A simplified mathematical model including nonequilibrium two-phase transient flow was developed based on experimental results with a heated annular tube in a low pressure system. The model determines operating limits to avoid flow instability transients and also predicts system behavior during a transient. Correlations describe two-phase flow characteristics within controlling parts of the system subject to flashing and subcooled nucleate boiling. Two-phase flow experimental data can be used to extend this model to other one-dimensional systems. Accordingly, the model should be useful to designers and analysts of systems subject to two-phase flow for which some experimental data are available.