1,721,043 research outputs found
Surface Energy Modification of Filter Media to achieve optimal Performance Characteristics in select Applications
No full textThe surface modification of modern filter media is examined from the perspective
of the energetic properties and how they influence select filtration applications. In
contrast to the known mechanical filtration mechanisms, which are mainly applicable
to the solid-liquid separations, new findings strongly suggest that direct interaction
forces between the filter and the functional fluids must be taken into account in order
to achieve sufficient efficiencies. Separation processes of liquid phases such as liquidliquid
coalescence (LLC) or the treatment of process gases with liquid-gas coalescence
(LGC) require special properties of filter media with regard to the degree of interaction
with these phases. These include but are not limited to surface energy, wettability,
chemical resistance, etc. The focus falls increasingly on eliminating the undesired
interactions of modern filters with the fluid to be filtered. Filtration with modern
fine filter media can result in undesired additive removal (ADDREM), particularly of
those additives that are not fully dissolved in carrier fluid. Specifically this refers to
the removal of antifoamants from gear oils, which lead to serious consequential damage
of those systems. The interfacial interactions between the filter media and the
functional fluids are also responsible for other effects such as the highly undesirable
phenomenon of electrostatic charging/discharging (ESC/ESD) during the filtration of
low-conductivity oils. In this work, the effect of the surface energy modification, in
particular, is examined in greater detail. Ultimately, the surface energy of modern
filter media is characterized and modified in order to optimize their performance in
select applications. The work also presents some examples that illustrate the importance
of surface energy in highly challenging filtration applications
Evaluation of synergy effects of central storage systems in low-voltage grids by integrative modeling.
Get PDFSecurity of supply, affordability, and sustainability form the pillars of a new energy policy towards renewable generation and decarbonization. However, the dynamics of the power generation due to the increasing amount of renewable energies cause temporal and local discrepancies between generation and consumption. Resulting energy transports between grid sections and different voltage levels cause additional load flows. To ensure grid stability, the grid operator provides system services and grid extension measures. With the help of energy storage systems with grid-serving control and placement strategies, the flexibility of the electricity supply can be increased. Besides, a high amount of renewable energy can be used locally while maintaining grid stability. A centralized installation approach focussing single grid sections, instead of many decentralized home storage units, offers economic and environmental advantages. Furthermore, the operation strategy can be optimized by the global view of the grid operator and thus be adapted to local conditions. This research evaluates synergy effects of central storage systems by integrative computational analysis using a rural low-voltage grid section in Luxembourg. Three linked simulation levels are used to calculate operational strategies, storage dimensioning as well as placement based on 15-minute smart meter data. The operation strategy is developed within a power system simulation and is used to control a parameterizable simulation model of a vanadium-redox-flow-battery. The operating strategy focuses on reducing the maximum power flow at the transformer and reactive power compensation to maintain voltage stability. A future photovoltaic scenario is being adopted by doubling the status quo photovoltaic generation. The simultaneous optimization of storage utilization and power reduction at the transformer provides the storage design parameters power and capacity. Storage placement is determined by the system boundary and the resulting data selection. A final sensitivity analysis evaluates an optimized storage placement while enhancing the voltage profiles. The results of this work are a differentiated active as well as reactive power related operating strategy, automated calculation algorithms to determin control parameters, optimized battery design parameters as well as the methodical approach to transfer calculation algorithms to further grid sections
Energy Efficiency Optimization of Low Grade Waste Heat Recovery via a Carbon Dioxide Rankine Cycle
No full textGlobal energy consumption is continually increasing, and intelligent energy supply is
a critical concern for our generation. Rising concerns about climate change, as well
as rising greenhouse gas emissions from the usage of fossil fuels, highlight the need of
clean energy generation. Waste heat recovery might be one solution to this problem.
Waste heat is generated as a byproduct of various processes and is discharged into
the environment and the majority being generated at temperatures below 250°C. This
lost energy has the potential to be absorbed and turned into useable energy, notably
electricity. A Rankine cycle is capable of converting heat energy into electricity. Organic
Rankine cycles employ working fluids with low evaporation temperatures (41-78°C),
but are harmful to the environment due to their high global warming potential and
greenhouse gas factor. CO2, on the other hand, has become a popular refrigerant in
recent years due to its environmental friendliness and strong heat transfer capabilities.
For medium to high temperatures about 250°C, converting waste heat to power works
effectively. However, the efficiency is quite poor at temperatures below 100°C.
