393 research outputs found
Numerical investigation of adiabatic growth and detachment of a gas bubble injected from a submerged orifice at various surface inclinations
No full textThe heat transfer mechanism of Nucleate Boiling (NB) is widely used in technological applications. However,there is an incomplete understanding of the fundamental physics of bubble dynamics, at small scales as well as at non-trivial geometrical configurations. In order to investigate bubble dynamics an adiabatic approach is often used, where gas/vapor bubbles are injected into liquids at saturation conditions. So far, there is a great deal of experimental, theoretical and numerical investigations on adiabatic gas/vapor injected bubble growth dynamics. However, according to the authors' best knowledge the majority of these works deal with gas/vapor bubbles injected upwards into stagnant liquid domains, with the injection axis being parallel to the gravitational acceleration direction, using mainly water and air as the working fluids. In the present investigation, an improved algebraic VOF (Volume of Fluid) based interface capturing approach, originally developed in OpenFOAM®, is applied for the conduction of axisymmetric and 3D numerical experiments on adiabatic bubble growth dynamics. The investigation focuses on the influence of different fluid properties and inclination angles of the gas/vapor injection orifice, on the bubble growth and detachment characteristics. Prior to the application, the predictions of the numerical model are validated against literature available experimental data. From the overall results it can be concluded that the bubble growth and detachment characteristics, are strongly dependent not only on the fluid properties but also on the orifice inclination angles due to the induced asymmetry, which mainly results from the formation of asymmetric and unsynchronized vortexes during the bubble growth process
A numerical investigation of the solid surface material influence on flow boiling within microchannels
No full textFlow boiling within microchannel heat sinks constitute a promising solution for the cooling of high-performance electronic devices, dissipating high values of heat flux. Yet still, due to the complexity of flow boiling in small scales, the effect of important parameters is not clearly defined. In the present study a numerical investigation on the effect of solid surface thermophysical properties on flow boiling heat transfer characteristics within micro- channels, is conducted. For the proposed investigation an enhanced, custom VOF-based numerical model that has been developed in OpenFOAM is used. The utilised computational domain consists of a top rectangular fluid domain in contact with a bottom rectangular solid domain. The solid domain is heated at its bottom boundary by the application of a constant heat flux. In total five different solid surface materials were examined, focusing on the first transient stages of the confined two-phase flow development. The findings indicate that the investigated effect has a significant influence on the resulting two-phase regimes and in the associated heat transfer char- acteristics. High thermal conductivity materials such as silver, aluminium and copper exhibited the highest values of the time-averaged heat transfer coefficient with more than 35% increase compared to the single-phase stage of the simulations, whereas the brass and silver channels resulted in a lower increase of<30%. The flow boiling process in the brass and silver channels, was characterised by frequent bubble break-ups and a lower total vapour fraction values within the channels. The rest of the examined material cases were characterised by thicker liquid films and higher values of total vapour fraction. Finally, a new correlation for the global Nusselt number is proposed that takes into consideration the thermophysical properties of the solid domain.</p
Unraveling low nucleation temperatures in pool boiling through fluctuating hydrodynamics simulations
Full text linkWhen dealing with numerical simulations of boiling phenomena, the spontaneous appearance of vapor bubbles is one of the most critical feature to be addressed. Capturing bubble formation during the dynamics, instead of patching vapor regions as initial conditions, is crucial for the correct evaluation of nucleation rates and nucleation site density, two of the most important parameters characterizing boiling. In this work the Diffuse Interface modeling for vapor–liquid systems is coupled with Fluctuating Hydrodynamics Theory to properly address this aspect and to analyze the detailed nucleation mechanism during boiling inception on a hot surface. The simulations revealed a new enhancing mechanism of bubble formation that is able to explain the low onset temperature measured in boiling experiments on ultra-smooth, wettable surfaces: the interaction and coalescence between sub-critical vapor embryos plays a fundamental role in lowering the onset temperature, increasing the lifetime of the embryos and their probability to trigger the phase change.</p
An Enhanced VOF Method Coupled with Heat Transfer and Phase Change to Characterise Bubble Detachment in Saturated Pool Boiling
