1,721,087 research outputs found
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
Heat transfer coefficient and pressure drop of flow boiling in microchannels using R134a : simulation and validation with empirical correlations
No full textLAUREA MAGISTRALEIl flusso multifase con evaporazione (flow boiling) in microcanali
racchiude fenomeni complessi e differenti.
Sfide riguardanti la stima delle prestazioni, degli schemi dei flussi
e dei meccanismi di scambio termico nascono dai complessi fenomeni
fisici che governano questi flussi.
Dispositivi bifase sono utilizzati a scopo di refrigerazione e
trasferimento di calore a causa del più alto coefficiente di scambio
termico rispetto a soluzioni tradizionali monofase, e quindi migliori
proprietà di asportazione del calore, grazie al fatto di poter sfruttare
sia il calore sensibile che quello latente.
Per quanto concerne il flow boiling in microcanali, una soluzione
bifase attiva di trasferimento del calore, ci sono molti aspetti da
considerare quali capillarità, fenomeni di dry-out e meccanismi di ebollizione.
Alcuni di questi non sono ancora stati del tutto compresi e
ostacolano il passaggio dall’uso prettamente di ricerca alla diffusione
in commercio. Tra questi è possibile menzionare l’assenza di strumenti
affidabili per predirre il coefficiente di scambio termico e la caduta di
pressione in un meccanismo di ebollizione di tale tipo. Inoltre la mancanza
di una definizione unanime di microcanale, dalle cui dimensioni
alcuni parametri come rugosità superificiale, iniziano a giocare un ruolo
non trascurabile, è uno dei temi più controversi tra i ricercatori.
Per tale scopo, il presente lavoro si focalizza sulla descrizione Euleriana
di un flusso costituito da R134a che evapora in un microcanale
riscaldato. É stato usato un modello multifase all’interno del software
ANSYS Fluent, attivando il modello RPI per quando riguarda
l’evaporazione e implementando gli effetti dovuti alla frazione di volume
costituita da fase liquidi e gli effetti legati al film sottile.
I risultati ottenuti attraverso la simulazione numerica in termini di
coefficiente di scambio termico e perdita di carico sono stati confrontati
con alcune correlazioni presenti in letteratura e adattate al caso in
esame, in maniera tale da validare la simulazione numerica.
L’intero lavoro è stato svolto a supporto di un apparato sperimentale
per il flow boiling presente nel Advanced Engineering Centre della University of Brighton. Quindi le condizioni al contorno e di lavoro globali
utilizzate nella simulazione numerica sono state selezionate in maniera
tale da ottimizzare l’apparato sperimentale proposto. Mini e microcanali
ricoperti dal film di riscaldatori ITO verranno utilizzati in futuro
per permettere di visualizzare il fluido all’interno del canale proposto.
É degno di menzione il fatto che, oltre all’obiettivo principale di
questo lavoro e cioè le simulazioni CFD e il loro confronto con correlazioni
empiriche, è stata svolta una parte sperimentale addizionale attraverso
una telecamera a infrarossi per verificare l’uniformità termica
dei riscaldatori ITO.Flow boiling in microchannels involves different and complex phenomena.
Many challenges in terms of performance estimation, flow patterns
and heat transfer mechanisms arise from the extreme physical phenomena
governing such flows.
Two-phase devices that utilise boiling and condensation phenomena,
are used for refrigeration purposes and heat transfer due to their higher
heat transfer coefficient compared to traditional single-phase solutions,
and this results in better heat dissipation properties by means of the
possibility of exploiting both the sensible and the latent heat.
Regarding flow boiling in microchannel, a two-phase active heat
transfer solution, there are many aspects to be taken into account
such as capillarity effects, dry-out phenomena, boiling and condensation
mechanisms. Some of these are still not properly understood and
hinder the passage from research to commercial use.
Among them, it is possible to point out the absence of a reliable tool
to foresee the heat transfer coefficient and the pressure drop for different
geometrical configurations and operating conditions. Moreover, the
lack of a unanimous definition of microchannel from whose dimensions
some parameters, e.g. surface roughness, start to play a non negligible
role, is one of the most controversial topics among the researchers.
