Session T15: Flow Instability: Multiphase Flow (8:00am - 8:45am CST)

From Waste to Power: Pulsing Biosludge Atomization for Efficient Energy Conversion

Efficient conversion of human waste to usable energy is sought by introducing a highly concentrated non-Newtonian biosludge to a steam boiler via direct spray injection. Compared to other methods of energy conversion that require dilution and/or drying, using a more concentrated biosludge increases energy conversion efficiency, reduces water usage, and reduces fossil fuel emissions by decreasing the transportation load. The use of steam as the assisting gas for a twin-fluid atomizer reduces the viscosity of the biosludge, enabling more effective atomization; however, the steam reduces boiler efficiency. Therefore, two objectives must be balanced in an atomizer design: minimizing steam usage and effective atomization of the viscous biosludge. CFD simulations provide preliminary assessments of inverted twin-fluid atomizer designs when the steam flow is reduced to a desirable rate. Typical designs involve the steam flowing outside of the slurry stream, while the inverted design has the steam entering inside of the slurry stream. It is shown that the inverted design produces superior atomization and is robust in that droplet size does not change significantly for a range of acceptable steam flows. Additional difficulties arise because the viscosity of boisludge varies widely; if viscosity levels are too high, an undesirable pressure drop restricts the flow, and atomization quality suffers. To improve the robustness of this system, two PID controllers are added. The first automates the flow of biosludge based on pressure drop, and the second compensates for phase momentum ratio and controls the flow of steam based on droplet size distribution.   

On a Gas-Particle Analogue to the Richtmyer-Meshkov Instability. Part 1: Evolution of a Corrugated Particle Curtain

An emerging area of research in the multiphase flow community is the study of Shock-Driven Multiphase Instability (SDMI) which is a gas-particle analog of the traditional two-fluid Richtmyer-Meshkov instability (RMI). We study the interaction of a planar air shock with a corrugated glass particle curtain through the use of numerical simulations with an Eulerian-Lagrangian approach. The simulations track the computational particle trajectories as well as the evolution of the curtain of gas which is initially trapped inside of the particle curtain. This work focuses on the evolution of the particle curtain after interacting with the shock. Two shock Mach numbers, 1.21 and 1.5, are studied along with perturbation wavelengths of 3.6 and 7.2 mm to analyze the roles of these parameters as the evolving curtain undergoes SDMI both before and after reshock. The effect of particle loading in the curtain evolution is also investigated. A dilute curtain (roughly 0.1{\%} volume fraction) replicates the Atwood number of an air-SF$_{\mathrm{6}}$ RMI experiment and a denser curtain at 26{\%} volume fraction is used to introduce additional multiphase coupling effects. This study also looks at the validity of comparing the fluid-only and multiphase Atwood numbers in the two-way coupled flow regime.   

Fluid Capture Patterns in Hairy Surfaces

Interfacial flows past dense arrays of flexible hairs are highly coupled: capillary forces of the moving interface can be strong enough to deform fibres, which in turn modifies local flow geometry. As a result, the ability to drain a pre-existing layer of fluid over the deformable surface can significantly differ from the undeformed reference case. Here we fabricate “hairy” elastoporous media by casting a curing elastomer and mounting the arrays in a 1D Hele-Shaw cell. Upon displacing a defending phase of oil with water, elastocapillary bundling causes preferential invasion into certain pores at a regular interval. We characterize these patterns and then study pore drainage dynamics and bundle size as a function of capillary number $\mathbf{Ca}$. Models for depth-averaged fluid flow and the deflection of elastic beams are adapted to our problem to describe the deformation of the host medium due to interfacial and viscous forces. These experiments are then upscaled to higher dimensions, where oil is displaced from a Hele-Shaw cell textured with a 2D carpet of fibres arranged in a lattice; we present results for oil drainage and pattern formation in both rectangular and radial geometries.   

On a Gas-Particle Analogue to the Richtmyer-Meshkov Instability. Part 2: The Initially Entrapped Gas

We study a planar numerical shock tube containing a corrugated particle curtain. The curtain is about 4mm thick and has a peak volume fraction of about 26{\%}. It is composed of spherical particles of 115$\mu $m in diameter with a density of 2500kg.m$-$3, thus mimicking glass particles commonly used in multiphase shock tube experiments or multiphase explosive experiments. Under these conditions the gas-particle flow that follows the shock interaction with the curtain is two-way coupled. Using a Eulerian-Lagrangian approach, we track trajectories of computational particles in the three-dimensional planar shock tube as well as the air initially trapped inside the particle curtain. This work focuses on the latter. We characterize the evolution of the gas cloud inside the particle curtain for two Mach numbers, M$=$1.21 and M$=$1.50, and two dominant wavelengths, l$=$3.6mm and l$=$7.2mm, as it is advected downstream and undergoes Richtmyer-Meshvov instability with features corresponding to the initial perturbation imposed on the particle curtain. We also analyze the behavior of the gas after re-shock.   

