A series of: experiments has been conducted to study mixing and hydrodynamic behaviour of a downward facing sparger in a turbulent fluidized bed reactor. Using pressure measurement techniques, two flow discharge modes were identified around the sparger by injecting a gas tracer into the bed. These are bubbling and jetting conditions. Experimental results show that, under bubbling conditions, bubbles tend to keep their identity, while under jetting conditions a highly turbulent heterogeneous area is formed around the injection point. Due to attrition and erosion of internal heating or cooling surfaces in industrial reactors, the dominant discharge mode is the bubbling pattern. Therefore, in this Investigation, the bubbling pattern is studied by measuring the radial and axial dispersion of gas tracer injected to a hot fluidized bed reactor of 20 cm diameter of FCC and sand particles. A three-phase model is also proposed in order to predict the mixing length. In addition, the effect of sparger configuration on tracer gas mixing was examined for FCC particles.