@article{DresiaKurudzijaDeekenetal.2023,
  author    = {Dresia, Kai and Kurudzija, Eldin and Deeken, Jan and Waxenegger-Wilfing, G{\"u}nther},
  title     = {Improved wall temperature prediction for the LUMEN rocket combustion chamber with neural networks},
  series = {Aerospace},
  volume    = {10},
  journal   = {Aerospace},
  number    = {5},
  issn      = {2226-4310},
  doi       = {10.3390/aerospace10050450},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-319169},
  year      = {2023},
  abstract  = {Accurate calculations of the heat transfer and the resulting maximum wall temperature are essential for the optimal design of reliable and efficient regenerative cooling systems. However, predicting the heat transfer of supercritical methane flowing in cooling channels of a regeneratively cooled rocket combustor presents a significant challenge. High-fidelity CFD calculations provide sufficient accuracy but are computationally too expensive to be used within elaborate design optimization routines. In a previous work it has been shown that a surrogate model based on neural networks is able to predict the maximum wall temperature along straight cooling channels with convincing precision when trained with data from CFD simulations for simple cooling channel segments. In this paper, the methodology is extended to cooling channels with curvature. The predictions of the extended model are tested against CFD simulations with different boundary conditions for the representative LUMEN combustor contour with varying geometries and heat flux densities. The high accuracy of the extended model's predictions, suggests that it will be a valuable tool for designing and analyzing regenerative cooling systems with greater efficiency and effectiveness.},
  language  = {en}
}
@phdthesis{Brendel2018,
  author    = {Brendel, Harald},
  title     = {W{\"a}rmetransport in keramischen Faserisolationen bei hohen Temperaturen},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-157917},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2018},
  abstract  = {Das Ziel dieser Arbeit ist eine umfassende numerische und experimentelle Charakterisierung des W{\"a}rmetransports in oxidkeramischen Faserisolationen im Hochtemperaturbereich. Zugleich sollen neue Konzepte f{\"u}r eine optimierte technische Auslegung von Faserisolationen erarbeitet werden. Oxidkeramiken zeigen im Infrarotbereich ein semitransparentes Verhalten. Das bedeutet, ein Teil der Strahlung gelangt durch die Probe, ohne gestreut oder absorbiert zu werden. Durch die Ausgestaltung als disperses Medium mit Abmessungen der Fasern im \$\mu m\$ Bereich wird jedoch eine starke Wechselwirkung mit infraroter Lichtstrahlung erzeugt. Man befindet sich im optischen Resonanzbereich. Technisch relevante Faserisolationen besitzen eine Rohdichte zwischen \$50 \mathrm{kg/m^3}\$ und \$700 \mathrm{kg/m^3}\$ und k{\"o}nnen als optisch dichtes Medium betrachtet werden. Eine Optimierung hinsichtlich der D{\"a}mmwirkung gegen W{\"a}rmestrahlung bedeutet eine massenspezifische Maximierung des Lichtstreuverm{\"o}gens im relevanten Wellenl{\"a}ngenbereich. Hierzu werden in einer numerischen Studie keramische Hohlfaserisolationen mit konventionellen Fasern verglichen. Diese Abhandlung unter Ber{\"u}cksichtigung anwendungsnaher Aspekte gelangt zu der Schlussfolgerung, dass die Strahlungsw{\"a}rmestromdichte in Hohlfaserisolationen, im Vergleich zu konventionellen Isolationen, signifikant erniedrigt wird. Hinsichtlich der Gesamtw{\"a}rmeleitf{\"a}higkeit ist eine Reduzierung um den Faktor zwei zu erwarten. \\ Trotz moderner Rechner ist die Anwendung der vollen Maxwellschen Streutheorie, insbesondere im Rahmen von Optimierungsaufgaben mehrschichtiger Streuk{\"o}rper, ein zeitaufwendiges Unterfangen. Um sinnvolle Parameterkonfigurationen bereichsweise eingrenzen zu k{\"o}nnen, wird eine N{\"a}herungsmethode f{\"u}r die Lichtstreuung an mehrschichtigen Zylindern weiterentwickelt und mit der vollst{\"a}ndigen Maxwellschen Streutheorie verglichen. Es zeigt sich, dass das Modell f{\"u}r kleine bis moderate Brechungsindizes sehr gute Vorhersagekraft besitzt und auch zur n{\"a}herungsweisen Berechnung der Streueffizienzen f{\"u}r r{\"a}umlich isotrop angeordnete Zylinder herangezogen werden kann. \\ Neben den numerischen Studien wird im experimentellen Teil dieser Arbeit eine kommerzielle Faserisolierung aus Aluminiumoxid hinsichtlich ihrer W{\"a}rmetransporteigenschaften charakterisiert. Die optischen Transportparameter Albedo und Extinktion werden mittels etablierter Messmethoden bestimmt. Bei bekannter Faserdurchmesserverteilung k{\"o}nnen diese Messwerte dann mit den theoretischen Vorhersagen der Maxwellschen Streutheorie verglichen werden.