@misc{BorciaBorciaBestehornetal., author = {Borcia, Ion-Dan and Borcia, Rodica and Bestehorn, Michael and Richter, Stefan and Xu, Wenchao and Harlander, Uwe}, title = {Horizontal Faraday instability and parametric excitation in a circular channel}, series = {GAMM 2019, 90th Annual Meeting of the International Assoociation of Applied Mathematics and Mechanics, February 18-22, 2019, Vienna, Austria}, journal = {GAMM 2019, 90th Annual Meeting of the International Assoociation of Applied Mathematics and Mechanics, February 18-22, 2019, Vienna, Austria}, publisher = {TU Verlag}, address = {Wien}, isbn = {978-3-903024-84-7}, pages = {338}, language = {en} } @misc{NowakMerboldEgbersetal., author = {Nowak, Torsten and Merbold, Sebastian and Egbers, Christoph and Schacht, Ralph}, title = {Experimentelle Studie mit numerischer Validierung von thermi-schen Kopplungs-Ph{\"a}nomenen mit in Flip-Chip-Technologie aufge-bauten Testchips auf Leiterplatten}, series = {Experimentelle Str{\"o}mungsmechanik : 27. Fachtagung 3.-5. September 2019 Erlangen}, journal = {Experimentelle Str{\"o}mungsmechanik : 27. Fachtagung 3.-5. September 2019 Erlangen}, editor = {Delgado, Antonio}, publisher = {Deutsche Gesellschaft f{\"u}r Laser-Anemometrie - German Association for Laser Anemometry GALA e.V.}, address = {Karlsruhe}, isbn = {978-3-9816764-6-4}, pages = {27.1 -- 27.8}, abstract = {In aktuellen mikro-elektronischen Systemen m{\"u}ssen verschiedene aktive und passive Bauteile auf engstem Raum miteinander auf dem Substrate (bspw. FR4-Leiterplatte) kombiniert werden. Die dabei stetig steigende Leistungsdichte bedarf einer optimierten K{\"u}hlstrategie und der Definition von konduktiven und konvektiven Entw{\"a}rmungspfade, die bereits w{\"a}hrend der Layout-Designphase der Baugruppe ber{\"u}cksichtigt werden sollten. Str{\"o}mungsmechanische Simulationen (CFD) k{\"o}nnen dabei ein erstes Abbild zur Analyse von Kopplungseffekten zwischen den W{\"a}rmequellen und der Umgebung liefern. Diese sind jedoch oft zeitaufwendig und es bedarf zumindest ein gewisses Grundverst{\"a}ndnis, um die Fluid-mechanischen Parameter, das Vernetzen der Geometrien und die gewonnenen Ergebnisse korrekt zu interpretieren. Erste analytische Ans{\"a}tze zur reduzierten Beschreibung von ungeh{\"a}usten Chips auf Leiterplattensubstrat k{\"o}nnen durch thermische Widerstandsnetzwerke abgebildet und durch Aufteilung der W{\"a}rmestr{\"o}me in Richtung Substrat (W{\"a}rmeleitung) und in Richtung des bewegten Fluides (Konvektion) n{\"a}her beschrieben werden.}, language = {de} } @misc{SzaboZaussingerHaunetal., author = {Szabo, Peter and Zaussinger, Florian and Haun, Peter and Travnikov, Vadim and Meier, Martin and Egbers, Christoph}, title = {Complementary numerical and experimental study in the baroclinic annulus for the microgravity experiment AtmoFlow}, series = {EGU General Assembly 2020, EGU2020-20956, Online | 4-8 May 2020}, journal = {EGU General Assembly 2020, EGU2020-20956, Online | 4-8 May 2020}, doi = {10.5194/egusphere-egu2020-20956}, pages = {2}, language = {en} } @misc{HallolYousryMerboldetal., author = {Hallol, Zeinab and Yousry, Mohamed and Merbold, Sebastian and Egbers, Christoph}, title = {Time-resolved PIV measurements in fully developed turbulent pipe flow}, series = {25th International Congress of Theoretical and Applied Mechanics Milan (Italy), 22-27 August 2021}, journal = {25th International Congress of Theoretical and Applied Mechanics Milan (Italy), 22-27 August 2021}, isbn = {978-83-65550-31-6}, pages = {1374 -- 1375}, language = {en} } @misc{HasanuzzamanEgbers, author = {Hasanuzzaman, Gazi and Egbers, Christoph}, title = {Flow control of a turbulent boundary layer using uniform blowing}, series = {Progress in Turbulence X : Proceedings of the iTi Conference on Turbulence 2023}, journal = {Progress in Turbulence X : Proceedings of the iTi Conference on Turbulence 2023}, isbn = {978-3-031-55924-2}, doi = {10.1007/978-3-031-55924-2_36}, language = {en} } @misc{GaillardSzaboEgbers, author = {Gaillard, Yann and Szabo, Peter and Egbers, Christoph}, title = {AtmoFlow: Thermo-electrohydrodynamic convection in the thermally driven spherical shell with differential rotation}, series = {EGU General Assembly 2023}, journal = {EGU General Assembly 2023}, doi = {10.5194/egusphere-egu23-1841}, language = {en} } @misc{SimonYaraslauSzaboetal., author = {Simon, K{\"u}hne and Yaraslau, Sliavin and Szabo, Peter and Carter, R. and Krebs, Andreas and Egbers, Christoph}, title = {Simultaneous particle image velocimetry and background oriented schlieren measurements of convective flows}, series = {Experimentelle Str{\"o}mungsmechanik, Hrsg.: B. Ruck et al.