Dynamic characterization of Fiber Bragg Grating temperature sensors. - In: Experimental thermal and fluid science, Bd. 0 (2024), 111222, insges. 17 S.
To reliably characterize fast dynamic heat transfer mechanisms, fast-response temperature sensors are crucial, including knowledge about the temporal response. In this paper, the dynamic behavior of a Fiber Bragg Grating temperature sensor is investigated and compared to different types of fast-response thermocouples using two different experimental dynamic characterization methods. A temperature step is generated by either plunging the sensor into a fluid or exposing it to a fluid droplet at different temperatures. The step response is evaluated to determine the sensor response time. Calibration runs are performed for a silica-based 0.1mm FBG sensor, as well as for 0.16mm and 0.8mm exposed tip and 0.25mm sheathed tip type K thermocouples. Water, glycerin, oil and GaInSn were used to cover a broad range of applications regarding different thermal diffusivities and viscosities. The FBG sensor showed the shortest response times compared to the thermocouples, ranging from 60ms in oil down to 3ms in liquid metal, which is 20% up to 70% faster compared to a 0.25mm sheathed tip type K thermocouple. Additional plunging calibration runs of the FBG sensor were performed in a ternary nitrate molten salt mixture (HITEC) to determine its overall and dynamic behavior in corrosive fluids at elevated temperatures. It turns out that the FBG sensor is not affected by the molten salt and shows similar response times to those measured in water. Regarding the characterization methods, both techniques show reproducible results, even though the droplet method is inapplicable for sensors with higher heat capacity or lower thermal conductivity than the calibration fluid. Furthermore, splashing effects for fluids with low viscosity reduce the reliability of the droplet method. The results also show that a dynamic characterization is indispensable for temperature measurements with high temporal resolution because the response time depends on the sensor size and the heat transfer coefficient between sensor and surrounding, which in turn depends on the sensor type, fluid properties and the flow parameters.
https://doi.org/10.1016/j.expthermflusci.2024.111222
A dynamic method for online measurement and calibrating with Lorentz force velocimetry. - In: Measurement science and technology, ISSN 1361-6501, Bd. 35 (2024), 6, 065008, S. 1-9
Our previous study (Zheng et al 2020 Metall. Mater. Trans. B 51 558–69; Zheng et al 2020 Acta Metall. Sin.56 929–36) reports a non-invasive in-situ measurement technology using Lorentz force velocimetry (LFV) to quantitatively measure the meniscus velocity of molten steel online. However, effective signal recognition from complex environment noise and determination of zero-point calibration in harsh metallurgical processing is an essentially challenging task and indeed needs further exploration. In this paper, a method of combining double probe arrangement with real-time differential processing technology was proposed, the twin design structure not only enables the measurement of ∼mN Lorentz forces, but also has significant characteristics of environmental tolerance. The Lorentz force signal caused by conductor motion can be accurately calculated through a differential method, meaning that the problem of zero compensation in industrial online measurement can be effectively overcome. Moreover, based on the functional correlation between the Lorentz force and the parameter of the conductor to be measured, a method of the probes moving variably and actively and their data difference ratio processing was adopted, so as to achieve dynamic calibration during online measurement. This measurement strategy provides a new approach for LFV to achieve online dynamic measurement and online calibration, and provides technical support for electromagnetic measurement technology towards engineering applications.
https://doi.org/10.1088/1361-6501/ad2f06
Integration of multi-junction absorbers and catalysts for efficient solar-driven artificial leaf structures : a physical and materials science perspective. - In: Solar RRL, ISSN 2367-198X, Bd. 0 (2024), 0, S. 1-88
Artificial leaves could be the breakthrough technology to overcome the limitations of storage and mobility through the synthesis of chemical fuels from sunlight, which will be an essential component of a sustainable future energy system. However, the realization of efficient solar-driven artificial leaf structures requires integrated specialized materials such as semiconductor absorbers, catalysts, interfacial passivation, and contact layers. To date, no competitive system has emerged due to a lack of scientific understanding, knowledge-based design rules, and scalable engineering strategies. Here, we will discuss competitive artificial leaf devices for water splitting, focusing on multi-absorber structures to achieve solar-to-hydrogen conversion efficiencies exceeding 15%. A key challenge is integrating photovoltaic and electrochemical functionalities in a single device. Additionally, optimal electrocatalysts for intermittent operation at photocurrent densities of 10-20 mA cm^-2 must be immobilized on the absorbers with specifically designed interfacial passivation and contact layers, so-called buried junctions. This minimizes voltage and current losses and prevents corrosive side reactions. Key challenges include understanding elementary steps, identifying suitable materials, and developing synthesis and processing techniques for all integrated components. This is crucial for efficient, robust, and scalable devices. Here, we discuss and report on corresponding research efforts to produce green hydrogen with unassisted solar-driven (photo-)electrochemical devices. This article is protected by copyright. All rights reserved.
