The mean velocity of the flame front (in mm s-1) towards the unburnt gas mixture (usually vertically downwards). The quantity applies to gas mixtures and not to injection burners, and depends on the flame temperature and the solvent nebulized.
Source:
PAC, 1986, 58, 1737. 'Quantities and units in clinical chemistry: Nebulizer and flame properties in flame emission and absorption spectrometry (Recommendations 1986)' on page 1742 (https://doi.org/10.1351/pac198658121737)