Thermal performance and mechanistic role of U-bend number in microchannel catalytic hydrogen combustion: A CFD-Based study

Hydrogen is a promising energy carrier for compact energy conversion systems, and catalytic hydrogen combustion (CHC) in microchannel reactors enables stable heat release under fuel-lean, low-temperature conditions. Although U-bend microchannels are known to enhance convective transport, the role of bend number in governing CHC performance remains insufficiently understood. In this study, a computational fluid dynamics (CFD) model incorporating detailed kinetics is employed to investigate CHC in multi-U-bend microchannels. Parametric analysis reveals that increasing U-bend number (U) increases the average wall temperature from 824 K to 842 K and hydrogen conversion by 0.9% as U increases from 0 to 2, with a moderate pressure drop of 1651 Pa; further increases in U from 2 to 5 yield marginal thermal gains, while the pressure drop rises to 7208 Pa. Correlation results show that U primarily controls hydrogen conversion, which drives the nonlinear thermal response. Multi-objective optimization identifies U = 2 as the most balanced design. Within mass flow rates of 4.5 × 10−5 to 2.25 × 10−4 kg s−1 and equivalence ratios of 0.3 to 0.5, a trade-off operating condition yields enhancements of 13% in average wall temperature and 0.36% in hydrogen conversion relative to the baseline. Mechanistic analysis shows that U-bends promote upstream relocation of the dominant heat-release region by enhancing near-wall transport, whereas further increasing U beyond 2 induces reaction saturation associated with surface-state evolution. This work provides a systematic mechanistic characterization of U-bend number effects on CHC thermal performance, delivering physically grounded geometric design guidance for compact hydrogen energy conversion systems.