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An experimental study of heat transfer and flow dynamics in a multi-layer Rayleigh-Bénard convection cell.

Will, J.B. (2016) An experimental study of heat transfer and flow dynamics in a multi-layer Rayleigh-Bénard convection cell.

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Abstract:The physics group at CuHK is starting investigation of Rayleigh-Bénard convections cells with two fluid layers. A RB-cell is a system with a heated bottom plate and a cooled top plate. In between a working fluid is located. Due to the buoyancy instability the system will, for Ra ⌧ 104, become unstable and convection will occur. For higher values of Ra this instability evolves into chaos and turbulence. This research is currently being extended into systems with multiple fluid layers. This introduces a dynamic boundary condition at the fluid on fluid interface resulting in new and interesting physics. The Rayleigh-Bénard system has a few characteristic features. The first is the presence of the large scale circulation (LSC). This is a conveyor belt for heat transfer in the cell. The hot liquid gets carried up and the cool liquid down on opposite sides of the cell. The direction of this circulation is unstable and can reverse direction. This is for instance relevant in oceanic flow or even the flow of molten iron in the earths core causing reversals in the geomagnetic field. This instability depends on a number of system parameters which are being examined. A second feature is thermal plumes. In the bulk of the fluid the temperature of the liquid is near constant, no gradients are present. Almost all of the transported heat is in the form of plume-like structures detaching from the thermal boundary layers and traveling along the LSC to the opposite side. The boundary layers are the final feature. These contain almost all of the thermal gradients in the system. The physics of plume formation is based on their interaction with the LSC. The two layer system being investigated is quasi two-dimensional because it is located in a thin cell. The LSC has only a single plane of orientation. This was done in order to have two modes per fluid layer only: the LSC can not rotate around the vertical axis. The most important aspect here is the dynamic boundary. When the flow on the fluid interface of both layers is in the same direction the system state is called viscously coupled (minimizing shear stress) and when the flow is going in opposite direction it is called thermally coupled (maximizing heat exchange). The frequencies and presence of the coupling modes as a function of Ra is one of the aspects we want to investigate. An experimental setup to examine the behavior was constructed and tested. A number of long term experiments were performed in order to obtain initial results which can give insight into what to investigate next. Several avenues of analysis were tried to best interpret the acquired data. It was found that for different values of Ra the coupling mode changes. For lower temperature differences the thermal mode is dominant and for higher temperature differences the viscous mode is most often present. Contrary to cylindrical cells no flow reversals in this system were detected when the system was in a steady state. Also different heat transfer scaling laws are suspected for the two coupling modes. More data needs to be gathered to verify this result. The results are significantly different from experimental data for the single layer convection cell. A number of difficulties pertaining to the experimental setups were found. Making the results less accurate when compared to the single layer system. Also the detection of the circulation direction appears to be more difficult, this could be because it is not clear yet how the fluid in the system is behaving. PIV experiments can possibly help uncover what is going on in the system.
Item Type:Internship Report (Master)
Chinese University of Hong Kong, China
Faculty:ET: Engineering Technology
Subject:52 mechanical engineering
Programme:Mechanical Engineering MSc (60439)
Keywords:Rayleigh-Bénard convection, Multi-layer, Turbulence, Heattransfer, Fluid-dynamics, Experimental
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