1,720,960 research outputs found
Evaluation of the Potential for Commercial Space Industry Growth: an Entrepreneurial Environment Analysis of Russia
No full textA Parametric Study of the Effect of Liquid Fuel Entrainment on the Combustion Characteristics of a Paraffin-Based Hybrid Rocket Motor
No full textThe interaction between a melting hybrid rocket fuel grain and the combustion processes inside the combustion chamber of a hybrid rocket motor was investigated using a CFD model, ultimately evaluating how this affects the motor’s performance. The motor’s working conditions were simulated by changing the global oxidizer to fuel mass flow ratio, the fraction of the total fuel entrained by the oxidizer, and the size distribution of the entrained droplets. Due to the transport of the fuel droplets the reaction zones are extended farther downstream, which results in an incomplete evaporation and causes significantly lower combustion efficiency comparing to the ideal well-mixed mixture
A Wide Characterization of Paraffin-based Fuels Mixed with Styrene-based Thermoplastic Polymers for Hybrid Propulsion
Full text linkDevelopment of an Experimental Lab-Scale Set up for Transient Combustion Investigation in Hybrid Rockets
No full textEntrainment effects on combustion processes in paraffin-based hybrid rocket motors
Full text linkDurante gli ultimi anni i motori ibridi dei razzi rappresantano un campo molto attivo di ricerca per la loro capacità di unire gli aspetti positivi dei motori liquidi e dei motori solidi. Rispetto ai motori tradizionali a combustibile liquido o solido, i motori ibridi offrono una serie di vantaggi come la riduzione della complessità meccanica, la possibilità di throttling e riavviamento del motore, e la riduzione del rischio di esplosione e incendio. Nonostante questi vantaggi e molti anni di ricerca, i motori ibridi sono stati inutilizzati per l’esplorazione dello spazio poichè la velocità di regressione del combustibile solido è troppo bassa.
Recentemente, è stato scoperto un nuovo tipo di combustibile al base di paraffina. Le prove di laboratorio di questo combustibile hanno ottenuto una velocità di regressione di 3-4 volte superiore rispetto ai combustibili convenzionali. La causa di questo miglioramento è ritenuta essere il risultato della fusione del combustibile e della formazione di uno strato liquido che verrà poi trascinato sotto forma di goccioline nel ossidante che scorre sulla sua superficie ad alta velocità. Queste goccioline trascinate creano una superficie di combustibile piu’ grande con cui l'ossidante può reagire. Tuttavia, ciò comporta anche una significativa perdita massa di combustibile attraverso l'ugello del motore, prima che questa venga bruciata. Inoltre, i combustibili alla base di paraffina possono non avere la resistenza meccanica per essere utilizzati nelle applicazioni reali. Per rafforzare il grano del combustibile si usano gli additivi termoplastici, che pero’ possono influenzare la viscosità dello strato liquido del combustibile e ridurre l’effetto di entrainment. Questi processi e le interazioni non sono ben chiari e prima che i benefici di questi combustibili possono essere tradotti in design di applicazioni concrete, i loro effetti devono essere ben compresi.
Nel presente lavoro vengono studiati gli effetti che influenzano il design di un razzo ibrido in due fasi. La prima fase riguarda più test sui materiali per determinare come la composizione del combustibile (i.e il percento dell’addittivo termoplastico) possa cambiare la resistenza del combustibile solido, la temperatura di fusione e la viscosità liquida. Nella seconda fase, è stato sviluppato un modello CFD per le simulazioni di un motore ibrido nella scala di laboratorio. Il modello CFD e’ stato utilizzato per studiare la sensibilità del rendimento del motore alle variazioni delle caratterisitiche del flusso trascinato: ovvero le dimensioni delle gocce, rapporto combustibile-ossidante e la frazione di combustibile trascinato. Queste quantità sono state calcolate utilizzando relazioni empiriche e di analisi disponibili in letteratura e imposte come condizioni al contorno nel modello CFD. I risultati mostrano che la presenza delle gocce di combustibile in particolare di quelle più grandi, spinge la zona delle reazioni chimiche più a valle. Di conseguenza, ai fini della progettazione e dell’ottimizzazione dell’efficienza, la massa delle particelle incombuste deve essere considerata oppure la lunghezza del motore dovrebbe essere modificata per consentire una più completa evaporazione/combustione delle gocce.
Questo lavoro dà un contributo significativo alla modellazione computazionale della combustione nei motori ibridi di razzi, particolarmente alla descrizione del fenomeno di entrainment, che è l'aspetto fisico più rilevante nel comportamento della combustione per migliorare la tecnologia dei razzi ibridi.Hybrid rocket motors have been an active area of research for many years due to the potential best-of-both-worlds which they offer by leveraging the favorable aspects of both liquid and solid rockets respectively. Compared to traditional liquid- and solid-fuel rocket motors, hybrid rocket motors offer a number of distinct advantages such as reduced mechanical complexity, the possibility of throttling and restart, and reduced fire and explosion risk. In spite of these advantages and many years of research, hybrid rocket motors have gone largely unused in space exploration due in large part to the inherently low regression rate of the solid fuel grain. While the regression rate effect can be partially ameliorated through geometrical changes in the fuel grain’s design, this can lead to problems like inadequate structural strength of the fuel grain and insufficient volumetric efficiency.
Recently, a new type of hybrid rocket fuel has been proposed which uses paraffin as its base. Laboratory studies of this fuel have shown 3-4 times better regression rate performance compared to conventional fuels. The reason for this improved performance is believed to be the result of the fuel melting to form a liquid film layer which can then be entrained as droplets by the high-speed oxidizer flowing across its surface. These entrained fuel droplets may offer increased fuel surface area with which the oxidizer can react but the fact that they are entrained may mean that significant fuel mass is lost through the motor nozzle before it can be burned for thrust. Additionally, paraffin-based fuels may lack the mechanical strength and toughness to be considered for space flight applications. Fuel additives that strengthen and toughen the fuel grain may affect the viscosity of the melted fuel layer and affect how it becomes entrained in the oxidizer. These processes and interactions are not yet well understood and they must be investigated thoroughly before the benefits of these fuels can be translated to real-world design applications.
This work attempts to resolve several of the key unknowns affecting hybrid rocket design in two phases: first, materials testing to determine the effect of composition changes on the strength of the solid fuel, its melting point, and its viscosity once melted. Second, a CFD modeling strategy was developed for simulating a lab-scale hybrid rocket motor. The CFD model was used to study the sensitivity of the motor’s performance to changes in the characteristics in the entrained flow like droplet size, global mixture oxidizer-to-fuel ratio, and the fraction of fuel entrained. These quantities were calculated using empirical and analytical relations available in the literature and imposed as boundary conditions in the CFD model. The results show that the presence of the fuel droplets pushes the reaction zones farther downstream, particularly for larger particles. As a consequence, for design purposes the either the mass of the unburned particles would have to be accounted for or the length of the motor would have to be changed to allow for more complete droplet evaporation/combustion thus obtaining higher combustion efficiency.
This work gives a significant contribution in the computational modeling of hybrid rockets flowfields, with special attention to the entrainment phenomenon description, which is the most relevant physical aspect in the combustion behavior of low melting temperature fuels recently proposed to improve hybrid rocket technology.DIPARTIMENTO DI SCIENZE E TECNOLOGIE AEROSPAZIALI27DI LANDRO, LUCA ANGELOVIGEVANO, LUIG
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
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