Research On Models And Nolinear Characteristics Of Natural Convection In Dual-layer Porous Media

Adopting experimental tests and numerical calculation,this paper carried out solutions and nonlinear characteristics of natural convection in dual-layer porous media about the way of thermal load,the aspect-ratio,different proportion of materials.The solutions of experiments and simulations show that:the paper puts forward to the experimental models and the experimental conditions which can contribute to enhancing and weakening heat transfer;Analyzes the nonlinear characteristics affected by material proportion and calculates Rac,Rab of nonlinear static bifurcation and oscillation;Puts forward to the phenomenon that there is thermal stratification in contact surface and calculates the influence of parameter changing on the solutions which can reorganize distribution of thermal fluid value,attain the goal of thermal stratification,and provide a reference for local enhancing and weakening heat transfer.First of all,the effect of the way of thermal load,the aspect-ratio,different proportion of materials on solutions is explored by experiments.Experiments show that:the temperature of the measuring points with the extension of time eventually tends to be the steady value when the model is thermal loading in horizontal direction,and the temperature of the measuring points with the extension of time appears the nonlinear oscillation when the model is thermal loading from bottom.The greater the proportion of the high thermal conductivity material in the model is,the smaller the temperature difference between interior point is.When the model is thermal loading in horizontal direction,temperature difference of interior measuring points when the aspect-ratio is 4 is smaller than that when the aspect-ratio is 1/4;the temperature of the measuring points in contact surface is determined by the high thermal conductivity material.Secondly,the effect of the way of thermal load,the aspect-ratio,different proportion of materials on solutions is explored by numerical method.numerical results show that:under thermal loading in horizontal direction,(?) is smaller when the aspect-ratio is 4,and (?) is greater when the aspect-ratio is 1/4.When the aspect-ratio is 4,(?) is greater when the model is thermal loading from bottom than (?) when the model is thermal loading in horizontal direction.The solutions with the extension of time appear the nonlinear oscillation when the model is thermal loading from bottom,and different initial fields can lead to nonlinear static bifurcation.Proportion of material is the factor of the corresponding critical Ra of static bifurcation and oscillation,and the critical Rac,Rab is calculated.Hb=2:1,Rac=20.625,Rab=31.875;Hb=1:1,Rac=23.125,Rab=40.625;Hb=1:2,Rac=40,Rab=45.In other words,the proportion of the high thermal conductivity material becomes great,which leads to bigger H_b,the smaller corresponding critical Ra of static bifurcation and oscillation,the smaller intensity of fluid flow at the state of bifurcation and oscillation,and earlier nonlinear characteristics of bifurcate and oscillation.Next,the paper explains the phenomenon that there is thermal stratification in contact surface and calculates the differences of diameter and porosity in the dual-porous media leading to stratification in flow field and temperature field:The influence of the porosity’s difference is largest on solutions in thermal fluid field,and the diameter takes second place.The influence of them is greater on solutions in flow field than that on solutions in temperature field.The way of changing the temperature field is caused by heat-fluid coupling,that is to say,the way of changing the temperature field is caused by changing the fluid field first.The differences of density and specific heat capacity have no effect on numerical solution.Besides,only when the difference of thermal conductivity in the dual-porous media is big enough,the influence of that does have an effect on solution.