Cavitation erosion damage of scroll steel plates by high-speed gas working fluid

A steel plate is one of the critical components of a scroll expander system that can experience cavitation micro-pitting while in service. The content of the present paper consists of two distinct but interrelated parts. The first part aims to highlight that the use of Computational Fluid Dynamics (CFD) simulations in conjunction with experimental measurements can constitute a quite promising tool for the prediction of cavitation erosion areas in scroll expander systems. For this purpose a three-dimensional CFD, steady state numerical simulation of the refrigerant working fluid is employed. Numerical results revealed the critical areas where cavitation bubbles are formed. These numerical critical areas are in direct qualitative agreement with the actual eroded regions by cavitation, which were found by microscopic observations across the steel plate on an after use, scroll expander system. The second part of the paper, aims to further investigate the behaviour and the durability of the steel plate of the studied scroll expander system subjected to cavitation erosion by using an ultrasonic experimental test rig. Scanning Electron Microscopy (SEM) and optical interferometer micrographs of the damaged surfaces were observed, showing the nature of the cavitation erosion mechanism and the morphological alterations of the steel plate samples. Experimental results are explained in terms of the cavitation erosion rates, roughness profile, accumulated strain energy, and hardness of the matrix. The experimental study can serve as a valuable input for future development of a CFD numerical model that predicts both cavitation bubbles formation as well as cavitation damage induced by the bubbles that implode on the steels plates

ΠΕΙΡΑΜΑΤΙΚΗ ΠΡΟΣΟΜΟΙΩΣΗ ΣΧΗΜΑΤΙΣΜΟΥ ΔΙΝΩΝ ΣΤΗΝ ΕΛΕΥΘΕΡΗ ΕΠΙΦΑΝΕΙΑ ΧΟΑΝΗΣ ΕΙΣΟΔΟΥ ΜΙΚΡΟΥ ΥΔΡΟΗΛΕΚΤΡΙΚΟΥ

Η προσαγωγή του νερού προς τους στροβίλους ενός μικρού υδροηλεκτρικού ενδέχεται να οδηγήσει σε δημιουργία δινών (στροβιλισμών), που όταν είναι πολύ ισχυρές, συνεπάγονται την εισρόφηση αέρα και τη δυσλειτουργία του στροβίλου. Στην εργασία αυτή έγινε εργαστηριακή προσομοίωση της δημιουργίας δινών στο μικρό υδροηλεκτρικό Γιτάνης, στον ποταμό Καλαμά. Τα πειραματικά αποτελέσματα υπό αδιάστατη μορφή συμπληρώνουν την διεθνή βιβλιογραφία, ιδίως όσον αφορά την επίδραση του αριθμού Reynolds στη δημιουργία δινών

Security challenges of small cell as a service in virtualized mobile edge computing environments

Research on next-generation 5G wireless networks is currently attracting a lot of attention in both academia and industry. While 5G development and standardization activities are still at their early stage, it is widely acknowledged that 5G systems are going to extensively rely on dense small cell deployments, which would exploit infrastructure and network functions virtualization (NFV), and push the network intelligence towards network edges by embracing the concept of mobile edge computing (MEC). As security will be a fundamental enabling factor of small cell as a service (SCaaS) in 5G networks, we present the most prominent threats and vulnerabilities against a broad range of targets. As far as the related work is concerned, to the best of our knowledge, this paper is the first to investigate security challenges at the intersection of SCaaS, NFV, and MEC. It is also the first paper that proposes a set of criteria to facilitate a clear and effective taxonomy of security challenges of main elements of 5G networks. Our analysis can serve as a staring point towards the development of appropriate 5G security solutions. These will have crucial effect on legal and regulatory frameworks as well as on decisions of businesses, governments, and end-users

AN EXPERIMENTAL STUDY OF THE PLANE SUBMERGED HORIZONTAL BUOYANT JET

Βασικός σκοπός της εργασίας αυτής είναι η παρουσίαση πειραματικών αποτελεσμάτων για την αξονική αραίωση και την ένταση της διακύμανσης της Θερμοκρασίας σε μια διδιάστατη οριζοντίως εκτοξευόμενη ανωστική φλέβα, η παραγωγή μιας ημιεμπειρικής εξίσωσης για την αξονική αραίωση χρήσιμη για τις πρακτικές εφαρμογές και η κατα6ειξη σχέσης μεταξύ έντασης της διακύμανσης της θερμοκρασίας και καμπυλότητας στην καμπύλη αυτή ροή. Η προτεινόμενη ημιεμπειρική εξίσωση συγκρίθηκε με τα πειραματικά αποτελέσματα αυτής της εργασίας καθώς και άλλων ερευνητών και είναι εφαρμόσιμη σε οποιαδήποτε 6ιόιαστατη ανωστική φλέβα, κατακόρυφη ή υπό γωνία με την κατακόρυφο, και ισχύει για οποιοδήποτε αρχικό πυκνομετρικό αριθμό.The basic objective of this paper is the presentation of experimental data on the centerline dilution and intensity of temperature fluctuations of a plane horizontal buoyant, the derivation of a semi-empirical equation for the dilution to be used by the design engineer and to indicate the relationship between intensity of temperature fluctuations and curvature in this curved flow. The produced semi-empirical formula for the axial dilution was compared with experimental results of this work and of previous investigators and is valid for any plane buoyant jet, vertical or at an angle 6 to the vertical, and for any initial densimetric number