Session L09: Bubbly Flows

Bubble plumes beneath an inclined wall

Bubbles injected beneath a submerged inclined wall can form a two-dimensional plume, entraining surrounding liquid and spreading out laterally as they rise along the wall. These plumes are being considered as a technique for preventing biofouling in marine environments, for example on ship hulls. We use high-speed imaging experiments to obtain ensemble statistics and bubble trajectories, allowing us to investigate how the bubble plume structure and velocity depend on flow rate, nozzle size, and inclination angle. The results are compared to integral plume theory and previous studies of axisymmetric bubble plumes.   

Bubble trapping in a stirred vessel

We study a highly unexpected focusing phenomenon encountered in high-viscosity stirred fluids. In this project, we aim to uncover the fundamental focusing mechanism theoretically and experimentally. A fully three-dimensional direct numerical simulation is performed where the interface solver is based on a hybrid Front Tracking/Level Set method and designed to handle highly deforming interfaces with complex topology changes including pinch-off and coalescence. The form of the six-bladed Rushton turbine is constructed by means of a module for the definition of immersed objects via a distance function that takes into account the object’s interaction with the flow and treated as a fictitious fluid in the Navier-Stokes solution and its velocity is corrected in order to satisfy rigid body motion constraint. Bubbles are introduced into the system at various locations, and we demonstrate the conditions under which the motion of the bubbles is focused near the turbine impeller. The mechanisms underlying the bubble ‘trapping’ are also elucidated   

Complex vortex formation and aeration in air-water mixing flows in stirred tanks

Three-dimensional (3D) direct numerical simulations are carried out of air-water mixing flows using pitched blade turbine in an open vessel. We useahybrid front-tracking/level-set solver for 3D parallel simulations of multiphase flows. The turbine is constructed through a module that defines immersed objects using a distance function that accounts for the objects interaction with the flow and with appropriate boundary conditions in the Navier-Stokes solution the velocity is corrected in order to satisfy a rigid body motion constraint. In addition to ordinary primary vortices occurring in any kind of rotating flow, our configuration also features several secondary vortices, vortex breakdown, blade-tip, and end-wall corner vortices. For certain turbine rotational speeds, aeration takes places; this corresponds to the breakup of at the air-water interface leading to the entrainment of bubbles into the water phase. The mechanisms underlying the entrainment, and bubble formation processes, are elucidated.    

Liquid-Gas Flow-Induced Vibration in Flexible Pipe

Liquid-gas flows propagating through a pipe may evolve into a variety of flow patterns governed by the flow-pipe parameters and interfacial characteristics. Mechanisms of liquid-gas flow-induced vibrations in a long flexible pipe with a high length-to-diameter ratio are poorly understood owing to the lack of studies dealing with such complex flow phenomena inside the bendable geometries. Previous studies have mostly focused on a rigid inflexible pipe with a short span, small diameter or fixed inclination. Here, the most problematic slug flows with fluctuating liquid-gas volumetric fractions, velocities and pressure are studied through a hierarchy of mathematical, phenomenological and computational fluid dynamics models. We predict liquid-gas flow features and effects on flexible pipes with variable inclinations, multiple bends and multi-directional flows, and assess the pipe dynamic deformations, stresses and frequencies. The effects of slug properties (length, velocity, frequency, intermittency) are investigated. Numerical results reveal several key fundamental aspects of flexible pipe vibrations caused by travelling liquid-gas slug flows.   

Abstract Withdrawn

An experimental study on the steam-water direct contact condensation in an unstable bubbling condensation oscillation regime was conducted to investigate the effects of non-condensable gas on the heat transfer rate. As a working fluid, purified DI water though the reverse osmosis method was utilized. The experiments were performed at pool temperatures of 70 to 95°C ± 1°C, with different steam/air mixture mass fluxes between 5 and 50 kg/m²-s with the inner diameter of 10 mm gas injection nozzle. The steam and air mixing ratio was varied from 1.0:0.0 to 0.25:0.75. The pool temperature was measured using thermocouples, and the surface area of the bubble and the volume flow rate were measured with high-speed camera image processing to obtain the heat transfer coefficient. As a result of the presence of the non-condensable gas in the steam, decrease of heat transfer coefficient and change in the pattern of condensation were observed. The correlations between the heat transfer coefficient, the steam mass flux, and non-condensable gas mixing ratio were analyzed.   

The effects of non-condensable gas on cavitating flow over a cylinder

The effects of non-condensable gas (NCG) in cavitating flow over a circular cylinder are investigated using a homogeneous mixture model. We consider Reynolds number = 200 at different cavitation numbers (σ = 0.7 to σ = 1.0). The numerical method developed by Gananaskadan and Mahesh (2015) for the mixture of water and vapor is extended to include the effect of non-condensable gas. It is observed that the non-condensable gas has a stabilizing effect on the cavity being formed. The decrease in sound speed when the gas is added to the flow creates a damping effect, effectively decreasing the magnitudes of shock-waves generated during the cavity collapse. Thus, weaker pressure waves impinge on the body. In the bubbly shock regime (σ = 0.7), it is shown that the NCG substantially decreases the strength of the condensation shock. A shock speed equation is derived from the Rankine-Hugoniot jump conditions to explain the phenomenon. We show that this decrease in condensation shock speed in the presence of NCG is due to the decrease in the pressure ratio across the shock as it moves.   

Influence of thermal sensitivity on Venturi cavitation characteristics in a blow-down type tunnel with controlled dissolved gas content

nfluence of dissolved gas content (measured by dissolved oxygen, DO) on Venturi cavitation (10mm-scale test section) in water at different temperature, i.e., different degree of thermal sensitivity, is investigated in a blowdown-type cavitation tunnel. Investigations on the cavitation length and the unsteady cavitation behavior are presented. It is shown that the cavitation length decreases with increasing temperature (or the degree of the thermodynamic effect). In addition, the DO content has little influence on the mean cavitation length at thermally sensitive conditions.