The University of Kentucky civil and mechanical engineering department has developed comprehensive and inexpensive hydraulic programs dealing with slurry and network flow as well as transient flow (surge analysis). They offer regularly scheduled training sessions and siminars on the use of the programs.
For simple water/air or "black-oil" two-phase flow modeling, there are several published correlations (Lockhart-Martinelli, Dukler-Eaton, Beggs-Brill, etc.) that can give good results on simple systems. If you need compositional analysis, heat balance, and/or network analysis, try one of the major commercial packages (PipePhase, PipeSim, PipeFlo, etc.)