@misc{DurstZanoun, author = {Durst, Franz and Zanoun, El-Sayed}, title = {Laminar pipe flow instability : a theoretical-experimental perspective}, series = {Fluids}, volume = {10}, journal = {Fluids}, number = {8}, publisher = {MDPI}, address = {Basel}, issn = {2311-5521}, doi = {10.3390/fluids10080216}, pages = {1 -- 21}, abstract = {This paper revisits the theoretically predicted inherent stability of fully developed laminar pipe flow, which remains unconfirmed by experimental evidence. A recently developed theory of pipe-flow stability/instability addresses the gap between experimental observations and classical theoretical predictions by accounting for a parallel secondary flow through the pipe's roughness layer that accompanies the main stream. This secondary flow alters the near-wall velocity profile in the rough-wall region, creating an inflection point that promotes shear-driven instabilities and triggers the laminar-to-turbulent transition. A stability factor 𝑆=𝐷𝑐/𝐷 is introduced, where D is the nominal pipe diameter and 𝐷𝑐 refers to the critical pipe diameter. The pipe flow remains laminar and stable for 𝑆>1.0, and becomes unstable for 𝑆<1. Various experimental findings are theoretically derived, and the laminar-to-turbulent transition is identified at 𝑆=1.0. Particular attention is paid to the dependence of flow transition on both pipe diameter and pipe length. Rather than relying on a critical Reynolds number 𝑅𝑒𝑐, this study proposes the critical pipe diameter 𝐷𝑐 as the key parameter governing the laminar pipe flow instability, where 𝑅𝑒𝑐 refers here to the condition-dependent threshold at which laminar pipe flow becomes unstable and transition to turbulence occurs. The present analysis further suggests that instability arises only if the pipe length L exceeds a critical threshold 𝐿𝑐, that is, 𝐿>𝐿𝑐. The theoretical treatment presented provides deeper physical insights into the onset of laminar pipe flow instability including the phenomenon of reverse transition. It also distinguishes between natural and forced flow transitions, providing a refined understanding of the transition process. Finally, suggestions for future experimental work are made to further validate or challenge this new theoretical perspective on pipe flow instability.}, language = {en} } @misc{ZanounBauerWagneretal., author = {Zanoun, El-Sayed and Bauer, Christian and Wagner, Claus and Durst, Franz and Egbers, Christoph and Bellani, Gabriele and Talamelli, Alessandro}, title = {Cross-validation of numerical and experimental data in turbulent pipe flow with new scaling correlations}, series = {Journal of turbulence}, journal = {Journal of turbulence}, publisher = {Taylor \& Francis}, address = {London}, doi = {10.1080/14685248.2025.2560314}, pages = {1 -- 22}, abstract = {The dependence of turbulence statistics and wall friction on Reynolds number in fully developed turbulent pipe flow remains a fundamental subject in fluid mechanics. This paper cross-validates experimental and numerical results, focusing on the scaling of turbulence statistics at the pipe centerline and across the inner-outer flow region. Pipe flow experiments were reviewed for friction Reynolds numbers 810≀Reπœβ‰€55Γ—103, where Re𝜏=π‘’πœβ’π‘…/𝜈, π‘’πœ is the wall friction velocity, 𝑅 the pipe radius, and 𝜈 the kinematic viscosity. Complementary DNS data for 180≀Reπœβ‰€2880 provide detailed insight into near-wall turbulence. A novel friction correlation, Re𝜏=0.048Re0.923 c is introduced, predicting pipe-wall friction across a wide range of Re𝑐 with accuracy better than Β±2.06\%, where Re𝑐 is the Reynolds number based on the centerline streamwise mean velocity component π‘ˆzc. This correlation enables reliable friction estimates from centerline single-point measurements or DNS data without requiring near-wall or streamwise pressure-gradient information and is validated by consistent agreement with both experiments and DNS. The monotonic decrease in centerline turbulence intensity βŸ¨π‘’β€²π‘§2⟩1/2/π‘ˆzc with increasing Re𝑐 is explained using the streamwise mean momentum equation. Finally, azimuthal spatial filtering of DNS data highlights the limitations of hot-wire resolution near the wall. For Re𝜏β‰₯2880, higher-order experimental statistics agree well with DNS for 𝑦+β‰₯30 and into the logarithmic region, with both datasets equally well described by logarithmic or power-law correlations, while near-wall discrepancies remain due to resolution limits.}, language = {en} } @misc{HamedeSliavinMotuzetal., author = {Hamede, Mohammed Hussein and Sliavin, Yaraslau and Motuz, Vasyl and Egbers, Christoph}, title = {The effect of flow initial conditions and geometry on the saturation of the thermo-electrohydrodynamic instability within microgravity conditions}, series = {Physics of fluids}, volume = {37}, journal = {Physics of fluids}, number = {9}, publisher = {AIP Publishing}, address = {College Park, Maryland}, doi = {10.1063/5.0287547}, pages = {1 -- 10}, abstract = {The present study investigates the thermo-electrohydrodynamic-induced convection in a dielectric fluid confined between two concentric differentially heated cylinders under microgravity conditions through experimental means. The experiments were conducted during parabolic flight campaigns. In the present study, our primary focus was on the saturation of perturbations induced by thermo-electrohydrodynamic instability. The effect of initial flow conditions on perturbation growth rates is examined when entering the microgravity phase. Furthermore, a series of experimental setups with varied lengths and aspect ratios were considered to investigate the impact of geometry on flow behavior. The present study quantitatively investigated the flow using particle image velocimetry in the meridional and radial-azimuthal planes. Furthermore, direct measurements were taken of the global response of the flow, in this case heat transfer, using a heat flux sensor attached to the outer cylinder wall.