Session J35: Bubbly Flow Physics I

Dynamics of compressible displacement in a capillary tube

The displacement of viscous liquid in a confined geometry due to injected gas occurs in CO₂ sequestration, battery operation, and the reopening of pulmonary airways. Expelling liquid from a capillary tube by injecting air is perhaps the simplest example of such a flow, if not the simplest example of any two-phase flow. Here, however, we demonstrate that complex dynamical regimes arise in this system due to the compressibility of the injected air. In experiments, we drive silicone oil from the tube by compressing a connected reservoir of air at a fixed volumetric rate. We observe unsteady flow dynamics due to the interaction between spring-like air compressibility and diminishing viscous resistance in the draining oil slug. A simple mathematical model predicts these dynamics over a broad range of compression rates and air reservoir volumes. In the limit of large air reservoirs, the model predicts two distinct dynamical regimes, separated by a critical value of a dimensionless ‘compressibility’ number. The low- and high-compressibility regimes are associated, respectively, with quasi-steady and burst-like expulsion. Remarkably, we also observe these regimes in our experiments.   

The propagation of air fingers into an elastic branching network

We study experimentally the propagation of an air finger through the Y-bifurcation of an elastic, liquid-filled Hele-Shaw channel, as a benchtop model of airway reopening. With channel compliance provided by an elastic upper boundary, we can impose collapsed channel configurations into which we inject air with constant volume-flux. We typically observe steady finger propagation in the main channel, which is lost ahead of the Y-bifurcation but subsequently recovered in the daughter channels. At low levels of initial collapse, steady finger shapes and bubble pressure in the daughter channels map onto those in the main channel. At higher levels of initial collapse, experimentally indistinguishable fingers in the main channel can lead to multiple states of reopening of the daughter channels. The downstream distance at which steady propagation is recovered in the daughter channels also varies considerably with injection flow rate and initial collapse, which is governed by the change in the relative importance of viscous and elastic forces.   

Launching Liquid Slugs Through Tubes: Investigating Multiphase Liquid Slug Acceleration Behaviors in Spacecraft Propulsion Systems using 3D Multiphase VOF CFD Simulations and High-Speed Flow-Visualization Validation Experiments

Propellant management in crewed and robotic spacecraft propulsion systems is complex. Unintended and unavoidable scenarios can lead to liquid "slugs" of propellant accumulating in unwanted locations and thereafter being rapidly accelerated through complex tubing networks otherwise filled with vapor or gas. To understand and characterize the multiphase fluid dynamics of these "slug launch" scenarios, NASA Marshall Space Flight Center's ER42 branch – propulsion fluid dynamics research – conducted 3D, transient, multiphase Volume of Fluid CFD simulations using an in-house tool called LOCI-Stream VOF on canonical tube geometries with straight sections, bends, and bend complexes. In addition, high-speed flow visualization experiments were conducted on similar tube geometries at the NASA White Sands Test Facility. These simulations and experiments found a variety of unique multiphase fluid dynamic effects involving the leading and trailing edges of the slug in straight sections and bends as well as the progressive erosion, acceleration, and eventual "punch-through" of the liquid slug. Understanding these multiphase effects has led to significant and counter-intuitive observations of critical relevance to important engineering concerns associated with these "slug launch" scenarios.   

Experimental study of air-water two-phase slug flow in horizontal pipe using high speed imaging

In the present study air-water two-phase slug flow in 220m horizontal pipe flow loop is investigated experimentally using high speed imaging and gamma densitometers. The aim of the study is to obtain insight into the local flow processes of slug development to characterize the dynamics of slug flow including the role of secondary flows, mixing, and entrainment across the multi-phase interface. Additionally, the influence of adding different surfactant concentrations on the slug flow is analysed. The superficial velocities of water and air range from UL=0.2 to 2.5 m/s and UG=0.2 to 2.5 m/s, in a 55mm inner diameter pipe. Measurements are performed for 28 different flow conditions of air and water phases. Additionally, the same measurements are repeated for two different surfactant concentrations. High quality images of slug flow are captured with a synchronized four high speed camera system allowing filming at a frame rate up to 5400fps on the maximum resolution. The optical section in the flow loop consists of a 2m long transparent acrylic correction box, positioned between the camera system and the LED light system. An image processing algorithm is developed to analyse slug flow characteristics (length, speed, and frequency) and the influence of surfactants on the slug flow behaviour including slug front stability. The results are validated with measurements from gamma densitometers. 

