Session T40: DFD VIII

Transparent superconductivity in lithiated indium tin oxide thin films

Indium tin oxide (Sn-doped In2O3; ITO) is a well-studied transparent conductor where doping can be used to stabilize a superconducting state. In this work, we use a combination of thin film deposition of ITO and soft-chemistry using n-BuLi (C4H9Li) to realize superconductivity in Li-doped ITO. Using x-ray diffraction, atomic force microscopy, electronic transport, and optical transmission measurements, we characterize the optical transparency and superconductivity of lithium intercalated ITO thin films. After 72 hours of lithium intercalation, we find a critical temperature, Tc, of 0.49 K and an optical transparency of at least 70% in the visible optical range–all while maintaining crystallinity.   

Particle-fluid interaction force in nonuniform particle laden flows

Particle-fluid interaction force is of the primary importance in modeling particle laden flows. Most models for the force are developed from studies of uniform flows, while in most practical multiphase flows, the particle volumes are not uniform. In this work we study the effect of gradient of the particle volume fraction on the force. We find the particle-fluid interaction force can be decomposed into three terms. The first term is the average fluid force on a pair of nearest particles. Since this force component is the same on the nearest pair, it does not cause relative motion between the pair; and therefore, is not responsible for formation and deformation of particle clusters. In the second term we introduce a diffusion stress. The second term is the product of the diffusion stress and the volume fraction gradient.  This diffusion stress is similar to a tensor diffusion coefficient, but instead of producing a mass flux, it produces a force. The third term is the divergence of particle-fluid-particle (PFP) stress. Both the diffusion stress and PFP stress are results of the fluid mediated particle-particle interactions.  The diffusion stress is only important when there is a gradient in the particle volume fraction. The PFP stress can also be important in uniform particle distributions but with spatially varying velocity fields.  Definitions, properties, numerical results, and physical meanings of these terms will be presented.   

Modeling Cryogenic Two-Phase Fluid Transients: A Numerical Investigation of Rapid Valve Closure

A fluid transient occurs when abrupt changes in flow characteristics happen within a fluid network, often due to rapid valve opening or closure or the failure of fluid handling components. This leads to a pressure surge, followed by pressure wave oscillations. When these pressure fluctuations approach or drop below the vapor pressure, cavitation may occur. Cryogenic fluid handling systems frequently face the risk of such fluid transients. Cavitation is marked by the formation and collapse of bubbles caused by high-pressure gradients. Bubbles near solid surfaces collapse rapidly, generating high-velocity jets and pressure waves. The resulting pressure can exert stresses on materials exceeding their yield strength, potentially causing damage.

This study aims to numerically model two-phase fluid transient-induced cavitation resulting from the abrupt closure of a valve. It employs the finite volume computational fluid dynamics (CFD) approach. Additionally, a one-dimensional numerical model is developed using the Method of Characteristics (MOC) and incorporates the Rayleigh-Plesset equation for bubble dynamics. Comparing results from both models reveals that CFD outperforms MOC due to its ability to account for radial variations and visualize two-phase fluid transients more effectively. However, it is worth noting that the MOC method is computationally more cost-effective than CFD.   

Effect of Design Parameters on Flame Flashback Phenomenon Utilizing a Strut-Cavity Flame Holder :: A Numerical Study

Hypersonic air-breathing propulsion is now a viable and cost-effective method for launching payloads into orbit. This technology has significant advantages for both military operations and high-speed civilian transportation. However, challenges such as inefficient mixing, poor combustion, and flame stabilization issues arise due to short flow residence times. Addressing flame stability and combustion enhancement is crucial for effective scramjet designs. The underlying physical mechanism of flame flashback remains unclear, necessitating intensive research to understand its origins. In this study, we investigated the impact of design parameters on flame flashback using a newly developed configuration of a strut-cavity flame holder. By examining the influence of upper wall divergence, strut half angle, and cavity offset ratio, we gained insights into the mechanism of flame flashback.  The two-equation k-ω SST turbulence model was utilized for these simulations. The shear layer, originating from the cavity corner’s leading edge, impinges on the transverse fuel injection from the rearward step of the offset cavity. This impingement results in a complex flow field around the strut-cavity flame holder, characterized by interactions between shear-layer-shock phenomenon, cavity recirculation zones, and the trajectories of both the shear layer and fuel. During transient flow, a pre-combustion shock train reduces the flow speed, allowing heat release to influence the flow field and upstream flame propagation. Wall divergence contains the heat release and counteracts flashback speed caused by flow acceleration. Additionally, factors like thermal choking, low-speed zones, and boundary layer separation contribute to the interplay between the flow and flame speeds. Reducing the height of the rear wall of the cavity in relation to the cavity offset ratio created low-speed zone. This adjustment improved the interaction between the fuel jet and the surrounding fluid. The duration the fuel spent within the cavity, influenced by both the cavity size and the way the fuel jet interacted, was a key factor. It was noted that the cavity with an offset ratio was more likely to cause early unstart due to an increased flame flashback speed, disrupting the core supersonic flow.    

A wall-less waveguide solution for enhanced propagation of shallow water wave

The efficient transmission of shallow water waves over long distances poses a significant challenge due to the inherent spreading loss experienced during propagation. Conventional methods, including reflection-based waveguides, are limited by modal dispersion, restricting their operational bandwidth. We are introducing a novel approach to overcome these limitations through the utilization of a broadband wall-less waveguide engineered for shallow water waves. Inspired by graded-index optical fibers, the proposed waveguide utilizes a visco-elastic sea-bed carpet to achieve a tunable effective gravitational acceleration. This mechanism enables the control of wave propagation without physical boundaries, allowing the focused transmission of water waves over long distances with significantly reduced attenuation of wave energy. Numerical simulations reveal that water waves disperse in the transverse direction due to flat bottom topography, resulting in a large drop in wave amplitude. The waveguide, on the other hand, demonstrates better wave confinement with notably less reduction in wave amplitude. These findings present a promising solution for applications in various fields, including the design of artificial surf zones, wave energy farms, and coastal protection mechanisms.   

CO2 capillary trapping under varying wettability scenarios

Geological sequestration involves the injection of CO₂ in either (dissolved) gas, liquid, or supercritical phase, into the pore space of subsurface rock formations. The pore space in sedimentary rocks could suffice to store all the CO₂ removed from the air, making geological sequestration a promising carbon storage technology.  Once there, CO₂ can become trapped due to a series of physical and/or chemical mechanisms, some relating directly to the pore scale, such as capillary trapping inside the rock’s microscopic pore channels.  In this work, we apply pore-scale flow simulations to the study of CO₂ storage in geological formations modeled as a network of connected capillaries with spatially varying radii.  Multiphase flow simulations were employed to study the physics behind residual storage by analyzing the infiltration and retention of CO₂ inside the capillary network of a porous sandstone rock sample under varying fluid and rock parameters. We found that the conditions for maximum CO₂ storage through capillary trapping in a water filled reservoir greatly depends on the fluid interface contact angle and on the applied pressure gradient, followed by the absolute temperature of the reservoir, as it affects the viscosity of the fluids. The fluid interface contact angle is a manifestation of the wettability of the rock and may be modulated with additives in the injected fluid. Our simulation showed that beyond a pressure threshold, a contact angle approaching 90 degrees maximizes the storage of super-critical CO₂ as the effect of capillary pressure is minimized, leaving viscosity as the dominant forces limiting the displacement of the resident fluid from the pores.