1,720,966 research outputs found
Pseudo-boiling and heat transfer deterioration while heating supercritical liquid rocket engine propellants
No full textHeating of liquid propellants used as the coolant in rocket engines may lead to undesired phenomena such as pseudo-boiling or heat transfer deterioration under specific conditions. This can be an issue for propellants characterized at the same time by relatively low critical pressure and temperature. Light hydrocarbons, as for instance methane, belong to this family. In the present paper, a critical review is made of the main results obtained by Authors and their coworkers for the present application. Focus is on the correlations and trends inferred by their numerical simulations mainly carried out considering methane as the coolant, perhaps the most challenging one
Aerothermodynamic analysis of high-altitude space launcher with direct-simulation Monte Carlo method
No full textAerothermodynamics of space launchers operating at high altitudes (above 100 km) is characterized by complex physical processes that are related to the substantially higher air rarefaction with respect to lower altitudes. Consequently, the use of continuum methods to numerically simulate such flow conditions is questionable. In this study, the problem of proper assessment of the aerothermodynamic behavior of space launchers operating at high altitudes is analyzed considering Vega in its ascent flight, an actual space launcher that has already performed a number of successful flights. The direct simulation Monte Carlo molecular method is used to carry out the simulations. After a convergence analysis of the adopted numerical approach, a comparison of different air models is presented to highlight the effect of chemical reactions and molecules vibrational excitation. Then, a comparison of the flowfield around the Vega launcher is reported at two altitudes (113 and 127 km), where the third stage of the space launcher is not propelled. The analysis describes the effects of the atmospheric air rarefaction, which has significant repercussions on the bow-shock structure and the drag coefficient. Finally, the achieved solutions are compared with computational fluid dynamics (CFD) continuum solutions to show that a standard CFD approach is not able to represent the physical phenomenon under examination, due to the model’s breakdown caused by the effects of rarefaction
Oxygen–methane rocket thrust chambers: Review of heat transfer experimental studies
No full textNowadays oxygen–methane propulsion is considered of great potential for the next generation of operational rockets. In the development of a new liquid rocket engine with high-performance, an essential assessment is the thermal behavior of the thrust chamber. In fact, the flow of combustion gas inside the thrust chamber induces a level of wall temperature and heat flux such as to limit the life of this component. Furthermore, the heat transfer within the thrust chamber directly impacts the performance of the rocket engine. In the present study, a thorough survey of the open literature concerning the experimental activities focused on the heat transfer within oxygen–methane thrust chambers is carried out in order to identify the most consolidated results and the aspects that would require further investigations. The results are divided into three main categories: hot-gas side heat transfer; coolant heat transfer and wall compatibility; thrust chamber life
A Simplified Thermal Analysis Model for Regeneratively Cooled Rocket Engine Thrust Chambers and Its Calibration with Experimental Data
Full text linkAn essential part of the design of a liquid rocket engine is the thermal analysis of the thrust chamber, which is a component whose operative life is limited by the maximum allowable wall temperature and heat flux. A simplified steady-state thermal analysis model for regeneratively cooled rocket engine thrust chambers is presented. The model is based on semi-empirical correlations for the hot-gas and coolant convective heat transfer and on an original multi-zone approach for the wall conduction. The hot-gas heat transfer is calibrated with experimental data taken from an additively manufactured water-cooled nozzle that is connected to a combustion chamber either fed with decomposed hydrogen peroxide or decomposed hydrogen peroxide and automotive diesel. The thrust chamber (i.e., combustion chamber and nozzle) is designed to produce about 450 N of thrust when operating with a chamber pressure of 11 bar. For this application, the calibrated model predicts the total wall heat transfer rate very accurately and the temperature distribution within the wall structure with an uncertainty of a few tens of kelvins. This level of accuracy can be considered more than adequate for the design, and generally for engineering-type thermal analysis, of similar thrust chambers
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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