Numerical research for the effect of magnetic field on convective transport process of molten salt in Rayleigh-Bénard system. - In: International journal of thermal sciences, ISSN 1778-4166, Bd. 195 (2024), 108605, S. 1-21
The effects of external applied magnetic field on heat and momentum transfer of Rayleigh-Bénard convection in a closed cavity filled with electrically conductive molten salt are investigated by direct numerical simulation. Such arrangements are of strong interest in the context of thermal energy storage systems from renewable resources. To discretize the governing equations, the Chebyshev collocation spectral method is developed. A series of numerical results for 5000 ≤ Ra ≤ 10^6, 5 ≤ Pr ≤ 20 and 0 ≤ Ha ≤ 150 are obtained. First, we conduct two-dimensional numerical simulations to investigate the effect of Pr without and with magnetic field and find that Pr has little influence on heat and momentum transfer. Then, taking Pr as a fixed value of 7 and considering the effects of Ra and Ha, 2D and 3D direct numerical simulations are conducted. From both 2D and 3D numerical results, we conclude that, the heat and momentum transfer are enhanced with Ra at Ha = 0 and the fluid motion is stabilized by magnetic field at Ha 0. More phenomena of heat transfer and fluid flow, together with scaling correlations of Nu ∼ Ra, Nu ∼ Re for Rayleigh-Bénard convection without magnetic field, and, Nu ∼ RaHa and Re ∼ RaHa for Rayleigh-Bénard convection with magnetic field, are revealed under specified ranges of Ra and Ha.
Spatial prediction of the turbulent unsteady von Kármán vortex street using echo state networks. - In: Physics of fluids, ISSN 1089-7666, Bd. 35 (2023), 11, 115141, S. 115141-1-115141-15
The spatial prediction of the turbulent flow of the unsteady von Kármán vortex street behind a cylinder at Re = 1000 is studied. For this, an echo state network (ESN) with 6000 neurons was trained on the raw, low-spatial resolution data from particle image velocimetry. During prediction, the ESN is provided one half of the spatial domain of the fluid flow. The task is to infer the missing other half. Four different decompositions termed forward, backward, forward-backward, and vertical were examined to show whether there exists a favorable region of the flow for which the ESN performs best. Also, it was checked whether the flow direction has an influence on the network's performance. In order to measure the quality of the predictions, we choose the vertical velocity prediction of direction (VVPD). Furthermore, the ESN's two main hyperparameters, leaking rate (LR) and spectral radius (SR), were optimized according to the VVPD values of the corresponding network output. Moreover, each hyperparameter combination was run for 24 random reservoir realizations. Our results show that VVPD values are highest for LR ≈ 0.6, and quite independent of SR values for all four prediction approaches. Furthermore, maximum VVPD values of ≈ 0.83 were achieved for backward, forward-backward, and vertical predictions while for the forward case VVPDmax = 0.74 was achieved. We found that the predicted vertical velocity fields predominantly align with their respective ground truth. The best overall accordance was found for backward and forward-backward scenarios. In summary, we conclude that the stable quality of the reconstructed fields over a long period of time, along with the simplicity of the machine learning algorithm (ESN), which relied on coarse experimental data only, demonstrates the viability of spatial prediction as a suitable method for machine learning application in turbulence.
Data-driven estimation of scalar quantities from planar velocity measurements by deep learning applied to temperature in thermal convection. - In: Experiments in fluids, ISSN 1432-1114, Bd. 64 (2023), 12, 191, S. 1-18
The measurement of the transport of scalar quantities within flows is oftentimes laborious, difficult or even unfeasible. On the other hand, velocity measurement techniques are very advanced and give high-resolution, high-fidelity experimental data. Hence, we explore the capabilities of a deep learning model to predict the scalar quantity, in our case temperature, from measured velocity data. Our method is purely data-driven and based on the u-net architecture and, therefore, well-suited for planar experimental data. We demonstrate the applicability of the u-net on experimental temperature and velocity data, measured in large aspect ratio Rayleigh-Bénard convection at Pr = 7.1 and Ra = 2 x 10^5, 4 x 10^5, 7 x 10^5. We conduct a hyper-parameter optimization and ablation study to ensure appropriate training convergence and test different architectural variations for the u-net. We test two application scenarios that are of interest to experimentalists. One, in which the u-net is trained with data of the same experimental run and one in which the u-net is trained on data of different Ra. Our analysis shows that the u-net can predict temperature fields similar to the measurement data and preserves typical spatial structure sizes. Moreover, the analysis of the heat transfer associated with the temperature showed good agreement when the u-net is trained with data of the same experimental run. The relative difference between measured and reconstructed local heat transfer of the system characterized by the Nusselt number Nu is between 0.3 and 14.1% depending on Ra. We conclude that deep learning has the potential to supplement measurements and can partially alleviate the expense of additional measurement of the scalar quantity.
