Steam discharged into subcooled water is investigated experimentally to demonstrate the direct contact condensation phenomena in nuclear reactor safety system and underwater propulsion apparatus. The steam jet condenses to various shapes at different thermal hydraulic conditions. A condensation regime diagram is drawn to classify the regime for different flow patterns, among which there are three typical shapes of steam plume characterizing the chugging, condensation oscillation, stable condensation regime (Figure 1). The flow region can be separated into three parts—vapor, water and two-phase regions, and the white patch in the image indicating the two-phase region is a mixture of condensed vapor and subcooled water. Three typical stages of bubble motions—growth (subimage 1 to 6, Figure 2), necking (subimage 7 to 10, Figure 2), and detachment (subimage 11 to 13, Figure 2)—are demonstrated. The bubble diameter reaches the maximum at the necking stage and remains approximately invariant with the connecting neck prolonging for a period. A series of sequent photos exhibits shape transformations at the stable condensation regime, implying that the steam plume grows and shortens periodically due to comprehensive effects of injection, viscosity damping and condensation (Figure 3). The dimensionless penetration length, defined as the ratio of penetration length to nozzle diameter, is in the range of 8.23–11.67 in the Figure 3. The majority of previous literatures present the average dimensionless penetration length which is closely related with time-averaged heat transfer characteristic. However, variations of steam plume are proven to account for pressure oscillation phenomena by the transient visualization investigations, in which the first dominant frequency acquired from the FFT domain graph of pressure signal is consistent with the period of steam plume variations. The second dominant frequency is verified to be caused by oscillations of detached bubbles (subimage 8 and 9, Figure 3) in the research.

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