Dies ist ein Kapitel der 12. Auflage des VDI-Wärmeatlas.

Dies ist ein Kapitel der 12. Auflage des VDI-Wärmeatlas.

Dies ist ein Kapitel der 12. Auflage des VDI-Wärmeatlas

This thesis focuses on the investigation of the local wall heat flux by applying a new measurement technique in the field of Rayleigh-Benard 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.

We report highly resolved measurements of the local wall heat flux in turbulent Rayleigh–Bénard convection using an infrared camera. The measurements have been undertaken in a Rayleigh–Bénard cell with rectangular base of 2.50m×0.65m and a height of 2.5m which is filled with air. First of all, it could be demonstrated that in a Rayleigh–Bénard cell with rectangular cross-section the time-averaged wall heat flux locally deviates by 30% from its mean. Furthermore, a strong correlation between the global flow structure inside the cell and the distribution of the local wall heat flux could be identified.

In our talk we present a visualization of the flow field inside the boundary layer in highly turbulent Rayleigh-Bénard convection in air. Sequences were captured at various positions along the heated bottom plate in a rectangular cell of 2.5×2.5×0.6 m3 at a Rayleigh number of Ra = 1.3×1010. They demonstrate that the velocity field may not be considered as uniform over the entire surface of the plate and, therefore, the distribution of the local heat flux becomes non-uniform as well. Local heat flux measurements using an infrared camera show that in the specific case this quantity varies by at least ±10% depending on the local flow condition at distinct areas of the plate surface.

A simple analytical model has been developed to estimate the error of an invasive temperature measurement technique in a non-isothermal environment. The error depends significantly on the sensor geometry and the temperature distribution of the surrounding fluid. The problem is described in such a way that the model can easily be adapted to other sensor geometries.

Rayleigh-Benard cells are one of the simplest systems for exploring the laws of natural convection in the highly turbulent limit. However, at very high Rayleigh numbers (Ra greater than or similar to 10(12)) and for Prandtl numbers of the order of one, experiments fall into two categories: some evidence a steep enhancement of the heat transfer while others do not. The origin of this apparent disagreement is at present still unexplained. This puzzling situation motivated a systematic study of the triggering of the regime with an enhanced heat transfer, originally named the 'Ultimate Regime' of convection. High-accuracy heat transfer measurements have been conducted in convection cells with various aspect ratios and different specificities, such as altered boundary conditions or obstacles inserted in the flow. The two control parameters, the Rayleigh and Prandtl numbers, have been varied independently to disentangle their relative influence. Among other results, it is found that (i) most experiments reaching very high Ra are not in disagreement if small differences in Prandtl number are taken into account, (ii) the transition is not directly triggered by the large-scale circulation present in the cell and (iii) the sidewalls of the cell have a significant influence on the transition. The characteristics of this Ultimate Regime are summarized and compared with the R Kraichnan prediction for the asymptotic regime of convection.