\\ Um technische Optimierungsmaßnahmen experimentell zu verifizieren, besteht die Notwendigkeit, die Temperaturleitf{\"a}higkeit bzw. die W{\"a}rmeleitf{\"a}higkeit auch bei hohen Temperaturen oberhalb von \$1000^\mathrm{o}\mathrm{C}\$ zuverl{\"a}ssig bestimmen zu k{\"o}nnen. Zu diesem Zweck wird ein Versuchsaufbau realisiert, um in diesem Temperaturbereich erstmals die sogenannte Thermal-Wave-Analyse anzuwenden. Durch Abgleich mit einem gekoppelten W{\"a}rmetransportmodell und einem etablierten Messverfahren wird die besondere Eignung der Thermal-Wave-Analyse f{\"u}r ber{\"u}hrungsfreie Hochtemperaturmessungen gezeigt.},
  subject      = {W{\"a}rme{\"u}bertragung},
  language  = {de}
}
@article{SpitznagelDurigZimanowski2013,
  author    = {Spitznagel, N. and Durig, T. and Zimanowski, B.},
  title     = {Trigger - and heat-transfer times measured during experimental molten-fuel-interactions},
  series = {AIP Advances},
  volume    = {3},
  journal   = {AIP Advances},
  number    = {102126},
  doi       = {10.1063/1.4827023},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-128625},
  year      = {2013},
  abstract  = {A modified setup featuring high speed high resolution data and video recording was developed to obtain detailed information on trigger and heat transfer times during explosive molten fuel-coolant-interaction (MFCI). MFCI occurs predominantly in configurations where water is entrapped by hot melt. The setup was modified to allow direct observation of the trigger and explosion onset. In addition the influences of experimental control and data acquisition can now be more clearly distinguished from the pure phenomena. More precise experimental studies will facilitate the description of MFCI thermodynamics.},
  language  = {en}
}
@article{BollazziRoces2010,
  author    = {Bollazzi, Martin and Roces, Flavio},
  title     = {The thermoregulatory function of thatched nests in the South American grass-cutting ant, Acromyrmex heyeri},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-68225},
  year      = {2010},
  abstract  = {The construction of mound-shaped nests by ants is considered as a behavioral adaptation to low environmental temperatures, i.e., colonies achieve higher and more stables temperatures than those of the environment. Besides the well-known nests of boreal Formica wood-ants, several species of South American leaf-cutting ants of the genus Acromyrmex construct thatched nests. Acromyrmex workers import plant fragments as building material, and arrange them so as to form a thatch covering a central chamber, where the fungus garden is located. Thus, the degree of thermoregulation attained by the fungus garden inside the thatched nest largely depends on how the thatch affects the thermal relations between the fungus and the environment. This work was aimed at studying the thermoregulatory function of the thatched nests built by the grass-cutting ant Acromyrmex heyeri Forel (Hymenoptera: Formicidae: Myrmicinae). Nest and environmental temperatures were measured as a function of solar radiation on the long-term. The thermal diffusivity of the nest thatch was measured and compared to that of the surrounding soil, in order to assess the influence of the building material on the nest's thermoregulatory ability. The results showed that the average core temperature of thatched nests was higher than that of the environment, but remained below values harmful for the fungus. This thermoregulation was brought about by the low thermal diffusivity of the nest thatch built by workers with plant fragments, instead of the readily-available soil particles that have a higher thermal diffusivity. The thatch prevented diurnal nest overheating by the incoming solar radiation, and avoided losses of the accumulated daily heat into the cold air during the night. The adaptive value of thatching behavior in Acromyrmex leaf-cutting ants occurring in the southernmost distribution range is discussed.},
