Overview of geometrical effects on the critical flow rate of sub-cooled and saturated water

• A new correlation for the idealized critical mass flow rates was suggested.
• An overview of geometrical effects on critical flow rate was investigated.
• A transition criteria from non-equilibrium to equilibrium choking was suggested.
• A slip ratio was proposed as one of reasons for the over-measurement of test data.

A new correlation predicting the idealized critical mass flow rates of sub-cooled and saturated water was suggested, which can be applicable for wide ranges of stagnation pressures, e.g., 0.5–20.0 MPa. The suggested correlation will be instructive and helpful for related studies and/or engineering works. In addition, an overview of the geometrical effects on the critical flow rate of sub-cooled and saturated water was investigated, especially on the length and diameter aspects. As transition criteria from non-equilibrium to equilibrium choking, a length to diameter (L/D) ratio of 25 was suggested for the nozzle or pipe flows. In the case of an orifice, an increase of the L/D ratio induces a decrease of the form loss coefficient and thus results in an increase of the critical flow rate. Moreover, the trend of the critical flow rate at the orifices was found to be dependent upon the form loss coefficient, but the nozzle and pipe are dependent upon the length. In a nozzle or pipe, the entrance shape also affects the critical mass flow rate, e.g., smaller form loss coefficient and larger critical mass flow rate. Diameter effects occurred regardless of the diameters, lengths and upstream pressures under the same L/D ratio. A further discussion was conducted for the over-measurement of critical mass flow rate under lower sub-cooling temperatures, which was found in smaller throat diameters of the orifices, and the slip ratio between two-phases at the choking location was proposed as one of reasons for the over-measurement.