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Der Neubau eines Hochwasserrückhaltebeckens (HRB) erfordert umfangreiche hydraulische Untersuchungen. Möglichkeiten zur Überprüfung und Optimierung einer wasserbaulichen Planung sind sowohl der physikalische Modellversuch als auch eine hydrodynamisch-numerische Strömungsmodellierung. Fragestellungen zur Wirkungsweise und zu den Wechselwirkungen der Betriebseinrichtung eines HRB können so beantwortet werden. Der vorliegende Beitrag beschreibt das Zusammenwirken von Planung und hydraulischen Untersuchungen am Neubauprojekt HRB Weilers/Bracht in Hessen.
During heat recovery of exhaust gas energy in a shell and tube heat exchanger, the exhaust gas is cooled down and the water vapour content may condense when the corresponding temperature is reached. In this case, a condensation heat exchanger allows recovering the latent heat in addition to the sensible heat and is therefore beneficial for a preferably high heat recovery. The database and algorithms for the design of this kind of apparatus are still nowadays quite week because only little experimental work is realized with condensation of water vapour from a mixture with a very high proportion of non-condensable gas in horizontal tubes. The actual research work improves this database with a considerable amount of experimental data and proposes a correlation for the prediction of the heat transfer coefficient in an exhaust gas heat exchanger in which parts of the gas humidity condense. All experiments were carried out under conditions, which are typical for this kind of heat exchangers, hence the presented results can be used for the design of condensation heat exchangers in industrial heat transfer systems. A gas inlet temperature of 120 °C, a range of the air-steam mixture Reynolds number 11,000 Re 31,000, three tube lengths between 1.87 m and 3.04 m, water vapour volume fractions of 7 % to 14.5 % as well as tube diameters of 22 mm and 28 mm were selected to derive the correlation. Furthermore, additional tube lengths (0.574 m, 0.973 m, 1.373 m) were investigated to determine the parameters influencing the condensation rate and comprehensive location profiles over the tube length. A correlation for a correction factor is derived, which enables to calculate the increase of the heat transfer coefficient due to condensation from the dry heat transfer coefficient without condensation. This dry heat transfer coefficient is calculated by the measured values in the inlet and outlet for a purely dry air-steam mixture cooling without condensation. The correction factor depend on the water vapour volume fraction, the Reynolds number and the geometry ratio between the inner diameter and the tube length.
For circular finned tubes often used in heat exchangers, the wake regions behind the tubes are regions with low heat transfer coefficient. Delta-winglet vortex generators on the fin surfaces reduce the width of the wake regions and improve the overall heat transfer rate. The present work studies numerically the effect of enhancing the performance of the finned-tubes using delta-winglets. Simulations were carried out on a model of three circular tube rows with circular fins for Reynolds numbers of 500, 700, and 900 (based on the inlet velocity and the hydraulic diameter of unit cell). Eighteen basic configurations with different number and positions of delta-winglets were investigated. The flow and heat transfer behaviors are analyzed for certain configurations. Finally, the optimal configurations are presented based on the Pareto optimality strategy.
The Oder River, situated along the border between Poland and Germany, is regularly affected by ice-jam events and their associated hazards, such as a sudden rise in water level and the endangerment to flood-protection infrastructure. The existing databases on past ice-jam events lack substantial information considering ice formation, blockage origins or the spatiotemporal evolution of the ice cover needed for a comprehensive understanding of relevant ice processes. Within this study, the evaluation of satellite and Uncrewed Aerial Vehicle (UAV) data was carried out in order to analyze the capabilities of enhancing river ice information in the study area. Satellite imagery was proven to be a valuable source of investigating ice-jam phenomena on all scales, leading to the identification of initial ice-jam locations, surveying spatiotemporal ice cover evolution or monitoring the maximum ice-cover extent. A simplified approach for river ice classification of satellite radar data using the K-Means Cluster Analysis is introduced, enabling the differentiation between river ice formations. Based on UAV data taken in this study, workflows were presented, allowing for measurements of ice floe velocities and the localization of flooded and ice-covered flow control structures.
AbstractThe application of hydrogen proton exchange membrane fuel cells (PEMFC) in greenhouse gas emission free heavy-duty vehicles requires extremely durable PEMFC components with service lives in the range of 30,000 h. Hence suitable test and analysis methods are required that reflect realistic operation scenarios, but significantly accelerate aging. For this purpose, a dynamic accelerated stress test was developed, which is coupled with a comprehensive in-depth in-situ and ex-situ analysis program to determine the aging processes of a PEMFC membrane electrode assembly (MEA). The test comprehends dynamic cycling between low, moderate and high load, different temperature and humidity conditions as well as recovery sequences to distinguish between reversible and irreversible failure modes. All phases of the PEMFC system (i.e. solid, liquid and gaseous) are monitored on-line during aging by sophisticated electrochemical, mass spectrometric and ion chromatographic analytical methods. The structural and elemental composition of the MEA before and after the aging program (post-mortem) are investigated by X-ray fluorescence, scanning and transmission electron microscopy. This program was able to age a commercial PEMFC to end-of-life in 1000 h, while providing an accurate picture of the aging processes involved.
