This numerical study systematically investigates the influence of initial injection conditions of reactants on flame characteristics from a parallel multi-jet burner in a laboratory-scale furnace. In particular, varying characteristics from visible flame to invisible Moderate or Intense Low-oxygen Dilution (MILD) combustion is explored. Different parameters examined include the initial separation of fuel and air streams (S), air nozzle diameter (D-a), fuel nozzle diameter (D-f), and air preheat temperature (T-a). The present simulations agree qualitatively well with previous measurements reported elsewhere for two reference cases investigated by experiment. A number of new and significant findings are then deduced from the simulations. For instance, all S, D-a and D-f are found to play significant roles in achieving a proper confluence location of air and fuel jets for establishing the MILD combustion. Particularly, varying D-a is most effective for controlling the combustion characteristics. It is also found that the stability limits of the non-premixed MILD combustion varies with different combustor systems and inlet reactant properties. Moreover, for the first time, several analytical approximations are obtained that relate the flue-gas recirculation rate and the fuel-jet penetration to D-a, D-f, S and also reactant properties. (C) 2011 Elsevier Ltd. All rights reserved.