The aim of this study was to develop a population balance model to predict the resultant bubble size distribution and gas void fraction in the wake region below a ventilated cavity in down-flowing liquid inside a pipe. The model includes (1) the gas entrainment process from the ventilated cavity; (2) fluid flow and specific energy dissipation rate in the wake region, comprising both wall-jet and recirculation-vortex zones; (3) bubble breakup and coalescence events, determined by a critical Weber number criterion and bubble interaction and film drainage times, respectively; and finally (4) gas void fraction in the wake and recirculation of gas back into the base of the ventilated cavity. The model predictions are compared with experimental measurements for an air-water system. The model successfully predicts the length of the wake region and the gas void fraction within it, and the average and resultant bubble size distributions immediately downstream of the wake. The average bubble diameter was of the order 2-3 mm for each of the cases studied. In applying the model, attention was focused on the specific energy dissipation rate distribution between the wall-jet and recirculating-vortex zones within the wake. As part of the analysis Fluent computational fluid dynamics simulation was carried out and the results from this work are also presented in the paper.