Experimental Study On Turbulent Rayleigh-bénard Convection Under Linear Non-uniform Heating Boundary Conditions

Turbulent Rayleigh-Bénard(RB) convection has become a fruitful paradigm for understanding the physical nature of a wide range of complicated convection problems occurring in nature and in engineering problems. For the standard case, the temperatures of the top and bottom plates are assumed to be distributed homogeneously. However, in many applications a certain degree of inhomogeneity is present in the thermal forcing at the boundaries. Furthermore, even in a fully developed RB system, a certain degree of inhomogeneity can also be found in the thermal boundary due to the large-scale circulation(LSC). The objective of this thesis is to study the influence of non-uniform heating boundary conditions on turbulent heat transport.In the first part of this thesis, we present a standard turbulent RB convection experiment as a contrast group. We carry out high-precision measurements of the Nusselt number Nu as a function of the Rayleigh number Ra in a water-filled rectangular RB convection cell of aspect ratio Γx=0.99, Γy= 0.25. During the measurements, Ra varies from 9′108 to 1′1010 and the Prandtl number Pr is fixed at Pr = 5.4. Our result shows a scaling law of Nu~Ra0.286, which is in good agreement with previous theories and experiments.In the second part of this thesis, by adjusting the spatial distribution of the heating power on the lower plate, different heating conditions were achieved. The ratio of the power difference between adjacent heaters over the total power is defined as δ, which can be used to describe the inhomogeneity of heating conditions. Major findings are summarized as follow:(1)The temperature distribution of the lower plate are found to be linearly distributed.(2)An enhancement up to 8% of the Nusselt number has been found, suggesting that the non-uniform heating can enhance the global heat transport efficiency of the system.(3)When Ra is below 7′109,the higher δ or Ra is,the higher Nu increased.(4)With non-uniform heating condition, the asymmetric lower temperature boundary layer can promote the formation of hot plumes effectively, and thus enhance the heat transport efficiency. When Ra is higher than 7′109, the frequency of the formation of hot plumes is limited by the finite conductivity effect, and the enhancement of the Nu stays the same.