Detailed information about the reacting flow field in a double concentric-flow burner equipped with a circular disc of large blockage ratio (67%) was obtained by a laser-Doppler velocimetry. Meanwhile, fluorescence spectra of the hydroxyl radical, excited with PLIF, were used to probe the reacting features of the nonpremixed flame. From these data we propose four typical flow patterns, which are designated Types A, B, C, and D. The flow features were dominated by the annular air at small fuel-air velocity ratio (gamma), whereas the influence from the central fuel jet gradually became dominant when the velocity ratio was promoted. Regions with strong turbulence intensity also shifted with the variation of fuel-air velocity ratio from the outer recirculation zone toward the inner area. At small gamma, the combustion reaction mainly occurred around the outer shear layer of the downstream recirculation zone, which was associated with maximum turbulence intensity. Most reaction, nevertheless, spread over all the recirculation bubble at a large gamma; regions of reaction seemed to correlate poorly with turbulence intensity. From the information about the flow field and thermal structure, four typical mixing mechanisms are illustrated, which reveal a positive role played by the large-scale motion of the recirculated vortices. The recirculation bubble provides a low-velocity environment of prolong the stagnation of reactants, which is a key factor to stabilize the flame as the fuel-air velocity ratio becomes very large.