Dissertations / Theses on the topic 'Packet drops'

The mass transfer and the pressure drop characteristics for countercurrent rotating packed bed (RPB) with a continuous gas phase for the removal of ethylene dichloride (EDC) from water using air stripping have been investigated. The aim of this research was to understand the behaviour of the mass transfer performance and the pressure drop behaviour in a centrifugal environment. The mass transfer results showed that the height of a transfer unit (HTU) in a liquid film limited system can be in the range between 30 to 70mm at moderate centrifugal acceleration between 44 to 280g. Three packings of different packing densities ranging from 870 to 2300 m2/m3 were tested. The HTU values were found to vary with the centrifugal acceleration as HTU a g-0.11 to -0.28. It has been shown that the packing density may not have a notable effect on the separation performance of the bed. The pressure drop results indicated that in RPB the pressure drop is relatively higher than its equivalent packed beds operate at 1g. The usual flooding restrictions were relaxed thus high hydraulic capacities can be achieved per unit size of equipment. At the flooding point, experimental findings indicated that a part of the liquid is not accelerated by the bed. A model to predict the pressure drop was developed. The model was verified against experimental data and good agreement was obtained.

Thesis (MScEng (Process Engineering))--University of Stellenbosch, 2010.
ENGLISH ABSTRACT: Distillation continues to be the most widely used method of separation in the processing industry, in spite of its inherently low thermodynamic efficiency. Two of the critical distillation research needs that arose from the US-Initiative Vision 2020 were to develop a better understanding of the physical phenomena as well as developing better predictive models. Also, characterisation of modern packing materials is required to assist in the CO2 capture optimisation. This thesis deals with both these aspects by establishing a facility that can accurately measure the hydraulic capacity of packed columns. This setup eliminates mass transfer and specific attention can be given to the hydrodynamic behaviour of packed columns. Two phenomena that have a large impact on the mass transfer efficiency of packing materials are the loading and flooding point. The loading point is signified by the following: a.) where the packed column hold-up increases, b.) higher increase in pressure drop, and c.) a decrease in Height Equivalent to a Theoretical Plate (HETP). The onset of flooding is where the shear forces between the gas and liquid become so large (relative to the gravitational forces) that a net upwards movement of liquid occurs, resulting in liquid droplets being heavily entrained. This is normally accompanied by a sharp increase in HETP, pressure drop and liquid hold-up. The prediction of these operating limits is of great value but, despite the many contributions that were made from 1960 to 2010, there is still room for improvement. The operating region of particular interest is between the loading and flooding point, especially for fluids with physical properties significantly different from that of water. In the past, this operating region was not of great importance, but industries are constantly striving to increase their production with minimal capital expenditure. Thus, packed columns are being pushed to their limits and a good understanding of the phenomena occurring near these operational limits is now required. A 400 mm diameter glass packed bed setup (with a bed height of 3000 mm) was designed and constructed to test the effect of the following parameters on packed bed pressure drop and liquid hold-up: · Gas and liquid physical properties · Gas and liquid rates · Type of packing (either random or structured) The experimental setup has been designed so that in the future the influences of the above mentioned parameters on entrainment can also be measured. Initially, hydrodynamic tests on random packing materials (1.5” Pall® Rings, 1.5” IMTP®, 1.5” Intalox® Ultra™) were conducted over a liquid range of 6 - 122 m3/(m2·h). Through a thorough literature study it was found that the most likely semi-theoretical model, that would be able to predict the pressure drop and the liquid hold-up over most of the random packing test range, was the model developed by Billet [1991; 1993; 1995; 1999]. The other models found throughout the literature had at least one of the following deficiencies: · Limited to only the pre-loading region. · Tested (and thus applicable) only over a very select group of packing materials with no attempt to generalise. · Lacked the proper validation of significantly variable fluid properties over multitudes of liquid and gas rates especially, at higher gas and liquid rates. The experimental setup was successfully commissioned, noting the following maximum experimental errors: Vapour flow factor - 2.6 %; liquid rate - 0.75 %; packed bed pressure drop - 0.75 %; liquid hold-up - 1.25 % and entrainment - 1.05 %. Significant deviations were observed between the experimental hold-up and the hold-up from the predictive model of Billet (using Pall® Rings). Careful inspection revealed that this predictive model potentially uses two definitions for hold-up at flooding, one which has a theoretical basis and the other purely empirical. Upon substituting the theoretical value with the empirical value, a significant improvement was observed between the measured and predicted results. Deviations were still observed near the flooding point and were attributed to the difficulty of obtaining reliable flooding data. The range of liquid hold-up prediction by Billet was only verified up to a liquid rate of 82 m3/(m2·h) and the pressure drop prediction only verified up to a liquid rate of 60 m3/(m2·h). This reinforces the need for high liquid, high gas rate data. Due to the empirical nature of the liquid hold-up at flooding prediction, and since pressure drop prediction is directly linked to liquid hold-up, another model was used to compare the experimental pressure drop data. The KG-TOWER® simulator was used to predict IMTP® data and compare it to the experimentally measured values. It was found that the experimental IMTP® data followed the same trends as those from KG-TOWER® within the operating limits of the program. Thus, since the experimental data follows similar trends as models found in the literature, as well as falling within their reliable limits, the experimental setup can correctly measure the parameters in question. The experimental data from the different random packings were compared to one another by using a statistical method to determine the loading point and onset of flooding. This method uses prediction confidence intervals by fitting empirical curves to each operating region and was found to be useful in determining these critical points from experimental hydraulic data (in the absence of HETP data).The only useful comparison was between IMTP® and Intalox® Ultra™ as they both have roughly the same density, size and void fraction. It was found that, on average, the pressure drop of Intalox® Ultra™ is 20 % lower than that of IMTP® over the entire operating range. The hydraulic operating range of Intalox® Ultra™ was found to be on average 16 % larger than that of IMTP®. It is recommended that further testing should be done to investigate the influence of fluid properties (specifically liquid viscosity and to a lesser extent surface tension) on the hydraulic capacity of packed columns. Also, high gas and high liquid rate data should be generated to assist current modelling techniques. Lastly, a comparative characterisation between Intalox® Ultra™ and Raschig Super-Rings would serve as a benchmark for fourth generation random packings.
