Experimental study of steam chugging phenomena in direct contact condensation

The focus of the study is chugging phenomenon occurring during direct contact condensation (DCC) of steam in the wetwell of BWRs. Chugging is a depressurization induced by the bubble implosion at the pipe outlet that causes a cyclic suction of water inside the pipe. The presence of high pressure peaks is characteristic of this regime. The purpose of this study is to investigate the mechanisms of every phase composing a chugging cycle and to provide experimental results useful for the validation and development of CFD approaches. In order to achieve these objectives, an experimental campaign was performed. The steam mass flux range was 5.5-19.5 kg/(m2s) while the pool temperature one was 19-46.5 °C. Two different pipe materials were used, stainless steel and transparent polycarbonate, with 27 mm as the inner diameter. Pressure pulses were measured with two semiconductor pressure transducers located inside the pipe at 3 cm and 50 cm from the outlet and a high speed video camera was used for supporting the investigation of the phenomena. From the analysis of collected data a chugging condensation regime map was created. Different bubble collapse behaviours were noticed. A qualitative interpretation of this observation, might be the different effects of the interfacial area and turbulences on the condensation process. Moreover it was found out that internal condensations generate pressure spikes higher than 1.2 MPa because of the condensation induced water hammer (CIWH) phenomena. Formation of small encapsulating bubbles and occurrence of pressure peaks happens for the same conditions of low steam mass flux and low pool temperature. Such a link between the size of the bubbles and the frequency of the pressure peaks gives an effective method for easily predicting chugging characteristic in small scale tests where the adoption of stainless steel pipes do not allow visualizing the phenomena.