  subject      = {Acromyrmex heyeri},
  language  = {en}
}
@phdthesis{Scheibner2007,
  author    = {Scheibner, Ralf},
  title     = {Thermoelectric Properties of Few-Electron Quantum Dots},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-26699},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2007},
  abstract  = {This thesis presents an experimental study of the thermoelectrical properties of semiconductor quantum dots (QD). The measurements give information about the interplay between first order tunneling and macroscopic quantum tunneling transport effects in the presence of thermal gradients by the direct comparison of the thermoelectric response and the energy spectrum of the QD. The aim of the thesis is to contribute to the understanding of the charge and spin transport in few-electron quantum dots with respect to potential applications in future quantum computing devices. It also gives new insight into the field of low temperature thermoelectricity. The investigated QDs were defined electrostatically in a two dimensional electron gas (2DEG) formed with a GaAs/(Al,Ga)As heterostructure by means of metallic gate electrodes on top of the heterostructure. Negative voltages with respect to the potential of the 2DEG applied to the gate electrodes were used to deplete the electron gas below them and to form an isolated island of electron gas in the 2DEG which contains a few ten electrons. This QD was electrically connected to the 2DEG via two tunneling barriers. A special electron heating technique was used to create a temperature difference between the two connecting reservoirs across the QD. The resulting thermoelectric voltage was used to study the charge and spin transport processes with respect to the discrete energy spectrum and the magnetic properties of the QD. Such a two dimensional island usually exhibits a discrete energy spectrum, which is comparable to that of atoms. At temperatures below a few degrees Kelvin, the electrostatic charging energy of the QDs exceeds the thermal activation energy of the electrons in the leads, and the transport of electrons through the QD is dominated by electron-electron interaction effects. The measurements clarify the overall line shape of thermopower oscillations and the observed fine structure as well as additional spin effects in the thermoelectrical transport. The observations demonstrate that it is possible to control and optimize the strength and direction of the electronic heat flow on the scale of a single impurity and create spin-correlated thermoelectric transport in nanostructures, where the experimenter has a close control of the exact transport conditions. The results support the assumption that the performance of thermoelectric devices can be enhanced by the adjustment of the QD energy levels and by exploiting the properties of the spin-correlated charge transport via localized, spin-degenerate impurity states. Within this context, spin entropy has been identified as a driving force for the thermoelectric transport in the spin-correlated transport regime in addition to the kinetic contributions. Fundamental considerations, which are based on simple model assumptions, suggest that spin entropy plays an important role in the presence of charge valence fluctuations in the QD. The presented model gives an adequate starting point for future quantitative analysis of the thermoelectricity in the spin-correlated transport regime. These future studies might cover the physics in the limit of single electron QDs or the physics of more complex structures such as QD molecules as well as QD chains. In particular, it should be noted that the experimental investigations of the thermopower of few-electron QDs address questions concerning the entropy transport and entropy production with respect to single-bit information processing operations. These questions are of fundamental physical interest due to their close connection to the problem of minimal energy requirements in communication, and thus ultimately to the so called "Maxwell's demon" with respect to the second law of thermodynamics.},
  subject      = {Quantenpunkt},
  language  = {en}
}