AFRIKAANSE OPSOMMING: Distillasie is vandag nog die skeidingsproses wat die meeste gebruik word in the prosesnywerhede ten spyte van ‘n lae termodinamiese effektiwiteit. Twee van die kritieke distillasie navorsing behoeftes wat vanuit die US-Initiative Vision 2020 ontstaan het, was om die fisiese verskynsels beter te verstaan, asook om beter voorspellende modelle te ontwikkel. Die karakterisering van moderne pakking materiale is ook nodig vir die optimering van die verwydering van CO2 uit uitlaatstrome. Hierdie tesis spreek beide van hierdie faktore aan deur ‘n fasiliteit op te rig wat die hidrouliese kapasiteit van gepakte kolomme akkuraat kan meet. Hierdie opstelling elimineer massa-oordrag en dus kan spesifieke aandag gegee word aan die hidrodinamiese gedrag van gepakte kolomme. Twee verskynsels wat ‘n groot impak het op die massaoordrag effektiwiteit van pakkingsmateriale is die ladingspunt en die vloedpunt. Die ladingspunt word deur die volgende gekenmerk: a.) waar die vloeistof inhoud in die gepakte bed toeneem, b.) ‘n toename in drukval en c.) ‘n afname in die hoogte ekwivalent aan ‘n teoretiese plaat (HETP). Die vloed gebied word gekenmerk waar die skuifkragte tussen die vloeistof en gas so groot raak (relatief tot die gravitasionele kragte), dat daar ‘n netto opwaartse beweging van vloeistof druppels in die kolom is. Hierdie gaan normaalweg gepaard met ‘n skerp toename in HETP, drukval en vloeistof inhoud. Die voorspelling van hierdie bedryfslimiete is baie waardevol, maar ten spyte van die bydrae wat tussen 1960 en 2010 gemaak was, is daar nog steeds ruimte vir verbetering. Die spesifieke bedryfsgebied van belang is die gebied tussen die ladingspunt en die vloedpunt en spesifiek vir sisteme waar die fisiese eienskappe van die vloeistowwe drasties verskil van die van water. In die verlede was hierdie gebied van minder belang gewees, maar maatskappye probeer deesdae hul produksie opstoot met minimale kapitale uitleg. Dus is ‘n goeie kennis van massa-oordrag verskynsels naby aan die bedryfslimiete van kardinale belang. ‘n 400 mm Diameter gepakte kolom (met ‘n bed hoogte van 3000 mm en bestaande uit glas) opstelling is ontwerp en gebou om die effek van die volgende parameters te toets op gepakte bed drukval en vloeistof inhoud: · Gas en vloeistof fisiese eienskappe · Gas vloeistof vloeitempos · Tipe pakking (beide ongeordend en gestruktureerd) Die eksperimentele opstelling is ontwerp om die bogenoemde eienskappe op vloeistofmeesleuring te meet vir toekomstige navorsing. Hidrodinamiese toetse op ongeordende pakkingsmateriale (1.5” Pall® Ringe, 1.5” IMTP®, 1.5” Intalox® Ultra™) is uitgevoer vir vloeistof vloeitempos tussen 6 en 122 m3/(m2·h). Vanuit ‘n deeglike literatuurstudie is daar gevind dat die mees toepaslike semi-teoretiese model, wat die drukval sowel as die vloeistof inhoud kan voorspel oor al die bedryfsgebiede, is die model wat deur Billet [1991; 1993; 1995; 1999] ontwikkel is. Die ander modelle in die literatuur het ten minste een van die volgende tekortkominge gehad: · Is slegs van toepassing in die voor-ladings gebied. · Is slegs van toepassing vir ‘n paar pakkingsmateriale en geen poging is aangewend om dit te veralgemeen nie. · Is nie geldig waar die vloeistof eienskappe drasties verskil van ‘n lug/water sisteem nie, sowel as by hoë gas en vloeistof vloeitempos. Die eksperimentele opstelling is suksesvol in werking gestel met die volgende waargenome eksperimentele foute: Gas vloei faktor – 2.6 %; vloeistof vloeitempo – 0.75 %; gepakte bed drukval – 0.75 %; vloeistof inhoud – 1.25 %; vloeistof-meesleuring tempo – 1.05 %. Noemenswaardige verskille is waargeneem tussen die eksperimentele en teoretiese vloeistof inhoud (deur Pall® Ringe te gebruik). Na gelang van noukeurige inspeksie, is daar gevind dat die Billet-model twee moontlike definisies voorstel vir die voorspelling van vloeistofinhoud by die vloedpunt. Een van hierdie is teoreties van aard en die ander een suiwer empiries. ‘n Vervanging van die teoretiese waardes met die empiriese waardes het gelei tot ‘n merkwaardige verbetering tussen die eksperimentele en teoretiese voorspellings. Daar was nog steeds verskille naby aan die vloedpunt, maar dit kon toegeskryf word aan die feit dat min betroubare data naby aan die vloedpunt beskikbaar is. Die voorspelling van vloeistof inhoud deur Billet is slegs gekontroleer tot ‘n vloeistof vloeitempo van 82 m3/(m2·h) en die drukval slegs tot ‘n vloeistof vloeitempo van 60 m3/(m2·h). Die bogenoemde bewys dus die tekort aan hoë gas- en hoë vloeistofvloeitempo data. Die voorspellende model se drukval is gekoppel aan die vloeistof inhoud, en dus is ‘n ander model gebruik om die eksperimentele drukval data teen te vergelyk. Die KG-TOWER® simulasie program is gebruik om die IMTP® drukval te voorspel en dit het goed vergelyk met die eksperimentele data. Dus, aangesien die eksperimentele data dieselfde tendens toon as dié van die modelle in die literatuur en aangesien dit binne die modelle se foutbande val, kan die eksperimentele opstelling die verlangde parameters akkuraat meet. Die eksperimentele data van al drie pakkingsmateriale is teenoor mekaar vergelyk deur gebruik te maak van ‘n statistiese metode wat die ladings- en vloedpunt bepaal. Hierdie metode maak gebruik van voorspellings vertroue intervalle deur empiriese kurwes op die eksperimentele data in elke bedryfsgebied te pas. Hierdie metode is ontwikkel om toepaslike te wees in die afwesigheid van HETP data. Die enigste nuttige vergelyking is tussen IMTP® en Intalox® Ultra™ omdat albei dieselfde pakkingsdigtheid, grootte en pakkings oop ruimte het. Daar is gevind dat die drukval van Intalox® Ultra™ ‘n gemiddeld van 20 % laer is as dié van IMTP® oor die hele bedryfsgebied. Die hidrouliese bedryfsgebied van Intalox® Ultra™ is 16 % groter as dié van IMTP®. Daar word voorgestel dat bykomende toetswerk gedoen moet word om die invloed van vloeistof eienskappe (spesifiek vloeistof viskositeit en vloeistof oppervlak spanning) op die hidrouliese kapasiteit van gepakte kolomme te ondersoek. Bykomende toestwerk by hoë gas- en hoë vloeistofvloeitempo word benodig om die bestaande modelle aan te vul. Laastens, sal ‘n vergelykende studie tussen Intalox® Ultra™ en Raschig Super-Rings die grondslag lewer vir die karakterisering van vierde generasie ongeordende pakkingsmateriale.

