Abstract
This study experimentally investigates flame envelope morphologic characteristics of downward-orientated buoyant turbulent jet fires, where the initial jet momentum is opposite to flame buoyancy that an anti-buoyancy jet configuration is naturally formed. Such configuration exists practically in gas leakage jet fires, however, has not been quantified. Experiments were conducted employing four circular nozzles with propane as fuel. The flame downward distance from nozzle outlet, flame length and flame width were measured at various fuel velocities for such downward-orientated jets. The corresponding flame length and width for upward jets were also obtained for comparison. Results showed that the three flame dimensions increased with increasing fuel velocity or heat release rate. The ratio of flame downward distance to flame length correlated well to a dimensionless length scale, ξℓ, representing buoyancy to initial momentum of the jets. The flame width and flame downward distance normalized by nozzle diameter correlated well to non-dimensional heat release rate by 1/2 power and 3/5 power, respectively; and their ratio can be approached by a -1/10 power consequently. The ratio of flame width to flame length of downward jets increased with heat release rates, being much higher than that of upward jets which first increased then plateaued (0.145) finally deceased with increasing heat release rate. Both the flame downward distance and flame length of downward jet showed to be much smaller than flame length of corresponding upward jet. A formula was proposed to correlate flame downward distance as a function of flame Froude number (Frf). The flame downward distance was initially smaller then, however, increased faster and finally turned to be larger than flame length with increasing fuel velocity. The critical turning state as flame downward distance equaling flame length (namely flame upper tip descending to nozzle outlet level) of downward-orientated jet can be represented by ξℓ*=3.4 or Frf*=1.
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