Anzahl der Treffer: 828
Erstellt: Thu, 28 Mar 2024 23:06:10 +0100 in 0.0688 sec


Moller, Sebastian; Resagk, Christian; Cierpka, Christian
Long-time experimental investigation of turbulent superstructures in Rayleigh-Bénard convection by noninvasive simultaneous measurements of temperature and velocity fields. - In: Experiments in fluids, ISSN 1432-1114, Bd. 62 (2021), 4, 64, S. 1-18

Large-scale mean patterns in Rayleigh-Bénard convection, also referred to as turbulent superstructures, have mainly been studied by means of numerical simulations so far, but experimental investigations are still rare. However, the analysis of turbulent superstructures, which are of great importance due to their effect on the local transport of heat and momentum, require both numerical and experimental data. Therefore, within the scope of this study measurements were performed in the horizontal mid plane and in a horizontal plane closer to the top of a Rayleigh-Bénard cell with an aspect ratio of [Gamma]=l/h=25, thereby showing the initial formation of turbulent superstructures and their long-time rearrangement. The turbulent superstructures are investigated experimentally by noninvasive simultaneous measurements of temperature and velocity fields, using the color signal of thermochromic liquid crystals (TLCs) for the evaluation of the temperature and their temporal displacement for the determination of all three velocity components in the measurement planes via stereoscopic particle image velocimetry (stereo-PIV). Applying this measuring technique it is demonstrated that the time-averaging of instantaneous temperature and velocity fields uncovers the turbulent superstructures in both fields. Furthermore, the combination of the temperature and velocity data is used to characterize the local heat flux quantified by the local Nusselt number, which confirms that the turbulent superstructures strongly enhance the heat transfer in Rayleigh-Bénard convection.



https://doi.org/10.1007/s00348-020-03107-1
Angermeier, Sebastian;
Energy-efficient operation of vapor compression systems applied to the battery thermal management of electric buses. - Düren : Shaker Verlag, 2021. - XIV, 187 Seiten. - (Schriftenreihe des MAHLE Doktorandenprogramms ; Band 8)
Technische Universität Ilmenau, Dissertation 2021

ISBN 978-3-8440-8009-4

In dieser Arbeit wird eine energieeffiziente Betriebsstrategie eines Batterie-Thermomanagementsystems von Elektrobussen untersucht. Dabei wird sowohl die flüssigkeitsbasierte Batterietemperaturregelung als auch der ideale Betrieb von Dampfkompressionssystemen im Allgemeinen betrachtet. Um das thermische Verhalten der Batterie und die erforderliche Kühllast zu untersuchen, wird eine neuartige transiente Berechnungsmethode des thermischen Batteriesystems vorgeschlagen. Die Ergebnisse zeigen eine ausreichende Temperierung der Zelltemperatur über eine quasistationäre Regelung. Folglich kann die Batteriekühlanlage unter stationären Bedingungen betrieben werden, um die Anforderungen an die Batterietemperatur zu erfüllen. Zur Optimierung der Energieeffizienz der Batteriekühlanlage wird eine detaillierte theoretische und experimentelle Analyse des idealen stationären Betriebs der Kompressionskältemaschine (KKM) für eine Serienkühleinheit der Mahle GmbH durchgeführt. Basierend auf den Ergebnissen wird eine neue modellbasierte Sollwertoptimierungsmethode vorgeschlagen, um die KKM bei idealen Einstellungen zu betreiben. Für quasistationäre Anwendungen stellt die vorgeschlagene Methode eine vielversprechende Alternative zur Extremwertoptimierung oder anderen fortschrittlichen Methoden dar, die zwar genauer sind, aber eine hohe Konvergenzzeit benötigen.



Käufer, Theo; König, Jörg; Cierpka, Christian
Stereoscopic PIV measurements using low-cost action cameras. - In: Experiments in fluids, ISSN 1432-1114, Bd. 62 (2021), 3, 57, S. 1-16

Recently, large progress was made in the development towards low-cost PIV (Particle Image Velocimetry) for industrial and educational applications. This paper presents the use of two low-cost action cameras for stereoscopic planar PIV. A continuous wave laser or alternatively an LED was used for illumination and pulsed by a frequency generator. A slight detuning of the light pulsation and camera frame rate minimizes systematic errors by the rolling shutter effect and allows for the synchronization of both cameras by postprocessing without the need of hardware synchronization. The setup was successfully qualified on a rotating particle pattern in a planar and stereoscopic configuration as well as on the jet of an aquarium pump. Since action cameras are intended to be used at outdoor activities, they are small, very robust and work autarkic. In conjunction with the synchronization and image pre-processing scheme presented herein, those cameras enable stereoscopic PIV in harsh environments and even on moving experiments.



https://doi.org/10.1007/s00348-020-03110-6
Sachs, Sebastian; Fröhlich, Thomas; Schumacher, Jörg
Suppression of free convection effects for spherical 1 kg mass prototype. - In: International journal of heat and mass transfer, ISSN 1879-2189, Bd. 170 (2021), 121037, insges. 13 S.

