Experimental Studies Of Thermal Boundary Layer Properties And Statistical Characteristics Of Plumes In Turbulent Thermal Convection

Thermal convection is ubiquitous in nature and industry.Rayleigh-Benard convection(RBC)has been regarded as an idealized model to study thermal convection for decades.Thermal boundary layers play a crucial role in determining the heat transport of the system,which can be also manipulated via thermal plumes that detach from the thermal boundary layers.While it is generally believed that boundary layer properties exhibit significant differences with varying Pr numbers(a control parameter that describes fluid properties),few studies have been devoted into this issue.In this thesis,we present an experimental investigation of thermal boundary layer properties and the associated statistical characteristics of plumes in convective thermal turbulence.Four rectangular convection cells were used in the experiment with the same aspect ratio(?x=1.0,?y=0.3)but different heights(12.6 cm,25.2 cm,37.8 cm,50.4 cm)to ensure the same Ra number(a control parameter that represents the driven strength)range(1.93×109 ?Ra?1.73×1010)under varying Pr(11.7,20.5,49.3,81.6,145.7).The main body consists of the following two parts.In the first part,we present the results about the thermal boundary layer thickness and the properties of temperature profiles.The Ra(also Pr)dependent scalings of ther-mal boundary layer thickness show consistent behaviors with the heat transfer efficiency.It is found that the normalized temperature profiles approach to the Blasius type with increasing Pr and can be described excellently by the boundary layer equations derived by Shishkina et al.recently,demonstrating smaller turbulent fluctuation as Pr becomes larger.Furthermore,the shapes of both mean temperature and root-mean-square temper-ature profiles were examined in detail and their characteristics were summarized in the Ra-Pr diagram.Our results show that the logarithmic profiles tends to exist in larger Ra and small Pr cases.In the second part,we extracted plume information by time series of temperature from representative flow dynamics and obtained the related statistics.It is found that total and plume-caused temperature fluctuation are almost the same in the boundary layer,from where plumes develop and erupt.However,the temperature fluctuation and also the properties of plumes are different in the boundary layer and the mixing zone,where multiple force balance mechanisms coexist.We further found that there is connection between plume density and large-scale circulation in some way.All these results reveal that the behavior of plumes is not the deciding factor in characteristic temperature profiles.