Thermal boundary condition studies in large aspect ratio Rayleigh–Bénard convection

Käufer, Theo; Vieweg, Philipp P.; Schumacher, Jörg; Cierpka, Christian

doi:10.1016/j.euromechflu.2023.06.003

Cited by 13 publications

(11 citation statements)

References 59 publications

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“…The definition of the convective heat transfer is equal to the definition of the local Nusselt number [17,53]. However, since the local Nusselt number represents the overall heat transfer, the conductive heat transfer must be neglectable, which is only a valid assumption close to the horizontal mid-plane within the Boussinesq-approximation regime.…”

Section: Resultsmentioning

confidence: 99%

“…Another aspect is the development of a joint data assimilation technique to extract Eulerian fields that take advantage of the available temperature data. Finally, we want to apply the new method to study large aspect ratio RBC [17] and thermal energy storages [54].…”

Section: Discussionmentioning

confidence: 99%

“…Since the first systematic studies [13,14] thermal convection has been extensively studied by means of experimental, numerical and theoretical methods [15,16]. Nevertheless, the flow phenomena are not yet fully understood, and many open questions, for example, with respect to the thermal boundary conditions [17,18] remain.…”

Section: Introductionmentioning

confidence: 99%

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Volumetric Lagrangian temperature and velocity measurements with thermochromic liquid crystals

Käufer,

Cierpka

2023

Meas. Sci. Technol.

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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 = 104 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.

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Section: Resultsmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

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Volumetric Lagrangian temperature and velocity measurements with thermochromic liquid crystals

Käufer,

Cierpka

2023

Meas. Sci. Technol.

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“…Moller et al 26 compared the superstructure sizes in numerical and experimental studies for water for Ra = 2 − 7 × 10 5 and concluded that superstructures are larger in experiments. This has to do with the effect of smaller aspect ratios compared to numerics and non-ideal isothermal boundary conditions in the experiments 38 . Hence, a similar comparison is intended for the current study as well.…”

Section: A Spatio-temporal Dynamicsmentioning

confidence: 99%

On the role of vortices in turbulent Rayleigh-Bénard convection

Ghazijahani,

Cierpka

2024

Preprint

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Understanding turbulent thermal convection is essential for modeling many natural phenomena. This study investigates the spatio-temporal 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 = 9.4×105,2.0×106 and 5.5×106. For all three cases, the Prandtl number is Pr = 0.79 and ΔT ≈ 7K. Then, stereoscopic particle image velocimetry is conducted to measure the three velocity components in the horizontal mid-plane for 5.78×103 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 2D 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 life time, displacement, and traveled distance of these structures. The relation between life time 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.

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“…The motivation for the present study arose from investigations of turbulent thermal convection in liquid metals and the interest in how the resulting flow structures influence the heat transport in the system [1][2][3][4][5]. Both numerical and experimental works have shown that detailed temperature data in the bulk of the fluid provide crucial information about local heat transport, leading to a more accurate and advanced understanding of the studied system [6]. Having access to a temperature map within a layer of turbulent fluid is very attractive, as it allows one to characterize the thermal structures and track their dynamics.…”

Section: Introductionmentioning

confidence: 99%

In-Bulk Temperature Profile Mapping Using Fiber Bragg Grating in Fluids

Su,

Niu,

Vogt

et al. 2023

Sensors

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The capabilities of Fiber Bragg Grating (FBG) sensors to measure temperature variations in the bulk of liquid flows were considered. In the first step of our research project, reported in this paper, we investigated to what extent the use of thin glass fibers without encapsulation, which only minimally disturb a flow, can fulfill the requirements for robustness and measurement accuracy. Experimental tests were performed in a benchmark setup containing 24 FBG measuring positions, which were instrumented in parallel with thermocouples for validation. We suggest a special assembly procedure in which the fiber is placed under a defined tension to improve its stiffness and immobility for certain flow conditions. This approach uses a single FBG sensor as a reference that measures the strain effect in real time, allowing accurate relative temperature measurements to be made at the other FBG sensor points, taking into account an appropriate correction term. Absolute temperature readings can be obtained by installing another well-calibrated, strain-independent thermometer on the reference FBG. We demonstrated this method in two test cases: (i) a temperature gradient with stable density stratification in the liquid metal GaInSn and (ii) the heating of a water column using a local heat source. In these measurements, we succeeded in recording both spatial and temporal changes in the linear temperature distribution along the fiber. We present the corresponding results from the tests and, against this background, we discuss the capabilities and limitations of this measurement technique with respect to the detection of temperature fields in liquid flows.

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Thermal boundary condition studies in large aspect ratio Rayleigh–Bénard convection

Cited by 13 publications

References 59 publications

Volumetric Lagrangian temperature and velocity measurements with thermochromic liquid crystals

Volumetric Lagrangian temperature and velocity measurements with thermochromic liquid crystals

On the role of vortices in turbulent Rayleigh-Bénard convection

In-Bulk Temperature Profile Mapping Using Fiber Bragg Grating in Fluids

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