This work seeks to improve the energy efficiency of waste heat recovery at low temperatures.
Different techniques from the literature on how to increase the thermal cycle
efficiency were studied in order to evaluate the optimization potential. Cycle adjustments
such as reheated expansion, intercooled compression, and recovery were reported.
However, the majority of the existing work is limited to medium to high temperatures.
They were examined under equal operational settings to see if these cycle adjustments
can also be used at low temperatures. A thermodynamic simulation using Matlab and
EBSILON Professional was created for this purpose. For waste heat temperatures ranging
from 60 to 100°C and a heat sink temperature of 20°C, the evaluated power cycles
produced cycle efficiencies ranging from 2.35 to 8.16 percent. The aforementioned cycle
adjustments only had a minor impact on efficiency at low temperatures. As a result,
the basic cycle arrangement with no layout changes was determined to be the optimum
for the examined temperature range. Another significant discovery was that fluid compression
is the primary cause of poor efficiency. Because the compression of the fluid
consumes a considerable portion of the energy provided by the turbine, the net power
output is poor. Consequently, lowering the compression energy increases the net power production and thus the cycle efficiency.
A thorough examination of how to compress a fluid with less (electric) energy input
was carried out. Based on this, a thermal compression device (TCD) was designed,
which uses heat rather than electricity to increase the pressure of a fluid. FLOWNEX
and Matlab were used to simulate the TCD. A fluid flow due to gravity and pressure
differential was developed in this model. The isochoric heat addition (IHA), a batch
process, was modelled along with two buffer vessels upstream and downstream. For the
thermodynamic cycle, the buffer vessels should allow for a steady mass flow. The results
demonstrated that the pressure difference between the vessels has a significant influence
on system performance and must not be neglected. Furthermore, the simulation revealed
that the mass flow is declining during operation, indicating that the containers have not
been completely emptied. As a result, the TCD’s throughput is reduced. This issue
may be solved by adding a piston in the vessel, which allows for exact adjustment of
the vessel volume and fluid flow control. The enhanced TCD, which incorporates the
changes was also simulated in FLOWNEX. The main advantage of the TCD is that it
uses less electrical energy to pressurize a fluid by using waste heat, which is abundant.
Another key advantage is that the constraint of a conventional pump or compressor to a
distinct fluid phase - liquid or gaseous - is removed. The TCD can manage both of these
fluid phases, as well as a phase shift during the pressurization process. The economic
analysis contrasts the traditional ORC with the ORC using the TCD, revealing that the
cost of electricity production is cut in half and the investment payback time is cut by 11
years
Numerische Untersuchung zur Strömungsentwicklung im passiven Flutsystem des Reaktordesigns Kerena
Get PDFIn case of a loss-of-coolant accident in a nuclear power plant, the passive gravity-driven core flooding system recovers the reactor’s water inventory so that the core is covered at all times. It is essential to estimate, when passive flooding valves open, whether reverse flow occurs and to which extent this may delay flooding or even lead to initial mass displacement from the reactor to the flooding pools. Experimental investigations including passive safety systems of the KERENA-design are performed in a scaled-down facility. Such so-called integral tests are carried out with initial and boundary conditions of different design basis accidents. The behavior of the passive core flooding system is analyzed proofing the influence through other passive safety systems. Evidence is shown that a stable two-phase reverse flow occurs, due to incomplete condensation at the outlet of the emergency condenser and to adiabatic evaporation in the flooding line. The hydrostatic column is reduced during reverse flow but regained in all considered cases. As a result the onset of flooding is delayed only moderately. The influence on the flow development of the containment pressure, of the geometry of the flooding line, and of other peculiar boundary conditions of the considered experiments is discussed. Results from numerical simulations are presented and confirm the experimental results. The considered numerical model of the flooding system can predict fundamental phenomena governing the flow development in the flooding line with satisfactory accuracy
Investigation of condensation process inside inclined tube
No full textGeneration III+ reactor designs partially rely on passive safety systems. It aims to increase the plant safety standards and to reduce investment costs. Passive Decay Heat Removal Systems, such as the Emergency Condenser (EC) of the KERENA reactor design, plays an important role in the safety in nuclear power plants. As part of the emergency cooling chain, EC removes the decay heat from the reactor pressure vessel and transfers it to the flooding pool. For the successful design of the EC, the reliable prediction of the condensation heat transfer inside inclined pipes is one of the important factors.
