Modeling of transition from stratified flow to condensation-induced water hammer in horizontal steam-water pipes

Kelvin-Helmholtz (K-H) instability theory is extensively used to predict the transition from stratified flow to slug flow. This study developed a one-dimensional (1-D) model based on K-H theory and considered the variation of axial wave amplitude to predict the transition from stratified flow to condensation-induced water hammer (CIWH). The prediction results of the present model were compared with the experimental data and existing models. The comparison shows that the present model could reasonably predict critical instability, which is the boundary between stratified flow and CIWH. Critical instability is divided into three types according to the wave amplitude of critical instability. Critical instability occurs at the upper limit, middle, and starting point of the exponential growth of wave amplitude under low, medium, and high liquid levels. The effects of major parameters, including liquid level, water temperature, pipe diameter, and pipe length, on the critical inlet steam Froude number and position of critical instability were analyzed. When the liquid level increases, the critical inlet steam Froude number decreases, then increases, and then decreases, and the position of critical instability moves downstream, then upstream, and finally turns to the steam inlet. The effect of water temperature, pipe diameter, and pipe length on the critical instability depends on the liquid level and is complex.