Linear stability analysis of a plane Poiseuille flow in a multi-layer porous channel

We examine a pressure-driven, incompressible, fully developed flow through a multi-layer channel containing an anisotropic porous layer placed parallel to the channel walls. The channel is confined by impermeable walls and governed by the Darcy-Brinkman-Forchheimer equation in a porous layer along with the Navier-Stokes equation in liquid layers. The continuity of stress and velocity are used at the interface and no-slip condition at the impermeable walls. The effect of anisotropic permeability and orientation angle on the flow and on the skin friction are discussed. Furthermore, the linear stability analysis of aforementioned configuration by considering the nonlinear inertial term in the Darcy-Brinkman-Forchheimer equation is performed numerically. The results are validated with the linear stability results of a flow where the channel consists of a fluid layer sandwiched between two isotropic porous layers. The present results are consistent when we neglect the inertial effect of the porous medium. It is found that the major system parameters affect significantly the stability characteristics of the flow, and therefore the inertial effect provides a useful means to control the flow instability of a multi-layer porous system having anisotropic permeability.   

A new pathway for the amplification of distortions to the surface of liquid jets

The atomization process of liquid jets often begins with the linear amplification of small perturbations to the material interface. We identify and characterize a novel mechanism for the amplification of interface distortions of liquid jets. The mechanism is independent of the exponential instability of the flow and, depending on the parameters, can intensify small perturbations to the material interface by several orders of magnitude and at a faster pace than the exponential Kelvin-Helmholtz instability. Analysis of the budget of the perturbation kinetic energy sheds light on the underlying physics. It is shown that energy is extracted from the mean shear by the production term in the axial velocity component and redistributed to the radial velocity component where it is transferred into the surface-tension energy of the material interface. Parameter studies show that the gain in surface tension energy scales linearly with the Reynolds number. Furthermore, a critical Weber number is identified as a lower bound beyond which the mechanism becomes active. Asymptotic analysis establishes a power-law relationship of a critical Weber number to the Reynolds number which is confirmed by computational results.   

Experimental Investigation of Surface Temperature Changes due to Flow Instability in a Micro/Mini Channel During Flow-Boiling Heat Transfer with Non-Uniform Circumferential Heat Fluxes at Different Inclinations

Flow instability was analysed during flow-boiling of Perfluorohexane (FC-72) in a rectangular micro/mini channel at different inclinations with one-sided heating. The flow passage had a hydraulic diameter of 909~$\mu $m and an aspect ratio of 10 (5mm~x 0.5mm). Inclination angles ranged from -20\textordmasculine (downward flow) to 0\textordmasculine (horizontal flow), and up to $+$90\textordmasculine (vertical upward flow) at mass fluxes of 10, 20~and 40kg/m$^{\mathrm{2}}$s. Pressure measurements and highspeed images were used to determine the effect of instability events on the heated surface's temperature measured using infrared thermography. Inlet pressure measurements identified reverse flow events while outlet pressure measurements identified two-phase mixing events. The frequency of instability events was determined using pressure transducer data and an instability threshold pressure. Flow instability was shown to decrease surface temperatures by up to 14.1\textordmasculine C. Negatively inclined channels and $+$90\textordmasculine experienced negligible instability frequencies. Horizontal and positively inclined channels had instability frequencies of up to 1.8Hz and 2.1Hz at the inlet and outlet.   

Energy budget analysis of plane Poiseuille-Couette flow over a permeable surface

The stability of Poiseuille flow over a permeable surface has been studied extensively over the past years due to its vast range of engineering applications. We previously investigated how imposing a Couette flow could impact the stability of such a system by performing a linear stability analysis. It was observed that the Couette flow exerts destabilizing or stabilizing effects depending on the permeable surface properties. This study, using the energy budget analysis, aims to provide a physical interpretation of the behavior of the system. The results show that for unstable wavenumbers, the production term from the Reynolds shear stresses produces the required energy which allows the disturbances to grow. For stable wavenumbers, the disturbances are dampened since the rate of energy loss due to viscous dissipation and Darcy drag becomes greater than the production term. In general, increasing both the Couette component and the porous permeability, as well as decreasing the fluid layer thickness results in a higher production term, which consequently makes the flow less stable. We were also able to distinguish different modes by computing the kinetic energies of perturbation flows in the fluid and porous layers.   

Primary instability of an air-water mixing layer: convergence between simulations, experiments and linear theory

The Kelvin-Helmholtz instability occurring at the interface between a slow liquid and a fast gas is the first in a cascade of instabilities leading to spray formation. Recent progress made in the understanding of the nature of this instability and its driving mechanisms (confinement by the gas stream, surface tension, viscosity) allows now to reconcile linear theory and experiments. While numerical simulations can be used to deepen the analysis of such complex phenomena, the codes have to be carefully validated first. Multiphase code validation is made challenging by several factors: high density and momentum flux ratio, high Reynolds number, and complex topology changes. In this work, we present a systematic validation of our multiphase flow solver against experiments and linear theory for the canonical configuration of a two-dimensional air-water mixing layer. Particular attention is given to the characteristics of the instability in both its linear and non-linear regime. We discuss the accuracy of our results and the convergence of the statistics. Finally, we explore the effects of the injector geometry on the stability of the flow.