Functional Testing and Numerical Analysis of Initial Freezing in Overboard Discharge Valve at Small Lift

Local freezing may affect subsequent discharge when a cold fluid enters a water-containing passage in an overboard discharge valve. Compressed-air functional tests assessed prototype opening and discharge performance. A volume-of-fluid (VOF) model coupled with the enthalpy–porosity method examined initial freezing during low-flow operation at a small valve lift. The measured assembly opening pressure was 1.53 × 106 Pa. The overboard check valve discharged 0.67814–0.79061 kg·s−1 at 3.55 × 106–3.75 × 106 Pa. With constant gas properties, all cases were compared over 0–0.10 s. Increasing inlet mass flow from 0.0010 to 0.0020 kg·s−1 reduced the terminal ice-phase fraction from 0.70% to 0.14%. The time-averaged fraction decreased from 0.39% to 0.12%, with a local increase at an intermediate flow rate. Terminal and time-averaged fractions were lower at equivalent roughness of 0.6–0.7 μm. At 0.8–1.0 μm, the ice-phase fraction continued increasing near the end of the window. Within the prescribed model, these results describe how flow rate and roughness affect the initial thermal shock and phase-change history. Their scope is limited to the initial transient response and excludes predictions of steady-state ice accumulation or valve blockage.