Search results for " Bubble"
showing 10 items of 65 documents
Experimental analysis of bubble size distributions in 2D gas fluidized beds
2010
Abstract The present work focuses on the development of a novel statistical approach for the analysis of the behavior of a 2D fluidized bed operating under bubbling and slugging conditions. Experimental data were collected by means of a purposely built lab-scale 2D fluidized bed and an original digital image analysis technique (DIAT), allowing the acquisition of a number of data sufficient to perform a statistical analysis of bubble dynamics. In particular, measurements of bubble size distributions (BSD) along the bed were performed. The analysis allowed to characterize the shape of BSD for different particle systems and fluidization gas velocities; interestingly a bimodal shape of BSD is o…
An expansion–coalescence model to track gas bubble populations in magmas
2016
Abstract We propose a kinetic model that statistically describes the growth by decompression, exsolution and coalescence of a polydisperse population of gas bubbles in a silicate melt. The model is homogeneous in space and its main variable is a distribution function representing the probability to find a bubble of volume v and mass m at time t. The volume and mass growth rates are described by a simplification of the classical monodisperse bubble growth model. This simplification, which shortens computational time, removes the coupling between mass evolution and an advection–diffusion equation describing the behavior of the volatile concentration in the melt. We formulate three coalescence…
Probing Physical Properties of Confined Fluids within Individual Nanobubbles
2008
Spatially resolved electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) has been used to investigate as fluidic phase in nanoubbles embedded in a metallic Pd90Pt10 matrix. Using the 1s->2p excitation of the He atoms, maps of the He distribution, in particular of its density an pressure in bubbles of different diameter have been realized, thus providing an indication of the involved bubble formation mechanism. However, the short-range Pauli repulsion mechanism between electrons on neighboring atoms seems insufficient to interpret minute variations of the local local measurements performed at the interface between the metal and the He bubble. Simul…
The chemistry of acetone at extreme conditions by density functional molecular dynamics simulations
2011
Density functional molecular dynamics simulations have been performed in the NVT ensemble (moles (N), volume (V) and temperature (T)) on a system formed by ten acetone molecules at a temperature of 2000 K and density ρ = 1.322 g cm(-3). These conditions resemble closely those realized at the interface of an acetone vapor bubble in the early stages of supercompression experiments and result in an average pressure of 5 GPa. Two relevant reactive events occur during the simulation: the condensation of two acetone molecules to give hexane-2,5-dione and dihydrogen and the isomerization to the enolic propen-2-ol form. The mechanisms of these events are discussed in detail.
Risk transmission between Islamic and conventional stock markets: A return and volatility spillover analysis
2017
Abstract This paper contributes to the current debate on the empirical validity of the decoupling hypothesis of the Islamic stock market from its mainstream counterparts by examining return and volatility spillovers across the global Islamic stock market, three main conventional national stock markets (the US, the UK and Japan) and a number of influential macroeconomic and financial variables over the period from July 1996 to June 2016. To that end, the VAR-based spillover index approach based on the generalized VAR framework developed by Diebold and Yilmaz (2012) is applied. The empirical analysis shows strong interactions in return and volatility among the global Islamic stock market, the…
The distribution of stock returns: international evidence
1994
Although financial theory rests heavily on the normality assumption, daily stock returns display significant departures from normality. Different researchers have proposed alternative distributions. In this paper, the distribution of stock returns is examined in six stock markets. The empirical distributions present common features and are best represented by Student's t-distribution, while several alternative distributions are rejected.
Bubbles and Crowding-in of Capital via a Savings Glut
2017
This paper uncovers a mechanism by which bubbles crowd in capital investment. If capital formation is initially depressed by a binding credit constraint, a bubble triggers a savings glut. Higher returns in a new bubbly equilibrium attract additional savings, which are channeled to expand investment at the extensive margin, leading to permanently higher capital, output, and wages. We demonstrate that crowding-in through this channel is a robust phenomenon that occurs along the entire time path.
The role of stock markets vs. the term spread in forecasting macrovariables in Finland
2011
Abstract A substantial body of stylized facts and empirical evidence exists regarding the relationships between financial variables and the macroeconomy in the United States. However, the question of whether this evidence is consistent with the cases of small open economies is less known. This paper focuses on the forecasting content of stock returns and volatility vs. the term spread for GDP, private consumption, industrial production and the inflation rate in Finland. Our results suggest that during normal times, the term spread is a much better tool than stock market variables for predicting real activity. However, during exceptional times, such as the recent financial crisis, the foreca…
Bubble formation at variously inclined nozzles
2014
Bubble formation at variously inclined submerged nozzles, fed with a continuous gas flow rate, is investigated. Results confirm previous findings, such as a substantial independence of bubble size of nozzle diameter and its dependence on the cubic root of gas flow rate at sufficiently high values of this parameter. Nozzle inclination in the range between 0 and 135° from the vertical upward orientation is found to have a negligible effect on bubble size. Observed bubble rise velocities lead to an estimation of the average liquid velocity induced by bubbles motion, leading in turn to a very simple correlation for the estimation of the liquid velocities induced by the repeated passage of bubbl…
Free surface oxygen transfer in large aspect ratio unbaffled bio-reactors, with or without draft-tube
2015
Abstract It is widely accepted that animal cell damage in aerated bioreactors is mainly related to the bursting of bubbles at the air–liquid interface. A viable alternative to sparged bioreactors may be represented by uncovered unbaffled stirred tanks, which have been recently found to be able to provide sufficient mass transfer through the deep free surface vortex which takes place under agitation conditions. As a matter of fact, if the vortex is not allowed to reach impeller blades, no bubble formation and subsequent bursting at the free-surface, along with relevant cells damage, occurs. In this work oxygen transfer performance of large aspect ratio unbaffled stirred bioreactors, either e…