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Heat transfer, scale formation and its reduction in membrane distillation for the concentration of seawater
No full textDer Einsatz der Membrandestillation zur Meerwasserentsalzung ist insbesondere dann viel-versprechend, wenn sie zur weiteren Konzentrierung des Meerwasserkonzentrats aus konventionellen Entsalzungsanlagen unter Nutzung von regenerativen Energien oder Abwärme verwendet wird. Darüber hinaus wird die Membrandestillation als aussichtsreiches Verfahren für die Konzentrierung stark salzhaltiger Abwässer in Zero Liquid Discharge-Anwendungen betrachtet. Ein Hindernis für die großtechnische Umsetzung ist jedoch die Ablagerungsbildung auf der Membranoberfläche aufgrund sehr hoher Salzkonzentrationen.
Für die Prozesssimulation der Membrandestillation ist eine bessere Kenntnis des Wärme-übergangs in Strömungskanälen mit Abstandshaltern von großer Bedeutung. Die Bildung von Ablagerungen und die Wechselwirkungen mit den gekoppelten Wärme- und Stofftransport-vorgängen wurden bisher nur unzureichend untersucht. Auch Maßnahmen zur Verminderung der Ablagerungsbildung und zur Reinigung der Membranen wurden kaum betrachtet. Hier setzt die vorliegende Arbeit an.
Basierend auf Messungen des Wärmedurchgangs wurde eine Korrelation für die Nußelt-Zahl abgeleitet, die den Wärmeübergang in Flüssigkeitskanälen mit Abstandshaltern in einem Plattenmodul beschreibt. Die Korrelation wurde in Prozessmodellen des Direktkontakt- und Luftspaltverfahrens eingesetzt, um die Temperatur- und Konzentrationsprofile zu ermitteln. In Verbindung mit der hydrogeochemischen Modellierungssoftware PHREEQC konnte hierdurch die Neigung gelöster Salze zur Ablagerungsbildung entlang des Strömungsweges in großen Modulen in Abhängigkeit der Prozessparameter bestimmt werden.
Ferner wurden die Einflüsse der Prozessparameter der Membrandestillation auf die Ablagerungsbildung bei der weiteren Konzentrierung von Meerwasserkonzentraten experimentell untersucht. Dabei konnte festgestellt werden, dass höhere Temperaturen und treibende Temperaturdifferenzen zu einer schnelleren Konzentrierung der Lösung führen, was die Ablagerungsbildung beschleunigt. Es zeigte sich, dass erst die dem Calciumcarbonat nachfolgende Bildung von Calciumsulfat zu einer signifikanten Verringerung der Permeatstromdichte führt. Die Bildung der Ablagerungen, die zum größten Teil aus Calciumsulfat bestehen, wird von den Prozessbedingungen deutlich beeinflusst. Durch den Einsatz von Additiven kann die Ablagerungsbildung zu sehr hohen Konzentrationsfaktoren verschoben werden. Die sich dennoch bildenden Beläge können durch eine Modulreinigung mit Ethylendiamintetraacetat-Lösung und Methansulfonsäure relativ einfach entfernt werden
»Wer sagt denn jetzt, was richtig ist?!« oder Konflikte als Strukturmerkmal einer offenen Gesellschaft.
No full textPublikation der U Bremen und U Wien internationalen Studierendenkonferenz »Debattieren, Opponieren, Protestieren – Interdisziplinäre Perspektiven auf sprachliche Praktiken des Widersprechens« 2023.828
OER-Policy der Universität Bremen
No full textDie OER-Policy der Universität Bremen.
