Session X05: Multiphase Flows: General (10:45am - 11:30am CST)

Numerical simulation of gaseous-granular multiphase flow with phase change and external heat input

A new physical and numerical model for gas-granular multiphase flow of neutral gas, water vapor, ice, and dust with phase change between ice and water vapor was developed and tested. The objective is to investigate the ice formation process at the surface of the comet TEMPEL 1. A numerical test problem was set up to simulate a 1D vertical tube that can be constructed in a terrestrial laboratory. Conditions for the test problem are ~0.2 atm and 250K. Simulations were performed with and without cyclic external heat input. As expected, ice and dust particles separate and lead to a final state with ice on the top and dust at the bottom. Surprisingly, this result is insensitive to the period of the heat cycle. These conclusions are explained based on the physical limits of the sublimation model. The next steps are to test the model in a laboratory experiments and then extend it to conditions more descriptive of the surface of a comet.   

Direct Numerical Simulations of Frost Buildup Below Turbulent Flow

We present results from two-way coupled direct numerical simulations of frost buildup below a turbulent open-channel air flow at $Re_{\tau}=180$. We solve the conservation equations of mass, momentum, and energy along with a transport equation for vapor mass fraction. We use the immersed boundary method to account for the presence of the temporally and spatially evolving frost surface by imposing the no-slip, no-penetration boundary condition. The bottom plate, over which frost develops, is maintained at a uniform and constant temperature. Additionally, the mean bulk temperature and humidity of the turbulent flow are also kept unchanged for the duration of the simulation. By equating the temporally and spatially resolved convective and diffusive heat fluxes to the conductive flux between the frost surface and the bottom plate, we extract the time-dependent temperature at the frost surface. Furthermore, we use mass conservation of water vapor to measure the growth rate of the frost thickness over time. Our simulations are validated against laminar experimental data and could be used for comparison with future simulations or experiments of frost buildup below turbulent flow.   

Seawater desalination using ion removal kinetics of MOF-incorporated alginate composite

Water scarcity is one of the global issues resulted from water pollution and population growth. To resolve this problem, various seawater desalination technologies, such as reverse osmosis, multi-stage flash, and solar evaporation, have been developed, because abundant seawater is easily accessible. Adsorptive desalination method can remove pollutants or ionical matters without consuming energies. In this study, an adsorptive metal-organic framework (MOF)-embedded alginate hydrogel intertwined with poly(vinyl alcohol) (PVA) was proposed to get high ion removal efficiency and long durability. The adsorption performance of the fabricated hydrogel, including adsorption kinetics, ion removal rate, and adsorption/desorption cycle test, was examined by using NaCl solution and seawater. As a result, the adsorption equilibrium state was reached within 4 hours and the maximum ion removal rate was 33 {\%}. Furthermore, when the proposed hydrogel was reused for cycle tests, the total ion removal efficiency was slightly reduced and kept higher than half after 10 repeated cycles. These results imply that the proposed hydrogel would be utilized in an effective desalination with a high concentration.   

Experimental investigation of a vapor chamber featuring wettability-patterned surfaces

Wettability patterning has been shown to increase condensation performance and transport liquid microvolumes pumplessly at speeds O(0.1 m/s), even against gravity. Vapor-chamber heat spreaders are hollow heat sinks that house metal wicks inside to circulate a phase-changing liquid and spread heat more efficiently than solid-metal heat sinks. However, metal wicks, besides providing pumpless fluid transport, also come with capillary limitations due to high pressure drops. Wettability patterning does not pose the same limitations and has potential to replace metal wicks for transporting fluids faster and more efficiently, in addition to effectively regulating filmwise and dropwise condensation. Thus, an intriguing hypothesis is explored here by incorporating wettability patterning (i.e. eliminating wicks) inside vapor chambers. Multiple copper vapor chambers incorporating wettability patterns with different ratios of superhydrophilic to hydrophobic areas on the condenser side are experimentally investigated. The devices are tested at different configurations. The lowest thermal resistance achieved is 0.24 K/W at 87 W heat load. The current work demonstrates the promise of wettability patterning for inclusion in vapor chambers.   

Liquid entrainment and gas pull-through in horizontal, stratified gas-liquid flows

Flows of water-air and saturated water-steam mixtures in a horizontal cylindrical pipe with a single transverse cylindrical outlet were investigated computationally using the volume of fluid (VOF) method to separate the phases and the detached eddy simulation (DES) model to simulate turbulence. The time-dependent gas-liquid interface was resolved and the vertical distance $h$ between the interface elevation and the outlet was determined. The values $h_{\mathrm{ole}}$ at the onset of liquid entrainment and $h_{\mathrm{ogp}}$ at the onset of gas pull-through for different geometries and inlet flow rates were expressed as empirical functions of the Froude number and the ratios $h$/$h_{\mathrm{ole}}$ and $h$/$h_{\mathrm{ogp}}$ following onset of the corresponding incident were expressed as functions of the mass quality of the fluid. Available two-phase flow models appear to describe fairly well both liquid entrainment and gas pull-through. A combination of the Bond number and the Froude number was introduced into a model to represent the effects of surface tension and inlet liquid and gas flow rates. These studies are currently being extended to horizontal manifolds with multiple branches.   

