Effects of a recess on cryogenic flame stabilization

Flame stabilization is a central issue in propulsion applications. In cryog enic liquid rocket engines this process is controlled by a competition betw een liquid core breakup, atomization, vaporization, and reaction. It is kno wn from practical experience that cryogenic flame stabilization is improved by recessing the liquid oxygen (LOX) tube with respect to the injection pl ane. This effect is investigated in this article using model scale experime nts. A single jet-flame formed by a coaxial injector fed by LOX and gaseous hydrogen (GH(2)) is analyzed by imaging the light emitted by hydroxyl (OH) radicals. To characterize the mean reaction zone structure, the light emis sion images are averaged and the resulting image is treated by numerical to mography (based on the Abel transform). This yields the local volumetric li ght intensity distribution. This method is used to examine the modification s in the near flame structure due to the LOX tube recess. It is shown that when the LOX tube is recessed with respect to the injection plane, the flam e is stabilized inside the injector, the flame expansion angle is augmented , the thickness of the flame brush and the size of the volume where reactio n takes place is enhanced. Effects observed experimentally are quite signif icant. The phenomenon is interpreted with a simple model relying on a one-d imensional description. When the flame develops inside the duct, it produce s hot gases which occupy a certain fraction of the available duct area, the hydrogen stream is accelerated, and consequently the gas to liquid momentu m flux ratio J is augmented. The model provides the values of this quantity in terms of the fraction of vaporized oxygen. The augmented value of J, in turn, leads to a faster breakup of the liquid oxygen core, an improved pri mary atomization, and a corresponding augmentation of the flame blooming an gle and combustion volume. (C) 1999 by The Combustion Institute.