One-dimensional (1D) codes, such as ATHLET, RELAP and TRACE, are widely used today by engineers to predict the thermal hydraulic behavior of the system in nuclear power plant. However, state-of-the-art 1D codes are mainly validated for active components, and the qualification of passive systems is still a remaining problem. The goal of this thesis therefore is to investigate the condensation phenomena in EC using current advanced 1D code ATHLET (Analysis of Thermal-hydraulics of Leaks and Transients). The performance of ATHLET for prediction of condensation in slightly inclined tube was assessed and the results showed that the standard models in ATHLET code have significant deficiencies on the prediction of the condensation heat transfer coefficients. Thus, the new empirical model has been derived using experimental data from COSMEA (COndenSation test rig for flow Morphology and hEAt transfer studies) tests, condensation experiments for flow morphology and heat transfer studies in a single slightly inclined tube conducted by HZDR (Helmholtz-Zentrum Dresden-Rossendorf) and data sourced from literature.
The new model was developed using Machine Learning – Regression Analysis methodology in MATLAB, which consists of upper liquid film condensation and bottom convective condensation. It was further implemented in ATHLET with Python programming language and the modified ATHLET code was used to calculate the COSMEA experiments. The post-calculation results were compared to experiments in three aspects: heat flux, condensation rate and void fraction along the whole pipe. The outcomes showed that the modified ATHLET code can be used to recalculate the relevant heat transfer values of experiments under different pressure and mass flow rate conditions.PANA
Numerical Modeling of air-gap membrane distillation
Get PDFFresh water supply is a problem in large parts of the world and present on every continent. Many countries facing physical water scarcity, however, have access to the sea and lie in arid zones of the earth where solar energy is plentiful available. Membrane distillation (MD) describes an emerging desalination technology which has advantages when driven by solar energy or waste heat. In MD, seawater is thermally desalinated by generating a temperature gradient between hot salt water and produced fresh water which are separated by a membrane. In air-gap membrane distillation (AGMD) an insulating air-gap is introduced between membrane and distillate in order to minimize conductive losses. Despite its advantages, the permeate stream needs to be increased for large-scale application. To improve performance and energy efficiency, a detailed understanding of the highly coupled heat and mass transfer is crucial. However, for AGMD not many models exist and the existing models simplify the heat and mass transfer processes. The goal of this thesis is therefore to increase the understanding of the AGMD process and the predictive power of numerical models. A three-dimensional (3D) macro-scale model is developed with emphasis on the heat and mass transfer. It integrates aspects from multiphase flow modeling namely energy conservation over phase-change interfaces and the thermodynamic concept of moist air in the air-gap. Thereby, it computes the condensation mass flow independently from the evaporation mass flow, allowing to study the influence of convection on the heat and mass transfer in the air-gap. The model is accelerated for computation on graphical processing units (GPU). Employing the macro-scale model, a comparative analysis of the effects of module orientation on module performance and efficiency is performed. Vortexes in the air-gap are observed when using a module configuration where the hot feed flows below air-gap and membrane and the temperature gradient is opposing gravity. These vortexes lead to a significantly increased energy utilization also at low feed velocities. As the main advantage of AGMD is the reduction of heat losses, this configuration could bring further improvement. Furthermore, membrane transport properties are determined from high-resolution 3D membrane imaging combined with Lattice-Boltzmann simulation. Thereby, the 3D structure of membrane samples is obtained and porosity, tortuosity and permeability values are computed for the investigated membranes. Following the findings in the papers, further studies are suggested employing the modeling approaches developed in this thesis
Towards Swirl-Stratified Methane Slip Reduction in Stationary Lean-Burn Gas Engines: Numerical and Experimental In-Cylinder Cold-Flow Analysis
No full text
Experimentelle Untersuchungen und analytischen Modellierung von adiabaten Siedevorgängen in Naturumlaufsystemen
Get PDFModerne Nuklearreaktoren setzen vermehrt auf den Einsatz passiver Sicherheitssysteme,
damit die Integrität des Sicherheitsbehälters im Falle eines Unfalls sichergestellt werden
kann. Um eine passive Wärmeabfuhr aus dem Sicherheitsbehälter an die Umgebung zu
ermöglichen, werden Naturumlaufsysteme verwendet. Diese haben den Nachteil, dass
Instabilitäten auftreten, sobald es zu Verdampfungen im Naturumlauf kommt.
