1,720,958 research outputs found
Wear assessment of heavy-duty centrifugal fans: experimental and numerical analysis of particle erosion
No full textLe turbomacchine elaborano spesso flussi contaminati da particelle solide. Nella maggior parte dei casi, la presenza di particelle micrometriche disperse nel fluido non è desiderata ed è correlata alle condizioni in cui la macchina opera. Questo è il caso di motori aeronautici, che, aspirando grandi quantità di aria ingeriscono significative quantità di particelle solide. In altri casi, le macchine a fluido sono progettate proprio con lo scopo di processare aria contaminata da particelle. Questo è infatti il caso di ventilatori centrifughi di grande taglia impiegati in impianti di produzione, come acciaierie e cementifici, o in sistemi di ricircolo dell'aria, come quelli presenti in miniere o impianti energetici.
Indipendentemente dal fatto che la presenza di contaminanti solidi sia voluta oppure no, uno dei problemi che può scaturire in queste condizioni è l'erosione dei componenti fissi o rotanti della macchina. Questo problema concorre a ridurre l'affidabilità e la sicurezza dell'intero impianto, generando anche una riduzione di efficienza della macchina. In base alla severità del processo erosivo, la possibilità che si verifichino rotture premature e improvvise che mettano a rischio la sicurezza dell'uomo può diventare molto alta, soprattutto nei casi in cui le operazioni di manutenzione vengono ritardate o ignorate. Tuttavia, una corretta ed efficiente organizzazione delle operazioni di manutenzione non può essere fatta senza che non si abbia prima una chiara comprensione del fenomeno in gioco. In questo lavoro, l'attenzione è rivolta all'analisi dell'erosione di ventilatori di grossa taglia che processano aria contaminata da particelle solide. La severità del processo erosivo è stata studiata attraverso l'unione di tecniche di analisi numerica e sperimentale. Per prima cosa è stata effettuata un'analisi numerica in modo da comprendere le caratteristiche del fenomeno. In questa fase, sono stati utilizzati modelli e coefficienti di letteratura per studiare il processo erosivo all'interno della macchina. Successivamente, attraverso analisi sperimentali su materiali e contaminanti impiegati in condizioni operative nella macchina oggetto di studio, è stato possibile ottenere coefficienti numerici tarati appositamente sulle condizioni di interesse. In questa parte è stato utilizzato un banco prova appositamente realizzato. I risultati ottenuti dall'analisi sperimentale sono stati utilizzati per completare le analisi numeriche e stimare il danneggiamento dovuto all'erosione. Basandosi sui risultati ottenuti dall'implementazione dei coefficienti sperimentali nel modello numerico, è stato proposto un modello meccanicistico di natura semi empirica per fornire una stima dei tempi di manutenzione, in base a parametri geometrici della macchina, condizioni di impatto dei contaminanti, e resistenza all'erosione dei materiali. Il lavoro presentato in questa tesi è stato arricchito da un'ulteriore analisi sperimentale volta ad investigare l'accumulo del danneggiamento ad erosione su componenti aeronautici realizzata presso l'Institut fur Luftfahrtantriebe (Dipartimento di Sistemi per la Propulsione Aeronautica) dell'Università di Stoccarda.Turbomachinery operate often in conditions where solid particles are suspended into the processed gas flow. In most cases, the presence of micro-metric particles is not desired and it is related to the conditions where the machine operates. This is the case of aero-engines, which, swallowing a huge amount of air, may ingest, as a consequence, significant quantities of solid particles. In other cases, turbomachinery are specifically employed in operations for the transport of solid particles. Such is the case of heavy-duty centrifugal fans employed in production plants, for cement or steel production, or as a part of exhaust systems in mines or energy production plants.
Regardless of whether the presence of solid contaminants in the flow stream is desired or not, one of its consequences is the over-time wear of rotating and stationary parts of the machine. This issue contributes to reducing the reliability and the safety of the whole plant and leads to a decrease in the efficiency of the machine. The severity of the erosion, if maintenance operations are postponed or even ignored, can bring the machine to a premature failure, with potentially serious consequences for human safety. However, the proper scheduling of the maintenance operations can not be possible without a deep understanding of the mechanisms involved in the particle erosion phenomenon which affects rotating machines.
In the present work, the attention is focused on the investigation of the wear phenomenon in heavy-duty centrifugal fans. A combined experimental and numerical analysis has been carried out to assess the erosion severity on the machine. First, a preliminary numerical analysis has been carried out to investigate the characteristics of the phenomenon. Literature models and coefficients have been employed to assess the erosion behavior of the machine.
The need for more reliable coefficients, capable of predicting the erosion behavior, led to testing the particle and substrate materials, actually employed in the machine, on an on-purpose designed test bench. The results have been used in numerical simulations to properly assess the wear severity and erosion-related damages. A mechanistic semi-empirical relation is proposed to assess the time to overhaul based on the results of the present investigation.
The present work has been enriched with a further experimental investigation of the erosive damage accumulation of aeronautical components carried out at the Institut fur Luftfahrtantriebe (Institute for Aircraft Propulsion Systems) of the University of Stuttgart
Analysis of soil and soot deposits by X-ray computed microtomography
No full textEngineering systems continuously interact with nano-sized particles coming from natural or artificial sources. Considering nano-sized contaminants the detrimental effects are known as soiling and fouling phenomena. The objective of the present work is to gather morphological and three-dimensional quantitative information of discontinuities such as voids and pores within the nano-sized particle layer and in correspondence to the substrate/layer interface using microtomography. Deposits are obtained through impact tests using fine powder (soil and soot), and there are realized on a low-density material substrate characterized by different surface roughness values. The packing process and the discontinuities at the substrate-to-layer interfaces explain macroscopic effects as detachment and spallation but at the same time, give the possibility to understand more in detail the process related to the deposition mechanisms involved in this type of test defining general guidelines to predict the fouling phenomenon and to improve the capability of the removal process
Numerical Investigation of a Wood-Chip Downdraft Gasifier
No full textBiomass gasification is regarded as one of the most promising technology in the renewable energy field. The outcome of such operation, i.e. the synfuel, can be exploited in several ways, for example powering engines and turbines, and is considered more flexible than the biomass itself. For this reason, a careful analysis of the gasification performance is of paramount importance for the optimization of the process. One of the techniques that can be used for such a purpose, is the numerical analysis. CFD is indeed a tool that can be of great help in the design and study of the operation of the gasifier, allowing for an accurate prediction of the operating parameters. In this work, a downdraft gasifier is considered, and the biomass is made of wood chip. The present analysis is devoted to build the numerical model and simulate all the reactions that happen inside an actual gasifier, considering the drying of the wood chip, heating, pyrolysis, and combustion. Good match with experimental results is found, making the numerical model here presented a reliable virtual test bench where investigating the effects of variation in the working parameters
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
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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