Experiments were conducted to study the temperature field,flow field,3-D characteristics and self-sustained oscillation of the natural air convection in a 3-D shallow cavity which was heated from underneath. The experiments were conducted by the methods of laser holographic interferometry photography and smoke visualization. To ensure the temperature of both plates to be constant and the accuracy of laser interferometer,the instrument was calibrated and error has was analyzed. The results showed that the flow field was stable at lower Rayleigh numbers. When the Rayleigh number increased,the flow field became instable and the isotherms distorted. The rolls merged at Ra=12500 and formed along both axes when Ra was over 18500. The air rose from the middle and descended in the circumference while the flow field and heat transfer converted to 3-D characteristic from 2-D characteristic. When the Rayleigh number increased,the flow field became more instable. The rolls became irregular and time dependent when Ra≥Rac(=30500) ,which is nonlinear. When lateral walls were heated or cooled,the rolls merged along the long axis and two rolls formed along the short axis. Three rolls formed occasionally in the process.
Natural convection of air is numerically simulated in a 3-dimensional rectangular cavity heated from below using SIMPLE algorithm with a QUICK scheme.The results suggest that when all lateral walls are adiabatic,the fluid rolls occur along the long axis.When the Rayleigh number is smaller,the flow is of 2-dimensional character,and the rolls shapes are similar.The average Nusselt numbers in the central part of the cavity are similar.The average Nusselt numbers in the part near by the cavity are different.According to the comparison of 3-dimensional results with 2-dimensional results,the flow patterns and heat transfer in the central part of the cavity can be assumed as a 2-dimensional flow,While those in the part near by the cavity can not.With increasing Rayleigh number,the flow is 3-dimensional characteristic.The 3-dimensional result accords with the experimental result.When all lateral walls are adiabatic,the ten rolls occur along the long axis.But when lateral walls are heated or cooled,the rolls disappear along the long axis and two rolls occur along the short axis.The rotation direction of the rolls is reversed.When Rayleigh number is over some critical value,flow and heat transfer will be asymmetry,indicating unsteady oscillation occurs.By nonlinear analyses,it is shown that with increasing Rayleigh number,flow and heat transfer will change from steady state to unsteady state through HOPF bifurcation,and transition to chaos will occur through multi-periodical oscillation.
ZHAN NaiYan1,2,XU PeiWei1,SUN ShiMei,YANG Mo2 & LIU Jun3 1 Scool of Municipal and Environmental Engineering,Jilin Institute of Architectural and Civil Engineering,Changchun 130021,China
