Effects of a recess on cryogenic flame stabilization

Abstract Flame stabilization is a central issue in propulsion applications. In cryogenic liquid rocket engines this process is controlled by a competition between liquid core breakup, atomization, vaporization, and reaction. It is known from practical experience that cryogenic flame stabilization is improved by recessing the liquid oxygen (LOX) tube with respect to the injection plane. This effect is investigated in this article using model scale experiments. 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 emission images are averaged and the resulting image is treated by numerical tomography (based on the Abel transform). This yields the local volumetric light intensity distribution. This method is used to examine the modifications 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 flame is stabilized inside the injector, the flame expansion angle is augmented, the thickness of the flame brush and the size of the volume where reaction takes place is enhanced. Effects observed experimentally are quite significant. The phenomenon is interpreted with a simple model relying on a one-dimensional description. When the flame develops inside the duct, it produces hot gases which occupy a certain fraction of the available duct area, the hydrogen stream is accelerated, and consequently the gas to liquid momentum 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 primary atomization, and a corresponding augmentation of the flame blooming angle and combustion volume.