Research On Thermal Oscillation Effect Of Rayleigh-Bénard Convection In Cavity And Particle Packed Porous Media

The particle-packed porous media is one of the most important building insulation materials.The ideal porous insulating layer should transfer heat in the form of heat conduction.However,in practice,the interior natural convection reduces the thermal insulation performance of porous materials,which may result in the failed design for keeping warm.Therefore,the heat convection in porous material,and the influence of pore parameters on the system heat transfer,become the hot issues in the current researches on porous material.In this thesis,it takes the particle-packed porous media as the prototype to study the Rayleigh-Bénard convection?RBC?in porous materials,with the approaches of theoretical analysis,numerical simulation and experimental verification.And from the perspective of RBC thermal oscillation effects,the mechanism in which the pore parameters influence the system heat-transfer characteristics is mainly studied.The RBC thermal oscillation effects refers to the movement of the structures transferring heat in the flow field,which causes the oscillation of the system heattransfer capacity.By studying the features of thermal oscillation effects,to establish the relationship between the macro-scope heat transfer parameters and micro flow structures is the main idea of present works to analyze the mechanism of convective heat transfer in porous RBC system.This research is divided into three progressive levels: establishing the theory of thermal oscillation effects with the simulation study on standard RBC system;verifying this theory with the experimental study on standard RBC system;and applying this theory to analyze the heat transfer features of porous RBC system.In this paper,the standard RBC system in the square cavity filled with air is simulated in the large range of Ra number from laminar states to strong turbulence,with the Lattice Boltzmann Method?LBM?and the improved grid-setting strategy.The simulation results show that the dimensionless character-frequencies of the system character-signals present a discrete level-distribution with the increase of Ra number.According to each frequency level,several motions of temperature structures are recognized.It is confirmed that the thermal oscillation in laminar and strong turbulent RBC is dominated by the fluctuation of large-scale circulation,and the dimensionless character-frequency satisfies the scaling law of Ra^-1/6.Meanwhile,the thermal oscillation in weak turbulent RBC is dominated by plume motion,and the dimensionless character-frequency satisfies the scaling law of Ra^2/7 featuring two developing stages.According to the transition among the different scaling laws,the mechanism of several special phenomena in standard RBC system,such as the deflection of heat transfer capacity and the inversion of large-scale circulation,is revealed and detailed studied.Therefore,the theory of thermal oscillation effects in standard RBC is established based on the system of scaling laws.And the experimental results of standard RBC in quasi-2D cavities verify the validity of the core conclusions in this theory.Based on the theory of thermal oscillation effects in standard RBC system,the porous RBC system with varying porosity and different filler diameters is simulated in the range of Ra=10⁶?1.5×10⁸.The statistical results of the different samples with the same pore parameters show that the thermal oscillation effect in porous RBC system obeys to the basic forms of the scaling laws in standard RBC system.The offset coefficient in each scaling law decreases when increasing the specific surface area.The proportion coefficient only decreases in weak turbulent states when reducing the ratio of filler size to porosity.The results show that the pore parameters have a greater influence on the heat transfer characteristics of low-Ra systems.According to the rules of thermal oscillation effect changing with pore parameters,some abrupt changes of heat transfer capacity in the low-Ra porous RBC system are predicted when the filler size is too small or the porosity is too low.And the mechanism and conditions of these abrupt changes are detailed discussed.The thermal oscillation effects in standard RBC system and porous RBC system are deeply studied in present works,and the related conclusions are applied to the mechanism analysis of the heat transfer characteristics in porous media.It not only provides a theoretical basis for the structural optimization of porous insulation materials,but also provides a new research idea for the study of other heat transfer problems in porous media.