Theoretical Modeling and CFD Validation of Outlet Initial Conditions of a Pressure-Swirl Nozzle Under Microgravity

A reduced-order framework was developed to describe the outlet initial conditions of a pressure-swirl nozzle under reduced-gravity conditions and to clarify how gravity modifies the nozzle-exit state relative to the corresponding 1 g baseline. Under normal gravity, the baseline outlet state was established through CFD-informed identification of the discharge coefficient and initial spray cone angle, from which the outlet axial and tangential velocity components were reconstructed. Gravity-induced deviations were then introduced through compact correction relationships for the outlet descriptors and liquid-film geometry. The outlet parameters were evaluated over pressure drops of 0.1–0.5 MPa and gravity levels from 1 g to 10−5 g. The results indicate that operating pressure mainly determines the baseline outlet state, whereas reduced gravity acts primarily as a correction to that baseline. Under forward-gravity injection, decreasing gravity reduces the axial and tangential velocity components, discharge coefficient, and initial spray cone angle, while increasing the outlet liquid-film thickness. A CFD-based comparison at 0.3 MPa indicates that the framework captures the first-order trends of normalized outlet velocity components and liquid-film thickness within the investigated conditions. Independent experimental validation is not included in this study and remains necessary for future quantitative assessment.