newly developed method was then used to determine the mass-averaged axial velocity of a co-axial airblast spray in the near-field region. Results showed that the spray velocity increased linearly with distance for the range in which data was taken. Additionally, the slope of the velocity with downstream distance increased linearly with the gas Reynolds number. The results imply that it may be possible to predict the mass-averaged axial velocity of a spray in the near-field region.
The final study provides an in-depth analysis of spray breakup from white-beam X-ray radiographs. The time series images showed many structure formations such as ligaments, bags, bubbles, mushrooms, crowns, and webs. The images also showed breakup processes of each of these structures and how each breakup process resulted in different sizes of droplets or alternative structures.
Julie Kathleen Bothell
Bothell, Julie Kathleen, "Experimental studies of the near-nozzle region of a coaxial spray" (2020). Graduate Theses and Dissertations. 18282.