Experimental and Numerical Study of Catalyst Temperature in Passive Autocatalytic Recombiner at Elevated Hydrogen Concentrations

This study investigates hydrogen catalytic recombination in a passive autocatalytic recombiner (PAR) at elevated hydrogen concentrations of 6-8 vol % in air. A combined experimental and numerical approach was used to test and characterize a cylindrical-type PAR catalyst for thermal response, conversion efficiency, and ignition threshold. Catalyst temperature and hydrogen concentration were studied in a small-scale facility, called the recombiner section testing station (RSTS), and compared with computational fluid dynamics (CFD) simulations. The model was developed and validated against experimental data in STAR-CCM+ software and incorporated the detailed heterogeneous and homogeneous reaction mechanisms from CHEMKIN kinetics simulation software. Experimental results showed a stable catalyst surface temperature with a maximum of 370-440 °C and hydrogen conversion efficiencies of ∼70%. Verification of turbulence models (k-ε and k-ω families) demonstrated a notable influence on temperature and species distribution, while neglecting thermal radiation led to overprediction of temperatures. The proposed CFD model demonstrated prediction with a deviation of 5% for catalyst temperature and 15% for hydrogen conversion. The model enabled evaluation of the ignition threshold by comparing the hydrogen reaction rate on the catalyst surface and in the gas phase. The onset of gas-phase hydrogen combustion was numerically calculated at 6 vol % of inlet hydrogen, indicating the lower ignition threshold for the cylindrical-type PAR catalyst configuration. The findings confirm that catalysts ensure safe and effective operation under high hydrogen concentrations and provide validated modeling guidance for hydrogen safety studies.