1,721,983 research outputs found
Pool boiling critical heat flux in dielectric liquids and nanofluids
No full textWith the recent advances in consumer and power electronics, efficient thermal management of high heat flux components has taken on new urgency. While its inherent advantages have made air cooling the method of choice for the vast majority of electronic systems, the relatively poor thermophysical properties of air limit its ability to meet today's more demanding thermal requirements. Therefore, researchers have continued to investigate liquid cooling techniques such as microchannel forced convection, jet impingement, and pool boiling heat transfer. Although water has superior thermal properties, concern over its dielectric strength and chemical activity have directed attention to the use of the inert and dielectric perfluorocarbons as alternative cooling fluids, with particular emphasis on harnessing boiling and evaporative heat transfer processes. Passive pool boiling with these dielectric fluids, including the effects of subcooling, pressure, length scale, mixtures, surface enhancements, and nanoadditives, has been investigated by a large number of researchers. The present study focuses on the critical heat flux for pool boiling of these dielectric liquids, representing the upper limit of the nucleate pool boiling regime, and provides a review and summary of the work reported in more than 100 archival papers by research groups from around the globe
Tracking and Thrust Vectoring of E-Sail-Based Spacecraft for Solar Activity Monitoring
No full textThe Electric Solar Wind Sail (or E-sail) is a propellantless propulsion system conceived by Dr. Janhunen in 2004. An E-sail extracts momentum from the charged particles constituting the solar wind by means of long and electrically charged tethers, which are deployed and kept stretched by spinning the spacecraft about a symmetry axis. Trajectory analysis of an E-sail-based spacecraft is usually performed assuming that the tether arrangement resembles that of a rigid disc. However, this assumption may be inaccurate since the actual shape of each tether is affected by the chaotic interaction between the solar wind dynamic pressure and the centrifugal force due to the spacecraft spin. The first goal of this chapter is therefore to describe the thrust and torque vectors of a non-flat E-sail. Then, the orbital and the attitude dynamics of a spinning E-sail are analyzed separately due to the marked separation between their characteristic timescales, showing that the torque acting on the E-sail induces a perturbation on the orientation of the thrust vector. Accordingly, the modulation of the tether electrical voltage is proposed and investigated as a possible attitude control strategy. An effective control law is first obtained as a function of spacecraft attitude and time, and then validated through numerical simulations. Finally, two heliocentric mission scenarios (useful, for example, for the monitoring of near-Earth objects or the surveillance of solar activity) are analyzed, where the thrust vectoring of the E-sail is exploited for the generation of Earth-following orbits or the maintenance of a heliostationary equilibrium point
In memoriam Prof. Avram Bar-Cohen
No full textThe outpouring of emotion and appreciation for Prof. Avram (Avi) Bar-Cohen from his students and colleagues in the academic, industrial, and government communities in Heat Transfer and Electronics Packaging is a testament to the truly profound impact that he had on our professional and personal lives. Awards are being renamed in his honor, tributes are pouring in [1], special issues of the IEEE Transactions on Components, Packaging, and Manufacturing Technology (TCPMT) and the ASME Journal of Electronic Packaging are appearing, and articles have been crafted to honor his legacy [2], [3]
In memoriam Prof. Avram Bar-Cohen
No full textIt has been a little over a year since the passing of Prof. Avram, Avi as he is popularly known, Bar-Cohen. Eulogies from his students and colleagues in the academic, industrial, and government communities in Heat Transfer and Electronics Packaging still continue to come in. His impact on the electronic packaging community in general and thermal management community, in particular, is profound. The IEEE ITherm Conference renamed Best Paper awards in his honor. The ASME InterPACK Conference renamed its Achievement Award to honor him and is working diligently to elevate it from a Divisional Award to a Society-Level Award. The ASME Journal of Electronic Packaging will be publishing a Special Issue, edited mostly by his former students, in December 2021
Mechanical stretch influence on lifetime of dielectric elastomer films
No full textFilm pre-stretching is a widely adopted solution to improve dielectric strength of the DEA systems. However, to date, long term reliability of this solution has not been investigated. In this work it is explored how the dielectric elastomer lifetime is affected by film pre-stretching. The dielectric loss of soft polydimethylsiloxane (PDMS) films is studied for different stretch ratios by measuring tanδ. Additionally, time-to-breakdown was measured at DC electric stress for different stretch ratios. For this purpose, accelerated life test (ALT) were performed. The results obtained are compared with non-pre-stretched samples. This study suggests that no additional dielectric losses are caused by film stretching up to 80% of original dimensions.DC systems, Energy conversion & Storag
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Displaced Non-Keplerian Orbits for Sun and Inner Planet Observation
No full textA displaced non-Keplerian orbit is a trajectory whose orbital plane does not contain the center of mass of the primary body, so that its orbital maintenance requires the application of a suitable continuous thrust. Although the latter could be provided, in principle, by a low-thrust electric propulsion system, innovative propellantless propulsive technologies are well suited to such a mission scenario, due to their ability to generate thrust without requiring any propellant, thus significantly extending mission lifetime. This chapter focuses on the possibility of maintaining a displaced non-Keplerian orbit by means of both solar sails and electric solar wind sails (or E-sails). In fact, these advanced propulsion systems are both capable of generating a propulsive acceleration without consuming any propellant, by exploiting the solar radiation pressure (in case of solar sails) or the solar wind dynamic pressure (E-sails). This analysis uses recent models to provide a mathematical description of the propulsive acceleration generated by both propulsion systems, and different scenarios involving non-Keplerian orbits are analyzed. Particular focus is given to Type II displaced orbits, non-Keplerian orbits lying on the ecliptic plane, and heliostationary positions. Performance and attitude requirements are provided for each scenario. A linear stability analysis is also performed, in order to identify the combination of orbital parameters that characterize stable non-Keplerian orbits. The results suggest the feasibility of the mission scenarios discussed, but for most of them performance requirements are very demanding. A possible exception is non-Keplerian orbits lying on the ecliptic, which represent a very promising near-term scenario
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