https://doi.org/10.1002/solr.202301047
Spatio-temporal dynamics of superstructures and vortices in turbulent Rayleigh-Bénard convection. - In: Physics of fluids, ISSN 1089-7666, Bd. 36 (2024), 3, 035120, S. 035120-1-035120-19
Understanding turbulent thermal convection is essential for modeling many natural phenomena. This study investigates the spatiotemporal dynamics of the vortical structures in the mid-plane of turbulent Rayleigh-Bénard convection in SF6 via experiments. For this, a Rayleigh-Bénard cell of aspect ratio 10 is placed inside a pressure vessel and pressurized up to 1, 1.5, and 2.5 bar in order to reach Rayleigh numbers of Ra = 9.4 × 10^5, 2.0 × 10^6, and 5.5 × 10^6, respectively. For all three cases, the Prandtl number is Pr = 0.79 and Δ T ≈ 7 K. Then, stereoscopic particle image velocimetry is conducted to measure the three velocity components in the horizontal-mid-plane for 5.78 × 10^3 free fall times. For the given aspect ratio, the flow is no longer dominated by the side walls of the cell and turbulent superstructures that show a two-dimensional repetitive organization form. These superstructures show diverse shapes with faster dissipation rates as Ra increases. Out-of-plane vortices are the main feature of the flow. As Ra increases, the number of these vortices also increases, and their size shrinks. However, their total number is almost constant for each Ra through the measurement period. Furthermore, their occurrence is random and does not depend on whether the flow is upward-heated, downward-cooled, or horizontally directed. Vortex tracking was applied to measure lifetime, displacement, and traveled distance of these structures. The relation between lifetime and traveled distance is rather linear. Interestingly, in the vortex centers, the out-of-plane momentum transport is larger in comparison to the bulk flow. Therefore, these vortices will play a major role in the heat transport in such flows.
https://doi.org/10.1063/5.0191403
Experimentelle Untersuchung des Skalenverhaltens bei Kondensation und Verdampfen in einem generischen Fahrzeugscheinwerfer. - Ilmenau, 2024. - 1 Online-Ressource (xxiii, 178 Seiten)
Technische Universität Ilmenau, Dissertation 2024
Diese Promotionsschrift befasst sich mit der experimentellen Untersuchung des Wärmetransports und Stofftransfers feuchter Luft mit Phasentransition an einer Oberfläche. Eine Motivation hierfür gibt unter anderem das vermehrte Auftreten von unerwünschter Kondensation innerhalb von Automobilscheinwerfern. Hierzu wird ein experimenteller Aufbau vorgestellt, der die physikalischen Prozesse abbildet, die für die Tropfenkondensation und die Verdampfung von Tropfen relevant sind. Der in dieser Arbeit untersuchte Parameterbereich umfasst 200 < Re < 1300, bei 0 < Gr < 108 und relative Luftfeuchten von 0.19 < ϕein < 0.85 (bei 25 ◦C). Das Geschwindigkeitsfeld im Zellinneren wurde mittels tomografischer Particle Image Velocimetry gemessen. Die Ergebnisse einer probabilistischen Analyse und einer Hauptkomponentenanalyse werden präsentiert. Hierbei zeigt sich, dass die erzwungene Strömung den internen Wärme- und Stofftransport dominiert, höhere Temperaturgradienten jedoch zu einer Stabilisierung beziehungsweise Symmetrieerhöhung der großskaligen Strömung führen. Zur Bestimmung des Massentransfers beim Kondensieren und Verdampfen werden drei Messmethoden vorgestellt. Die erste bilanziert die Menge an Wasser an Ein- und Auslass, die zweite wiegt die Wassermasse direkt, während die dritte Konturen einzelner Tropfen mittels eines Mikroskops misst. Die ersten beiden liefern Aussagen über die globale Entwicklung der Masse auf der Kühlplatte. Die optische Methode eröffnet Einblicke in die lokale Tropfendynamik. Auf Basis dieser Messergebnisse wird die Skalierung der Sherwood-Zahl in Abhängigkeit von der Reynolds-Zahl und der relativen Luftfeuchte am Einlass untersucht. Zusätzlich wurde eine dimensionslose Beschreibung des Anwachsens und Schrumpfens von Einzeltropfen auf der Oberfläche formuliert. Die Messergebnisse des sensiblen und des latenten Wärmestroms werden abschließend in einem 1D-Modell reproduziert, validiert und auf den konkreten Betriebsfall eines Serienscheinwerfers angewendet.
