Thermal convection on rough surfaces
Final Report Abstract
Thermal convection on a rough or structured surface is a physical process that reflects reality much better in many problems, e.g. in technical systems for heat transfer, in the cooling of electronic components or in research into the urban climate, then the models usually used with smooth surfaces are able to do. Although there have been a large number of publications on this problem in the past, the local transport processes in the immediate vicinity of such a structured wall in particular are still largely misunderstood. This lack is due to the fact that there are only very few convection experiments in which the transport quantities near the wall can be measured with an adequate spatial and temporal resolution and that the computational effort for direct numerical simulations is currently still extremely high. In the research project, the local velocity and temperature fields on a rough surface were to be measured in the “Ilmenau Barrel” convection experiment. Due to the large dimensions of the convection cell, with a diameter of 7.1 m and a total height of 8.0 m, these measurements can be carried out not only at very high Rayleigh numbers (up to Ra = 1012 ), but also with a spatial resolution that has never been achieved before. In particular, the aim was to identify the mechanisms that lead to an increase in the convective wall heat flux compared to a smooth surface. It was shown that this increase only occurs when the thickness of the thermal boundary layer becomes smaller than the height of the roughness elements. If this limit is exceeded, both the profile of the mean temperature (it becomes flatter) and the profile of the temperature fluctuations (the peak of the fluctuations becomes smaller) change. The position of this peak falls with increasing Rayleigh number according to a power law with an exponent of -0.323. Between the roughness elements, the so-called VALLEYs, the profile of the temperature fluctuations also changes from an approximately linear to a non-linear dependency if the thickness of the thermal boundary layer falls below the height of the roughness elements. There are also significant variations in the velocity field compared to a smooth surface when the thickness of the boundary layer falls below the height of the roughness elements. While the time-averaged velocity field changes only insignificantly at this transition, the fluctuations of the wall-normal velocity component in particular show a marked increase. This increase is particularly prominent above the VALLEYs, which was also demonstrated in direct numerical simulations by Belkadi et al. (2021). The VALLEYs therefore contribute more to the increase in the convective wall heat flux than the roughness elements themselves do.
Publications
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Time-resolved measurements of the local wall heat flux in turbulent Rayleigh–Bénard convection. International Journal of Heat and Mass Transfer, 188, 122649.
du Puits, Ronald
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ERCOFTAC Classic Collection Database. Case95: Thermal Boundary Layers in Turbulent Rayleigh-Bénard Convection with Rough and Smooth Surfaces
R. du Puits
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Ilmenauer-Fass Database. Case 4: Thermal boundary layers in turbulent Rayleigh-Bénard convection with rough and smooth surfaces
R. du Puits
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Thermal boundary layers in turbulent Rayleigh-Bénard convection with rough and smooth plates: A one-to-one comparison. Physical Review Fluids, 9(2).
du Puits, Ronald
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Turbulent Rayleigh-Bénard convection with rough surface. 1st European Fluid Dynamics Conference, Aachen, Germany, 16. – 20. September 2024
R. du Puits
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Turbulent thermal convection at a rough surface. Proceedings of the 21st International Symposium on Application of Laser and Imaging Techniques to Fluid Mechanics, Lisbon, Portugal, 8. – 11. July 2024
R. du Puits
