Three-dimensional Numerical Simulation Of Double-diffusive Rayleigh-bénard Convection In A Cylindrical Container Of Aspect Ratio2for Positive Buoyancy Ratio

Double-diffusive convection is a phenomenon that usually coursed bytemperature and concentration gradients, and many steady flow structures can be foundin this convection. Investigating the formation of these flow structures and how theydevelop, can not only help people to understand some natural phenomena such as oceancurrent and the movement of the polluted air, but also provide important theory base tomany industrial processes, such as crystal growth, chemical gas depositing and drying.A three-dimensional numerical simulation was conducted to study the steady statesmultiplicity and flow pattern evolution of Rayleigh-Bénard convection in a cylindricalcontainer with aspect ration of2. In the simulation, we use a binary mixture of10%(mass fraction) isopropanol-water solution, also consider vertical temperature andconcentration gradients. Flow state multiplicity, transition among flow patterns, and thebasic characteristics of bifurcation sequences are analyzed. The stability ranges of allthe steady patterns are determined.The results show that:(1) in double-diffusive Rayleigh-Bénard convection, thesystem is much easier to lose its stability from conductive state and leads to convectivestate. The critical Rayleigh number decreases with the increase of the buoyancy ratio.(2)The coexisting phenomenon of multiple steady flow states in the same Rayleigh numberis observed in all different buoyancy ratios. As the buoyancy ratio increase, less steadyflow patterns are found in the system. Steady two-roll, three-roll and four-roll patternswhich are common in pure fluid disappear gradually. However, some other specialpatterns are observed.(3) The flow initial condition and the history of the patternevolution, as well as the equation parameters are significant to the pattern formation andevolution. We also observed the hysteresis phenomenon in the flow pattern transition.(4)In the condition of N=0.5and N=1, after the system lose its stability from conductivestate, with the increase of Rayleigh number, a period of unsteady flow occurs before thesystem turns into steady convective state because of the effect of the concentrationgradient. Also, when the flow pattern is turning into other patterns as the Rayleighnumber changes, oscillatory flow occurs near the critical point between differentpatterns. This phenomenon does not happen when N=0.(5) When N=0, every steadyflow pattern turns into many other flow patterns as the Rayleigh number changes.However, with the increase of buoyancy ratio, less flow patterns are shown as the Rayleigh changes. For example, one-torus and four-spoke pattern barely change whenN=1.(6) The increase of buoyancy ratio will lead to an enhancement of heat transfer ofthe system.