Rapid depressurization of a cold single-phase liquid leads to the onset of a vaporisation, i.e. the phase transition phenomena. Prior to the start and in the very first moment of the phase transition, pressure in the liquid may briefly drop to negative values, when initial liquid temperature is low enough. Metastable liquid in a state of tension is well known in static experiments, but is less known in the fluid dynamics. The present paper discusses some preliminary findings in the field of modeling of the negative pressures in transient water flows and subsequent phase transition with a single-pressure two-fluid model. A highly simplified single-pressure two-fluid model is used in the present work to describe the two-phase flow with negative liquid and positive vapor pressure. The assumption used at negative liquid pressures is equal temperature of liquid and newly generated vapor phase, while the gas pressure is assumed to be the saturation pressure at that temperature. Rather rough models seem to be sufficiently accurate due to the large uncertainty in the modeling of the single-to-two-phase flow transition, which strongly depends on the density of the nucleation sites in the liquid and at the wall.

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