1,720,956 research outputs found
Aero-Thermal Numerical Predictions of Trailing Edge and Leading Edge Cooling Channels
Full text linkState-of-art gas turbine are designed to operate at turbine inlet temperatures higher than 2000[K]. Such temperature levels are sustainable only by means of aggressive and efficient cooling of the components exposed to the hot gas path. It should be pointed out that not only must the maximum metal temperature be kept below safety limits, but the thermal field must be reasonably uniform too, in order to limit thermal stresses. Moreover, mod- ern blade cooling systems consist of a combination of internal cavities with cross-sections specifically developed for each different blade portion; therefore, specific studies are essen- tial to describe their performances in detail in order to provide designers with the most accurate knowledge.
The need for such detailed information is in conflict with some common practices in cool- ing system design: most of the studies deal with square or rectangular channels cross- section (hence resembling ducts in the central body of the blade), the link between coolant flow field and heat transfer rates is seldom analyzed; finally, the coupling of rotation and different channel orientations is rarely taken into account.
Over the last few years CFD (Computational Fluid Dynamic) has been exploited to pro- vide valuable information on complex flow fields and heat transfer in internal cooling passages; indeed, it is already used as an engineering tool in design and optimization processes of gas turbine cooling. On the other hand, the reliability of the numerical tools available at present is not sufficiently high and, hence, detailed experimental analyses are still required for numerical validation purposes.
The present thesis focuses on the aspects pertaining to the suitability of CFD for the prediction of the aero-thermal performances inside cooling channels designed for two es- sential portions of the blade, namely trailing edge and leading edge, whose sizes and shapes are quite different from those resembling cooling channel in the central portion of the blade.
The trailing edge cooling model is characterized by a trapezoidal cross-section of high aspect-ratio and coolant discharge at the blade tip and along the wedge-shaped trailing side, where seven lengthened pedestals are also installed. Three different configurations are taken into account, namely the smooth channel and two others characterized by the use of ribs in different portions of the duct. Firstly, an extensive comparison with detailed experimental data including local flow velocities, turbulence proprieties and local heat transfer coefficient in static (Ro = 0) and orthogonal rotating conditions (Ro = 0.23) is carried out using the Shear Stress Transport (SST) turbulence model. Moreover, for one rib-roughened configuration in static condition (Ro = 0) different turbulence models are tested in order to enhance all computational results. Finally, the CFD code is exploited to analyzed more engine-like conditions, namely Ro = 0.46 and γ = 22.5 − 45[◦]. The results show that rotation and channel orientation produce contrasting effects which are more significant in the rib-roughened configuration. In fact, on the radial central portion rotation/orientation generates an increase/decrease in the heat transfer; conversely, on the trailing side region, rotation/orientation has a negative/positive effect on the thermal field.
The leading edge cooling model consists of a straight, smooth channel with an equi- lateral triangle cross-section. Geometry and test conditions resemble those pertaining to the passages used for the internal cooling of gas turbine blades leading edge. On the same geometry and at comparable working conditions, heat transfer data are also available from literature. Experimental data are used for CFD validation purposes at Re = 20000 Ro = 0.2 and Re = 10000 Ro = 0.4. Consequently, a wide range of work- ing conditions, namely Re = 10000 − 40000 Ro = 0.2 − 0.6 are numerically explored by the SST turbulence model. The results show that the rotation-induced flow structure is rather complicated showing relevant differences compared to the flow models that have been supposed by the research community so far. Indeed, the secondary flow turned out to be characterized by the presence of two or more vortex cells depending on channel location and Ro number. No separation or reattachment of these structures is found on the channel walls but they are observed at the channel apexes. The stream-wise velocity distribution shows a velocity peak close to the lower apex and the overall flow structure does not reach a steady configuration along the channel length. This evolution is has- ten (in space) if the rotation number is increased while changes of the Re number have no effects. Moreover, thanks to the understanding of the flow mechanisms associated to rotation, it was possible to provide a precise justification for the channel thermal behav- ior. Finally, different channel orientations (namely γ = 22.5 − 45[◦]) are numerically investigated. The results further demonstrate that the variation of the channel orien- tation to more engine-like conditions significantly affects the flow field and, hence, the aero-thermal behavior
