Enhancing Thermal Confinement in Hydrogen-Fuelled Frustum Meso-Scale Combustors Through Outlet-Diameter Optimisation

Meso-scale combustors experience major challenges associated with flame instability, excessive wall heat losses, and limited reactant residence time due to their high surface-to-volume ratios. This study numerically investigates the thermo-fluid behaviour of hydrogen-fuelled vortex flames in a frustum meso-scale combustor under stoichiometric conditions (φ = 1.0). Three outlet-diameter configurations of 6 mm, 8 mm, and 10 mm were analysed under stoichiometric hydrogen–air conditions at air mass flow rates of 40, 80, and 120 mg/s, corresponding to Reynolds numbers of approximately 624–1780, with Computational Fluid Dynamics (CFD) used to evaluate the influence of combustor geometry on thermal confinement, wall temperature distribution, and flame stabilisation characteristics. The numerical simulations were performed in ANSYS Fluent 14.0 using the RNG k–ε turbulence model coupled with the Eddy Dissipation combustion model. The results indicate that reducing outlet diameter significantly enhances thermal confinement and recirculation behaviour within the combustor core. The temperature contours showed a maximum flame temperature of approximately 2.23 × 103 K, while the 6 mm outlet configuration produced a more compact and axially elongated high-temperature core compared with the 10 mm configuration. The 6 mm outlet enhanced thermal localisation by approximately 10.4% and increased residence time by 66.8% relative to the 10 mm outlet. The peak inner wall temperature ranged from approximately 752 K to 1085 K depending on outlet diameter and mass flow rate. The 6 mm outlet exhibited the highest average wall temperature of approximately 909 K, followed by the 8 mm outlet (879 K) and the 10 mm outlet (838 K). Compared with the 10 mm outlet, the 6 mm configuration increased the average wall temperature by approximately 8.5%, indicating improved thermal confinement and heat retention within the combustor. These results indicate that outlet diameter strongly influences the balance between thermal confinement, flame stabilisation, and flow resistance.