Sie finden die englisch-sprachige Version über folgenden Link: https://doi.org/10.26092/elib/3614
You can find the English version via the following link: https://doi.org/10.26092/elib/361
Exploring the transition gap - a processual model of contextual influences on students' engagement in entrepreneurial activities
No full textThis cumulative dissertation explores the contextual factors influencing why students often fail to act on their entrepreneurial intentions, despite taking entrepreneurship courses and developing near market-ready business ideas. While many students develop entrepreneurial aspirations through education, they do not always translate these intentions into real entrepreneurial activities. The study investigates this gap, focusing on why exposure to entrepreneurship education does not consistently lead to entrepreneurial action, particularly within the German context where this discrepancy is notably pronounced. The thesis examines contextual factors on four different levels: the university level, the course level, the team level and the individual level. While results provide insights at each level, the study also sheds light on the interdependencies among these factors, providing a comprehensive understanding of how various factors collectively influence students’ entrepreneurial decision-making
Organic carbon transfer between coral reef micro- and macro-organisms under environmental change
No full textEfficient organic carbon (OC) and nutrient cycling between coral reef-associated micro- and macro-organisms is vital for coral reef functioning and supports the ecosystems’ exceptional productivity. However, global and local anthropogenic stressors interfere with the transfer of OC within the reef ecosystem (i.e., through altered benthic-pelagic coupling) as well as within its keystone species, the coral holobiont (i.e., between coral host and algal endosymbiont). Excess OC inputs though wastewater and/or algal blooms can stimulate microbe-mediated hypoxia and disease, supporting coral mortality and resulting in shifts towards macroalgae or soft coral dominance on many degraded reefs. While soft corals are generally understudied, carbohydrates exuded by macroalgae may support reef degradation by stimulating bacterioplankton respiration, but the role of carbohydrates released by benthic primary producers in shaping coral reef community metabolism is still poorly understood. This thesis aims to improve our knowledge on OC transfer between coral reef micro- and macro-organisms under environmental change by: i) assessing the effects of global and local stressors on the physiology of the increasingly widespread soft coral Xenia umbellata, ii) developing an indicator tool for the detection of OC eutrophication in reefs, and iii) investigating the carbohydrate compositions of hard coral- and macroalgae exudates and their effects on bacterioplankton communities. The experimental studies in this thesis include physiological, biogeochemical, and microbial parameters, and were carried out in aquarium facilities in Bremen and on Curaçao, Dutch Caribbean. Findings revealed a high resistance of X. umbellata to warming, phosphate eutrophication, and acidification, while nitrate eutrophication reduced its resistance to warming. A newly developed microbial fuel cell (MFC) successfully detected OC pulses by electrically quantifying microbial degradation of OC in coral reef sediment. Compositional analyses of coral mucus carbohydrates and comparisons with previously reported data revealed a correlation with hard coral phylogeny. When comparing hard coral- with macroalgae exudates, differences in carbohydrate compositions and responses of bacterioplankton communities emerged. Coral exudates enriched opportunistic microbial taxa commonly considered as stress indicators, while macroalgae exudates were compositionally similar to ambient reef water and did not induce any shift in bacterioplankton communities. In conclusion, the soft coral X. umbellata may replace less resistant hard corals on many reefs in the future, and may therefore sustain some ecosystem functions and services provided by reefs. However, coastal water quality management will become more important for hard- and soft corals with ongoing ocean warming. As such, the here presented MFCs could be used to detect OC eutrophication before benthic community shifts occur. Changes in hard coral assemblages may influence benthic-pelagic coupling in yet unexplored ways through their compositionally distinct mucus carbohydrates. Macroalgae exudates will likely reduce trophic transfer of OC compared to coral exudates due to their inefficient incorporation and/or resistance to microbial degradation. Results further suggest that not the origin of OC (i.e., coral- vs. macroalgae-derived) per se, but rather an alteration in OC composition relative to ambient reef water disrupts the stable bacterioplankton community and supports the increase of opportunistic microbes. Overall, results presented here can be used by coral reef managers to target conservation strategies at a soft coral species with high resistance to environmental change and monitor OC eutrophication in reefs with the here developed MFC biosensors. Finally, this thesis provides new insights for understanding how benthic macro-organisms shape microbial communities by altering the OC composition of reef water. As shifts in benthic primary producers are not exclusive to coral reefs, altered OC compositions may influence microbial communities in a wide range of coastal ecosystems under environmental change
Carbon Nanotube/Polyimide Film Based Strain Microsensor for Carbon Fiber/Epoxy Composites
No full textThe work reports the development of strain sensors based on a 5 µm thick piezoresistive polyimide film. A direct mixing process of carbon nanotubes with polyimide precursor was proposed for the material fabrication. As a proof of concept, the sensor was embedded within carbon fiber composites for strain measurements.
For the development of the sensors, the following steps were performed:
- The adhesion between polyimide layers was investigated using surface treatments based on oxygen plasma and alkaline solutions.
- Modeling of the electrical percolation of films was proposed to predict the amount of nanoparticles required to achieve electrical conductivity of the polyimide films.
