Abstract: Considering the bubble coalescence and break-up,the CFD simulations for air/water two-phase flow in 3×3 rod bundles with grid spacers have been performed with MUSIG model.The simulation is sensitive with the bubble diameter size but not with the inlet void fraction.The small diameter bubbles are the main effect of void fraction distribution nearby the downstream of the space grid,while the large diameter bubbles are main effect of void fraction distribution at the farther downstream of the space grid.Considering the effect of inlet air-water flow quality,geometry and pressure on bubble max diameter,this paper gives a relation for simulating the bubble with maximum diameter based on which is dealt with numerical simulation for rod bundles with grid spacers is set,and the advice of simulation method and model setting is given.The calculation results show that the shape and peak of void fraction distribution accord well with the experiment,and this simulation method can predict rationally the distribution of two phase flow in complicated channels.