https://doi.org/10.22032/dbt.59510
Volumetric Lagrangian temperature and velocity measurements with thermochromic liquid crystals. - In: Measurement science and technology, ISSN 1361-6501, Bd. 35 (2024), 3, 035301, S. 1-11
We propose a Lagrangian method for simultaneous, volumetric temperature and velocity measurements. As tracer particles for both quantities, we employ encapsulated thermochromic liquid crystals (TLCs). We discuss the challenges arising from color imaging of small particles and present measurements in an equilateral hexagonal-shaped convection cell of height h = 60 mm and distance between the parallel side walls w = 10^4 mm, which corresponds to an aspect ratio Γ = 1.73. As fluid, we use a water-glycerol mixture to match the density of the TLC particles. We propose a densely-connected neural network, trained on calibration data, to predict the temperature for individual particles based on their particle image and position in the color camera images, which achieves uncertainties below 0.2 K over a temperature range of 3 K. We use Shake-the-Box to determine the 3D position and velocity of the particles and couple it with our temperature measurement approach. We validate our approach by adjusting a stable temperature stratification and comparing our measured temperatures with the theoretical results. Finally, we apply our approach to thermal convection at Rayleigh number Ra = 3.4 × 10^7 and Prandtl number Pr = 10.6. We can visualize detaching plumes in individual temperature and convective heat transfer snapshots. Furthermore, we demonstrate that our approach allows us to compute statistics of the convective heat transfer and briefly validate our results against the literature.
https://doi.org/10.1088/1361-6501/ad16d1
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. 31 (2024), 2, S. 401-415
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.
https://doi.org/10.1007/s42243-023-01062-9
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.
https://doi.org/10.1016/j.ijthermalsci.2023.108605
Formation of the inlet flow profile for passive control of a magnetohydrodynamic liquid-metal flow in a channel. - In: High temperature, ISSN 1608-3156, Bd. 61 (2023), 3, S. 417-428
The paper describes an experimental attempt to affect the flow of liquid metal using a relatively small perturbation at an inlet to a long channel. The purpose is to form a flow structure which is stable in a strong magnetic field at high heat loads, enhance heat transfer, and achieve more predictable flow parameters. It is demonstrated that an obstacle in the form of a rod located transverse to the flow and parallel to the applied magnetic field and installed at the inlet can induce perturbations in the form of regular vortices observed along the flow at lengths as great as several tens of channel hydraulic diameters. The experiments confirm that thus generated vortices considerably change the structure of the isothermal MHD flow. In the case of mixed convection, such vortices suppress the development large-scale thermogravitational fluctuations in the flow and enhance heat transfer under certain flow conditions.
https://doi.org/10.1134/S0018151X23030033
Electrically driven plane free shear flow in a duct under an oblique transverse uniform magnetic field. - In: Magnetohydrodynamics, Bd. 59 (2023), 2, S. 119-134
A mathematical model of electrically driven laminar free shear flows in a straight duct under the action of an applied oblique transverse uniform magnetic field is considered. The mathematical approach is similar to that used in [1]. A system of stationary partial differential equations with two unknown functions of velocity and induced magnetic field is solved. Three different cases of electric current supply to the liquid are considered. An electric current is introduced into the liquid first by one pair of linear electrodes, and in two other cases by two pairs of electrodes located on the upper and lower walls of the duct. The cases are analyzed when the angle of inclination of the magnetic field vector to these walls is ϕ0 = π/4. Depending on the direction of the electric current supplied to the pairs of electrodes, two coinciding in direction or two opposite inclined flows are driven in the zone between these walls. Increasing the magnetic field only leads to an internal rearrangement of the flows. The Hartmann number Ha ranges from 1 to 10, at which MHD effects distinctly enough are already displayed.
https://doi.org/0.22364/mhd.59.2.1