1,721,361 research outputs found
Focus on synthesis, characterization and applications of low dimensional nanomaterials
Full text linkThis Editorial summarizes the content of a focus collection on the synthesis, characterization and applications of low dimensional materials. The collection groups original research and review articles providing recent results and prospects in the field, with particular attention on optoelectronic applications, dielectric and magnetic properties, and electrochemical and biological performances
Anomaly-Based DNN Model for Intrusion Detection in IoT and Model Explanation: Explainable Artificial Intelligence
No full textIoT has gained immense popularity recently with advancements in technologies and big data. IoT network is dynamically increasing with the addition of devices, and the big data is generated within the network, making the network vulnerable to attacks. Thus, network security is essential, and an intrusion detection system is needed. In this paper, we proposed a deep learning-based model for detecting intrusions or attacks in IoT networks. We constructed a DNN model, applied a filter method for feature reduction, and tuned the model with different parameters. We also compared the performance of DNN with other machine learning techniques in terms of accuracy, and the proposed DNN model with weight decay of 0.0001 and dropout rate of 0.01 achieved an accuracy of 0.993, and the reduced loss on the NSL-KDD dataset having five classes. DL models are a black box and hard to understand, so we explained the model predictions using LIME.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Cyber Securit
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
Hybrid Model for Micro-channel Heat Exchangers used in sCO2 Brayton Power Blocks
No full textStudies have shown that Micro-channel Heat Exchangers (MCHEs) commonly referred to as Printed Circuit Heat Exchangers (PCHEs) are suitable candidates for recuperators and gas coolers used in sCO2 power blocks. CFD and 1-d unit-cell-based models have been proposed in the literature for the design and analysis of MCHEs. To estimate the heat exchanger size and arrive at an optimum channel configuration, CFD models are found to be computationally expensive and time-consuming, especially when full-scale MCHEs are to be modelled. On the other hand, 1-d models are inadequate for correctly predicting the performance. The thesis aims to address this gap by proposing a hybrid model.
The hybrid model incorporates a Thermal Resistance Network (TRN) framework combined with a unit-cell CFD model to investigate the thermo-hydraulic performance of the complete MCHE stack. CFD-based unit-cell models are developed for straight and non-straight flow paths to obtain the variation of local heat transfer coefficient and Fanning factor along the channel length. A stack optimization scheme based on the rate of heat loss from the external surfaces of the MCHE core is proposed and incorporated in the hybrid model to arrive at the optimum stack width, height, and number of rows. Additionally, the hybrid model also includes a model for the inlet and exit manifolds utilizing the concept of flow resistance balance to obtain optimum pressure drop across the entry and exit manifolds. The proposed manifold pressure balance scheme facilitates uniform flow distribution among the channels in the MCHE stack. The efficacy of the hybrid model is presented for MCHE based recuperator and a gas cooler used in a 1-MW scale sCO2 Brayton power block. For the recuperator, optimum stack volume and corresponding pressure drop are presented for a 5˚C pinch temperature differential. The improvement over the straight channel is demonstrated by using sinusoidal and zigzag flow path configurations. Channel pressure drop values obtained from the optimum stack and channel dimensions are used as inputs in the manifold model to obtain optimum overall pressure drop. The optimum overall pressure drop ensures uniform flow distribution among the channels in the stack.