Full text linkThe present numerical investigation identifies quantitative effects of fundamental controlling parameters on the detachment characteristics of isolated bubbles in cases of pool boiling in the nucleate boiling regime. For this purpose, an improved Volume of Fluid (VOF) approach, developed previously in the general framework of OpenFOAM Computational Fluid Dynamics (CFD) Toolbox, is further coupled with heat transfer and phase change. The predictions of the model are quantitatively verified against an existing analytical solution and experimental data in the literature. Following the model validation, four different series of parametric numerical experiments are performed, exploring the effect of the initial thermal boundary layer (ITBL) thickness for the case of saturated pool boiling of R113 as well as the effects of the surface wettability, wall superheat and gravity level for the cases of R113, R22 and R134a refrigerants. It is confirmed that the ITBL is a very important parameter in the bubble growth and detachment process. Furthermore, for all of the examined working fluids the bubble detachment characteristics seem to be significantly affected by the triple-line contact angle (i.e., the wettability of the heated plate) for equilibrium contact angles higher than 45°. As expected, the simulations revealed that the heated wall superheat is very influential on the bubble growth and detachment process. Finally, besides the novelty of the numerical approach, a last finding is the fact that the effect of the gravity level variation in the bubble detachment time and the volume diminishes with the increase of the ambient pressure
Numerical Investigation of Quasi-sessile Droplet Absorption into Wound Dressing Capillaries
Full text linkThe key concept in wound dressing design and development is the fact that keeping a wound moist accelerates healing. Therefore, the selection of the appropriate wound dressing type is vital. The absorption of wound exudate by wound dressings can be considered as a microfluidic phenomenon that can be investigated either by performing high resolution laboratory experiments or by utilizing high resolution Computational Fluid Dynamics numerical simulations. As an initial step, in the present paper, the effects of the pore size (wound dressing porosity), the liquid (wound exudate) viscosity, and the initial droplet diameter are numerically investigated using a simplified analog of the phenomenon that consists of a quasi-sessile droplet being absorbed by a single cylindrical pore. For this purpose, an enhanced Volume Of Fluid model, developed in the general context of OpenFOAM, is validated and applied. It is found that distinct droplet absorption rates exist with specific relationships derived using best-fit lines that can predict the absorption rates for particular values of pore size and liquid viscosity. For the examined Eo and Oh number ranges (0.0015 < Eo < 0.15 and 0.0035 < Oh < 0.095), these distinct droplet absorption rates are directly linked with four different droplet evolution regimes that are grouped in a well-defined flow map. Finally, it is shown that the resulting liquid absorption rates are not significantly affected by the initial droplet diameter and that an appropriate wound dressing porosity can be selected by an estimation of the wound exudate physical properties. </p
Break-up Mechanisms and Conditions for Vapour Slugs Within Mini-Channels
Full text linkIn the present investigation an enhanced Volume Of Fluid (VOF) based numerical simulation framework is applied for the conduction of parametric numerical simulations, aiming to investigate observed break-up phenomena of vapour slugs, within circular mini-channel branches of a hybrid thermosyphon / pulsating heat pipe device, during microgravity experiments. The simulation results identify three prevailing break-up regimes. The effect of fundamental controlling parameters in the resulting break-up characteristics is also examined. An entrainment of a liquid droplet at the trailing edge of the vapour slug, that is responsible for its subsequent “full” break-up, is identified from the simulations. Moreover, it is quite interesting that the value of the applied heat flux, does not seem to influence the break-up regime and its main characteristics
Formal language for statistical inference of uncertain stochastic systems
Full text linkStochastic models, in particular Continuous Time Markov Chains, are a commonly
employed mathematical abstraction for describing natural or engineered dynamical
systems. While the theory behind them is well-studied, their specification can be
problematic in a number of ways. Firstly, the size and complexity of the model can
make its description difficult without using a high-level language. Secondly, knowledge
of the system is usually incomplete, leaving one or more parameters with unknown
values, thus impeding further analysis. Sophisticated machine learning algorithms have
been proposed for the statistically rigorous estimation and handling of this uncertainty;
however, their applicability is often limited to systems with finite state-space, and
there has not been any consideration for their use on high-level descriptions. Similarly,
high-level formal languages have been long used for describing and reasoning about
stochastic systems, but require a full specification; efforts to estimate parameters for
such formal models have been limited to simple inference algorithms.