- The fabrication of a nanotube/polymer film was performed using ultrasonication and direct mixing approaches. As an alternative, graphene nanoplatelets were also tested.
- The piezoresistive characterization of the nanotube/polyimide films was performed to evaluate the material sensitivity under applied strain.
- The nanotube content of films and the minimum size for sensor structures were determined considering photolithography for the microfabrication of the sensors.
The surface modification of the polyimide indicated that oxygen plasma is the proper route for ensuring the adhesion between the sensor layers.
The direct mixing proposed for this work was the most efficient approach to produce polyimide films with 1.5 - 3 % nanotube contents. This particle content agreed with the predictions of the material percolation. On the other hand, graphene was not a suitable material due to its higher percolation threshold (>10 %).
The sensors were fabricated as meander structures, with an aspect ratio of length/width = 10 and a minimum linewidth of 50 µm. The embedded devices showed a gauge factor K = 10.7, which is five times higher than the sensitivity of commercial strain gauges (K = 2).
These sensors could be used as integrated sensors in different structural parts for online monitoring of forces or strain. The proposed material has additional potential applications such as pressure and temperature sensing, or polymer cure monitoring
Using educational escape games for game-based learning in science lessons.
No full textSchon in der frühkindlichen Phase schulen wir unsere motorischen, kognitiven und sozialen Fähigkeiten durch Spielen. Das Lernen erfolgt beiläufig und intrinsisch motiviert. Forschende und Lehrkräfte setzen Lernspiele ein, um der geringen Motivation und dem Engagement der Lernenden entgegenzuwirken. Ein neues Genre des spielerischen Lernens sind Educational Escape Games (EEG). In diesen müssen Lernende eine Vielzahl fachlicher Rätsel lösen, um schnellstmöglich aus einem Raum zu entkommen. Das Lösen der Rätsel gleicht einem kreativen Problemlöseprozess, wodurch sie besonders interessant für den naturwissenschaftlichen Unterricht sind. Zudem fördern kommerzielle Escape Rooms bereits interdisziplinäre Kompetenzen, wodurch das Spielkonzept überhaupt erst Anwendung in der Personal- und Schulbildung fand. Im Rahmen dieser Arbeit erfolgte eine theoretische Rahmung für den methodisch-
didaktischen Einsatz des Spielkonzepts. Um sich einen Überblick über die bereits vorhandenen EEGs zu machen, erfolgte zu Beginn eine literaturbasierte Analyse naturwissenschaftlicher EEGs. Folglich sind die meisten EEGs auf die tertiäre Bildung ausgerichtet. Der Großteil der EEGs kann nicht zeitgleich von der gesamten Lerngruppe gespielt werden und ist auf eine spezifische Schule/Lerngruppe ausgerichtet. Darüber hinaus liegen bisher wenige empirische Ergebnisse zu den kognitiven und emotionalen Effekten des Spielkonzepts vor. Auf dieser Grundlage wurden vier EEGs entwickelt und evaluiert. Acid Base Global kann zur
Wiederholung der Einheit Säure-Base-Chemie eingesetzt werden. Es handelt sich um eine analoge Tisch-/Brettspielversion mit inkludierten, experimentellen Rätseln. Das Expeditionsjournal ist ein analoges Educational Escape Buch zu den Sauerstoffübertragungsreaktionen. Der Fokus liegt hier auf dem Wissenserwerb. Ein EEG mit Nachhaltigkeitsbezug stellt der Green Chemistry Escape dar. Durch das digitale Lernspiel erhalten die Lernenden einen Einblick in die Prinzipien der grünen Chemie. Mit dem Crazy Safety Escape wird eine spielerische Alternative zur regulären Sicherheitseinweisung geboten. Dabei handelt es sich um ein hybrides EEG, das physische Rätselmaterialien mit digital eingebetteten Schlössern und Hinweisen vereint. Alle entwickelten EEGs lassen sich an die Kursgröße und die räumlichen Gegebenheiten anpassen. Außerdem sind sie auf die Sekundarstufen I und II ausgerichtet und curricular legitimiert. Die Ergebnisse der vier Fallstudien bestätigen, dass EEGs zum Erlernen von Fachinhalten sowie fächerübergreifenden Kompetenzen geeignet sind. Lernende stehen dem unterrichtlichen Einsatz von EEGs positiv gegenüber und wünschen sich einen häufigeren Einsatz. Zudem konnte ein positiver Effekt auf die Motivation der Lernenden festgestellt werden. EEGs stellen somit eine zukunftsweisende, kreative Lernmethode dar, die sich durch eine erhöhte Aktivität der Lernenden auszeichnet
Investigation on the pyruvate uptake, release and metabolism of cultured rat brain astrocytes
No full textAstrocytes play a pivotal role in brain metabolism and in neuroprotection. These cells are considered as very glycolytic, consuming large amounts of glucose that is mainly converted into lactate. The α-keto acid pyruvate is a key player of cellular metabolism that links cytosolic and mitochondrial metabolism as the end product of glycolysis. Pyruvate can also be exported from astrocytes and is in brain believed to have neuroprotective functions. In order to elucidate the pathways involved in astrocytic pyruvate metabolism, primary rat astrocyte cultures were used as model systems to investigate pyruvate consumption and release.
