Flow reversals in particle-dispersed natural convection in a two-dimensional enclosed square domain

Flow reversals in natural convection of particle-dispersed two-phase flow in a two-dimensional square box are studied by numerical simulation. The Rayleigh number based on the domain side length is set to 10(4). The domain accommodates 11(2) neutrally buoyant circular particles, and the thermal conductivity of the particle is set to 10(2) times higher than the ambient fluid. The particle-dispersed flow driven by buoyancy develops into circulating flow, which transports particles into a diagonal pair of corner regions of the domain. The particles vertically aligned in the corner regions are a strong source of moment of buoyancy in the counterconvective direction, and flow reversals take place at the intervals of several hundred convective-time scales. The mechanism is different from that of the reversals or oscillation in single-phase or particle-dispersed natural convection reported in the literature. Thermal effect of the vertically aligned particles in a corner is modeled by nondimensionalized heat flux, and a cross-coupled sum of those of the four corners is found to be a precursor indicator of the reversal events. The investigation on the effects of three major parameters (Rayleigh number, conductivity ratio, and average interparticle spacing) suggests that the reversals occur in a small region in the parameter space.