This paper presents a numerical comparative study on the ignition characteristics of straight-channel and U-bend micro catalytic combustors, with particular focus on the role of inlet velocity. A two-dimensional computational fluid dynamics model with coupled gas-phase and surface catalytic reaction kinetics for propane combustion is developed using a fluid simulation program ANSYS Fluent. The catalyst coating (Pt/Al₂O₃) is modeled as a zero-thickness reaction surface, and the U-bend design features an uncoated recirculating channel to ensure identical catalyst loading between the two configurations. Simulations are conducted over an inlet velocity range of 0.25–8 m/s. Key ignition and combustion metrics including ignition temperature, ignition time, maximum combustion temperature, heterogeneous reaction contribution, and thermal/species field distributions are systematically compared. Results reveal a crossover in relative performance depending on flow regime. At low velocities (≤2 m/s), the straight-channel combustor exhibits lower ignition temperatures; at high velocities (≥4 m/s), the U-bend design achieves superior ignition performance with lower ignition temperatures (e.g., 526 K vs. 555 K at 8 m/s) and higher combustion temperatures (1726 K vs. 1474 K at 8 m/s). However, the straight-channel combustor consistently yields shorter ignition times across all velocities (25.9–108.6 s) compared to the U-bend (52.6–145.2 s). The heterogeneous reaction contribution decreases with increasing inlet velocity for both designs, with the straight-channel maintaining higher values than the U-bend. The U-bend achieves higher maximum temperatures due to enhanced heat recirculation, particularly at high flow rates. The findings suggest that the U-bend configuration is advantageous for high-flow-rate applications requiring low ignition temperatures and high combustion temperatures, whereas the straight-channel design is preferable for rapid cold-start scenarios.

Catalytic micro-reactors offer a promising solution for portable power generation, yet catalyst deactivation during long-term operation remains a critical challenge to reliable ignition and combustion stability. In this work, a transient numerical model is developed to conduct a sensitivity study of ignition and combustion performance to catalyst deactivation, represented as prescribed reductions in catalytic activity, for a propane-fueled straight-channel micro-combustor. The present model explicitly resolves the porous catalyst coating layer as a separate computational domain, incorporating a combined Knudsen and molecular mixed-diffusion mechanism to capture species transport resistance within the microporous structure. The results demonstrate that increasing catalyst deactivation markedly deteriorates ignition performance. As the deactivation index rises from 0.1 to 0.9, the ignition temperature increases from 488 K to 588 K in a distinctly nonlinear manner, with the greatest sensitivity observed at high deactivation levels. At inlet temperatures 10 K above the respective ignition thresholds, the ignition delay time is prolonged from 191 s to 224 s when the deactivation index increases from 0.7 to 0.9, attributed to the dominance of kinetic deterioration over sensible heat compensation. The heterogeneous reaction contribution, although decreasing with deactivation, remains above 95% across all cases, indicating that the gas-phase pathway is largely suppressed due to the inherently low temperature environment within the microchannel. Spatial distributions of temperature and CO₂ mass fraction reveal that combustion is confined to the near-inlet region, with a peak temperature of approximately 1001 K at the highest deactivation level. These findings quantify the sensitivity of ignition and combustion characteristics to catalyst deactivation.

Geometric asymmetry in recirculating micro-burners remains underexplored in catalytic micro-combustion research, despite its potential to significantly influence ignition behavior. This numerical study investigates the ignition and combustion characteristics of four micro-channel catalytic burners with distinct geometric configurations, aiming to evaluate the role of asymmetry on combustion behavior. Burner 1 is a straight-channel design without heat recirculation. Burners 2–4 are U-bend recirculating configurations: Burner 2 with symmetric channel sizes, Burner 3 with a wider inlet channel, and Burner 4 with a narrower inlet channel. A two-dimensional computational fluid dynamics model with a one-step global propane oxidation mechanism and catalytic wall reactions is employed. The results show that Burner 1 exhibits the highest ignition temperature (555 K) and shortest ignition delay (28.5 s) due to its low thermal mass. Among U-bend burners, Burner 4 achieves the lowest ignition temperature (530 K) and the highest steady-state combustion temperature (1726 K), owing to reduced recirculation velocity and enhanced thermal feedback. Burner 2 shows moderate performance, while Burner 3 gives the weakest combustion intensity among recirculating designs. The heterogeneous reaction contribution is highest in Burner 1 (69.3%) and lowest in Burners 2 and 3 (~55%). Asymmetric channel sizing, particularly a narrow catalytic channel combined with a wide recirculation channel (Burner 4), significantly improves ignition and combustion performance under cold-start conditions.