Study Of The Stability Of RB Natural Convection In A Fluid With PCM Particles

In such an age as the shortage of energy becomes more and more serious, it is of great importance to advocate energy conservation. As a kind of material which can store the energy via phase-changing, the microencapsulated PCM slurry can also enhance the convective heat transfer performance. It has a promising potential in the practical application. The neutral instability of Rayleigh-Bénard convection in such a fluid is studied in this thesis.When the upper and lower boundaries are taken to be case of perfect heat conductors, the onset of Rayleigh-Bénard convection in a fluid layer with phase-change-material particles is analyzed using Galerkin method. An analytical solution has been obtained and a formula suitable for the engineering application to calculate the Rayleigh number Ra is deduced. Its correctness is proved by the comparison with the previous numerical result.When considering the influence of the boundaries, the critical Rayleigh number Racr is obtained through the solution of linear stability by a numerical differential method. At the same time the effects of various boundary conditions on Racr are investigated. The result shows that the tendency of Racr's variation with the phase angleΨand the amplitude b is the same as the situation without boundary conditions. But the value of Racr will vary wth different boundary conditions.Lattice Boltzmann Method is a new way of numerical simulation developed in the last few decades, which has many advantages over the traditional numerical method. Its basics including the background, models and boundary condition treatment are briefly introduced in this thesis. Several typical problems in fluid flow and heat transfer, such as the lid-driven cavity flow, Poiseuille flow, channel entrance flow with different thermal boundary condition and RB convection are numerically simulated by LBM, the results are validated by comparing with the previous work in literature. The Rayleigh-Bénard convection for fluids having a variable viscosity or specific heat is simulated. The result shows that it's feasible to use LBM to determine the critical conditions for fluids with large fluctuation of thermal properties with temperature, but some improvement are needed for the available LBM models instead of by changingτf orτg alone. Finaly, several suggestions about the future work in LBM method are proposed based on the problems encountered during the research.