}, 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 300Β°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 150Β°C. In order to overcome the technological challenges to reach 300Β°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 200Β°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 Β°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 Β°C and 300 Β°C and a heat source of -10 Β°C, the potential of different cycles were analyzed by varying the secondary inlet temperature of the heat sink from 100 Β°C to 200 Β°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 Β°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 170Β°C with a coefficient of performance (COP) of 1.39 using air at TIII = 15 Β°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{KabatJendeYueceletal., author = {Kabat, Nancy and Jende, Enrico and Y{\"u}cel, Fatma Cansu and Stathopoulos, Panagiotis}, title = {Thermodynamic analysis of a novel high-temperature heat pump cycle configuration based on the reversed Brayton cycle}, series = {International journal of sustainable energy}, volume = {44}, journal = {International journal of sustainable energy}, number = {1}, publisher = {Taylor \& Francis}, address = {Abingdon, Oxon}, issn = {1478-6451}, pages = {1 -- 37}, abstract = {High-temperature heat pumps offer a promising approach to deliver CO\$_2\$-neutral process heat by upgrading waste heat with electricity from renewable resources. In this work, a heat pump configuration including multiple heat sinks and sources is presented and compared to the recuperated Brayton cycle. An exergy analysis is conducted by using numerical simulations to provide process heat at 250Β°C and process cooling at -30Β°C simultaneously. The results show, that a multi-stage compression leads to an improved overall performance for simultaneous heating and cooling. A parametric study presents the influence of the isentropic efficiency, heat exchanger effectiveness and heat sink and source inlet temperature on the second law efficiency. This study serves as a basis to gain a better understanding on the physics of the novel cycle, highlighting the potentials of improving the efficiency of Brayton heat pumps with cycle layouts adaptation and identify the most important components for minimizing exergy losses.}, language = {en} } @misc{TranStathopoulos, author = {Tran, A. Phong and Stathopoulos, Panagiotis}, title = {Dynamic simulation and experimental validation of a high-temperature Brayton heat pump}, series = {Applied thermal engineering}, volume = {274, Part B}, journal = {Applied thermal engineering}, publisher = {Elsevier BV}, address = {Amsterdam}, issn = {1359-4311}, doi = {10.1016/j.applthermaleng.2025.126536}, pages = {1 -- 13}, abstract = {The decarbonization of industrial process heating will require widespread adoption of high-temperature heat pumps. Brayton cycle heat pumps are capable of providing heat at temperatures that currently cannot be achieved by conventional vapor-compression cycle heat pumps. However, significant challenges remain in adapting these systems to industrial applications, particularly with regard to operational safety, control strategies, and flexibility in response to varying operational conditions. This study presents a dynamic model of a closed-loop Brayton cycle heat pump capable of producing temperatures of 250 Β°C and higher, validated using experimental data. The physics-based model implemented in Modelica captures key thermodynamic processes and system dynamics, including thermal inertia and volume dynamics. An optimization-based method is used to calibrate model parameters, minimizing the error between measured and simulated data. Given the significant impact of the compressor on overall heat pump performance, a novel calibration method is introduced to adjust an existing compressor map using limited measurement data. This approach ensures that the compressor behavior is represented with sufficient accuracy, smoothness, and numerical robustness. The calibrated model achieves mean-normalized root mean squared errors (NRMSE) ranging from 0.12 \% to 1.46 \% for temperatures, pressures, and mass flow rates. The model is applied to examine the system's start-up and deceleration sequences, offering insights into compressor stability and heat exchanger temperature profiles. These results demonstrate the model's utility for control design, performance evaluation, and stability analysis.}, language = {en} } @misc{SchaffrathStathopoulosSchmitzetal., author = {Schaffrath, Robert and Stathopoulos, Panagiotis and Schmitz, Andreas and Nicke, Eberhard}, title = {Multistage turbomachinery optimization for high-temperature heat pumps with the reverse rankine cycle}, series = {Journal of turbomachinery}, volume = {147}, journal = {Journal of turbomachinery}, number = {111003}, publisher = {ASME}, address = {New York, NY}, issn = {1528-8900}, doi = {10.1115/1.4068480}, pages = {111003-1 -- 111003-12}, abstract = {The electrification of process heat generation will be a key to achieving carbon neutrality in the coming decades. One of the most promising approaches is to replace conventional heat supply systems with high-temperature heat pumps (HTHPs). A promising heat pump concept is based on the reverse Rankine cycle that uses water as its working fluid. By using turbomachinery for the compression process in this cycle, the performance of the HTHP can be increased compared to the volumetric displacement systems, like screw or piston compressors. Although the design of the compressor geometry can be done sequentially in relation to the HTHP cycle design, better results can be obtained by an approach that integrates turbomachinery and the thermodynamic cycle design. Against this background, an automated optimization method for a reverse Rankine HTHP with two radial turbo-compressors in series is presented. In contrast to the current state of the art, the presented novel optimization approach uses 3D computational fluid dynamics data to calculate the compressor's performance. Furthermore, the integration of low-fidelity compressor specific reduced-order models are used to accelerate the gradient-free optimization process by a CO-Kriging surrogate model. The advantages of the novel approach are justified by comparing the numerical effort and the final values of the optimization objectives.}, 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} }