    

Two-phase Mixing Through a Narrow Gap

Mixing of a two-phase flow through a gap connecting adjacent channels was investigated for a variety of gap heights and flow rates for both balanced and unbalanced flows. Flow could be unbalanced based on liquid flow alone, gas flow alone, or by both being unbalanced. The water inlet Reynolds number was varied from 40k to 100k, and volumetric gas quality from 0 to 0.35. Air was introduced by a needle array producing nominally monodispersed bubbles at lower gas flow rates with broader bubble size distributions at higher gas flow rates. The Sauter mean diameter varied from 3 to 8 mm, depending on flow rates. The liquid mixing through the gap was calculated based on the measured mass flow rates and dye tracer concentrations at the inlet and outlet of each channel. Net gas transfer, the void fraction distributions, bubble size distributions, and gas interface phase velocities were measured using dual-plane wire mesh conductivity sensors at both inlets and outlets. Additionally, imaging of the fluorescent tracer dye and air bubbles was used to further examine the mixing mechanisms. For the balanced flows large coherent structures dominated the mixing. However, 5% volumetric quality gas injection could significantly suppress these structures reducing mixing by up to 80%. For unbalanced flows, the mixing could be simply driven by the lateral pressure gradient, often leading to a large net mass transfer across the gap.   

Development of the bubble-trapping wake behind a bluff body in liquid-gas flows at intermediate Reynolds numbers

An upward bubbly liquid-gas flow around a cylinder results in a non-uniform distribution of the gas phase, called bubble trapping, under certain flow conditions. We have previously demonstrated that the bubble-trapping wake occurred due to strong inertial, pressure gradient, and lift forces acting on the bubbles just downstream of the cylinder when the Reynolds number (Re), based on the cylinder diameter of 9.5 mm, was 3,000. In this investigation, we show the evolution of the trapping forces at intermediate Reynolds numbers with mean bubble diameters of 3-4 mm. To resolve the forces acting on the bubbles, we performed phase-separated velocimetry of the bubbly flow. We simultaneously tracked the air bubble movement using Lagrangian Particle Tracking Velocimetry (PTV) and liquid movement by applying Particle Shadow Image Velocimetry (PSIV) to suspended tracer particles. The resolution of the two-phase velocities and bubble sizes allowed us to determine the buoyancy, inertial, drag, lift, and pressure gradient forces acting on each of the bubbles. Here, we illustrate how the trapping wake develops by comparing the force balances at Reynolds numbers ranging from 500 to 2,500, which lead up to the trapping conditions observed at higher Reynolds numbers.   

Effect of bubble-induced turbulence in an upward bubbly flow in a pipe with sudden expansion

The control of the reattachment length is very important in the flow around a backward-facing step, and in this study, we investigate such phenomena in the bubbly flow in the expansion pipe. When gas bubbles are injected into the liquid flow, they significantly modulate the liquid-phase flow characteristics, among which we focus is the turbulence enhancement, which is known to reduce the reattachment length in backstep flow. We experimentally study the bubble dynamics and the liquid-phase flow characteristics in an upward bubbly square pipe with a sudden expansion (area expansion ratios of ER = 4.0-9.0). The effect of ER on the change in the turbulence of the liquid-phase is compared at two Reynolds numbers of 420 (laminar) and 6000 (turbulent), based on the inlet bulk velocities of the single-phase (without bubbles) flow. The mean volume void fractions and averaged bubble size considered ranges from 0.31-1.57% and 2.72-3.74 mm, respectively, and we use the high-speed two-phase particle image velocimetry to measure the gas and liquid phases simultaneously. We will discuss how the bubbles induce the turbulence structure in the expansion pipe flow and explain the mechanism of the controlled flow.   

Full field measurement of turbulent bubbly jet by Lagrangian Particle Tracking Velocimetry

This study proposes a method for simultaneous measurements of time-resolved three-dimensional velocity fields of the dispersed and continuous phases of a turbulent bubbly jet at a low void fraction using Lagrangian particle tracking (LPT) velocimetry with the Shake-The-Box algorithm. Bubbles are firstly tracked using intensity differences between tracer particles and bubbles, then the bubble images are removed from the camera images and all tracer particles are tracked using the residual images. Subsequently, FlowFit interpolation is applied to the LPT results obtained by phase separation to investigate turbulent characteristics of a bubbly jet. The bubbly jet was divided into two regions along the vertical direction: jet-like and plume-like regions. Streamwise vortex structures of continuous phase were generated mainly by the rising bubbles. The measured slip velocity in the radial direction was not constant but linearly increased. The classical assumption of self-similarity with Gaussian profiles for fluid velocity and bubble concentration is experimentally verified. The entrainment coefficient is obtained as a function of bubbly flow physics parameters based on three-dimensional measurements. The integral theory agrees well with experiments in the plume region.   

