Experimental Investigation Of Heat Transport And Flow Structures In Turbulent Rayleigh-bénard Convection In Annulus

Turbulent thermal convection is ubiquitous in nature and in many industrial applications,which is closely related to people's production and life.For instance,atmospheric and oceanic circulations tightly linked with global climate issue,convective flows associated with all kinds of heat exchangers.Turbulent Rayleigh-Bénard(RB)convection is a paradigm in the community of thermal turbulence.The study on this model is beneficial to physically understand natural phenomenon and provide theoretical support for engineering problems,such as heat-transport and heat-disspation.Turbulent heat transport and the dynamics of large-scale structures are key issues in thermal convection.In this thesis,we carry out experimental studies on thermal turbulence in annulus and systematically investigate heat transport and characterstics of turbulent structures.The main findings are summarized as follows:1)First of all,experimental apparatus of RB annular cells with the aspect ratio?=1,2 and 4 are designed and set up.The heat transport in annular RB convection are measured by the high-precision-temperature semiconductors method.Flow visualization is adopted to observe the process of large-scale flow structures.The horizontal temperature profiles of turbulent structures are studied by the multi-temperature-probe method.2)Heat transport in turbulent RB convection in annulus with?=1,2 and 4 cells are investigated.We experimentally analyze the scaling law between Nusselt number(Nu)characterizing the intensity of heat transport and the Rayleigh number(Ra)characterizing the vigor of RB turbulence.Experimental measurements show that the scaling behaviours Nu(Ra) respectively satisfies Nu=0.08Ra^0.32±0.01for the annular cell of?=1, Nu=0.14Ra^0.29±0.01 for?=2 and Nu=0.15Ra^0.29±0.01 for?=4.For the unit aspect ratio cell,we focus on the statistical properties of the local temperature.It is shown that the power-law between the normalized temperature standard deviations by the temperature difference?/?T and Ra are?/?T=0.02Ra^-0.10±0.03(upper),?/?T=0.24Ra^-0.14±0.03(mid-height) and?/?T=0.04Ra^-0.11±0.03(lower).The power-law exponents are in agreement with the results obtained in rectangular and cylindrica convection,but the prefactors of the Nu vs.Ra number differs.3)The scaling relationship of response parameter Reynolds number(Re) dependence on Ra are investigated.It is shown that the scaling behaviors Re(Ra)are experimentally measured as Re=0.11Ra^0.47±0.02(?=1), Re=0.16Ra^0.44±0.02(?=2)and Re=0.06Ra^0.46±0.01(?=4).Flow visualization reveals that plumes are ejected from thermal boundary layers and self- organized into the two-roll flow structure in unit aspect ratio cell.Based on multi-temperature-probe method,the two-roll flow structure is also captured in?=1 convection cell and the four-roll flow structure is found in?=2 and 4 convection cells.Cessations and reversals are absent in azimuthal motion of the large flow structure.Probability densit functions(PDFs)of amplitudes and orientations are satisfied with Gaussian distribution.Flow mode decomposition is applied to study the multi-roll structures.For?=1 convection cell,the first flow mode possesses the highest proportion of energy in all flow modes,which indicates that the two-roll flow structure plays a dominant role.As Ra increases,the first-order flow mode becomes stronger and the second- order flow mode becomes weaker.The reason is that the first-order flow mode extracts energy from the second-order flow mode.The second- order flow mode accounts for the highest ratio in?=2 and 4 convection cells.It implies that the four-roll flow structure is dominant.4)Finally,the comparsive study of annular cells with?=1,2 and 4 effects heat transport and flow structures.The effect of annular aspect ratios on Re number is investigated.Measured scaling behavior of Re(Ra,?)is Re?Ra^0.46?^-0.52.Dimensionless mean amplitudes(<?>/?T)of large scale structures as a function of Ra are measured.The power law fits are <?>/?T?Ra^-0.82(two-roll flow structure)and<?>/?T?Ra^-0.40(four-roll flow structure).It demonstrates that the flow structures have different dynamic characteristics.Flow patterns of turbulent structures in this system are different,but heat transport is insensitive to the change of flow structures.