Structure of plumes in Turbulent Rayleigh-Bénard convection

Abstract:

Rayleigh-Bénard convection can be seen everywhere in the environment - convection in the sun, in the earth atmosphere and in earth mantle between the hot core and surface of earth [1]. Typically any region which is heated from below and cooled from the top creates Rayleigh-Bénard convection. Plumes are generated as the fluid inside the cell exchanges heat between the hot and cold boundary layers. These plumes form additional large scale circulation of fluid inside the cell thereby playing crucial role in the convection. Hence it is necessary to study the structure and effect of these plumes on the Rayleigh-Bénard convection which is the main objective of this work.
The experimental setup consists of a transparent cylindrical cell which is heated from bottom by a copper plate and cooled from the top by circulating cooling water. Two color Laser Induced Fluorescence technique [2] is used to measure the temperature inside the cell and also to visualize the plume structure and their motion inside the cell. The cell is illuminated by the exciting laser sheet which is absorbed by the fluorescent dyes and the resulting fluorescent signal intensity gives the temperature inside the cell. Two temperature sensitive dyes are used. Fluorescein and Rhodamine B. Two narrow spectral band filters such as 540-560 nm and 625-635nm which have different sensitivity on temperature are placed in front of the camera lens in order to eliminate the effect of local laser intensity on the resultant signal. Experiments are being carried out at several top and bottom plate temperature differences to find out their effect on the resultant heat transfer and thereby on the plume structure and motion. Images obtained from these experiments can be visualized to characterize the plumes inside the cell. The kinetics of heat transfer inside the cell can be clearly explained by proper visualization and analysis of the plume structures. Also the plane of circulation of hot plumes ascending from one end of the wall and of descending cold plumes on the other end can be clearly visualized. Mechanisms causing this phenomenon can be explained by further analysis of the plume structures.
The shape, temperature and hydrodynamics of plumes will also be simulated numerically and compared with experimental observation.
A further objective of this work is to study plumes close to the boundary layers and also to investigate the effect of rotation on plumes in a rotating cell.

Bibilography :
1. Ahlers G., Grossmann S., Lohse D. (2009) Heat transfer and large scale dynamics in turbulent Rayleigh-Bénard convection, Rev. Mod. Phys. 81, 503:537 .
2. Chaze W., Cabalina O., Castanet G., Lemoine F., (2016) The saturation of the fluorescence and its consequences for laser-induced fluorescence thermometry in liquid flows , Exp. in Fluids, 58, 1-18.