For this purpose, the present work focuses on a multiphase Eulerian
description of an R134a flow boiling inside a heated microchannel. A
multiphase model in the ANSYS Fluent environment has been used,
enabling the RPI boiling model and implementing liquid volume fraction
effects and thin film effects.
The outcomes in terms of heat transfer coefficient and pressure drop
using the numerical tool were therefore compared with some correlations
available in literature adapted to the specific case, in order to
validate the numerical simulation.
The overall work was conducted in the general framework of the
preparation of a unique flow boiling experimental set-up in the Advanced
Engineering Centre in the University of Brighton. Therefore
the overall boundary and operating conditions for the numerical simulations were selected accordingly in order to aid in the optimisation of
the proposed experimental apparatus. Mini and micro-channels fitted
with special Indium Tin Oxide (ITO) heaters are going to be utilised
in the proposed set-up in order to allow for flow visualisation through
the heated regions of the channels. It should be mentioned that apart
from the main aim of the present thesis that is the aforementioned CFD
simulations and their comparison with empirical correlations from the
literature, some preliminary experimental work was also conducted additionally
checking the thermal homogeneity of the ITO heaters with
a high-speed IR Camera
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Reduced dimensionality model of annular two-phase flows in small channels
No full textLAUREA MAGISTRALEQuesta tesi fa parte di uno sforzo per estendere un modello numerico delle Pulsating Heat Pipes (PHPs) precedentemente sviluppato, tenendo conto dell'effetto della dinamica del film liquido sul canale nelle regioni occupate da vapor plugs.
L'efficienza computazionale è un requisito fondamentale perché il modello è inteso come modulo di uno strumento computazionale molto più ampio e complesso. Un modello 3D esistente per il flusso bifase nei tubi capillari è stato sviluppato nel gruppo di ricerca e convalidato rispetto a risultati sperimentali. Qui miriamo a eseguire una riduzione della dimensionalità al fine di ottenere risultati di accuratezza comparabile a una frazione del costo. Si procede, quindi, a derivare uno schema numerico con time-step esplicito, formulato per consentire l'implementazione parallela del codice.
Il modello a dimensionalità ridotta è costituito da due modelli 1D separati, rispettivamente per il flusso del vapore al centro del tubo e per il film liquido che circonda la bolla, separandola dalle pareti del tubo.
I due insiemi di equazioni che rappresentano tali flussi sono accoppiati tramite correlazioni semi-empiriche che descrivono vari fenomeni che si verificano all'interfaccia (i.e. attrito e tensione superficiale). La validità di tali correlazioni viene valutata disaccoppiando i sottosistemi nel modello e testando ciascun sottosistema con i dati ottenuti dal modello 3D completo come input. Infine, vengono eseguiti diversi test per valutare le prestazioni complessive del modello 1D, del corrispondente schema numerico e i risultati vengono presentati in modo sistematico e discussi criticamente.This thesis is part of an effort to extend a previously developed numerical model of Pulsating Heat Pipes (PHPs) by accounting for the effect of the dynamics of the liquid film on the channel in regions occupied by vapor plugs.
Computational efficiency is a key requirement because the model is intended as a module of a much larger and more complex computational tool. An existing 3D model for two-phase flow in capillary pipes has been developed in the research group and validated with respect to experiments. Here we aim to perform a dimensionality reduction in order to obtain results of comparable accuracy at a fraction of the cost. We, therefore, proceed to derive a numerical scheme with explicit time-stepping, formulated to allow parallel implementation of the code.
The reduced dimensionality model consists of two separate 1D models for the flow of the vapor in the center of the pipe and for the liquid film surrounding the bubble and separating it from the pipe walls.
The two sets of equations representing such flows are coupled via semi- empirical correlations describing various phenomena occurring at the interface (i.e. friction and surface tension). The validity of such correlations is assessed by decoupling the subsystems in the model and testing each subsystem with data obtained from the full 3D model as input.
Finally, several tests are performed to assess the overall performance of the 1D model and the corresponding numerical scheme and the results are presented in a systematic way and critically discussed
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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