Abstract: Population balance approach was presented for isothermalbubbly flows. An averaged bubble number density was introduced into thethree-dimensional two-fluid model to describe the evolution of the structureof the gas bubbles. Moreover, Yao and Morel theoretical models wereintroduced to account for the coalescence and breakage mechanisms ofbubbles. A numerical approach for five variables in bubbly flows ofdifferent locations in a tube: void fraction, Sauter mean diameter,interfacial area concentration, gas and liquid velocities, are carried out.The numerical results showed a good agreement with the experimental datafrom Hibiki et al.