CFD study of the influence of inlet geometry in non-premixed microreactors on the thermal performance of catalytic hydrogen combustion

Catalytic hydrogen combustion (CHC) provides stable, low-temperature heat release in compact microreactors without a freely propagating flame. In non-premixed catalytic reactors, inlet geometry affects near-wall reactant delivery, reaction-zone distribution, and wall heat extraction. Because these coupled transport-reaction effects are difficult to resolve experimentally, three-dimensional computational fluid dynamics simulations with detailed platinum surface kinetics and conjugate heat transfer were performed for a CHC microchannel reactor in which inlet geometries were explicitly resolved under non-premixed reactant supply. The effects of nozzle placement, nozzle length, premixing length, and porous-layer-assisted injection were evaluated using hydrogen conversion, average wall temperature, wall-temperature non-uniformity, pressure drop, total wall heat-transfer rate, and wall heat-transfer efficiency. The results show that shifting the nozzle upstream and introducing a finite premixing section redistribute the reaction zone, markedly reduce wall-temperature non-uniformity relative to the extreme inlet arrangements under non-premixed operation, and maintain high hydrogen conversion. Under the reference operating condition considered in this study, the 3 mm / 3 mm solid-nozzle configuration provided the most favorable thermal-hydraulic balance within the explored solid-nozzle design space. Relative to the corresponding solid-nozzle inlet case, the porous-layer-assisted inlet was associated with enhanced hydrogen depletion near the catalytic wall, an increase of about 40 K in average wall temperature, and the highest wall heat-transfer rate of about 83 W together with a wall heat-transfer efficiency of 57.65%. These findings highlight the importance of near-wall transport and reaction-zone redistribution for improving reactor-level wall heat extraction and thermal-hydraulic performance in non-premixed CHC microreactors under the modeled boundary conditions.