Two-Phase region, in condensing flow undergoing complete condensation inside a tube, acts as an amplifier of any small internal or external disturbances. A small, externally imposed change in the inlet vapor flow rate, or heat flux, leads to substantial surges in the outlet liquid flow rate, including the possibility of flow reversals. Also, if the conditions are right, slight internal disturbances as a result of vapor/liquid interaction, can lead to sustained oscillations of large amplitude, such as in the outlet liquid flow rate. Such surging characteristics coupled with rapid bubble collapse may lead to water hammer phenomenon. This paper will summarize both experimental observations and theoretical models as a result of externally imposed, or internally induced, flow changes in condensing flows. The physics of the processes, including liquid/vapor density ratio, vapor compressibility, bubble collapse, and liquid inertia will be highlighted. The condensing flow stability criterion will be used to provide a possible physical and an analytical basis for the catastrophic piping failure due to a condensation induced water hammer.

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