During this study an experimental test facility was designed and constructed with the purpose of conducting experiments to measure the pressure drop through packed beds of spheres with varying levels of structuredness. The test facility had to be designed so that the uncertainty in the measured friction factors would be below ±10% and commissioned to ensure that results with an acceptable degree of accuracy could be obtained. Experiments were done on a randomly packed bed and a structured packed bed in order to demonstrate the proper operation of the test facility. The resulting experimental data was compared with applicable correlations found from relevant literature. The nuclear safety standards commission (KTA (1981)) correlation, as well as the relation of Ergun (1952) was chosen for comparison with the experimental data obtained from the experiment on the randomly packed bed. The correlation of Wentz and Thodos (1963) was selected for comparison with the experimental data obtained from the structured packed bed experiment. The friction factors obtained from the experimental data was found to be higher than the friction factors calculated with the different correlations for the respective packing configurations. This could be attributed to the manufacturing process of the packing configurations that resulted in the surface finish of the particles to be extremely coarse. In order to obtain the desired porosity within the structured packed bed, cylindrical rods were utilised to separate the particles to prevent contact between them. Wentz and Thodos (1963) also made use of cylindrical rods, called distention rods, to make varying porosity possible within the structured packed beds. The cylindrical rods that were utilised during these experiments are larger (in diameter) than those described in the literature, which could have contributed to the higher pressure drop through the structured packed bed. Furthermore, it was found that the friction factors derived from the experimental data increased as the modified Reynolds number was increased. This is a phenomenon that is not fully understood at this time and further study is proposed. The operation of the experimental test facility was successfully demonstrated. The measurements were shown to be repeatable and the uncertainty of the friction factors derived from the measured data obtained from the test facility was less than 10%, which is satisfactory. Therefore, the ability to measure the pressure drop through packed beds of spheres with varying degrees of structuredness has now been established as a result of this research.
Thesis (MIng (Nuclear Engineering))--North-West University, Potchefstroom Campus, 2012

