The so-called resistance characteristic of fluidized bed refers to the relationship of the resistance of the air passing through the material layer  and the fluidization air in cold-state calculated according to section area. Within the material layer of bubbling bed, the area where the concentration of solid particles is very low and its behavior is similar to the rising bubbles referred to as a bubble phase. The local area with higher concentration of particles in the bubbling bed and still contains a small amount of gas and its behavior is similar to liquid is known as the emulsion phase.