High Void Fraction Experiments in Bubbly Vortex Shedding

Vortex shedding in bluff-body flows can affect the performance of many devices such as hydrofoils and heat exchangers. Past experiments have observed that introducing bubbles in the carrier liquid can increase the shedding frequency. Building off low void fraction (α < 0.01%) one-way coupled flow experiments, we increase the gas injection to achieve void fractions of up to 10% to observe the vortex street in a two-way coupled regime and further understand the mechanics behind the frequency shift. Flow in a 100mm square channel over a 25mm circular cylinder at Reynolds number up to 25,000 is examined. A needle array with liquid co-flow controlled independently from the bulk flow is used to control the diameter distribution of O(mm) nominally monodisperse bubbles, as previous findings suggest bubble diameter affects the shedding frequency. High-speed cameras and optical probes are used to characterize the preferential concentration of bubbles, while pressure transducers will record the shedding frequency. Additional bluff body geometries will also be used to explore the impact of flow separation on bubble preferential concentration.

    

Ultrasound Measurement for Prediction of Gas-Liquid Two-Phase Flow Patterns

A method for identifying the flow patterns of gas-liquid two-phase flow in a vertical pipe using ultrasound was developed in this study. The method of analyzing the gas-liquid two-phase flow rising in a vertical pipe can be applied to an immediate measurement method in an airlift pump which is potentially used for pumping resources such as rare earth mud and cobalt rich crusts existing in the deep-sea bottom. Two-phase flow pattern identification method was realized using ultrasound echo intensity and Doppler velocity distribution by unsupervised machine learning. Measuring flow patterns makes it possible to predict the subsequent flow based on a two-phase flow model in a vertical pipe and control a pump system. By statistically processing both distributions in the distance and time directions in this order several features such as the mean of all echoes and frequency components of the temporal variation of the flow velocity fluctuation were extracted. Flow characteristics were visualized by applying dimensionality reduction method to these features. Moreover, we compared the result with the existing flow pattern maps and human sensory identification and evaluated the performance.   

Three-dimensional flow characteristics of a turbulent bubbly jet near the source

Time-resolved Lagrangian particle tracking (LPT) based measurement of a bubbly jet from a round source is presented in this study (Re ~ 3,000, D = 20 mm, α ~ 1%). To achieve accurate measurement of both phases, illuminating devices for (1) fluorescent tracer particles for water and (2) spherical bubbles from a bubble generator are operated with a short delay. The measurement is conducted for a near-field of the bubbly jet. LPT data is reconstructed into a regular grid with 2 mm spacing with Vortex-in-Cell sharp (VIC#) method. Reconstructed data are validated with a bubbly jet DNS result with a similar slip velocity and Reynolds number. Near the jet source, there is a region with an alternation of acceleration and deceleration. The centerline streamwise velocity of the bubbly jet eventually reached around 1.7 times the velocity at the source, and the potential core of the bubbly jet collapsed. Alternating accelerations attribute to the formation of a series of ring-like vortices. The vortices are unstable and diminish as the jet flows downstream. Formation of the vortical structure occurs more frequently than large-scale meandering of the jet but less frequently than bubble-attributed instability.   

Gas Motion in a Vibrated Liquid-Filled Cantilevered Tube

When a closed tube containing liquid and gas is vibrated vertically, stable gas regions can form at the upper and lower ends. Acrylic tubes of 10 cm height and 1.27-2.54 cm diameter are filled with 20-cSt silicone oil and ~10% air and then vibrated vertically at 100-600 Hz and 20-40 G. Large lower gas regions are observed in experiments at conditions for which the downward vibration-induced Bjerknes force on bubbles is too small relative to the upward buoyancy force to allow these lower gas regions to form. The possibility that downward bubble motion is enhanced by lateral vibration caused by cantilever motion of the tube is assessed. This effect is shown to be small unless the driving frequency is close to the cantilever frequency. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government.