Wärmestromverteilung in turbulenter Rayleigh-Benard Konvektion

Kaiser, Robert GND

This thesis focuses on the investigation of the local wall heat flux by applying a new measurement technique in the field of Rayleigh-Bénard convection. The local wall heat flux at the interface between a solid and a gas was measured under different boundary conditions and in various geometries. Using a fast (30 fps) and high-resolution (640 px x 480 px) infrared camera, unprecedented spatial and temporal resolution was achieved.The first case studied in this thesis is a slender rectangular convection cell with a height of 2.5 m, a length of 2.5 m and a width of 0.65 m. Inside this cell, the large scale circulation is confined in a single plane. The local convective heat flux on the heating plate was measured in the range of 1.36e10≤Ra≤5.45e10. The measurements revealed a highly inhomogeneous distribution of the wall heat flux, whereas a variation up of to 37% compared to the globally averaged quantity became visible. Based on this observation the local wall heat flux is divided into three subregions (impingement, centre and corner flow) and the local scaling of the heat transport with respect to the Rayleigh number is calculated for each subregions.In a cylindrical cell with a width to height ratio of Γ=1.13, the dynamic of a three-dimensional convective flow was investigated in a range of 1e11≤Ra≤8e11. In coincidence with the quasi two-dimensional case, the time-averaged local wall heat flux varies up to 30% with respect to the globally averaged quantity. In the investigated range, the principal plane of the large scale circulation oscillated up to ±90°. This behaviour homogenizes the local wall heat flux in the centre of the heating plate. In addition, an enhancement of the local wall heat flux near the sidewall was observed with a uniform distribution in azimuthal direction.In a variable aspect ratio cell of 1.13≤Γ≤4, the influence of the sidewall on the global heat transport was analysed. It was shown, that the global heat flux at a constant Rayleigh number varied significantly up to 35%. In this context, a crucial mechanism is the collapse of the global flow structure, which occurs at Γ=1.65. For this reason, the experimental data of a convection cell with Γ=1 are not suitable to verify the theoretical approaches for a horizontally infinite extended fluid layer.

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Kaiser, Robert: Wärmestromverteilung in turbulenter Rayleigh-Benard Konvektion. 2015.

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