Experimental investigation of indoor air ventilation in a small-scale aircraft cabin model. - In: Proceedings of the 5th International Conference on Building Energy and Environment, (2023), S. 1935-1941
The velocity field of the large-scale circulations (LSC) in turbulent mixed convection is analysed by means of 2D2C particle image velocimetry (PIV). The experiments are carried out in a small-scale model room resampling a generic passenger cabin. To achieve wide ranges of dimensionless numbers, pressurized dry air is used in the SCALEX facility. Three different LSCs have been found, depending on the Archimedes number Ar.
Unsteady inherent convective mixing in thermal-energy-storage systems during standby periods. - In: PRX energy, Bd. 2 (2023), 4, 043001, S. 043001-1-043001-17
Recent studies on the flow phenomena in stratified thermal-energy-storage (TES) systems have shown that heat conduction from the hot upper fluid layer through the vertical tank sidewall into the lower cold fluid layer leads to counterdirected wall jets adjacent to the vertical sidewalls. It was shown that these phenomena destroyed half of the total exergy content in less than a tenth of the storage time constant of a 2-m3 stratified TES system. This paper investigates short-term fluctuations of the wall jets since these fluctuations can potentially mix the hot and cold zones of the thermal stratification that are separated by the thermocline region. Using particle-image velocimetry measurements in two regions of a TES model experiment (near-wall region and far-field region) and analyzing the frequency content of the velocity fields revealed characteristic oscillations for different regions. In the near-wall region, observed fluctuations agreed well with an adjusted boundary layer frequency from the literature, showing that the wall jet is transitioning from laminar to turbulent flow. In the far-field region, the oscillations are related to the Brunt-Väisälä frequency. It is shown that the fluctuations from the boundaries of the thermocline region are most dominant and propagate into deeper regions of the thermocline. A comparison to data from the large-scale test facility for thermal energy storage in molten salt at the German Aerospace Center in Cologne showed good agreement. The consensus between the two experiments proves firstly that a small-scale model experiment with water as a storage liquid can be used to analyze the physical phenomena of large-scale molten salt storage facilities and secondly that these fluctuations are relevant for exergy destruction in real-scale TES.
On the behavior of prolate spheroids in a standing surface acoustic wave field. - In: Microfluidics and nanofluidics, ISSN 1613-4990, Bd. 27 (2023), 12, 81, S. 1-19
The active manipulation of particle and cell trajectories in fluids by high-frequency standing surface acoustic waves (sSAW) allows to separate particles and cells systematically depending on their size and acoustic contrast. However, process technologies and biomedical applications usually operate with non-spherical particles, for which the prediction of acoustic forces is highly challenging and remains a subject of ongoing research. In this study, the dynamical behavior of prolate spheroids exposed to a three-dimensional acoustic field with multiple pressure nodes along the channel width is examined. Optical measurements reveal an alignment of the particles orthogonal to the pressure nodes of the sSAW, which has not been reported in literature so far. The dynamical behavior of the particles is analyzed under controlled initial conditions for various motion patterns by imposing a phase shift on the sSAW. To gain detailed understanding of the particle dynamics, a three-dimensional numerical model is developed to predict the acoustic force and torque acting on a prolate spheroid. Considering the acoustically induced streaming around the particle, the numerical results are in excellent agreement with experimental findings. Using the proposed numerical model, a dependence of the acoustic force on the particle shape is found in relation to the acoustic impedance of the channel ceiling. Hence, the numerical model presented herein promises high progress for the design of separation devices utilizing sSAW, exploiting an additional separation criterion based on the particle shape.
Melt flow, heat transfer and solidification in a flexible thin slab continuous casting mold with vertical-combined electromagnetic braking. - In: Journal of iron and steel research, international, ISSN 2210-3988, Bd. 0 (2023), 0, insges. 15 S.
During continuous casting of steel slabs, the application of electromagnetic braking technology (EMBr) provides an effective tool to influence solidification by controlling the pattern of melt flow in the mold. Thus, the quality of the final product can be improved considerably. A new electromagnetic braking (EMBr) method, named vertical-combined electromagnetic braking (VC-EMBr), is proposed to be applied to a flexible thin slab casting (FTSC) mold. To evaluate the beneficial effects of the VC-EMBr, the melt flow, heat transfer, and solidification processes in the FTSC mold are studied by means of numerical simulations. In detail, a Reynolds-averaged Navier-Stokes turbulence model together with an enthalpy-porosity approach was used. The numerical findings are compared with respective simulations using the traditional Ruler-EMBr. The results demonstrate that the application of the VC-EMBr contributes significantly to preventing relative slab defects. In contrast to the Ruler-EMBr, the additional vertical magnetic poles of the VC-EMBr preferentially suppress the direct impact of jet flow on the narrow face of FSTC mold and considerably diminish the level fluctuation near the meniscus region. For instance, by applying a magnetic flux density of 0.3 T, the maximum amplitude of meniscus deflection reduces by about 80%. Moreover, the braking effect of the VC-EMBr effectively improves the homogeneity of temperature distribution in the upper recirculation region and increases the solidified shell thickness along the casting direction. On this basis, the newly proposed VC-EMBr shows a beneficial effect in preventing relative slab defects for FTSC thin slab continuous casting.