The role of contact angle on sloshing and air entrainment in a confined domain

Sloshing in a confined domain exists in many engineering applications, and the dynamic changes of the interfacial area and air entrainment due to sloshing may result in undesired consequences. We study the sloshing dynamics and air entrainment in an enclosed, vertically positioned glass tube, with and without silicone surface coating. We introduce a sudden acceleration and deceleration to the tube along its axial direction by impacting the top of the container using a spring-preloaded rod. The height of airgap, the viscosity of the liquid, and the magnitude of acceleration/deceleration are varied. Interface deformation and breakup due to the intense movement are captured using a high-speed camera. The resulting air entrainment, which is characterized by the bubble size distribution, is obtained using an in-house developed edge recognition algorithm. The dynamics changes of the interfacial area and the degree of air entrainment at different test conditions are reported and compared, and the role of the surface coating and contact angle is analyzed.   

Radiation driven dust hydrodynamics in late-phase AGB stars

Over the years, scientists have sought to understand the formation of small- and large-scale hydrodynamic features in planetary nebulae. These visually striking features involve both ionized gas and dust and are formed from the end of Asymptotic Giant Branch (AGB) stars. Much remains unknown about how this dust is formed and processed by these late phase stars. At the end of the evolutionary stage of AGB stars, shock waves formed by stellar pulsations and radiation pressure push the gas and particles in layers, shells, out from the star. In our work we aim to study the role of dust in the formation of small-scale hydrodynamic features known as cometary knots. In contrast to previous research, which addressed to the mixture of dust and gas as a single mixed fluid, we investigate these flows using an Eulerian-Lagrangian method to track the phases separately. Simulations are performed using the FLASH CODE developed at the FLASH Center at the University of Chicago. The particle-in-cell method was used with the two-dimensional Euler equations and solved using directionally split piecewise-parabolic method. This method was then adapted for the astrophysics regime by implementing radiation and non-continuum drag models for the particle phase. Further, the gas phase was modified to enable a hydrostatic equilibrium to exist over stellar length scales. The effects of a perturbed radiation field and perturbed particle spatial distribution were investigated to determine if these could be responsible for the formation of cometary knots observed in planetary nebulae.   

Sensitivity of the Annular Flow Film Thickness to Inclination Angle

Annular flow is a multiphase flow regime characterized by a thin liquid film layer on the pipe wall that is surrounded by a fast-moving gas core. The liquid film thickness is thought to significantly impact pressure drop and liquid entrainment; however, the literature annular film thickness is scarce compared to other multiphase flow regimes. This is especially true with respect to the sensitivity of the film thickness to inclination angle. In this experimental work, planar laser induced fluorescence (PLIF) was used to study the bottom film thickness over a wide range of inclination angles. The measurements were validated by comparing results to established horizontal data from literature. This presentation will include these validation measurements, an overview of the findings at various inclination angles, and share preliminary work from a new setup.   

Solution Methods for the Liquid-Gas Adjoint Equations with Applications to Spray Control

Atomization appears in important engineering applications such as fuel sprays for combustion engines. The capability to control sprays has the potential to create new technologies as well as improve existing devices. An efficient way to achieve computational spray control is to formulate the control problem as a minimization exercise that can be solved using a gradient-descent algorithm. The gradient can be calculated by solving an auxiliary set of differential equations known as the adjoint equations. In our proposed formulation, the adjoint equations for liquid-gas flows comprise the adjoint Navier-Stokes equation and the adjoint level set equation. Both pose numerous challenges such as the surface-bound nature of the adjoint level set equation, non-trivial jump conditions and complex two-way coupling. In this study, we describe a tailored surface transport method for solving the adjoint level set equation. We then verify and validate the two-way coupled adjoint equations by performing a gradient checking exercise of model liquid-gas flow problems.   

Modeling Sunlight Inactivation of SARS-CoV-2 in Aerosols

Modeling the persistence of SARS-CoV-2 in aerosols is of paramount importance, especially as aerosols have been established as a key route for COVID-19 transmission. Recent experiments have demonstrated that SARS-CoV-2 is inactivated by simulated sunlight, with ultraviolet B (UVB) intensity assumed to be the determinant factor, since UVB affects RNA directly. Unfortunately, in sunlight, the integrated energy over the UVB range is only a few percent that of ultraviolet A (UVA), and is significant over a narrower daytime window. Intriguingly, UVA inactivation has previously been demonstrated for other enveloped RNA viruses. In this study, we use a model inclusive of both UVA and UVB to examine published data for SARS-CoV-2 inactivation. We find that a mechanism relying primarily on UVA provides better quantitative agreement with experiments. The UVA sensitivity that we deduce for SARS-CoV-2 is also in good agreement with experiments for UVA-only inactivation of SARS-CoV-1. Our analysis indicates the need for new experiments to separately assess the effects of UVA and UVB, and suggests that inexpensive and efficient UVA sources might be useful for disinfection. In addition, aerosol models for COVID-19 transmission may need to be expanded to include the inactivating effect of UVA.