}, journal = {Experimentelle Str{\"o}mungsmechanik, Hrsg.: B. Ruck et al.}, publisher = {German Association for Laser Anemometry 26.8 GALA e.V.}, address = {Karlsruhe}, isbn = {978-3-9816764-4-0}, language = {en} } @misc{KuehneSliavinSzaboetal., author = {K{\"u}hne, Simon and Sliavin, Yaraslau and Szabo, Peter and Carter, Richard and Krebs, Andreas and Egbers, Christoph}, title = {Phase-demodulation of interferograms of thermo-electrohydrodynamic convection in a differentially heated cylindrical annulus}, series = {Experimentelle Str{\"o}mungsmechanik : 31. Fachtagung, 3.-5. September 2024, Berlin / Herausgeber: C. Rauh, B. Ruck, A. Leder}, journal = {Experimentelle Str{\"o}mungsmechanik : 31. Fachtagung, 3.-5. September 2024, Berlin / Herausgeber: C. Rauh, B. Ruck, A. Leder}, publisher = {German Association for Laser Anemometry 26.8 GALA e.V.}, address = {Karlsruhe}, isbn = {978-3-9816764-4-0}, pages = {7.1. -- 7.8}, language = {en} } @misc{KrieseLockanSchaffrathetal., author = {Kriese, Maximilian and Lockan, Michael and Schaffrath, Robert and Stathopoulos, Panagiotis}, title = {Quasi-static simulation of a start-up procedure for a multistage centrifugal compression system}, series = {Proceedings of ASME Turbo Expo 2025 : Turbomachinery Technical Conference and Exposition, June 16-20, 2025, Memphis, Tennessee : Volume 4 : Controls, Diagnostics \& Instrumentation; Cycle Innovations; Education; Electric Power}, journal = {Proceedings of ASME Turbo Expo 2025 : Turbomachinery Technical Conference and Exposition, June 16-20, 2025, Memphis, Tennessee : Volume 4 : Controls, Diagnostics \& Instrumentation; Cycle Innovations; Education; Electric Power}, publisher = {American Society of Mechanical Engineers}, address = {New York, NY}, isbn = {978-0-7918-8880-3}, doi = {10.1115/GT2025-151935}, pages = {12}, abstract = {In order to achieve the European Union's climate protection targets by 2050, energy use in the private and industrial sectors must be switched from using fossil fuels to renewable energy. For industry, this might require that the production processes itself needed to be changed or new technologies to manufacture in a sustainable way needed to be introduced. One example of the latter mentioned are high-temperature heat pumps (HTHP), which will be capable to generate sustainable process heat up to 300°C e.g. for the pulp and paper or food industry. There are currently only a few HTHPs commercially available that can reach temperatures above 150°C. In order to overcome the technological challenges to reach 300°C, two HTHP prototypes are currently developed at the Institute of Low-Carbon Industrial Processes of the German Aerospace Centre (DLR). One of them is the pilot plant ZiRa which is based on the reversed Rankine process and uses steam as its working medium. Herein three centrifugal compressors are applied to achieve a saturated steam temperature of 200°C at the sink. The safe speed-up of these three compressors, titled as the start-up procedure, from the standstill of each compressors up to their respective nominal speed, is modelled in the flow chart software EBSILON Professional. In addition to the compressors, all necessary components, i.e. the intercoolers, the fittings and valves, the pipes, the separators and the bypasses are modelled so that all parameters influencing the thermodynamic inlet state of the respective compressor are considered. The start-up of the multi-stage compression system of the pilot plan ZiRa is shown as a quasi-static time series. As a first step, the speed increments are defined. Afterwards, absolute pressures between 1.25 and 2 bar upstream of the first compressor and their effect on the operation of the subsequent compressor stages, are analyzed. Therefore, the mass flow rates and rotational speeds are specified manually at first so that an executable, editable model is created. Based on these results, the operation of the compressors are optimized in order to ensure a surge margin of 15 \% and to avoid choke in each operating point.