Flame downwash behavior (flame pulled by the wake flow and attached to the leeward side of the nozzle) of gaseous fuel jets in cross-flow is of much practical importance in the design of burners as well as industrial flare, and thus important for energy conversion and conservation; however, the studies are still very limited. The critical condition (i.e., critical cross-flow air speed) for flame downwash occurrence as well as the evolution of flame downwash length for various fuel jet exit velocities has not been quantified yet. In this work, the flame downwash length evolution of non-premixed jets for different pipe nozzle diameters at various fuel discharge velocities and cross-flow air speeds as well as the critical condition under which the flame downwash occurs have been quantified comprehensively. Pipe nozzles with inner diameters of 8, 10, 13 and 15 mm were employed in the experiments, using propane as the fuel and with fuel jet velocities ranging between 0.38 and 2.42 m/s. The experimental results showed that the flame downwash length increased with increasing cross-flow air speed for a given fuel jet exit velocity. It was also found that, with increasing cross-flow air speed, the downwash length increased more rapidly for the higher fuel jet exit velocity than that for the lower fuel jet velocity. The critical cross-flow air speed, when flame downwash occurs, was found to be little dependent on the nozzle diameter but increase with the fuel jet exit velocity following a linear relationship. A new correlation for flame down wash length was proposed based on physically the coupling effects due to the competition of the fuel jet momentum to the cross-flow momentum and the total fuel mass supply. The proposed correlation was shown to well characterize the flame downwash length in terms of nozzle inner diameters, dimensionless fuel mass flow rates and the jet momentum ratio. The findings of this work provide basic knowledge and have potential practical applications for burner design and flare implementation, which allow predictions to be made regarding the possible threat and the establishment of the necessary safety length for the stack to prevent the damage for reducing the potential risk. (C) 2017 Published by Elsevier Ltd.