Interfacial instability of ferrofluid flow under the influence of a vacuum magnetic field

This study is to numerically test the interfacial instability of ferrofluid flow under the presence of a vacuum magnetic field. The ferrofluid parabolized stability equations (PSEs) are derived from the ferrofluid stability equations and the Rosensweig equations, and the characteristic values of the ferrofluid PSEs are given to describe the ellipticity of ferrofluid flow. Three numerical models representing specific cases considering with/without a vacuum magnetic field or viscosity are created to mathematically examine the interfacial instability by the computation of characteristic values. Numerical investigation shows strong dependence of the basic characteristic of ferrofluid Rayleigh-Taylor instability (RTI) on viscosity of ferrofluid and independence of the vacuum magnetic field. For the shock wave striking helium bubble, the magnetic field is not able to trigger the symmetry breaking of bubble but change the speed of the bubble movement. In the process of droplet formation from a submerged orifice, the collision between the droplet and the liquid surface causes symmetry breaking. Both the viscosity and the magnetic field exacerbate symmetry breaking. The computational results agree with the published experimental results.

Flow and heat transfer characteristics of high temperature continuous rising bubbles

In the movement process of continuous bubbles, different liquid flow state and initial temperature of bubble shave important influence on bubble movement and heat transfer. In this study, the flow and heat transfer characteristics of high temperature bubbles in normal temperature liquid are investigated. The rising process of and temperature of bubbles are studied by numerical simulation and compared with experimental results for verification. The results show that the shape, velocity and temperature of the bubble are affected by the initial position of the bubble. Because the closer the initial position of the bubble is to the bottom of the cylinder, the greater decrease of the bubble aspect ratio (w*),the faster decrease of bubble temperature, and the larger bubble velocity would be. The increase of the bubble diameter would mitigate the influence of liquid jet on the bubble motion. Finally, as the bubble surface tension increases, the liquid jet would have less influence on the bubble movement.

Using Tapered Channels to Improve LAD Performance for Cryogenic Fluids: Suborbital Testing Results

Improvement of cryogenic fluid storage and transfer technology for in-space propulsion and storage systems is required for long-term space missions. Screened channel liquid acquisition devices (LADs) have long been used with storable propellants to deliver vapor-free liquid during engine restart and liquid transfer processes. The use of LADs with cryogenic fluids is problematic due to low temperatures associated with cryogenic fluids. External heat leaks will cause vapor bubbles to form within the LADs that are difficult to remove in the existing designs. A tapered LAD channel has been proposed to reliably remove vapor bubbles within the device without costly thrusting maneuvers or active separation systems. A model has been developed to predict bubble movement within tapered LAD channels, and subsequent ground testing was completed with a simulant fluid to provide model validation data. Suborbital microgravity testing of tapered LAD technology was recently completed with two different simulant fluids and demonstrated that the concept can passively expel vapor bubbles within the channel. Two additional suborbital flights have been funded to further develop this technology by investigating the performance of larger scale versions of the design.

Numerical Study of Single Taylor Bubble Movement Through a Microchannel Using Different CFD Packages

A Computation Fluid Dynamics (CFD) study for micro-scale gas–liquid flow was performed by using two different software packages: OpenFOAM® and ANSYS Fluent®. The numerical results were compared to assess the capability of both options to accurately predict the hydrodynamics of this kind of system. The focus was to test different methods to solve the gas–liquid interface, namely the Volume of Fluid (VOF) + Piecewise Linear Interface Calculation (PLIC) (ANSYS Fluent®) and MULES/isoAdvector (OpenFOAM®). For that, a single Taylor bubble flowing in a circular tube was studied for different co-current flow conditions (0.01 < CaB < 2.0 and 0.01 < ReB < 700), creating representative cases that exemplify the different sub-patterns already identified in micro-scale slug flow. The results show that for systems with high Capillary numbers (CaB > 0.8) each software correctly predicts the main characteristics of the flow. However, for small Capillary numbers (CaB < 0.03), spurious currents appear along the interface for the cases solved using OpenFOAM®. The results of this work suggest that ANSYS Fluent® VOF+PLIC is indeed a good option to solve biphasic flows at a micro-scale for a wide range of scenarios becoming more relevant for cases with low Capillary numbers where the use of the solvers from OpenFoam® are not the best option. Alternatively, improvements and/or extra functionalities should be implemented in the OpenFOAM® solvers available in the installation package.