The purpose of this study was to investigate the velocity distribution through an annular bed packed randomly with equal sized spheres. Extensive research has been conducted on the velocity distribution inside packed beds packed with equal sized spheres, different sized spheres, deformed spheres, cylinders and Raschig-rings. A majority of these experimental and numerical studies focused on the cylindrical packed bed. These studies and numerical models are all confined to the velocity profile once the fluid flow is fully developed. The development of the velocity through the inlet region of the bed and the fluid flow redistribution in the outlet of the bed is thus neglected. The experimental investigation into the velocity distribution down stream of the annular packed bed of the HTTU indicated that the velocity profile was independent of the mass flow rate for a particle Reynolds number range of 439 £ Re £ 3453 . These velocity profiles did not represent the distribution of the axial velocity due to shortcomings associated with the single sensor hot wire anemometry system used to measure the velocity distribution. A numerical investigation, using the RANS CFD code STAR-CCM+®, into the velocity distribution downstream of an explicitly modelled bed of spheres indicated that the axial velocity distribution could be extracted from the experimental velocity profiles by using an adjustment factor of 0.801. This adjusted velocity profile was used in the verification of the implicit bed simulation model. The implicit bed simulation model was developed in STAR-CCM+®. The resistance of the spheres was modelled using the KTA (1981) pressure drop correlation and the structure of the bed was modelled using the porosity correlation proposed by Martin (1978), while the effective viscosity model of Giese et al. (1998), adjusted by a factor of 0.8, was used to model the velocity distribution in the near wall region. It was found that the structure in the inlet region of the bed, where two walls disturb the packing structure, can be modelled as the weighted average of the radial and axial porosity while the structure in the outlet regions can be modelled by letting the radial porosity increase linearly to unity. The basic shape of the velocity profile is established immediately when the fluid enters the bed. The amplitude of the velocity peaks however increase in magnitude until the velocity profile is fully developed at a distance approximately of five sphere diameters from the bed inlet. The profile remains constant throughout the bed until the outlet region of the bed is reached. In the outlet region a significant amount of fluid redistribution is observed. The amplitude of the velocity peaks is reduced and the position of the velocity peaks is shifted inwards towards the centre of the annular region. The fully developed velocity profile, predicted by the simulation model is in good agreement with profiles presented by amongst others Giese et al. (1998). The current model however also offers insight into the development of the profile through the inlet of the bed and the fluid redistribution, which occurs in the outlet region of the bed.
Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2010.

