Abstract
Flame height and lift-off distance of vertically oriented rectangular source natural gas jet fires with buoyancy-momentum flame Froude number up to 3.06 are investigated by a series of experiments. The jet fires are produced by rectangular equal-area nozzles (314 mm2) with 6 aspect ratios (length to width of the nozzle exit) ranging from 5:1 to 50:1. It is found the flame height (excluding lift-off) gradually decreases as the aspect ratio increases for a given heat release rate due to an improvement of buoyant entrainment. With refinement values of the entrainment strength constant C1 for different source aspect ratios being obtained, the application of the classic correlation of Quintiere & Grove can be extended to characterize the flame height of jet fire in transition regime from buoyancy-controlled to momentum-controlled. The value of C1 can be considered as constant (0.216) due to varying slightly with increasing aspect ratios. Moreover, it is revealed that increased aspect ratio reduces the lift-off distance as a result of an improvement of mixing and entrainment in the orifice near field. A unified dimensionless correlation based on the Mixedness-Reactedness Flamelet Theory to predict lift-off distances is developed, which gives encouraging results comparing to the measured values for different source aspect ratios and different equivalent diameters.
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