Browsing by Subject "multiphase"

Now showing 1 - 3 of 3
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    Design Of Multiphase Rare Earth Aluminate Zirconate Thermal Barrier Coating Materials For Enhanced Reactivity With Molten Silicates.
    (2022-09) Godbole, Eeshani Paresh
    The hot-section components in turbine engines rely on ceramic thermal and environmental barrier coatings (T/EBCs) for insulation and corrosion protection. The ability of these coatings to mitigate premature failure caused by calcium-magnesium-aluminosilicate (CMAS) based corrosive deposits is critical to ensure the desired component lifetimes. This work proposes advanced multiphase rare earth (RE) aluminate zirconates as candidate coating materials to promote a more predictable and consistent coating-CMAS reaction response against a range of deposit compositions. An integrated process using experiments and thermodynamic modelling tools was used to accelerate coating design. Understanding reactions between coating materials and CMAS deposits is important to design next generation coatings that can withstand CMAS attack to higher temperatures. This need was addressed through three experimental thrusts. The first focused on understanding the intrinsic stability of multi-cation, mixed oxide/sulfate deposits. The results showed that specific reactions between sulfates and CMAS oxides drive rapid decomposition of the sulfate, implying that the deposits inducing coating degradation would be primarily oxides. The second thrust improved the understanding of the temperature- and composition-dependent extent of the rare earth aluminosilicate garnet phase field, and the influence of RE ion identity on the equilibrium transitions between silicate apatite and garnet phases in Gd/Y/Yb+CMAS systems. Guided by computational design tools developed using results from the early experiments, the third thrust evaluated the performance of RE- rich multiphase aluminate zirconate novel candidate coating materials. The inclusion of alumina in traditional Y or Gd zirconate coating compositions was hypothesized to stabilize garnet as a CMAS reaction product, promoting more consistent reaction sequences across a variety of deposit compositions, and improving the reliability of coating materials in different service environments. The extent of reactive melt spreading and coating-CMAS reaction depths in conjunction with microchemical analyses of reaction products were used to evaluate the response of sintered pellets of candidate coating materials against model deposit compositions. As hypothesized, the addition of alumina to the coating material resulted in garnet formation along with a range of crystalline products which maximized the reactive consumption of molten deposits. This work has established an efficient protocol towards utilizing targeted experiments integrated with thermodynamic computations to accelerate materials discovery.
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    Laboratory Investigation Of Dispere Multiphase-Turbulent Flows, Dilute & Dense Distributions Of Inertial Particles Settling In Air
    (2020-05) Petersen, Alec
    Turbulent multiphase flows are found throughout our universe, all over Earth and in many man-made systems. Despite surrounding us, their dynamics are still in many ways obscure and require further study. These chaotic systems are however quite complicated to both simulate or explore experimentally. In this thesis, we present our laboratory investigation of particle-laden turbulent flows in air. We first focus on the statistical dynamics of dilute multiphase turbulence. Utilizing a zero-mean-flow air turbulence chamber, we drop size-selected solid particles and study their dynamics with particle imaging and tracking velocimetry at multiple resolutions. The carrier flow is simultaneously measured by particle image velocimetry of suspended tracers, allowing the characterization of the interplay between both the dispersed and continuous phases. The turbulence Reynolds number based on the Taylor microscale ranges from 200 – 500, while the particle Stokes number based on the Kolmogorov scale varies between O(1) and O(10). Clustering is confirmed to be most intense for Stokes ≈ 1 , but it extends over larger scales for heavier particles. Individual clusters form a hierarchy of self-similar, fractal-like objects, preferentially aligned with gravity and sizes that can reach the integral scale of the turbulence. Remarkably, the settling velocity of Stokes ≈ 1 particles can be several times larger than the still-air terminal velocity, and the clusters can fall even faster. This is caused by downward fluid fluctuations preferentially sweeping the particles, and we propose that this mechanism is influenced by both large and small scales of the turbulence. The particle-fluid slip velocities show large variance, and both the instantaneous particle Reynolds number and drag coefficient can greatly differ from their nominal values. Finally, for sufficient loadings, the particles generally augment the small-scale fluid velocity fluctuations, which however may account for a limited fraction of the turbulent kinetic energy. We also investigate denser particle-laden flows, specifically plumes driven by the downward buoyancy of inertial particles. With similar tools, we conduct two experiments: one to capture the particle-phase behavior and another to measure the ambient air velocity. Our first focus is on the assumption of self-similarity, which unlike single-phase plumes is not a trivial assumption. We also characterize the mean plume properties observed: the particle-phase velocity and the plume spread comparing their evolution with axial distance from the plume source. From our measurements of the ambient air flow we calculate the entrainment velocity into the particle-laden plumes and using the time-averaged value we estimate the entrainment coefficient along the plume. We find a relatively stable entrainment rate, as expected in the assumption used to formulate many integral plume models. Lastly we compared our experimental results to single and multiphase plume models with the same initial conditions as our experiments. Our multiphase plume model, inspired by the work of Liu (2003) and Lai et al. (2016), well described our velocity measurements, which single phase models were completely unequipped for.
