Numerical investigation of abrupt pipe cross-sectional area changes on the occurrence of condensation-induced water hammer

Condensation-induced water hammer (CIWH) is a transient and potentially damaging phenomenon that is commonly encountered in phase-change thermal systems. Abrupt changes in pipe cross-sectional geometry are critical factors that influence the onset and severity of CIWH. By using a one-dimensional simulation code with an improved abrupt area change model, this study investigates the behavior of CIWH in pipelines that experience abrupt cross-sectional area changes. Results demonstrate that abrupt pipe expansion delays CIWH initiation, and the severity of CIWH intensifies due to vapor layer formation. Notably, the expansion ratio that corresponds to the pressure peak increases from 1.027 to 1.110 with rising flow rates. Abrupt pipe contraction exerts a negligible influence on CIWH initiation. However, once the contraction ratio exceeds a critical threshold, the formation of new isolated vapor bubble during liquid column reflux leads to CIWH with significantly larger-amplitude pressure oscillation. The contraction ratio that causes the pressure surge increases from 0.890 to 0.973 with rising flow rates. Furthermore, a momentum theorem-based theoretical formula that incorporates the pipe diameter abrupt change ratios is developed to predict the magnitude of CIWH. These findings provide critical insights into CIWH mitigation strategies for pipeline systems undergoing abrupt geometrical transitions.