Influence of air velocity and powder bed height on local density and float–sink of spheres in a gas–solid fluidized bed

•Local fluidized bed density is either minimal at mid-height or gradually increases with height at higher air velocities.•Spheres float–sink behavior changes dramatically with the air velocity and the powder bed height.•Local variation of the motion of the air bubbles and the fluidized medium would affect the fluid force acting on the sphere.