Dissertation Defence: Clustering characteristics of large-Stokes-number sprays interacting with turbulent co-flows

Ali Rostami, supervised by Dr. Sina Kheirkhah, will defend their dissertation titled “Clustering characteristics of large-Stokes-number sprays interacting with turbulent co-flows” in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Mechanical Engineering.

An abstract for Ali Rostami’s dissertation is included below.

Examinations are open to all members of the campus community as well as the general public. Registration is not required for in-person exams.

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Abstract

Clustering characteristics of large-Stokes-number sprays interacting with turbulent swirling and non-swirling co-flows are investigated experimentally. Stereoscopic Particle Image Velocimetry, hotwire anemometry, Interferometric Laser Imaging for Droplet Sizing, Astigmatic Interferometric Particle Imaging, Mie scattering, shadowgraphy, and Particle Tracking Velocimetry are utilized. A new diagnostic, referred to as the Paired Astigmatic Interferometric Particle Imaging, is developed and used. For non-reacting conditions, the tested sprays are dilute with measured volume fraction varies from about 0.3 × 10⁻⁶ to 2.2 × 10⁻⁶. The Taylor-length-scale-based Reynolds number varies from 10 to 47, and the Stokes number estimated based on the Kolmogorov time scale varies from 3 to 142. For reacting sprays, the experiments are conducted for a fixed combustor nominal power of 10 kW and for three global fuel-air equivalence ratios of 0.5, 0.6, and 0.7. The results show that in droplet-laden flows with large Stokes numbers and small values of the Taylor-length-scale-based Reynolds number, the degree of droplet clustering plateaus at 0.3–0.4. It is shown that the mean length scale of the clusters normalized by the Kolmogorov length scale follows a power-law relation with the Stokes number, and the mean void length scale normalized by the integral length scale plateaus at about 1.5 at large Stokes numbers. The results show that the number density of the droplets in the clusters is about one order of magnitude larger than that for the voids. It is concluded that the ratio of the number density of droplets within clusters (voids) to the global number density is about 5.5 (0.8), independent of the presence of swirling flow, the type of diagnostics (2-D versus 3-D), and for Taylor-length-scale-based Reynolds number varying from 20 to 40. For the first time, three-dimensional clustering characteristics of reacting Jet-A1 sprays stabilized inside a gas turbine model combustor are obtained. The results show that the interaction of the turbulent swirling flow with the droplets influences the PDFs of axial and tangential droplet velocity, while the PDF of the droplet radial velocity remains almost unchanged. The results are of significant importance for engineering applications that aim to understand the 3-D clustering characteristics of large-Stokes-number droplets sprayed into turbulent swirling and non-swirling co-flows.