Regenerative heat exchangers are used in a wide range of industries and in the technical equipment of buildings. These heat exchangers play an important role in saving thermal energy and removing volatile organic compounds from flue gases. The theoretical part of the work deals with the division of regenerative exchangers into rotary and switching exchangers and the possibilities of their use. These types of heat exchangers are used in many applications, e.g. as a heat exchanger using waste heat to preheat the process gas (regeneration layer), or as catalysts to accelerate the reaction required to remove volatile organic compounds (catalytic layer), or as integrated equipment where both the regeneration layer and the catalytic layer. The aim of the diploma thesis is experimental support in the development of a computer program for the design of a specific integrated device. The program allows the calculation of the regeneration and catalytic bed, or both beds simultaneously, i.e. integrated equipment. The diploma thesis deals with the support of a mathematical model for the calculation of the regeneration bed. Pressure loss and heat transfer play an important role in the selection and subsequent calculation of a suitable bed. To calculate them, it is possible to find more available computational relationships that differ significantly in their accuracy. It is therefore necessary to select the most suitable ones for the computational model. The practical part of the work then deals with research, analysis, and assessment of the suitability of methods used to calculate pressure losses based on a comparison with the values measured on experimental equipment. Subsequently, the work deals with computational methods for determining the heat transfer coefficient of the packed bed. A significant part of the practical part deals with the modification of the experimental equipment for the verification of computational relations for the determination of heat transfer with measured data.

