Experimental Studies Of Prandtl-dependence Of Plume And Large Scale Circulation In Rayleigh-Bénard Convection

Thermal convection is not only widely found in nature,but also often used in the fields of heat exchanger and electronic component cooling,crystal growth and nuclear reactor design because of its excellent heat transfer effeciency.The Rayleigh-Bénard?RB?convection system is a physical model abstracted from those natural phenomena and industrial applications.The Prandtl?Pr?number,a system control parameter that describes the characteristics of the working fluid,plays an important role in the RB convection.However,in the past work,experimental studies of the Pr number effects in this area remain rare.In this thesis,we conducted experimental studies on how the Pr number affects the plume,large-scale circulation,and their related properties in RB convection.We used five different kinds of dimethicone?Pr=11.7,20.5,49.3,81.6,145.7?as the working fluid,and four kinds of convection cells whose dimensions are different,but the geometric ratios are the same?length to height ratio?_x=1.0,aspect ratio?_y=0.3?to ensure that the range of Rayleigh?Ra?number remains unchanged while the Pr number varies.The content of this thesis mainly consists of two parts.In the first part,we used standard shadowgraph technology to visualize the plume and large-scale circulation structure in RB convection under different Pr numbers,and then applied the optical flow method to analyze the flow field semi-quantitatively.The range of the Ra number in the first part of the experiment is 2.4×10⁹?Ra?2.1×10¹⁰.We found that as Pr number increases,the plume becomes more slender and tends to gather along the sidewall.This makes the plume-dominant zone in the flow field becomes more concentrated,and the area of turbulent bulk flow region becomes wider.We further found that while the large-scale circulation becomes more coherent,its motions slow down as Pr number increases.These changes could be attributed to increase in both the viscosity of fluid and the thermal diffusion time.Since plume and large-scale circulation are the main turbulent structures that largely determine the dynamics and transport properties of the system,so in the second part of this thesis,we investigated how the Pr number influences the turbulence intensity and heat transport of RB convection.Because more data is needed to fit curves in the second part of the experiment,the range of the Ra number is 4.1×10⁸?Ra?7.1×10¹⁰.We mainly measured the Reynolds?Re?number,heat transfer efficiency,i.e.the Nusselt?Nu?number,and temperature fluctuations??/?T?in the cell center and near the sidewall region with the scaling law of Pr number and Ra number,and the corresponding results are:Re?Pr^-0.81±0.07Ra^0.57±0.01;the temperature fluctuations in the bulk??/?T??Pr^-0.19±0.02Ra^{-0.280±0.003};the temperature fluctuations near the sidewall region??/?T??Pr^0.10±0.03Ra^{-0.200±0.007};Nu?Pr^{-0.026±0.004}Ra^0.295±0.001.These results quantitatively demonstrate that as the value of Pr increases,the plume tends to move along the sidewall of the system rather than through the central region of the cell,and the motion of the large-scale circulation becomes slower.On the other hand,although the scaling law of the Re number and the Nu number are basically the same with the previous results obtained by the convection system of other configurations,the temperature fluctuation is quite different from the existing literature and the mainstream theoretical prediction,indicating that the configurations of the convection cell have little influence on the global transport of the system,but have a great impact on local characteristics,which needs to be fully considered in future theoretical models.