| Natural convection heat transfer existed widely in all kinds of natural phenomenaand the engineering technology fields. The natural convection in the closed cavitybecomes more important due to its important applications in engineering. Up to now,many works have been performed for Boussinesq fluid that the density of the fluid wasconsidered as a linear function of temperature. The research of natural convection ofnon-Boussinesq fluid with density maximum is mainly focused on rectangle cavity.Only a few investigations about natural convective of water near its density maximumbetween horizontal cylinders have been reported. This paper takes the naturalconvection of cold water in horizontal annulus as a question for discussion, which isusually used in the cool-thermal energy storage (CTES) system. Both experiment andnumerical simulation are performed to determine the flow patterns under thesteady-state convection and the evolution laws of various flow patterns after losingstability. The effects of geometry parameters, density inversion parameters and Rayleigh(Ra) number on natural convection are discussed. The mechanisms of flow instabilityare revealed and the new heat transfer correlation is obtained. Result can provide areference for the energy conservation of CTES system in engineering and optimaldesign.Physical and mathematical models of natural convection of cold water inhorizontal concentric and eccentric annulus are established when the inner and outerwalls of the horizontal annulus are isothermally held at constant uniform temperatures.Two-and three-dimensional numerical simulations are systematically conducted. Thedistributions of the temperature and the flow fields are obtained and the effects ofvarious parameters, such as the radius ratio, eccentricities, gap aspect ratio, densityinversion parameter and Ra number, on the natural convection are analyzed. The basiccharacteristics of steady and oscillatory convection of fluids with density maximum areascertained. The results indicate that,(1) the flow pattern mainly depends on the densityinversion parameter Θmand Ra number. When the density inversion parameter is small,Rayleigh-Bénard convection appears near the top of annulus, which is similar with theresults of common fluids. However, Rayleigh-Bénard convection appears near thebottom of annulus at a large density inversion parameter. The reverse bi-cellular flowstructure is observed at medium value of the density inversion parameter. The radius ratio has slight effect on flow patterns. The number of Bénard cells decreases with theincrease of the radius ratio.(2) The mechanisms of oscillatory convection underdifferent density inversion are different. The vertical converse density gradients in thetop or bottom of annulus drive respectively the oscillatory flow when the densityinversion parameter is small or large. The horizontal density gradient in the middle ofannulus plays an important role for the formation of oscillatory flow when the densityinversion parameter is in a moderate range.(3) The effect of eccentricity on flowpatterns is small. The eccentricity results in that the flow pattern has the characteristicsof coupling the narrow-gap flow at the narrow region with the large-gap flow at thelarge region of annulus. Therefore, the critical Ra number for the Bénard cells changeswith the eccentricity.(4) The average Nusselt (Nu) number of inner wall increases withthe increase of Ra number and radius ratio. Around Θm=0.5, the reversal bi-cellularflow structure results in that the flow near inner wall is suppressed by the flow cell nearthe outer wall, therefore, the average Nu number is always the least at the same Ranumber. Furthermore, the heat transfer correlations for average Nu number on the innerwall are obtained.(5) The typical flow patterns for different density inversion parameterare observed by the three-dimensional numerical simulation. The results show that thevelocity field and isotherms for R-θ plane is consistent with two-dimensional results.Due to the effect of insulated walls, the critical Ra number for the Bénard cells isdifferent from that for two-dimensional results. The appearing law of cells on R-Z planewhich appear in the top and bottom of annulus under different density inversionparameter is the same as that for two-dimensional results. When density inversionparameter is small with low Ra number, the pathline for different radius ratio is thesame as that for air. When density inversion parameter is large with low Ra number,only the middle region of annulus for large-gap annulus represents the two-dimensionalflow. Whereas the flow for narrow-gap and moderate-gap exhibits three-dimensionalflow. Furthermore, the gap aspect ratio mainly determines the number of cells incross-section plane. However, the gap aspect ratio plays a weak role in the improvementof the heat transfer capability.An experimental system has been designed and built to investigate the naturalconvection heat transfer of cold water in horizontal annulus. In order to enhance heattransfer, the inner cylinder is adopted three-dimensional external finned tube. The innerand outer walls of the horizontal annulus are isothermally held at constant uniformtemperatures. The effect of different parameters of fins on natural convection heat transfer characteristics of cold water near its density maximum in horizontal annulus areanalyzed. The comparison between the experimental results for three-dimensionalexternal finned tube and the bared tube under the same conditions is carried out. Theresults show that three-dimensional external finned tube can enhance the naturalconvection of cold water in horizontal annulus when considering finned ratio. When thetemperature difference is close to8℃, the average Nu number is the least. With theincrease of fin height and the longitudinal fin pitch, the average Nu number increases.Furthermore, the average Nu number decreases with the increase of fin width. However,the effect of fin number is smaller. Especially, the effect of fin width is bigger. |
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