}, language = {en} } @misc{VankelekomStathopoulosDemeyeretal., author = {Vankelekom, Christophe and Stathopoulos, Panagiotis and Demeyer, Frederiek and De Paepe, Ward}, title = {Thermodynamic performance assessment of recuperated Brayton cycle high-temperature heat pumps for combined heat and cold roduction}, series = {Proceedings of ASME Turbo Expo 2025 : Turbomachinery Technical Conference and Exposition, June 16-20, 2025, Memphis, Tennessee : Volume 4 : Controls, Diagnostics \& Instrumentation; Cycle Innovations; Education; Electric Power}, journal = {Proceedings of ASME Turbo Expo 2025 : Turbomachinery Technical Conference and Exposition, June 16-20, 2025, Memphis, Tennessee : Volume 4 : Controls, Diagnostics \& Instrumentation; Cycle Innovations; Education; Electric Power}, editor = {Vankelekom, Christophe}, publisher = {American Society of Mechanical Engineers}, address = {New York, NY}, isbn = {978-0-7918-8880-3}, doi = {10.1115/GT2025-153988}, pages = {14}, abstract = {Producing heat at high temperatures for industrial processes like chemistry, food, or paper is still achieved using fossil fuels. To reach net-zero CO2 emissions by 2050, as desired by the European Union, technology driven by renewable energy must emerge to electrify these sectors. For this reason, high-temperature heat pumps, based on a Reversed Brayton Cycle working with air, offer a promising solution. Indeed, nowadays, most high-temperature heat pumps are vapor compression cycles but they face several issues when a higher temperature is necessary (200 °C or more). The selection of refrigerant is challenging (high compression ratio and temperature limitation), and it has a non-negligible environmental impact. Nevertheless, the Reversed Brayton cycle still suffers from a low Coefficient Of Performance (COP) that could be improved by valorizing the cold heat flux produced in the cycle. In the literature, few works are related to this combined use of heating and cooling for sub-MWth applications, which is very important for the food industry (cooking and freezing). With this work, we aim to fill this gap by proposing an analysis of the achievable potential based on the 2nd law of thermodynamics using a generic model in Aspen Plus. Using a heat sink of 250 °C and 300 °C and a heat source of -10 °C, the potential of different cycles were analyzed by varying the secondary inlet temperature of the heat sink from 100 °C to 200 °C. For low inlet temperatures, the heating and total COP can reach a value of up to 1.3 and 1.66 respectively with an exergetic efficiency up to 54\%. The COP decreases when the secondary inlet temperature of the heat sink increases while the exergy efficiency stays almost constant. A further increase does not allow to provide cooling anymore for lower outlet heat sink temperature.}, language = {en} } @misc{OehlerYuecelStathopoulos, author = {Oehler, Johannes and Y{\"u}cel, Fatma Cansu and Stathopoulos, Panagiotis}, title = {Experimental performance analysis of a 50 kW Brayton turbomachine heat pump demonstrator}, series = {Proceedings of ASME Turbo Expo 2025 : Turbomachinery Technical Conference and Exposition, June 16-20, 2025, Memphis, Tennessee : Volume 4 : Controls, Diagnostics \& Instrumentation; Cycle Innovations; Education; Electric Power}, journal = {Proceedings of ASME Turbo Expo 2025 : Turbomachinery Technical Conference and Exposition, June 16-20, 2025, Memphis, Tennessee : Volume 4 : Controls, Diagnostics \& Instrumentation; Cycle Innovations; Education; Electric Power}, publisher = {American Society of Mechanical Engineers}, address = {New York, NY}, isbn = {978-0-7918-8880-3}, doi = {10.1115/GT2025-152419}, pages = {10}, abstract = {The high-temperature heat pump "CoBra" is the first demonstrator of a closed-loop Brayton heat pump realizing sink temperatures above 150 °C. The experimental results can proof the feasibility of a turbomachine-driven Brayton heat pump delivering heat and cold simultaneously. The German Aerospace Center (DLR) developed the Cottbus Brayton cycle heat pump "CoBra", a closed-loop Brayton heat pump utilizing dry air as the working medium. The CoBra is fully operational and has been used for experiments since spring 2024. This work evaluates the steady-state performance of the CoBra heat pump demonstrator experimentally for the first time. In the current setup, the heat pump achieves a heating capacity of up to 55 kW and can supply process heat up to 170°C with a coefficient of performance (COP) of 1.39 using air at TIII = 15 °C as sole heat source. The heat pump's operational envelope and performance in terms of supplied thermal power, supply temperature and efficiency is analyzed for different operating conditions. Increasing compressor speed raises both heating power and supply temperatures. Using recuperation raises both supply temperature and COP. Fluid inventory control is experimentally confirmed to be a promising control strategy for closed-loop Brayton machines. It widens the operational envelope towards part load operation at constant supply temperatures while maintaining constant efficiency. Operating points in the compressor map remain on the same operating line, thus compressor stability is not impacted by this control method.}, language = {en} } @misc{AbuKhassCristofaroKopparthyetal., author = {Abu Khass, Omar and Cristofaro, Marco and Kopparthy, Saketh Bharadwaj and Kl{\"o}ppel, Steffen and Nicke, Eberhard and Stathopoulos, Panagiotis}, title = {Numerical investigation of two-phase water ejectors for high-temperature heat pumps : insights into flow behavior and shock wave dynamics}, series = {Proceedings of ASME Turbo Expo 2025 : Turbomachinery Technical Conference and Exposition, June 16-20, 2025, Memphis, Tennessee : Volume 4 : Controls, Diagnostics \& Instrumentation; Cycle Innovations; Education; Electric Power}, journal = {Proceedings of ASME Turbo Expo 2025 : Turbomachinery Technical Conference and Exposition, June 16-20, 2025, Memphis, Tennessee : Volume 4 : Controls, Diagnostics \& Instrumentation; Cycle Innovations; Education; Electric Power}, publisher = {American Society of Mechanical Engineers}, address = {New York, NY}, isbn = {978-0-7918-8880-3}, doi = {10.1115/GT2025-151706}, pages = {11}, abstract = {Two-phase water ejectors can serve as a secondary steam compression mechanism in high-temperature heat pump (HTHP) systems. By integrating an ejector, high-pressure water can be combined with hot steam from the compressor, achieving simultaneous cooling and pressure increase. This integration offers the potential to reduce both the power and the number of stages required to obtain the specified compression. However, the complex flow behavior within two-phase water ejectors, especially under high-pressure, high-temperature conditions, remains insufficiently explored in the literature. This study addresses this gap by conducting a detailed numerical investigation of two-phase flow and shock wave behavior using the compressible two-phase mixture approach in computational fluid dynamics (CFD) commercial solver Ansys Fluent. The ejector geometry and critical operating points were derived from a prior study using one-dimensional (1D) analysis, which provided the design and operational conditions used in the current simulations. The ejector was modeled in two-dimensional (2D) axisymmetric configurations. The Lee model, in conjunction with the water saturation curve, was applied to capture non-equilibrium mass transfer processes, including evaporation and condensation. During the simulations, the presence of a liquid-vapor mixture may cause the speed of sound to drop, making the flow locally supersonic. Initial simulations revealed a series of shock waves in the mixing section, which elevated the mixture flow pressure to the designated set back-pressure value, thereby achieving an ejector pressure ratio of approximately 1.3. These findings provide crucial insights into how thermodynamic conditions influence two-phase flow behavior, particularly in the generation and intensity of shock waves. This work advances the modeling procedures for two-phase ejectors and enhances the understanding of the physical phenomena occurring within two-phase water ejectors designed for HTHP applications.