Flow analysis of liquid oxygen phase transition under subcritical conditions in regenerative cooling channels for methane rocket engines: A preliminary study using CFD

초록

Subcritical liquid oxygen (LOX) phase transition in regenerative cooling channels (RCC) is critical in variable-thrust methane rocket engines, where throttling, chilldown, startup/shutdown, and landing operations in reusable systems can reduce chamber and coolant pressures below the LOX critical pressure. Under these conditions, the strong variations in thermophysical properties and localized liquid-vapor transitions may significantly alter heat transfer and pressure drop characteristics. Circumferential flow asymmetry and inlet-manifold-induced flow structures can further amplify phase-transition non-uniformity under subcritical pressures. A detailed understanding of these effects is therefore essential for reliable thermal design and structural integrity. In this study, a coupled combustion–conjugate heat transfer–RCC simulation framework using the non-adiabatic Steady Diffusion Flamelet model and a wall-temperature profile-based thermal decoupling method was applied to investigate LOX phase-transition behavior over a wide range of equivalence ratios (ϕ = 0.4–2.66). The proposed approach was validated against experimental measurements and showed reasonable agreement in both combustion and cooling channel predictions. The analysis captured non-uniform inner-wall temperature, flow recirculation, and approximated local phase transitions in each cooling channel. The results showed that as ϕ increased, the inner-wall and coolant-side wall temperatures increased, promoting an earlier LOX phase transition. Channels 1–14 (0°–45° in azimuthal direction) experienced an earlier phase transition than channels 15–28 (45°–90° in azimuthal direction), primarily due to the recirculation and lower pressure at the channel entrances. The transition location shifted downstream with increasing ϕ, from the converging to the diverging nozzle region, and eventually re-approached the nozzle throat at the highest ϕ = 2.66. Centerline analysis of channels 8 and 22 revealed that the earlier phase transition was associated with greater pressure drop, lower coolant density, and higher coolant temperature. These findings underscore the strong coupling between combustion, wall temperature, and coolant flow for the manifold-induced flow structures in RCC design. As a result, equivalence ratios in the range of 0.727 ≤ ϕ ≤ 1.33 are identified as favorable for achieving uniform flow and phase transitions across all channels, providing practical guidance for engine control during throttling or landing maneuvers. © 2026 Elsevier Masson SAS.

키워드