The function of volume fraction η versus flow rate Q was determined. It is found that η decreases exponentially with increase of Q. The volume fraction η will reach saturation when Q is reduced or increased　to certain extent. The increase of η slows down rapidly with decrease of Q when D/d decreases to a critical value. The decrease of η slows rapidly with increase of Q when d decreases to a critical size. The phenomenon is explained by the dissipation of granules, effect of air, effect of bottleneck and collision theory.Theoretical result about η-Q is in agreement with the experimental result.