1,720,975 research outputs found
Mechanisms and enhancement of CO2 condensation heat transfer
No full textThe aim of this thesis is to increase the understanding of the mechanisms governing Carbon Dioxide (CO2) condensation, and in that way finding a pathway towards a higher condensation efficiency. Liquefaction of CO2 occurs e.g. in industrial processes such as in heat exchangers for thermal management. Compact geometries have become increasingly important in thermal management following the decrease in size of electronic equipment. CO2 is a suitable solution as a refrigerant in such heat exchangers due to its thermophysical properties. It is also a viable substitute as a refrigerant in conventional heat transfer equipment in the quest for eliminating fluorine based refrigerants, due to their negative environmental impact. Liquefaction of CO2 also occurs when processing the gas prior to ship transportation of CO2 being captured from e.g. power plant exhaust. Reduction of the anthropogenic emissions of CO2 is required for reducing the global warming and achieving the 2-degree scenario set forth by the International Panel of Climate Change.
The main reason for resistance to heat transfer during condensation is the restricted conduction through the condensate film that is formed on the condenser surface. The research on more efficient condensation currently focuses on suppressing the formation of this condensate film and in stead achieving dropwise condensation. An introduction of droplets on the surface will reduce the heat transfer resistance and a more efficient condensation process emerges. The condensation heat transfer coefficient could be increased by up to an order of magnitude. For CO2 this alternative has not yet been considered and the research on CO2 condensation has hitherto focused on understanding how the flow properties affects the heat transfer in flow condensation inside tubes and channels. These are relevant geometries for heat exchangers and the flow in microchannels must be deciphered to gain control of the condensation in compact equipment. The condensation of CO2 on flat and structured surfaces has, however, not yet been studied, and the models for predicting the behavior do not include the influence of condenser material or surface structures on the heat transfer. We have through this work tried to answer if condensation of CO2 could be achieved in the dropwise mode, and if not, how the efficiency of CO2 condensation otherwise can be increased. We have also explored how the condenser material and/or surface structures influence the heat transfer during condensation.
For solving these issues we have designed a methodology and built an experimental facility for measuring the CO2 condensation heat transfer coefficient on various sur faces. By achieving elevated pressures and low temperatures CO2 has been liquefied on vertical surfaces and the heat transfer has been measured.
We have in this work established a first set of experimental data for CO2 condensation on vertical walls. We have found that surface properties such as roughness and surface energy affect the condensation heat transfer. With surface roughness, the material’s thermal conductivity also comes into play, as the uniformity of the temperature within the liquid will be influenced by the conductivity of the peaks of the material when the liquid film is thin. We have also found that structuring the surface with micro- and nanosized features will alter the heat transfer efficiency. A combination of micro- and nanostructures could result in a thinner liquid film on the surface, consequently reducing the heat transfer resistance through the condensate. On the other hand, the structures could also cause significant liquid retention and therefore a thicker liquid film. The results indicate that a careful design and fabrication of a combination of micro- and nanostructures will enhance the condensation heat transfer by increasing the surface area, reducing liquid retention and increasing the effective thermal conductivity of the composite surface of liquid and nanostructures similar to wicking condensation.
The results in this thesis show that there are pathways to increasing condensation heat transfer of CO2, which will lead to reduced costs and energy demands of the liquefaction process. Two main parameters have been identified: 1) roughness control and 2) optimization potentials for structured surfaces. This will result in e.g. lower CO2 transport costs during carbon capture and storage, and in improved thermal management of small electronic devices
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
Mechanisms and enhancement of CO2 condensation heat transfer
No full textThe aim of this thesis is to increase the understanding of the mechanisms governing Carbon Dioxide (CO2) condensation, and in that way finding a pathway towards a higher condensation efficiency. Liquefaction of CO2 occurs e.g. in industrial processes such as in heat exchangers for thermal management. Compact geometries have become increasingly important in thermal management following the decrease in size of electronic equipment. CO2 is a suitable solution as a refrigerant in such heat exchangers due to its thermophysical properties. It is also a viable substitute as a refrigerant in conventional heat transfer equipment in the quest for eliminating fluorine based refrigerants, due to their negative environmental impact. Liquefaction of CO2 also occurs when processing the gas prior to ship transportation of CO2 being captured from e.g. power plant exhaust. Reduction of the anthropogenic emissions of CO2 is required for reducing the global warming and achieving the 2-degree scenario set forth by the International Panel of Climate Change.
The main reason for resistance to heat transfer during condensation is the restricted conduction through the condensate film that is formed on the condenser surface. The research on more efficient condensation currently focuses on suppressing the formation of this condensate film and in stead achieving dropwise condensation. An introduction of droplets on the surface will reduce the heat transfer resistance and a more efficient condensation process emerges. The condensation heat transfer coefficient could be increased by up to an order of magnitude. For CO2 this alternative has not yet been considered and the research on CO2 condensation has hitherto focused on understanding how the flow properties affects the heat transfer in flow condensation inside tubes and channels. These are relevant geometries for heat exchangers and the flow in microchannels must be deciphered to gain control of the condensation in compact equipment. The condensation of CO2 on flat and structured surfaces has, however, not yet been studied, and the models for predicting the behavior do not include the influence of condenser material or surface structures on the heat transfer. We have through this work tried to answer if condensation of CO2 could be achieved in the dropwise mode, and if not, how the efficiency of CO2 condensation otherwise can be increased. We have also explored how the condenser material and/or surface structures influence the heat transfer during condensation.
For solving these issues we have designed a methodology and built an experimental facility for measuring the CO2 condensation heat transfer coefficient on various sur faces. By achieving elevated pressures and low temperatures CO2 has been liquefied on vertical surfaces and the heat transfer has been measured.
We have in this work established a first set of experimental data for CO2 condensation on vertical walls. We have found that surface properties such as roughness and surface energy affect the condensation heat transfer. With surface roughness, the material’s thermal conductivity also comes into play, as the uniformity of the temperature within the liquid will be influenced by the conductivity of the peaks of the material when the liquid film is thin. We have also found that structuring the surface with micro- and nanosized features will alter the heat transfer efficiency. A combination of micro- and nanostructures could result in a thinner liquid film on the surface, consequently reducing the heat transfer resistance through the condensate. On the other hand, the structures could also cause significant liquid retention and therefore a thicker liquid film. The results indicate that a careful design and fabrication of a combination of micro- and nanostructures will enhance the condensation heat transfer by increasing the surface area, reducing liquid retention and increasing the effective thermal conductivity of the composite surface of liquid and nanostructures similar to wicking condensation.
The results in this thesis show that there are pathways to increasing condensation heat transfer of CO2, which will lead to reduced costs and energy demands of the liquefaction process. Two main parameters have been identified: 1) roughness control and 2) optimization potentials for structured surfaces. This will result in e.g. lower CO2 transport costs during carbon capture and storage, and in improved thermal management of small electronic devices
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
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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