Numerical study on the effect of modular structures on hydrogen-air combustion for micro thermophotovoltaic applications

In the present study, modular two-tube and four-tube micro-combustors are investigated as potential heat sources for thermophotovoltaic (TPV) systems. Three-dimensional numerical simulation is conducted using hydrogen as the fuel to evaluate both combustion stability and thermal efficiency under variations in inlet velocity, equivalence ratio, and wall thermal conductivity. Results show that increasing inlet velocity shifts the flame downstream, enlarges the flame front distance from the inlet (Li), and lowers total efficiency, which directly reduces useable radiative output for TPV conversion. Larger Li also increases heat loss and the risk of flame extinction, limiting stable operation. In the four-tube combustor (B1 configuration), total efficiency decreases from 8.11 % at 4 m/s to 0.89 % at 18 m/s. Using walls with low thermal conductivity (e.g., 1.18 W/m·K) causes further downstream flame displacement and greater flame stretch (Ls), both of which promote instability. The four-tube modular structure significantly enhances flame stabilization by reducing Li and Ls. For instance, at inlet velocity of 16 m/s, Li is about 14 mm in the two-tube combustor (A4) but only 8 mm in the four-tube combustor (B1) resulting in nearly double TPV conversion efficiency. Adopting a four-tube modular structure reduces both Ls and Li, ultimately leading to improved flame stability. Moreover, applying non-uniform inlet velocities in the four-tube configuration substantially improves performance, yielding up to twofold efficiency enhancement at high flow rates compared to uniform inlet conditions.