Experimental study of the critical flashing flow through a relief line: evidence of the double-choked flow phenomenon

New experimental results are presented for steady-state critical steam–water flows through a horizontal relief line involving an abrupt cross-sectional area change. In the stagnation conditions, the pressure is fixed to 6 bars and the temperature is varied in the range of 110–150°C. Two geometrical singularities are tested: the sudden enlargement and the circular orifice. For both geometrical configurations, the occurrence of the so-called double-choked flow phenomenon is demonstrated on the basis of experiments performed by varying the back-pressure. For a small enough value of the back-pressure, two simultaneous stable locations of critical sections are highlighted: the first one is located close to the singularity and the other is located at the outlet of the relief line. The maximum possible value of the mass flowrate is limited only by the first critical section. The occurrence of the second critical section does not affect the mass flowrate any more. The interpretations of the associated transfer phenomena (the locations of critical sections, the critical mass flowrate and pressure ratio) are supported by the visualisation of three stable flashing flow patterns close to the singularity: the confined free jet, expanded jet and submerged jet patterns.