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    A numerical and theoretical study of drag reduction using superhydrophobic surfaces
    (2018-09) Li, Yixuan
    Motivated by the potential drag reduction benefits of superhydrophobic surfaces (SHS), direct numerical simulation (DNS) and theoretical analyses are used to explore the interaction between SHS and turbulent channel flow. First, DNS is used to study the drag reduction by SHS in laminar channel flow. Resolved multi-phase simulations using the volume of fluid (VOF) methodology are performed to study the effects of groove geometry, interface shear rate and meniscus penetration independently. An analytical solution for the flow in a laminar channel with grooved surface with gas-pocket within is obtained. The solution accounts for both the groove geometry and the trapped fluid properties, and shows good agreement with simulation results. The solution is used to propose a scaling law that collapses data across fully wetted to fully gas-filled regimes. The trapped gas is simulated as both flat and meniscal interfaces. The drag reduction initially increases with interface deflection into the groove and then decreases for large deflections as the interface velocity approaches zero due to the proximity to the bottom of the groove. Next, the geometric effect of SHS in turbulent flow is studied by performing DNS at friction Reynolds number $\Rey_\tau = 400$ over longitudinal grooves whose size is comparable to the viscous sublayer thickness. It is found that despite the bulk flow being close to that of a flat-wall channel, the slip effect of the grooves causes some differences within the viscous sublayer. Spectral analysis of the velocity transfer function between the interior and the exterior regions of the grooves shows that the grooves suppress the energy at low frequencies. The DNS reveals negligible Reynolds shear stress near the grooves, which motivates an unsteady Stokes flow model. It is assumed that the flow in the vicinity of the grooves is governed by the unsteady Stokes equations, forced by an oscillating external flow. The effects of streamwise, spanwise and vertical velocity, freestream wavenumber and the height of freestream perturbation above the groove are studied. The non-dimensional parameter $\omega L^2/\nu$ obtained from this model problem ($L$ is half of the groove span, $\omega$ is the frequency of the external turbulent signal and $\nu$ is the kinematic viscosity) is used to relate the model to the current DNS. Good agreement is seen with DNS at low frequencies. It is suggested that higher frequency disturbances are produced by smaller spanwise structures near the wall, and when this effect is accounted for, good agreement is also observed at higher frequencies. Finally, we study multiphase flow within grooved textures exposed to external unsteadiness. We derive analytical expressions for multiphase unsteady Stokes flow within periodic grooves driven by oscillating streamwise/spanwise freestream velocity. Good agreement is obtained between the analytical solution and DNS performed with the VOF method. The effect of oscillation frequency, Reynolds number, and the multiphase interface location on the transfer function between the input signal external to the groove and output near the interface, is examined. Also, the effective slip length and the shear stress over the grooved plane are studied.