We investigate the free convection processes in the vicinity of a spherical 1 kg mass standard by two- and three-dimensional direct numerical simulations using a spectral element method. Our focus is on the determination and suppression of updraft forces in a high-precision mass comparator which are caused by temperature differences between mass standard and its environment in the millikelvin range - a source of systematic uncertainties in the high-precison mass determination. A two-dimensional model is presented first, which obtains a good agreement with previous laboratory measurements for the smaller temperature differences up to 15 mK. The influence of different boundary conditions and side lengths of the square domain is discussed for the mass standard positioned in the center of the chamber. The complexity is increased subsequently in configurations with additional built-ins for counter heating in form of planar plates or hemispherical shells above the mass standard. The latter ones lead to a full compensation of the updraft force. Three-dimensional simulations in a closed cubic chamber confirm the two-dimensional findings and additionally reveal complex secondary flow patterns in the vicinity of the mass standard. The reduction of the heat transfer due to the built-ins is also demonstrated by a comparison of the Nusselt numbers as a function of the Rayleigh number in the chosen parameter range. Our simulations suggest that such additional constructive measures can enhance the precision of the mass determination by suppression of free convection and related systematic uncertainties.



https://doi.org/10.1016/j.ijheatmasstransfer.2021.121037
Pyatnitskaya, Natalia; Luchinkin, Nikita; Belyaev, Ivan; Kolesnikov, Yuri; Krasnov, Dmitry; Listratov, Yaroslav; Zikanov, Oleg; Sviridov, Evgeniy
Liquid metal flat jet transformation under influence of transverse magnetic field. - In: ICNRP Volga 2020, (2020), 012002, insges. 7 S.

The article is devoted to an experimental study of a submerged flat jet flow in a transverse magnetic field. Two different approaches to the experimental study of jet flows are described. Detailed information about the experimental program and measuring methods presented here. The flow of a flat jet 6 mm high in a square channel with a side of 56 mm is considered. The channel is positioned so that the plane of the jet is perpendicular to the magnetic field induction. The results of measuring velocity profiles and waveforms by swivel-type probe with potential sensor are presented. Effects that can be interpreted in different ways are found: strongly unstationary flow regimes, mean flow reorganization, and development of near-wall jets. Additional experiments are prepared to obtain more detailed information about the restructuring and development of the jet. In particular, continuous measurements along the channel will be made in the presence of a slight main flow.



https://doi.org/10.1088/1742-6596/1689/1/012002
Angermeier, Sebastian; Karcher, Christian
Model-based condenser fan speed optimization of vapor compression systems. - In: Energies, ISSN 1996-1073, Bd. 13 (2020), 22, 6012, S. 1-26

Vapor compression systems (VCS) cover a wide range of applications and consume large amounts of energy. In this context, previous research identified the optimization of the condenser fans speed as a promising measure to improve the energy efficiency of VCS. The present paper introduces a steady-state modeling approach of an air-cooled VCS to predict the ideal condenser fan speed. The model consists of a hybrid characterization of the main components of a VCS and the optimization problem is formulated as minimizing the total energy consumption by respectively adjusting the condenser fan and compressor speed. In contrast to optimization strategies found in the literature, the proposed model does not relay on algorithms, but provides a single optimization term to predict the ideal fan speed. A detailed experimental validation demonstrates the feasibility of the model approach and further suggests that the ideal condenser fan speed can be calculated with sufficient precision, assuming constant evaporating pressure, compressor efficiency, subcooling, and superheating, respectively. In addition, a control strategy based on the developed model is presented, which is able to drive the VCS to its optimal operation. Therefore, the study provides a crucial input for set-point optimization and steady-state modeling of air-cooled vapor compression systems.



https://doi.org/10.3390/en13226012
Krasnov, Dmitry; Kolesnikov, Yuri; Belyaev, Ivan A.; Listratov, Yaroslav I.; Zikanov, Oleg
Liquid metal swirling flow affected by transverse magnetic field. - In: Magnetohydrodynamics, Bd. 56 (2020), 2/3, S. 121-129

