The bubble size is a key parameter appearing in closure relations for the Euler-Euler two-fluid model. The bubble size distribution is established as a result of bubble coalescence and breakup processes. These processes are very complex, and therefore, a two-step procedure is adopted for model validation where, in a first step, measured values are substituted for the bubble size distribution. In this way, the uncertainties of the less developed modeling for bubble coalescence and breakup are bypassed and a validation of the other parts of the overall model becomes possible. In a second step, the previously qualified models for bubble forces and bubble-induced turbulence are used without any change and the validity of the models for bubble coalescence and breakup can be assessed.