A brief description of the physics of instability arising from vibrations imposed on a two-fluid system with an initial density gradient is given. The instability is contrasted with the instability that arises when two immiscible layers of fluids are shaken.
Particle accumulation structures (PAS) in liquid bridges are considered in a didactic approach. After reviewing the experimental results on PAS we explain PAS as a limit cycle of the particle motion in the frame of reference co-rotating with the hydrothermal wave. The limit cycles representing PAS are created by perturbations of the motion of perfect tracer particles. Therefore, the topology of the underlying incompressible flow field is of key importance. Perturbation by particle–free-surface collisions as well as by a density mismatch between particle and liquid are
discussed. It is argued that for typical experimental parameters the effect of particle–free-surface collisions leads to formation times compatible with the experimental results while inertial effects are much slower.
Aiming at observing stable particles accumulation structures (PAS), a set of experiments have been performed for studying dynamics of solid particles in an unsteady flow in a non-isothermal liquid bridge made of n-decane with Pr = 13.5. The particles are 8% denser than the liquid; they range from approximately 20 microns to 100 microns in radius. Some of these particles have successfully formed a PAS when the liquid bridge was subject to a temperature difference of about 1.5 times larger than the critical value, at which the flow becomes oscillatory.
The accumulation of the particles has allowed us to visualize the structure of the supercritical flow. It is described by m = 2 wave number, which was confirmed by computer simulations. Modeling of particles’ motion in the thermocapilary flow has been performed using the simplified Maxey-Riley equation. The results of the modeling obtained in the present work are in a very good agreement with the experimental observations.
Conventional modeling of two-phase flow, including homogeneous, drift-flux and two-fluid models, is based on timeaveraged properties, even in the case of simulation of transient behavior. Then the inherent fluctuations of two-phase flow are neglected so as to ensure smoothed and continuous properties of parameters, such as void fraction and velocity. Gasliquid two-phase flow is a typical complex system, and thus to simulate inherent fluctuation, e.g. of slug and churn flow, is rather hard with above-mentioned conventional modeling. This paper describes one of the approaches to such simulation aiming at pattern formation and/or evolution of void fraction fluctuation of two-phase flow. The present simulation is conducted based on the void propagation equation and additional limited number of momentum effects, so that the complex properties are successfully realized on a computer.
Recent developments in our understanding of gravity effects on pool boiling heat transfer are discussed. An experimental apparatus using a fast response microheater array was used to obtain data throughout the continuously varying gravity levels during the transition from hypergravity (1.8 G) to low-G (~0.01 G) using low-G aircraft. A similar heater was used to obtain boiling data in the very quiet microgravity environment of the International Space Station as part of the Microheater Array Boiling Experiment. This data has allowed the development of a unified model to predict the boiling heat transfer at any gravity level if the heat transfer at a reference gravity level (e.g., earth gravity) is known. The model is first discussed and validated against experimental data from the International Space Station, then used to explain previous low-G data from other groups.