The limited lifetime and severe degradation after long-term usage of polymer electrolyte membrane fuel cells (PEMFCs) are challenges that have to be overcome, if PEMFCs are to play the anticipated major role in a sustainable energy and transportation system based on green hydrogen. Therefore, analytical scanning methods to precisely characterize and understand the degradation and ageing mechanisms with high spatial resolution are essential. We are using and validating scanning electrochemical microscopy (SECM) as tool for detecting descriptors of the electrochemical performance of PEMFCs treated under different activation and ageing protocols. For this purpose, a pristine Nafion™ membrane, which is typically used as proton exchange membrane (PEM) in low temperature fuel cells, was activated and deactivated after well-defined protocols to demonstrate the fundamental suitability of SECM for investigating PEMFC components. Our results indicate that the impedance associated with the proton conductivity of the Nafion™ membrane as measured by SECM was dependent on the pretreatment of the membrane and increased in the following order: hot water < hydrochloric acid < sulfuric acid. The partial and complete deactivation of the membrane using Fenton’s reagent and barium hydroxide, respectively, could be spatially visualized with SECM as well. In addition, we investigated a PEM as part of a complete membrane electrode assembly in a fully functional PEMFC after accelerated ageing tests and could identify an increased local PEM impedance by SECM. All trends could be confirmed by electrochemical impedance spectroscopy (EIS). Moreover, we investigated a typical gas diffusion layer (GDL) of a PEMFC with SECM. As a result, the microporous side of the GDL showed a much lower impedance than its macroporous side, which is comprehensible since it agrees with the additional carbon black coating on the former side.
In this work, a spatially resolved analytical method based on scanning electrochemical microscopy (SECM) to distinguish different degradation phenomena in polymer electrolyte membranes was developed. SECM was combined with a Franz diffusion cell to distinguish between radical-induced aging of a sulfonated tetrafluoroethylene based fluoropolymer-copolymer due to deactivation of the sulfonic acid groups followed by a decreased proton conductivity, and the radical-induced formation of cracks and holes in the polymer. The experiments were performed with ferrocyanide as redox mediator to detect holes and cracks, and protons (sulfuric acid) to determine the through-plane proton conductivity, respectively. A pristine Nafion™ membrane, a pristine Nafion™ membrane with an artificial pinhole and a Nafion™ membrane aged with Fenton's reagent were investigated to prove the measurement principle. It could be shown that holes and cracks can be reliably detected with this approach and discriminated from a change in proton conductivity. The presence of holes in the investigated aged membranes was confirmed by scanning electron microscopy, whereas the loss of sulfonate groups could be supported by infrared spectroscopy measurements.
Delta-winglet vortex generators (VGs) are known to enhance the heat transfer between the energy-carrying fluid and the heat transfer surfaces in plate-fin-and-tube banks. In this study optimal angles of attack of the delta-winglets are investigated based on the Pareto optimal strategy. The optimization process combines a CFD analysis, genetic algorithms and the response surface methodology. The angle of attack of a pair a delta-winglet-type VGs mounted behind each tube is varied between β = −90° and +90°. Three circular tube rows with inline and staggered tube arrangements are investigated for Reynolds numbers from 200 to 1200 (based on the inlet height and inlet velocity). The flow structure and heat transfer behavior is analyzed in detail for certain cases and the staggered and the inline tube arrangements are compared. Finally, for each of these arrangements the optimal sets of angles of attack for different Reynolds numbers are presented.
This paper introduces a model predicting the unsteady conditions in vertical pipes of a steam power station during start up of the plant. Knowing the time depending variation in the rate of condensation in the pipe is helpful for optimizing the design of the drainage system, which removes the condensate collected from the pipes. At present this system is designed on the basis of the total amount of condensate produced in a particular pipe during start up of the power plant. The model presented will provide a design based on the true maximum flow rate during said period. The paper describes the mass and energy balances for the gas and the condensate as well as the energy balance for the wall. All equations are transformed into a set of differential and algebraic equations and solved at once. The model was subsequently applied on genuine industrial scenarios. The results obtained from the model are discussed in detail and compared with experimental data. In all situations considered, the maximum condensate flow rate calculated was less than 50% of the estimated value using standard design methods.