Study on flow dynamics and phase transition characteristics of two-phase ejection in confined small channels

This research presents a fundamental investigation of two-phase critical flow in confined geometries, focusing on gas–liquid phase transition and coupled flow dynamics. Experimental investigations and numerical simulations were performed using subcooled water, with pressures and temperatures reaching up to 15.5 MPa and 603 K, and back pressures as high as 8 MPa. The study analyzed the influence of temperature, pressure, and back pressure on ejection behavior in narrow channels with diameters of 1.29–2.58 mm and length-to-diameter ratios (L/D) of 1.5–3.56. To overcome limitations of traditional one- and two-dimensional models, a three-dimensional numerical framework integrating cavitation and evaporation phase-change mechanisms was developed. The results suggest that the onset of even a small vapor fraction may be associated with a marked reduction in critical mixture velocity. A nonlinear relationship between channel diameter and mass flow density was also observed. Furthermore, the phase-change intensity, modulated by temperature and pressure, induces non-monotonic flow behavior. Phase transitions predominantly occur near the inlet wall, with cavitation prevailing at lower temperatures and a combination of cavitation and flashing manifesting at elevated temperatures. Local inlet vortices exacerbate flow instability, whereas a smooth transition incorporating a small-diameter, large-radius channel suppresses flow separation and vortex development, thereby increasing the mass flow rate. Quantitative assessment was facilitated through dimensionless parameters η and p′, which, respectively, characterize variations in flow rate and phase distribution. Collectively, these findings elucidate the synergistic influences of geometric parameters and thermal–hydraulic conditions on critical flow and phase-change characteristics in confined two-phase systems.