}, language = {en} } @misc{KabatOehlerStathopoulos, author = {Kabat, Nancy and Oehler, Johannes and Stathopoulos, Panagiotis}, title = {Experimental exergy analysis of a high-temperature Brayton heat pump}, series = {Proceedings of ASME Turbo Expo 2025 : Turbomachinery Technical Conference and Exposition, June 16-20, 2025, Memphis, Tennessee : Volume 4 : Controls, Diagnostics \& Instrumentation; Cycle Innovations; Education; Electric Power}, journal = {Proceedings of ASME Turbo Expo 2025 : Turbomachinery Technical Conference and Exposition, June 16-20, 2025, Memphis, Tennessee : Volume 4 : Controls, Diagnostics \& Instrumentation; Cycle Innovations; Education; Electric Power}, publisher = {American Society of Mechanical Engineers}, address = {New York, NY}, isbn = {978-0-7918-8880-3}, doi = {10.1115/GT2025-153648}, pages = {16}, abstract = {High-temperature heat pumps (HTHP) are a promising technology to provide emission free process heat at high temperatures. HTHPs available on the market can provide process heat up to 150°C. However, many industrial processes, especially in the food industry, require higher temperatures, so the industrial sector remains one of the largest emitters of greenhouse gases due to the burning of fossil fuels such as coal, natural gas and oil. To advance the decarbonization of industry towards climate neutrality and drive forward the development of HTHPs for the provision of industrial process heat, challenges such as the temperature level to be achieved while maintaining high efficiency must be addressed to ensure applicability for the end user. Despite the knowledge of the relevance of heat pumps for the decarbonization of industry, there are only a few experimental heat pumps that work with high temperature lifts at sink temperatures above 150°C. This paper shows first experimental results of a HTHP demonstrator based on the reversed Brayton cycle with air as working medium for the simultaneous provision of process heat above 150°C and process cooling below 0°C. The design of the heat pump and the experimental test procedure are explained, followed by the execution of an exergy analysis to evaluate the efficiency of the system and the components, taking into account exergy losses in order to identify optimization potential. The HTHP demonstrator called "CoBra" at the Institute of Low-Carbon Industrial Processes of the German Aerospace Center (DLR) in Cottbus consists of two radial compressors, one turbine and three shell and tube heat exchangers. One of the heat exchangers is used as a recuperator. Dry ambient air is used as the working medium in the primary and secondary cycles. The tests are performed without and with recuperation reaching heat sink temperatures of 158°C and 168°C and heat source temperatures of 11°C and -3°C. A temperature lift of 130 K and 134 K is achieved and COPs of 1.37 and 1.4, respectively. The exergy analysis indicates the compressor to be the component with the highest optimization potential with a relative irreversibility of around 50\%. The high-temperature heat exchanger has the highest exergy efficiency of all components with around 75\%. The advantages of using a recuperator are confirmed by the increase in the overall heat pump performance. The present study provides new insights into the actual behavior of a heat pump that simultaneously provides process heat above 150 °C and process cooling below 0 °C, while also demonstrating the feasibility of such a system. This work contributes to the benefits and feasibility of Brayton heat pumps for industrial applications.}, language = {en} }