Of the different substrates tested (pyruvate, lactate, β-hydroxybutyrate, alanine and acetate) that are known to be metabolized in mitochondria, pyruvate was consumed most efficiently by cultured astrocytes incubated in the absence of glucose. Astrocytes exhibited a nearly time-proportional, concentration-dependent consumption of extracellular pyruvate with apparent Michaelis-Menten kinetic [KM = 0.6 ± 0.1 mM, Vmax = 5.1 ± 0.8 nmol/(min x mg protein)]. Lactate and alanine generated and released in pyruvate-fed astrocytes accounted for approximately 60 % and 10 %, respectively, of the pyruvate consumed within 3 h. The presence of AR-C155858, a monocarboxylate transporter 1 (MCT1)-inhibitor, or the application of 10-fold excess of the MCT1 substrates lactate and β-hydroxybutyrate strongly impaired the astrocytic consumption of extracellular pyruvate. Inhibition of the mitochondrial pyruvate carrier (MPC) by UK5099, as well as inhibition of the respiratory chain by the complex III inhibitor Antimycin A also prevented pyruvate consumption. In contrast, BAM15, a mitochondrial uncoupler, strongly accelerated pyruvate consumption in glucose-deprived astrocytes.
In the presence of glucose, astrocytes established a transient extracellular steady-state concentration of pyruvate between 150 µM and 300 µM, while lactate in contrast was continuously released and accumulated to millimolar concentrations. In DMEM culture medium, the extracellular pyruvate concentration remained almost constant for days. In amino acid-free incubation buffer, this almost constant extracellular pyruvate level was established within 5 h, with an initial pyruvate release rate of around 60 nmol/(h x mg), and was maintained for several hours. By consumption of excess extracellular pyruvate in the presence of glucose, astrocytes established similar extracellular pyruvate concentrations. Furthermore, pyruvate release was observed in glucose-free incubation buffer after application of mannose, lactate, fructose, sorbitol or alanine. MCT1 inhibition by AR-C155858 decreased the extracellular pyruvate concentration, while MPC inhibition by UK5099 strongly increased the release of glycolytically derived pyruvate. Both antimycin A and BAM15 application resulted in a complete loss of extracellular pyruvate accumulation.
The data presented demonstrate that MCT1 is the main transporter involved in pyruvate consumption and release. Modulation of mitochondrial processes revealed a strong involvement of the mitochondrial metabolism in pyruvate utilization. Overall, alterations of pyruvate metabolism presumably modify the intracellular pyruvate concentration, thereby influencing pyruvate consumption and release, as astrocytes seem to establish an equilibrium between their extracellular and intracellular pyruvate concentration
Ground-based remote sensing of aerosol properties using the emission FTS in Ny-Ålesund, Spitsbergen (78°N)
No full textArctic amplification, the rapid warming of the Arctic compared to the global average, re
mains partially understood. Key processes include temperature feedback, surface albedo feedback, and cloud and water vapor feedback, with aerosols playing a critical role. Since 2019, a Fourier-Transform Infrared Spectrometer (FTS) at the AWIPEV research base in Ny-Ålesund, Spitsbergen, has been used to measure aerosol components. An algorithm based on the Line-by-Line Radiative Transfer Model and DIScrete Ordinate Radiative Transfer model (LBLDIS) was developed for retrieving aerosol composition.
