Direct Numerical Simulation Of Natural Convection In A Square Cavity Filled With Fixed Particles

The phenomenon of particle flow with heat transfer is common in nature and various industry fields. However, studies on this complex internal flow and its heat transfer characteristics are still insufficient. In this paper, the problem of natural convection in square cavity with fixed particles was direct simulated based on the fictitious domain method and two cases with different boundary conditions were investigated.For the problem of natural convection in porous medium, the structure of porous media was approximated by distributing solid particles in the cavity. The influences of the particles' number, distributing pattern and particle shape on the heat transfer of the internal flow were analyzed. The results reveal that, with a high Rayleigh number, the heat of natural convection is mainly transferred by the circumfluence which is induced by the heat convection around the boundary. There is a high-speed flow area near the vertical walls. Its velocity increases with the Ra number, while the width of this area decreases with the Ra number. When the Ra number and the solid volume fraction are kept constant, the mean Nusselt number of the boundary decreases with the increase of the particles number in the particle regular distributing cases, which indicates the decrease of the heat transfer efficiency. Further analysis on the flow fields shows that the effects of the distance between the outermost row of particles and the vertical boundary on the heat transfer efficiency are significant. For the random particle distributing cases, the heat transfer efficiency is higher than that of the regular particle distributing cases at a high Ra number. In addition, the influence of the particle shape on heat transfer is weak.The spontaneous flow reversals were studied for the Rayleigh-Benard convection. The periods of reversals are exponential growth as the Ra number increase when there is no particle, and the reversals occur only in a certain range of Ra number and Pr number. The reversal periods are shortened if a particle is fixed in the cavity center. It increases linearly as the increase of the Ra number, and decrease when the particle radius increases. Changing the particle position along the X=0.5, the reversal periods is largest when the particle is fixed in the cavity center. The periods decrease no matter increasing or decreasing the particle's vertical position, and its trend is symmetrical with the distance from the center. Diagonal arrangement of two particles which located at the two vertices of the diagonal corner eddies in the cavity can prevent the occurrence of the flow reversals.