| Summary: | 博士 === 國立成功大學 === 機械工程學系 === 87 === The present investigation consists of numerical prediction and/or experimental visualization on the transition of natural convection of cold water in vertical enclosures. Using a finite difference method, numerical simulations of the transient natural convection of cold water contained in vertical enclosures of height/width ratio 8 (cylindrical annulus or rectangular enclosure) have been performed by increasing the Rayleigh number step by step under two different values of the density inversion parameter of 0.4 and 0.5, respectively. The range of the Rayleigh number considered is from to . The radius ratio of the cylindrical annulus is fixed at 2. The three dimensional numerical simulation focus on the natural convection of cold water in the rectangular enclosure under the density inversion parameter of 0.5 to explore the effect of the depth/width ratio of the enclosure on the instability mechanism. The range of the Rayleigh number considered for the three-dimensional simulations is from to . The evolutions of fluid flow structure and temperature distribution of cold water in the enclosures are visualized numerically by means of contour maps of streamline and/or velocity vector field and isotherm. Furthermore, the instability mechanism responsible for the transition to unsteady natural convection of cold water and its spatial distribution within the enclosures are illustrated by means of the contour maps of fluctuation magnitude of flow variables or compositions of the fluctuating kinetic energy. The experimental work in the present study mainly focuses on the flow and temperature visualizations, by means of a thermochromic liquid crystal, in a test cell, which was designed and constructed to mimic the physical configuration considered in the numerical simulations.
The results of numerical simulation clearly reveal that the value of density inversion parameter predominantly determines the profile of maximum density contour, which demarcates the two contra-rotating circulation flow structures of cold water prevailed in the enclosure. The instability mechanism appears to initiate by the Kelvin-Helmholtz instability due to shear imbalance along the sinking jet flow structure emanating from the ceiling of enclosure, which results in wavy distortion of the maximum density contour of cold water. As a result of the wavy maximum density contour, unstable density stratification (the Rayleigh-Benard instability) is induced. The vertical boundary layer flows along the vertical walls are thereby perturbed and the Tollmien-Schlichting instability arises accordingly. The laminar transition to self-sustained oscillatory convection has a strong bearing with the density inversion phenomenon of cold water. The results of two- or three-dimensional numerical simulations reveal similar mechanism for the unsteadiness of cold water in the rectangular enclosure. The production of fluctuating kinetic energy is completely contributed by the buoyancy.
The results of experimental visualization show that growing, merging and upward-drifting of the secondary vortex is accompanied wavy movement with the maximum density contour. Comparison between the numerical prediction and the experimental visualization reveals a fair agreement.
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