Subsequently, the hybrid approach is extended to model the gas cooler performance and obtain stack volume with water as the secondary heat transfer fluid. Unlike the recuperator where the mass flow rates across the hot and cold sides are identical, the water flow rate and the corresponding Reynolds number in the case of a gas cooler are dictated by the sCO2 side. For the gas cooler, a range of Reynolds numbers for both water and sCO2 are obtained, ensuring temperature pinch at the cold inlet. The effect of Reynolds numbers (both sCO2 and water) on stack volume and performance of gas cooler is demonstrated for a temperature pinch of 3˚C pinned at the cold inlet. Multi-objective optimization is performed with water pumping power and gas cooler stack volume as objective functions to arrive at optimum channel dimensions and stack geometry. The dimensions are used to optimize the manifold flow passages in a similar way as done for the recuperator. The procedure is extended to double and quadruple banking configurations to assess the impact of banking on gas cooler performance and stack volume. Multi-objective optimizations are performed to arrive at an ideal banking arrangement for the gas cooler which provides minimum stack volume without compromising performance
Analysis of Transcritical CO2 and CO2+ Propane Mixtures for Tropical Cooling Applications
No full textThe increasing global average ambient temperatures coupled with stringent environmental regulations has turned the focus on natural working fluids such as Carbon dioxide (CO2), and hydrocarbons for use in the refrigeration industry. CO2 being a natural, non-flammable and non-toxic fluid with a triple point temperature of -56.6°C at pressure of 5.2 bar makes it a good choice for wide range of refrigeration applications including deep freezing. The use of CO2 for refrigeration applications is not new, it was perhaps the only refrigerant in the early 20th century for deep freezing applications. However, the advent of efficient synthetic refrigerants with lower operating pressures led to a gradual decline of CO2 for refrigeration applications. CO2 has a low critical temperature of 31.1°C at pressure of 73.8 bar, making it unsuitable for operating in the subcritical mode for cooling at high ambient conditions. Therefore, CO2 refrigeration systems work in transcritical mode for ambient conditions greater than 30°C. Consequently, the standstill pressures of transcritical CO2 (T-CO2) systems are not only higher, but also require substantial energy for compressing CO2 to higher pressures. Compressing CO2 to high pressures yields high gas cooler temperatures with significant amount of heat rejection. As a result, T-CO2 systems suffer from lower COP’s compared to conventional subcritical refrigeration systems.
There has been considerable work reported in the literature for improving the COP of T-CO2 systems. Among the many methods that have been proposed, meaningful utilization of gas cooler heat seems to be the most preferred choice. The thesis proposes a host of novel concepts like cascaded gas cooler heat-driven vapor ejector refrigeration systems (VERS) to CO2 propane mixtures to reduce compression work. The first part of the thesis presents a comprehensive analysis of variable geometry ejector systems for reducing the compressor work in standard T-CO2 systems. The developed ejector model is verified by experimental measurements obtained from IIT-Madras T-CO2 refrigeration test system. The proposed T-CO2 systems can be simultaneously used for different applications: deep freezing (-40 to -25°C), refrigeration (-8 to 2°C), air-conditioning (5 to 9°C) and heating applications till 80°C. These systems can be used in supermarkets, hospitals, hotels, dairy industries or shipboard cooling. Subsequently, a comprehensive analysis of a novel T-CO2-VERS hybrid system is presented which is entirely driven by the heat rejected in the gas cooler. The performance and cooling capacity of the VERS not only depends on the compressor discharge temperature of CO2 entering the generator but also on the quantity of heat available at this temperature. Both factors limit the choice of working fluids for the ejector refrigeration system. The hybrid system shows a significant improvement in cooling capacity over the baseline T-CO2 system in the range of 10% -50% for the range of evaporator temperatures. The system provides the highest COP at an evaporating temperature of 12.5°C, which is ideal for chilled water-based data center cooling or centralized air conditioning applications. Finally, a zeotropic mixture of CO2+Propane (C3H8) is proposed for reducing the operating pressure of the system. The presence of CO2 in the mixture suppresses the flammability of C3H8, while the presence of C3H8 reduces the critical pressure of the CO2+C3H8 mixture. Amidst the growing environmental concerns, improving the system performance of zeotropic hydrocarbon mixture has been the biggest challenge in recent years. Addressing this concern, this work aims to arrive at an optimum CO2+C3H8 mixture composition with significantly reduced working pressures to allow subcritical operation without adversely affecting system performance. In this case, two evaporation models are proposed based on constant pressure operation and constant temperature operation, both of which utilize the temperature glide to provide cooling. The performance of the mixture is analyzed for the various mass compositions of CO2 permissible within the flammability suppression envelope. The analysis shows that 15% CO2 in the mixture is ideal for most refrigeration and air-conditioning applications
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