In this work we study numerically liquid metal flow in a squareduct under the influence ofa transverse magnetic field applied in a spanwise direction (coplanar). The key interest of thepresent study is an attempt of passive control of flow regimesdeveloped under magnetic fieldand thermal loads by applying specially shaped conditions,such as swirling, at the duct inlet.In this paper, we report results of numerical simulations ofthe interaction of swirling flow andtransverse magnetic field in a square duct flow. Analysis of the obtained regimes might beimportant for the development of an experimental setup, in order to design corresponding inletsections.



https://doi.org/10.22364/mhd.56.2-3.3
Leng, Xueyuan; Kolesnikov, Yuri; Krasnov, Dmitry; Li, Benwen
MHD Taylor-Couette flow with insulating walls at periodic condition and low magnetic Reynolds number. - In: Magnetohydrodynamics, Bd. 56 (2020), 2/3, S. 103-112

This work studies turbulent behavior in Taylor-Couette flow of an electrically conducting fluid between two co-axial and infinitely long insulating cylinders in the presence of an axial magnetic field at a low magnetic Reynolds number. The inner cylinder rotates and the outer one is kept stationary. Direct numerical simulation was conducted to study the problem with Reynolds numbers of 4000 and 8000 with different Hartmann numbers. The results show a continuous suppression of turbulence in the flow under the applied magnetic field. The mean flow profile is not directly affected by the magnetic field, but its transformation depends on the decrease of turbulent fluctuations and wall normal momentum transport. With increasing Hartmann number, the observed decrease of Taylor vortex flow is accompanied by the elongated axial wavelengths, confirming the theoretical prediction of linear stability theory. A comparison of the considered case of insulating cylinders with a previous study with conducting cylinders also indicates a difference between these two cases and highlights a significant impact of the electric boundary conditions on turbulence.



https://doi.org/10.22364/mhd.56.2-3.1
Angermeier, Sebastian; Ketterer, Jonas; Karcher, Christian
Liquid-based battery temperature control of electric buses. - In: Energies, ISSN 1996-1073, Bd. 13 (2020), 19, 4990, S. 1-20

Previous research identified that battery temperature control is critical to the safety, lifetime, and performance of electric vehicles. In this paper, the liquid-based battery temperature control of electric buses is investigated subject to heat transfer behavior and control strategy. Therefore, a new transient calculation method is proposed to simulate the thermal behavior of a coolant-cooled battery system. The method is based on the system identification technique and combines the advantage of low computational effort and high accuracy. In detail, four transfer functions are extracted by a thermo-hydraulic 3D simulation model comprising 12 prismatic lithium nickel manganese cobalt oxide (NMC) cells, housing, arrestors, and a cooling plate. The transfer functions describe the relationship between heat generation, cell temperature, and coolant temperature. A vehicle model calculates the power consumption of an electric bus and thus provides the input for the transient calculation. Furthermore, a cell temperature control strategy is developed with respect to the constraints of a refrigerant-based battery cooling unit. The data obtained from the simulation demonstrate the high thermal inertia of the system and suggest sufficient control of the battery temperature using a quasi-stationary cooling strategy. Thereby, the study reveals a crucial design input for battery cooling systems in terms of heat transfer behavior and control strategy.



https://doi.org/10.3390/en13194990
Belyaev, Ivan; Krasnov, Dmitry; Kolesnikov, Yuri; Biryukov, Dmitry; Chernysh, Denis; Zikanov, Oleg; Listratov, Yaroslav
Effects of symmetry on magnetohydrodynamic mixed convection flow in a vertical duct. - In: Physics of fluids, ISSN 1089-7666, Bd. 32 (2020), 9, 094106, S. 094106-1-094106-21

Magnetohydrodynamic convection in a downward flow of liquid metal in a vertical duct is investigated experimentally and numerically. It is known from earlier studies that in a certain range of parameters, the flow exhibits high-amplitude pulsations of temperature in the form of isolated bursts or quasi-regular fluctuations. This study extends the analysis while focusing on the effects of symmetry introduced by two-sided rather than one-sided wall heating. It is found that the temperature pulsations are robust physical phenomena appearing for both types of heating and various inlet conditions. At the same time, the properties, typical amplitude, and range of existence in the parametric space are very different at the symmetric and asymmetric heating. The obtained data show good agreement between computations and experiments and allow us to explain the physical mechanisms causing the pulsation behavior.



https://doi.org/10.1063/5.0020608