De nos jours, les systèmes de contrôle-commande temps-réel distribués à travers un réseau de communication sont de plus en plus utilisés dans les secteurs de l’automobile, de l’avionique, de la robotique mobile, de la télécommunication et plus généralement de la conduite de procédés industriels. En comparaison avec les systèmes de contrôle point-à-point conventionnel, un système contrôlé en réseau (SCR) permet non seulement de réduire le câblage et les coûts d’installation, mais offre aussi plus de flexibilité pour faire évoluer une installation existante et favorise les actions de diagnostic et de maintenance. Comme le réseau est partagé par plusieurs boucles de contrôle et par d’autres applications, la conséquence est que le trafic de communication est difficilement maîtrisable, ce qui peut conduire à des pertes de messages et engendre des délais aléatoires. Par ailleurs, le diagnostic et la tolérance aux défauts sont des enjeux importants pour les SCR, particulièrement dans le cas ou le domaine d’utilisation exige une grande sécurité. Il est évident que la théorie et l’application des approches classiques de diagnostic et de tolérance aux défauts doivent être révisées lorsqu’il s’agit de SCR.L’étude des SCR, reposant sur des compétences en automatique, en informatique et en réseau propose naturellement des solutions propres à chaque domaine. La problématique du diagnostic des SCR consiste non seulement à détecter et localiser des défauts affectant l’ensemble du système mais aussi à distinguer, les perturbations et défaillances affectant le réseau de communication de celles du système contrôle. L’objectif de nos travaux de thèse est de proposer des modèles intégrés permettant de représenter le comportement des SCR et de contribuer à leurs diagnostics
Real-time control systems distributed across communication networks are increasingly used in automotive, avionics, mobile robotics, and telecommunications and more generally in the conduct of industrial processes. Compared with point- to-point conventional control systems, a networked control system (NCS) can not only reduce wiring and installation costs, but also offer more flexibility to expand an existing facility and promote actions of diagnosis and maintenance. As the network is shared by multiple control loops and other applications, the result is that the communication traffic is difficult to control, which can lead to loss of messages and generate random delays.Diagnosis and fault tolerance are important issues for NCS, especially in the case where the area of application requires security. It’s obvious that the theory and application of conventional approaches to diagnosis and fault tolerance need to be revised when it comes to NCS.The study of the NCS, based on automatic skills, computer science and network competences naturally provides adequate solutions to each area. The problem of NCS diagnosis is the one hand to detect and locate faults affecting the whole system and on the other hand to distinguish, disturbances and malfunctions affecting the communication network of the control system.Our work aims is to propose integrated models to represent the behavior of NCS and contribute to its diagnosis

Denna rapport behandlar prestandaproblem i den nyligen standardiserade Mesh kommunikationsstandarden (IEEE 802.11s). I denna rapport, undersöker och förbättra vi ett förhållande som resulterar i reduktion av genomströmningen i en kedja av noder topologi. IEEE802.11s är mycket lovande med många fördelar för både IoT-systemen och trådlösa nätverk i båda hemmet och arbete.Vi arbetar med frågan om orättvisa när CSMA/CA tillämpas, vilket orsakar genomströmningsreduktion på grund av paketförluster och indikerar svältning. Vi analyserar konsekvenserna av Collision Avoidance (CA) mekanism och föreslår en ersättning för CA som är både rättvist och samtidigt kan upprätthålla undvikande av kollisioner. Vi implementera detta i en simulator och resultatet visar på betydligt högre end-to-end-genomströmning än standard CSMA/CA och inga paketförluster på grund av buffertspill.
This thesis rapport deals with the performance issues of the newly standardized Wireless mesh protocol (IEEE 802.11s). In this thesis, we work on improving the conditions that results in throughput degradation in a chain of nodes topology. The mesh standard is very promising with many advantages for both IoT systems and home wireless networks.We work on the issue of unfairness when CSMA/CA is applied, which causes throughput degradation due to packet loss and indicates starvation. We analyze the implication of the Collision Avoidance (CA) mechanism and propose a replacement for the CA that is both fair and able to maintain collision avoidance. We implement this in a simulator and the result shows significantly higher end-to-end throughput compared to the original CSMA/CA and no packet loss due to buffer overflow.