Beitrag des Grundlagenfachs Technische Thermodynamik zur Thematik Technische Bildung für eine nachhaltige Entwicklung. - In: Technische Bildung für eine Nachhaltige Entwicklung, (2023), S. 59-68
Die Technische Thermodynamik versteht sich heutzutage als eine allgemeine Energielehre. In vielen Ingenieurstudiengängen gilt sie als Grundlagenfach, dem die Aufgabe zukommt, den Studierenden die vielfältigen Umwandlungsmöglichkeiten von Energieformen aufzuzeigen. Des Weiteren werden die Studierenden über die Einschränkungen bei den Umwandlungsprozessen unterrichtet, anhand derer sie die Effizienz der Prozesse beurteilen können. Die Aussagen der Thermodynamik sind methodisch in vier Hauptsätzen zusammengefasst. Trotz dieses klaren inhaltlichen Aufbaus ist die Technische Thermodynamik bei vielen Studierenden ein eher unbeliebtes Fach, wohl, weil die sichere Beherrschung abstrakter, fachspezifischer Größen wie Entropie und Exergie notwendig ist. Der vorliegende Beitrag setzt sich zum Ziel zu zeigen, dass die Kernaussagen der Thermodynamik mit dem Gedanken der Nachhaltigkeit verknüpft sind. Dadurch ist fundiertes Fachwissen in dieser Disziplin von zentraler Bedeutung für die Umsetzung nachhaltiger Ansätze in der Anwendung. Alle künftigen technischen Lösungsvorschläge im Rahmen der viel zitierten Energiewende kommen an dem grundlegenden Verständnis der thermodynamischen Zusammenhänge nicht vorbei. Weiterhin wird anhand von Praxisbeispielen analysiert, welche Herausforderungen sich daraus für die Lehre in der universitären Ingenieurausbildung ergeben. Neben den klassischen Lehrinstrumenten wie Vorlesung und Seminarübung sind auch Erfahrungssammlung durch Laborversuche und Exkursionen wichtige Schritte im Lernprozess.
Electrically driven free shear flows in a duct under a transverse uniform magnetic field. - In: Magnetohydrodynamics, Bd. 59 (2023), 1, S. 3-22
A mathematical model of two-dimensional electrically driven laminar plane free shear flows in a straight duct under the action of an applied spanwise uniform magnetic field is considered. The mathematical approach is like that used in the research of Hunt and Williams (J. Fluid. Mech., 31, 705, 1968) and Kolesnikov and Kalis (Magnetohydrodynamics, vol. 57, 2021, no. 2). A system of stationary partial differential equations with two unknown functions of velocity and induced magnetic field is solved. The electric current is injected into the liquid by means of two couples of linear electrodes located vis-à-vis on opposite duct walls, perpendicular to the magnetic field. Three cases are considered. One pair of electrodes is current supplied and, depending on the direction of electric current injection on the electrode pair, two coinciding or two counter flows are also driven. At Hartmann numbers Ha >> 1, quasi-potential cores are formed in these flows, bounded by lateral Shercliff free boundary layers parallel to the field and two Hartmann layers on the walls perpendicular to the field. As a result, almost all of the injected current passes through these layers. An increase of the magnetic field leads only to an internal rearrangement of the potential cores of the flows. The Hartmann number varies in the range from 1 to 100.
Thermal boundary condition studies in large aspect ratio Rayleigh-Bénard convection. - In: European journal of mechanics, ISSN 1873-7390, Bd. 101 (2023), S. 283-293
We study the influence of thermal boundary conditions on large aspect ratio Rayleigh-Bénard convection by a joint analysis of experimental and numerical data sets for a Prandtl number Pr=7 and Rayleigh numbers Ra=105−106. The spatio-temporal experimental data are obtained by combined Particle Image Velocimetry and Particle Image Thermometry measurements in a cuboid cell filled with water at an aspect ratio Γ=25. In addition, numerical data are generated by Direct Numerical Simulations (DNS) in domains with Γ=25 and Γ=60 subject to different idealized thermal boundary conditions. Our experimental data show an increased characteristic horizontal extension scale ÜÞλ of the flow structures for increasing Ra , which due to an increase of the convective heat transfer also leads to an increase of the Biot number (Bi) at the cooling plate. However, we find the experimental flow structure size to range in any case in between the ones observed for the idealized thermal boundary conditions captured by the simulations: On the one hand, they are larger than in the numerical case with applied uniform temperatures at the plates. On the other hand, they are smaller than in the case of an applied constant heat flux, the latter of which leads to a structure that grows gradually up to the horizontal domain size. We are able to link this observation qualitatively to theoretical predictions for the onset of convection. Furthermore, we study the effect of the asymmetric boundary conditions on the heat transfer. Contrasting experimental and numerical data reveals an increased probability of far-tail events of reversed heat transfer. The successive decomposition of the local Nusselt number Nuloc traces this effect back to the sign of the temperature deviation ÜÞΘ, eventually revealing asymmetries of the heating and cooling plate on the thermal variance of the generated thermal plumes.