Research On Thermal Convection Of Fluid Near Its Density Maximum With Complex Boundary Conditions

Thermal convection exists widely in all kinds of natural phenomena andengineering technology fields. The natural convection in the closed cavity has become astudy hotspot due to its important applications in engineering. Up to now, many workshave been performed for common fluid that the density of which is considered as alinear function of temperature, and thermal convection of the fluid with the densitymaximum is mainly focused on horizontal annuli. However, research result on thermalconvection of the fluid near its density maximum with complex geometrical and thermalboundary conditions is lacked. In this thesis, a systematic study on thermal convectionof cold water near its density maximum with complex boundary conditions is performed,aiming at the thermal convection in the equipment of phase change cool-thermal energystorage system. The formation and evolution of the flow structures at different Rayleighnumber (Ra), density inversion parameter and geometrical configuration in annularenclosures with various complex configurations, rectangular and cylindrical cavities arepresented. The effect of the main parameters on the natural convection of cold water inannular enclosures with various complex configurations and the Rayleigh-Bénard (R-B)convection of cold water in rectangular and cylindrical cavities is analyzed anddiscussed. The study results can not only has important academic significance ondeveloping the theory of natural convection of the fluid with the density maximum withcomplex configuration and extending the research field of R-B convection, but alsoprovide a reference for design and optimization of phase change cool-thermal energystorage system.Firstly, a numerical investigation on the natural convection of cold water in ellipticannular enclosures is carried out to obtain the flow and thermal fields and heat transfercharacteristic along the inner wall at different Ra, density inversion parameter, aspectratio, elliptical ratio and inclination angle. Effects of various main parameters on thecharacteristic of flow and heat transfer are discussed. The results indicate thatconduction is the dominant mode of heat transfer at a small Ra, and the convection heattransfer becomes dominant with the increase of Ra. When Ra exceeds to a certain value,the flow is out of stability, but the mechanisms of flow instability are different forvarious density inversion parameters. The flow structure and heat transfer ability dependstrongly on the density inversion parameter. When the density inversion parameter is around0.5, there is a bi-cellular flow pattern consisting of two cells that suppressedeach other, which results in the weakest heat transfer ability. The average Nusseltnumber (Nu) increases with the decrease of the aspect ratio and the elliptical ratio. Theinclination angle modifies the flow and thermal field apparently. The secondary cells donot appear with a pair when the enclosure is tilted, which enhances the local heattransfer ability in the lower region while the overall heat transfer ability is nearlyunaffected.Secondly, the difference of flow and thermal field and heat transfer ability amongvarious annular enclosures with complex configurations is compared and discussed. Itshows that the R-B convection occurs in the top or bottom region for the R-C, T-C, E-Sand E-T configurations and the cell splitting phenomenon happens at a large aspect ratioin the R-C, D-C, T-C, E-S and E-T configurations. As expected, the wall configurationhas an importanr effect on the heat transfer on the inner wall. For the differentconfiguration of the outer wall, the heat transfer ability of the T-C configuration is thelargest among these configurations at a small Ra while R-C configuration becomes thelargest in heat transfer at a large Ra. For the different inner wall, the E-D configurationachieves the best heat transfer performance in the whole range of the Ra at small densityinversion parameter, and E-T and E-D configurations have the best heat transfer abilityat the large density inversion parameter. Furthermore, the heat transfer correlations forvarious configurations have been proposed by the multi-factor linear regression.In addition, the thresholds for the onset of convection, the formation and evolutionof complex flow pattern for the R-B convection of cold water with density maximum inrectangular and cylindrical cavities are assured. Results show that the density inversionplays a key role in the R-B convection. The critical Ra for the onset of convection islarger than that of the common fluid, and it increases with the increase of the densityinversion parameter. The flow structures are much more different from each other atdifferent inversion parameters. Conductive sidewall enhances the stability of the R-Bconvection, and the flow structures and bifurcation series are different with the twothermal boundary conditions of sidewalls. The increase of aspect ratio reduces thestability of the flow. When the aspect ratio is small, most of the flow patterns aresingle-roll or two-roll structure. The R-B convection in cylindrical cavity is more stablethan that in rectangular cavity. However, many multiple rolls flow patterns appear, andthe flow structures of cylindrical cavity are much more different from that ofrectangular cavity when the aspect ratio is large. Furthermore, center symmetric flow patterns exist for the R-B convection in cylindrical cavity. At the same Ra, thecoexistence of multiple stable states is observed and the heat transfer rates for thesestates are slightly different from each other.Finally, an experimental investigation on the heat transfer of the R-B convection ofcold water with the density maximum in the cylindrical cavity is carried out. The resultsof experiment are in good agreement with those of numerical simulation. The averageNu decreases with the increase of density inversion parameter. The increase of Ra canremarkably enhance the heat transfer on the wall while the increase of aspect ratio justenhances the heat transfer ability to a certain extent. Furthermore, the heat transfercorrelation for the R-B convection in the cylindrical cavity has been proposed by themulti-factor linear regression.