Comparative Analysis of Ignition and Combustion Characteristics in Straight-Channel and U-Bend Micro Catalytic Combustors: Numerical Investigation of Inlet Velocity Effects

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.