Numerical study on flame balls in NH3/H2/air mixture

Flame balls are stationary spherical flames known to exist in low-Lewis-number mixtures only under microgravity near-limit conditions. Whether such flame balls can form in NH₃ and NH₃/H₂ mixtures, and what characteristics they would exhibit, are questions of fundamental importance. In the present study, flame balls in NH₃/H₂/air mixtures are investigated numerically for the first time. Flame ball solutions are obtained for H 2 molar ratios in the mixed fuel ( a H 2 = X H 2 / ( X H 2 + X N H 3 ) ) of 0.079 or more. When a H 2 < 0.079, no flame ball solution is obtained, indicating the critical a H 2 for flame ball formation. With increasing a H 2 , the flammable range of flame ball expands toward that of the H 2 /air mixture. The flame radius exhibits an upward-convex variation with equivalence ratio, while the flame temperature exhibits a downward-convex variation. This inverse relationship arises because a smaller flame radius intensifies the Lewis number effect through increased curvature and simultaneously reduces total radiative heat loss. The boundary equivalence ratio for fuel-lean and fuel-rich combustion differs from 0.23 to 0.71 from a H 2 = 1.00 to 0.079, rather than ϕ = 1. This shift arises from differences in the diffusivities of the fuel and oxidizer species. An extended formulation of the critical equivalence ratio for binary fuel spherical flames is proposed. The new formulation successfully captures the trend of fuel-lean and fuel-rich combustion boundary with different a H 2 . The non-equidiffusive characters of flame ball also affect NO formation: near the lean limit, NO at the flame ball center increases with decreasing equivalence ratio, in contrast to the equilibrium conditions. This is because the Lewis-number-driven flame temperature rise enhances NO production. The lean limit of flame ball for the a H 2 = 0.30 case ( ϕ = 0.2374) is substantially lower than both counterflow flame ( ϕ = 0.3206) and the standard limit ( ϕ = 0.5280). The flame ball is expected to survive beyond the lean limit of propagating flames.