L'hydrodynamique et les transferts thermiques dans des réacteurs types lits fixes font l'objet d'études depuis une soixantaine d'années. Les informations fournies par ces travaux sont importantes pour le dimensionnement des réacteurs industriels. En effet, une étroite relation entre les propriétés de transports et le design du procédé (réacteur et support catalytique) est mise en avant dans l'état de l'art. Mais force est de constater que la complexité des phénomènes entraine des divergences entre les observations et les différentes conclusions faites par les auteurs. Deux pilotes à échelle laboratoire ont été conçus et mis en place au cours de cette thèse. L'un est dédié à l'étude hydrodynamique et l'autre à l'étude thermique. La caractérisation hydro-thermique des réacteurs passe également par une étape de développement de modèles mathématiques associés aux pilotes. Ils représentent les transferts opérant au sein d'une colonne garnie. Ils permettent également d'estimer les paramètres effectifs du milieu poreux. Les quatre grandeurs caractéristiques étudiées dans cette thèse sont les pertes de charges, la dispersion axiale, la conductivité effective globale et le coefficient d'échange thermique à la paroi. Les résultats obtenus à partir des modèles sont comparés aux travaux de la littérature. Ceci permet de valider les méthodes d'estimation développées dans cette thèse. Il est remarqué que les corrélations prédictives de la conductivité effective sont peu adaptées aux morphologies de supports les plus complexes. Nous proposons donc aussi de nouvelles corrélations basées sur nos mesures
Hydrodynamics and heat transfers in packed bed reactors have been studied for about sixty years. The information supplied by this work is important for the design of industrial reactors. Indeed, there is a close relationship between the heat and mass transfers and the design of the process (reactor and catalytic support); this is highlighted in the state of the art. It is clear that the complexity of the phenomena is the cause of divergences between the observations and the conclusions made by the authors. Two laboratory-scale pilots were designed and set up during this PhD thesis. One is dedicated to the hydrodynamic studies and the other to the thermal studies. The hydro-thermal characterization of the reactors includes a step of development of mathematical models associated with the pilots. They represent transfers occurring within the bed of particles. Coupling experimental data with the models allows estimating the effective parameters of the porous medium. The four characteristic quantities studied in this PhD thesis are the pressure drop, the axial dispersion, the overall effective conductivity and the heat transfer coefficient at the wall. The results are compared with the data provided by the open literature. This validates the estimation methods developed during this PhD thesis. It is noticed that the predictive correlations of the effective thermal conductivity are not adapted to the morphologies of the most complex supports. Based on our measurements, new correlations were developed

The introduction of programmability and dynamic protocol deployment in routers, there would be an increase in the potential vulnerabilities and attacks . The next- generation Internet promises to provide a fundamental shift in the underlying architecture to support dynamic deployment of network protocols. In this thesis, we consider the problem of detecting malicious packet drops in routers. Specifically, we focus on an attack scenario, where a router selectively drops packets destined for another node. Detecting such an attack is challenging since it requires differentiating malicious packet drops from congestion-based packet losses. We propose a controller- based malicious packet detection technique that effectively detects malicious routers using delayed sampling technique and verification of the evidence. The verification involves periodically determining congestion losses in the network and comparing the forwarding behaviors of the adjoining routers to affirm the state of a router in the network. We provide a performance analysis of the detection accuracy and quantify the communication overhead of our system. Our results show that our technique provides accurate detection with low performance overhead.

With the thriving technology and the great increase in the usage of computer networks, the risk of having these network to be under attacks have been increased. Number of techniques have been created and designed to help in detecting and/or preventing such attacks. One common technique is the use of Network Intrusion Detection / Prevention Systems NIDS. Today, number of open sources and commercial Intrusion Detection Systems are available to match enterprises requirements but the performance of these Intrusion Detection Systems is still the main concern. In this paper, we have tested and analyzed the performance of the well know IDS system Snort and the new coming IDS system Suricata. Both Snort and Suricata were implemented on three different platforms (ESXi virtual server, Linux 2.6 and FreeBSD) to simulate a real environment. Finally, in our results and analysis a comparison of the performance of the two IDS systems is provided along with some recommendations as to what and when will be the ideal environment for Snort and Suricata.