Coriolis Effects on the Flow Field Inside a Rotating Triangular Channel for Leading Edge Cooling
No full textThe flow field inside a rotating smooth radial channel with a triangular shaped cross section is investigated. Test conditions resemble those pertaining to the passages used for the internal cooling of the gas turbine blade's leading edge. Heat transfer data are also available from the literature on the same geometry and at comparable working conditions and have been profitably used for a combined aerothermal analysis. The model consists of a straight smooth channel with an equilateral triangle cross section. The rotation axis is aligned with one of the triangle bisectors. Two dimensional particle image velocimetry (PIV) and stereo-PIV were used in order to characterize the inlet flow (in static conditions) and the rotation-induced secondary flow in the channel cross section at Re = 20,000, Ro = 0.2 and Re = 10,000, Ro = 0.4. A wider range of working conditions (Re = 10,000-40,000, Ro = 0.2-0.6) was explored by means of Reynolds averaged Navier-Stokes (RANS) simulations carefully validated by the available PIV data. The turbulence was modeled by means of the shear stress transport (SST) model with a hybrid near-wall treatment. The results show that the rotation-induced flow structure is rather complicated and show relevant differences compared to the flow models that have been considered thus far. Indeed, the secondary flow turned out to be characterized by the presence of two or more vortex cells, depending on channel location and Ro number. No separation or reattachment of these structures is found on the channel walls but they have been observed at the channel apexes. The stream-wise velocity distribution shows a velocity peak close to the lower apex and the overall flow structure does not reach a steady configuration along the channel length. This evolution is fastened (in space) if the rotation number is increased while changes of the Re number have no effect. Finally, due to the understanding of the flow mechanisms associated with rotation, it was possible to provide a precise justification of the channel thermal behavior
Numerical aero-thermal analysis of a rib-roughened trailing edge cooling channel at different rotation numbers and channel orientations
No full textThe present work considers the aero-thermal characterization of a rib-roughened cooling channel for the trailing edge of gas turbine blades, and is based on previous findings from a smooth channel configuration. The passage is characterized by a trapezoidal cross section with high aspectratio, radial inlet flow, and coolant discharge at both model tip and trailing side, where seven elongated pedestals are installed. In this study, heat transfer augmentation is achieved by placing inclined squared ribs on the channel central portion. RANS simulations with a SST turbulence model were performed using the commercial solver ANSYS CFX®v14. The numerical tool was first validated on the available experimental data and, subsequently, its capabilities were exploited in a wider range of working conditions, namely at higher rotation speed and different channel orientation. In this way it was possible to highlight the effects that ribs and working conditions have on the development of both flow and thermal fields. The results show that rotation and channel orientation produce contrasting effects. On the rib-roughened wall, rotation/orientation generates an increase/decrease of the heat transfer; conversely, on the trailing side region rotation/orientation has a negative/positive effect on the thermal field. © 2014 by ASME
Effects of Rotation at Different Channel Orientations on the Flow Field inside a Trailing Edge Internal Cooling Channel
Full text linkThe flow field inside a cooling channel for the trailing edge of gas turbine blades has been numerically investigated with the aim to highlight the effects of channel rotation and orientation. A commercial 3D RANS solver including a SST turbulence model has been used to compute the isothermal steady air flow inside both static and rotating passages. Simulations were performed at a Reynolds number equal to 20000, a rotation number (Ro) of 0, 0.23, and 0.46, and channel orientations of γ=0∘, 22.5°, and 45°, extending previous results towards new engine-like working conditions. The numerical results have been carefully validated against experimental data obtained by the same authors for conditions γ=0∘ and Ro = 0, 0.23. Rotation effects are shown to alter significantly the flow field inside both inlet and trailing edge regions. These effects are attenuated by an increase of the channel orientation from γ=0∘ to 45°
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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