湍流热对流中温度边界层和羽流统计特性的实验研究

EXPERIMENTAL STUDIES OF THERMAL BOUNDARY LAYER PROPERTIES AND STATISTICAL CHARACTERISTICS OF PLUMES IN TURBULENT THERMAL CONVECTION

田野

11849038

硕士

力学

黄仕迪

2020-05-28

2020-07-13

哈尔滨工业大学

深圳

热对流在自然界和工程应用中广泛存在。Rayleigh-Bénard（RB）对流是研究热对流现象的经典理想模型。以往的研究表明，这个系统的传热性能主要由温度边界层控制，并且可以通过从边界层脱离生成的羽流进行调控。尽管人们通常认为改变Prandtl（Pr）数（表征工作流体物性的控制参数）会导致边界层特性发生显著变化，但关于RB 系统在不同Pr 数下温度边界层和羽流统计特性的研究还比较缺乏。为此，本文采用Pr 数分别为11.7、20.5、49.3、81.6、145.7 的二甲基硅油作为工作流体，对RB 系统在不同Rayleigh（Ra）数（表征流场驱动强度）下的温度剖面开展了一系列实验工作。实验采用了高度分别为12.6 cm、25.2 cm、37.8 cm 和50.4 cm 的四个对流槽，但所有装置的几何比例保持不变（长高比Gamma_x = 1.0，宽高比Gamma_y = 0.3），从而保证在不同Pr 数的实验中Ra 数均保持在1.93E9

Thermal convection is ubiquitous in nature and industry. Rayleigh-Bénard 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 (Gamma_x = 1.0, Gamma_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 (1.93E9 < Ra < 1.73E10) 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 thermal 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 derivedby Shishkina et al. recently, demonstrating smaller turbulent fluctuation as Pr becomes larger. Furthermore, the shapes of both mean temperature and root-mean-square temperature 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.

湍流热对流 温度边界层 温度剖面 羽流

turbulent convection thermal boundary layer temperature profile plume

中文

联合培养

南方科技大学

田野. 湍流热对流中温度边界层和羽流统计特性的实验研究[D]. 深圳. 哈尔滨工业大学,2020.