碩士
國立交通大學
電信工程系
88
Most router mechanisms designed to achieve fair bandwidth allocations, such as Fair Queueing, have many properties for congestion control in Internet. However, most of those mechanisms need to maintain states, manage buffer, and packet scheduling on a per flow basis. The complexity of the per flow basis will cause cost-ineffective implementation and network scalability. In this paper, we employ the edge-core router architecture in Differentiated Services to achieve fair bandwidth allocation with lower complexity. Edge routers maintain per flow states and insert some flow information into each packet. According the information in the packet, core routers maintain no per flow buffer management which achieve approximately fair bandwidth allocations.

碩士
國立交通大學
電信工程系
88
In this paper, we study the TCP performance in the high-speed access network, including the TCP flow control schemes, efficiency and fairness of TCP, and dynamics of TCP traffics with various buffer management mechanisms. After reviewing some typical buffer management algorithms such as TailDrop, RED (Random Early Detection), FRED, FBA (Fair Buffer Allocation), DFBA, VQ (Virtual Queue), and LQD (Longest Queue Drop), we present some useful buffer management algorithms to improve the performance that TCP flows transport in the network nodes. Finally, We compare these algorithms by writing simulation program, and get a better idea of buffer management.

碩士
國立中興大學
資訊科學研究所
93
The Internet is increasingly supporting real time applications such as VOIP, video conference and online games. These new applications have more requirements in terms of throughput or delay. We propose a new AQM mechanism to improve the performance of Adaptive Random Early Detection (ARED). Our mechanism is based on the proportional integral derivative control, which can provide robust and predictive congestion avoidance and stabilizes the queue length

碩士
中原大學
化學工程研究所
99
In this study, a systematic investigation on the pressure drop was performed in a countercurrent-flow rotating packed bed packed. The pressure drop was measured as a function of gas flow rate, liquid flow rate, rotational speed, liquid viscosity, the inner and outer radii of the bed and the types of the liquid distributor. Results showed that the pressure drop increased with increasing gas flow rate and rotational speed, and was not significantly affected by liquid flow rate. The pressure drop in a dry bed was higher than that in an irrigated bed. The pressure drop decreased with increasing the radial depth of the bed. Compared with single-tube liquid distributor, using double-tube liquid distributor can reduce the pressure drop effectively. Considering all operation parameters that mentioned before, a new correlation of pressure drop was proposed. Besides, the flooding phenomenon in rotating packed bed was also investigated in this study. Results showed that Sherwood correlation overestimated the operation range of rotating packed bed. The mass transfer near the flooding point was also investigated using water – isopropyl alcohol absorption system. When the system approaching flooding point, the system became unstable and a few liquid was carried out from gas outlet, and was not favorable for mass transfer.

Pressure drops over a packed bed of pebble bed reactor type are investigated. Measurement of porosity and pressure drop over the bed were carried out in a cylindrical packed bed facility. Air and water were used for working fluids. There are several parameters of the pressure drop in packed beds. One of the most important factors is wall effect. The inhomogeneous porosity distribution in the bed and the additional wetted surface introduced by the wall cause the variation of pressure drop. The importance of the wall effects and porosity can be explained by using different bed-to-particle diameter ratios. Four different bed-to-particle ratios were used in these experiments (D/dp = 19, 9.5, 6.33 and 3.65). A comparison is made between the predictions by a number of empirical correlations including the Ergun equation (1952) and KTA (by the Nuclear Safety Commission of Germany) (1981) in the literature. Analysis of the data indicated the importance of the bed-to-particle size ratios on the pressure drop. The comparison between the present and the existing correlations showed that the pressure drop of large bed-to-particle diameter ratios (D/dp = 19, 9.5and 6.33) matched very well with the original KTA correlation. However the published correlations cannot be expected to predict accurate pressure drop for certain conditions, especially for pebble bed with D/dp (bed-to-particle diameter ratio)