In order to show this measurement technique in details, a case study for an aerosol-only
case is presented with data from the 10th of June 2020. In the aerosol-only case, the retrieval results show that sulfate (τ900cm−1=0.007 ± 0.0027) is the dominant aerosol during the whole day, followedbydust(τ900cm−1=0.0039 ± 0.0029) and black carbon (τ900cm−1=0.0017 ± 0.0007). Sea salt (τ900cm−1=0.0012 ± 0.0002), which has the weakest emission ability in the infrared waveband, shows the lowest AOD value. Such proportions of sulfate, dust and BC also show good agreement with MERRA-2 reanalysis data. Additionally, the comparison with a sun-photometer (AERONET) shows the daily variation of aerosol AOD retrieved from FTS to be similar with that of the sun-photometer. Based on this retrieval method, long time period observations dataset using FTS is retrieved and presented in this study.
Based on the observed data, the infrared radiation effects of different aerosol compo
sition are analyzed. The results show that the hygroscopic aerosols, such as sea salt and sulfate, have a warming effect in the Arctic during winter. These aerosols absorb atmospheric water vapor, leading to wet growth, increased size, and enhanced longwave downward radiation emission, defined as the Aerosol Infrared Radiation Effect (ARE). Observations of aerosols, especially their composition, are challenging during the Arctic winter. We use an emission Fourier Transform Spectrometer to measure aerosol composition. Observations show that the ARE of dry aerosols is limited to about 1.45 ±2.00 Wm−2. Wet growth significantly increases the ARE of aerosols. During winter, at relative humidity levels between 60% and 80%, wet aerosols exhibit the ARE approximately 10 times greater than dry aerosols. When relative humidity exceeds 80%, the effect can be up to 50 times higher (30- 100 Wm−2). Sea salt aerosols in Ny-Ålesund demonstrate high effect values, while non-hygroscopic aerosols like black carbon and dust show consistently low values. Reanalysis data indicates increased water vapor and sea salt aerosol optical depth in Ny-Ålesund after 2000, correlating with significant positive temperature anomalies in this area. Besides, wet aerosols can remain activated even in dry environments, continuously contributing high effects, thereby expanding the area affected by aerosol-induced warming. This warming effect may exacerbate Arctic warming, acting as a positive feedback mechanism.
Additionally, the rapid Arctic warming, which is occurring faster than in other regions,
leads to a meandering atmospheric circulation pattern. Recently, a new pathway for African dust transport to the Arctic has been identified. This study provides a detailed description of the Rapid Pathway (RP) and investigates the temporal variation of African dust influx into the Arctic via this route. Using GEOS-Chem model simulations, we demonstrate the RP’s enhanced efficiency in accelerating African dust transport to the Arctic within approximately one week, compared to other pathways. Our analysis reveals a significant shift in African dust transport routes after 2000, with a marked increase in dust transport through the central North Atlantic (RP region), particularly in March and April. ERA5 wind field data reveal significant positive anomalies in poleward winds over the North Atlantic in March and April after 2000, facilitating northward dust transport via the RP region. In contrast, negative wind anomalies over Europe suggest a diminished role for the European pathway in Arctic dust transport
Individuelle Blueprints für summative Assessments - In fünf Schritten zur Vorlage für Prüfungsplanung und Aufgabenerstellung
No full textEin wichtiges Instrument für die adäquate Berücksichtigung aller intendierten Lernergebnisse (Intended Learning Outcome, ILO) in Prüfungen ist ein Blueprint, also eine Blaupause mit Quoten für Aufgaben und Punktvergabe, gegliedert nach Themen und Lernzielstufen. Dieser Aufsatz macht einen Vorschlag für eine pragmatische Vorgehensweise zur Konstruktion eines Blueprints in fünf Schritten: (1) Festlegung der Themen der Lerneinheit, (2) Festlegung der ILO für jedes Thema, (3) Klassifizierung der ILO nach Lernzielniveau und Domäne (= Lernzielfeld) sowie Gewichtungsfaktor, (4) Konstruktion des Blueprints mit den Spalten Thema, ILO, Lernzielfeld und Gewichtung und (5) Ableitung der prozentualen Quoten für jedes intendierte Lernergebnis. Die Besonderheit dieses Konzeptes gegenüber bisherigen Vorschlägen zur Erstellung von Blueprints besteht darin, dass die Quoten nicht vorab und pauschal festgelegt, sondern erst im Prozess und individuell aus den fachlich klassifizierten ILO der betreffenden Lerneinheit abgeleitet werden