CFD-Based Evaluation of Nozzles for Plasma Based H₂O₂ Production in Microgravity

Introduction.
For long-term space exploration, in-space propellant production with easily storable and accessible resources has emerged as a key direction for next-generation spacecraft systems. Plasma reactors are particularly attractive for producing hydrogen (H2) and hydrogen peroxide (H2O2) without cold-start limitations, enabling instantaneous operation [1]. Even if H2O2 concentrations are promising, efficient chemical conversion requires good plasma-liquid contact [2]. Moreover, the reported synthesis mostly takes place under earth gravity.
Therefore, in this work, we develop and investigate nozzle geometries with enhanced mixing performance within the reactor. Since experimental validation of nozzle performance under microgravity environment is costly and time-intensive, computational fluid dynamics (CFD) is employed to evaluate and compare the mixing performance of different nozzle configurations.

Methods.
A three-dimensional CFD model is developed to evaluate mixing performance using single-phase and multiphase flow models under gravity and microgravity conditions. The simulations are implemented in COMSOL MultiphysicsTM[3], which couple the Reynolds-averaged Navier-Stokes (RANS) equations with phase transport equations for two-phase flow. ... mehr
The critical parameters in nozzle configurations are diameter, tip shape, and orifice position.
A two-step simulation strategy is adopted: first, single-phase flow simulations are used to perform preliminary screening of nozzle designs based on residence time and flow field analysis. Subsequently, multiphase flow simulations are performed for selected designs to analyze mixture fraction distribution and concentration and export mixture data for future reactive and plasma simulations.

Results and discussion.
Variations in the tip shape led to distinct velocity, fields, directly affecting fluid residence time, as can be seen in Figure 1. The open-ended nozzle produces a direct flow toward the reactor bottom, while the closed-ended nozzles extend residence time by modifying the internal flow pattern. The sharp-tip nozzles configurations exhibit the strongest impact on the overall flow field, suggesting superior mixing performance.
In contrast, the effect of orifices position is less distinguishable in single-phase simulations. Consequently, two-phase flow models are introduced to evaluate volumetric mixture fraction and concentration as additional quantitative measures for evaluating mixing performance.