Three-dimensional Numerical Simulation Of Double-diffusive Rayleigh-bénard Convection In Cylindrical Enclosure

Double-diffusive convection is one kind of widespread physical phenomena. Investigation on the complex dynamic behavior of the flow, can not only help people to gain a clearer vision of some natural phenomena, such as the movement of the polluted air and thermohaline ocean circulation, but also provide important theoretical basis to many industrial processes, such as solar pond,crystal growth and reactor cooling system. Up to now, the previous research on double-diffusive convection are mainly focused on the cases of horizontal thermal and solutal gradients. However, research result on double-diffusive convection heated and soluted from below in a cylindrical enclosure is lacked, especially for the pattern formation and transformation. This thesis presented a series of three-dimensional numerical simulations on the double-diffusive convection in a cylindrical enclosure filled with the isopropanol/water mixture with the vertical temperature and concentration gradients. For different aspect ratio and buoyancy ratio, the critical Rayleigh numbers and flow patterns are determined. The diversity of fow state, transition process among flow patterns and the bifurcation sequences are analyzed. Furthermore, the stability range of every pattern is exhibited.The results show that:(1) the critical Rayleigh number at the primary threshold is strongly influenced by the aspect ratio ? and buoyancy ratio N in double-diffusive Rayleigh-Bénard convection. For cooperative buoyancy convection, the concentration gradient has an acceerative effect to the flow destabilizing, and the critical Rayleigh number decreases. By contrast, the system stability is enhanced and the critical Rayleigh number increases for opposing buoyancy convection.(2) The aspect ratio of the cylinder has a distinct influence on the diversity of the steady flow states, and the increase of the aspect ratio will prompt the increase of the number of rolls in fluid.(3) The buoyancy ratio has an important effect on bifurcation sequences. When N=0, the flow after the onset of convection is always steady. When N=1, a dipole pattern with non-periodic oscillation appears foremost, and then is followed by four typical steady flow states. When N=-0.5, the flow pattern after the onset of convection is the unsteady three-roll state, which is a periodic flow with the constant finite amplitude.(4) When N=0, using different flow patterns as the initial condition, when the Rayleigh number changes, many other flow patterns are determined. When the buoyancy ratio is positive, less flow patterns are detected when changing the Rayleigh number. For example, four-spoke and one-torus pattern remain stable Within the whole calculation when N=1. However, no steady flow pattern is obtained when N=-0.5, only five types of oscillatory flows are detected, which are characterized by the azimuthal rotation distinctly.(5) The pattern formation and transformation is not only depend on the controls parameter, but also affected by the initial condition, which give a reasonable explanation about the coexistence of patterns.(6) The average Nusselt number increases with the Rayleigh number, and is slightly influenced by the flow pattern.