This thesis explores how these two approaches can be brought together, drawing
ideas from the probabilistic programming paradigm. We introduce ProPPA, a process
algebra for the specification of stochastic systems with uncertain parameters. The
language is equipped with a semantics, allowing a formal interpretation of models
written in it. This is the first time that uncertainty has been incorporated into the syntax
and semantics of a formal language, and we describe a new mathematical object capable
of capturing this information. We provide a series of algorithms for inference which can
be automatically applied to ProPPA models without the need to write extra code. As
part of these, we develop a novel inference scheme for infinite-state systems, based on
random truncations of the state-space. The expressive power and inference capabilities
of the framework are demonstrated in a series of small examples as well as a larger-scale
case study. We also present a review of the state-of-the-art in both machine learning
and formal modelling with respect to stochastic systems. We close with a discussion of
potential extensions of this work, and thoughts about different ways in which the fields
of statistical machine learning and formal modelling can be further integrated
Compressible simulations of bubble dynamics with central-upwind schemes
Full text linkThis paper discusses the implementation of an explicit density-based solver, that utilises the central-upwind schemes for the simulation of cavitating bubble dynamic flows. It is highlighted that, in conjunction with the Monotonic Upstream-Centered Scheme for Conservation Laws (MUSCL) scheme they are of second order in spatial accuracy; essentially they are high-order extensions of the Lax–Friedrichs method and are linked to the Harten Lax and van Leer (HLL) solver family. Basic comparison with the predicted wave pattern of the central-upwind schemes is performed with the exact solution of the Riemann problem, for an equation of state used in cavitating flows, showing excellent agreement. Next, the solver is used to predict a fundamental bubble dynamics case, the Rayleigh collapse, in which results are in accordance to theory. Then several different bubble configurations were tested. The methodology is able to handle the large pressure and density ratios appearing in cavitating flows, giving similar predictions in the evolution of the bubble shape, as the reference
Accelerating Taylor bubbles within circular capillary channels:Break-up mechanisms and regimes
Full text linkIn the present paper, an enhanced Volume of Fluid model is applied for the conduction of parametric numerical simulations, to investigate break-up phenomena of accelerating, elongated, vapour bubbles, within circular mini-channels. The effect of fundamental controlling parameters in the resulting break-up characteristics is investigated. Four different series of parametric numerical simulations of isolated vapour bubbles within mini-channels are performed, examining the effects of the imposed pressure difference between the inlet and the outlet of the channel, the surface tension, the effect of the applied heat flux as well as the initial liquid film thickness between the bubbles and the channel, on the developed vapour/liquid interface dynamics. The overall dimensionless number ranges examined are 6.76 < We < 1474.7, 0.007 < Ca < 0.14, 694 < Re < 12541 and by introducing a modified Froude number in order to account for the flow acceleration, 1 < Fr* < 21.86. These dimensionless number ranges are selected in order to overlap with experimental observations in zero-gravity Pulsating Heat Pipe experiments that constitute the motivation for the present numerical investigation. The proposed simulation results identify three prevailing regimes. A “full break-up” regime, a “partial break-up” regime and a “no break-up” regime. The entrainment of liquid droplets at the trailing edge of the vapour slugs is in most cases responsible for their subsequent “full break-up”, into a leading and a trailing bubble, as it is identified from the numerical simulations. Moreover, the applied heat flux does not influence the resulting break-up regimes. Finally, these identified break-up regimes, are grouped together into a well-defined flow map with respect to the We and Fr* numbers.</p
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