Diese Arbeit beschäftigt sich mit der Untersuchung von Instabilitäten in einem zweiphasigen Naturumlaufsystem. Nach einer detaillierten Literaturrecherche über bestehende
Versuchsanlagen, wurden während des ersten Teils dieser Arbeit experimentelle Untersuchungen zum Stabilitätsverhalten von Naturumlaufsystemen durchgeführt. Hierfür
wurde die Versuchsanlage INTRAVIT designt und an der Universität Luxemburg errichtet. INTRAVIT bietet zum einen ein hohes Maß an Flexibilität für die Auslegung
der Rohrleitungen und zum anderen den Vorteil der direkten, elektrisch steuerbaren
Wärmezufuhr. Es wurden zwei Messkampagnen durchgeführt. In der ersten Messkampagne wurde, bei konstanter Steigrohrlänge, der Einfluss des Heizrohrneigungswinkels
auf die Instabilitäten untersucht. Für die zweite Messkampagne wurde, bei konstantem
Neigungswinkel des Heizrohrs, zum einen der Einfluss der Steigrohrlänge und zum anderen der Einfluss des Strömungswiderstands im Fallrohr auf die Instabilitäten untersucht.
Für die Untersuchungen wurden die Temperaturen, der Massenstrom und die Verteilung
des Dampfgehalts im Steigrohr analysiert. Zudem wurde der Druckverlauf während der
Instabilitäten gemessen, um die Druckstöße während den Kondensationsschläge zu untersuchen.
In einem zweiten Teil dieser Arbeit wurde ein analytisches Modell zur Beschreibung von
Verdampfungsvorgängen in adiabaten Rohren hergeleitet. Das Modell besteht aus mehreren Teilmodellen, die die Phasenoberflächendichte und die Verdampfungsrate in Abhängigkeit der Strömungsform berechnen. Das Einsetzen der Verdampfung wird hierbei über
ein integriertes Keimbildungsmodell berechnet. Das hergeleitete Modell wurde in den
Systemcode ATHLET implementiert. Die Versuche aus den INTRAVIT-Messkampagnen
wurden mit dem neuen Verdampfungsmodell und dem Standard-Verdampfungsmodell
modelliert. Anschließend wurden die Ergebnisse mit den Messdaten verglichen.
Mit der Realisierung der INTRAVIT-Anlage legt diese Arbeit einen Grundstein für weiterführende Untersuchungen an Instabilitäten in Naturumlaufsystemen. Zudem wurde
ein Verdampfungsmodell hergeleitet, das durch Anpassen einzelner Teilmodelle beliebig
weiterentwickelt werden kann.Modern nuclear reactors increasingly use passive safety systems to ensure the integrity
of the containment in the event of an accident. Natural circulation systems allow passive
heat removal from the containment to the environment. One of their disadvantages is
that instabilities develop as soon as evaporation occurs in the system.
This thesis investigates instabilities in two-phase natural circulation systems. After comprehensive literature review on existing test facilities, the first part of this work presents
experimental investigations on the stability behaviour of natural circulation systems. To
conduct these investigations, the INTRAVIT test facility was designed and constructed
at the University of Luxembourg. INTRAVIT offers both a high degree of flexibility in
the design of the pipelines and the advantage of a direct, electrically controllable heat
supply. Two measurement campaigns were carried out. During the first campaign, the
influence of the heating tube inclination angle on the instabilities was investigated at
constant riser pipe length. During the second measurement campaign, the influence of
the riser pipe length and the influence of the flow resistance in the downcomer pipe were
investigated at a constant inclination angle of the heating tube. For these investigations,
temperatures, mass flow and the distribution of void in the riser pipe were analysed.
In addition, pressure response during the instabilities was measured to investigate the
pressure shocks caused by water hammering.
The second part of this work develops an analytical model to decribe evaporation processes in adiabatic pipes. The model consists of several sub models that calculate the
interfacial surface density and the evaporation rate as a function of the flow pattern. An
integrated nucleation model calculates the onset of the evaporation. The derived model
was implemented in the system code ATHLET. Experiments from the INTRAVIT measurement campaigns were then modelled using the new evaporation model as well as the
standard evaporation model. Both were compared with the measurement data.
The design and construction of the INTRAVIT test facility is a foundation for future
research on instabilities in natural circulation systems. Moreover, a new evaporation
model is presented, which can easily be adapted and refined by modifying individual
sub models
- …
