A specially designed high-density downward gas-solids flow fluidized bed, consisting of a solids reservoir tank, a solid distributor, an acrylic downer (3.0 m high), a gas-solid separator and a solid receiver tank, has been developed to study the fundamentals of high-density downward gas-solids flow characteristics in the downer reactor. Experiments have been carried out using two types of sand particles (rho(p) = 2600 kg/m(3), d(p) = 80 and 320 mu m) as bed materials with superficial downward gas velocities (U-gd) ranging from 0 to 5 m/s. Particle diameter has a great effect on the solids mass flux (G(s)) and solids hold-up (epsilon(s)) in the downer. It is found that G(s) in a 50 mm diameter downer reached up to 1876 and 815 kg/m(2).s, with average epsilon(s) up to 0.16 and 0.07 in the developed region, for 80 and 320 mu m sand, respectively. The solids hold-up decreased with increasing U-gd at a fixed solids mass flux in the developed region. By considering the effects of downer diameters, operating conditions, and solids properties, a correlation for the prediction of dense solids hold-up (epsilon(s)*>0.07) in the developed region of the downer is proposed based on experimental data from the literature and this study.