Numerical methods for predicting poly-dispersed bubbly flows in bubble columns are indispensable in the design of bubble column reactors. The objectives of the present study are (1) to experimentally investigate the effects of a bubble size distribution on poly-dispersed bubbly flows in an open vessel and (2) to examine the applicability of an (N+2)-field model (NP2) to poly-dispersed bubbly flows. Distributions of void fraction and liquid velocity in air-water bubble plumes in the vessel are measured using an experimental setup with a bubble injection device by which the ratio of the volume flow rate of large bubbles to that of small bubbles is controlled to a desired value. The main conclusions obtained are as follows: (1) experimental data on the effects of a bubble size distribution on air-water bubble plumes are obtained, and (2) NP2 gives good predictions for poly-dispersed bubble plumes.