Hyperbolic model for transient high gas fraction gas-liquid flows in vertical pipes. Steady-state results were compared with 1500+ data points from eight sources. Pressure and flow rate transients were simulated in a 42-m long, 49-mm ID facility. Successful prediction of transient churn-annular behavior observed experimentally. Transient pressure drop and holdup were predicted with AAD of 14.5% and 7.9%.