The lifting limit of an axisymmetric, laminar, co-flow methane-air jet diffusion flame under normal earth gravity has been successfully predicted. Computations of the time-dependent full Navier-Stokes equations with buoyancy were performed using an implicit, third-order accurate numerical scheme and a detailed C_2 chemistry model. A one-step global chemistry model was also used to reveal its deficiencies and to demonstrate the need for "tuning" its kinetic parameters for the studies on flame lifting. The detailed chemistry model resulted in the standoff distance of the flame from the burner rim in good agreement with that measured previously. As the mean co-flow air velocity was increased, at a fixed fuel jet velocity under the near-limit condition, the calculated reaction kernel (peak reactivity spot) in the flame base broadened and rapidly shifted away downstream. As a result, a higher reactivity (heat-release rate, oxygen consumption rate, etc.) at the reaction kernel could be obtained to sustain combustion against a higher incoming flow velocity, or a shorter residence time. The reactivity augmentation is due to a "blowing" effect, which caused enhanced convective and diffusive fluxes of oxygen into the relatively low-temperature (～1550 K) fuel-lean (equivalence ratio ≈ 0.55) reaction kernel. Based on these new findings, a reaction kernel hypothesis is proposed for the diffusion flame stability, namely, that a subtle balance between the residence time and reaction time in the reaction kernel is maintained by its continuous movement in the downstream direction in response to the destabilizing effect caused by an increase in the co-flow air velocity, and the overall reaction time eventually exceeds the available residence time at the stability limit. If a secondary stabilizing point is obtained as a result of the transition to a turbulent flame base downstream, the flame lifts off, otherwise it blows off.
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