Ignition of Liquid Fuel Jets in a Supersonic Air Stream

SEVERAL propulsion concepts, such as scramjets and external burning, involve ignition and combustion at supersonic air velocities. Volume considerations have focused attention on liquid fuels; and fuel distribution, cooling, and drag studies indicate that fuel will be injected transversely from the walls. High air velocities lead to low residence time and low static temperature and pressure which means long chemical times, so spontaneous ignition becomes problematical. The primary, unclassified references are presented in tabular form in the backup paper. Results with CS2 (because of low auto-ignition temperature under no-flow conditions), kerosene, JP-5, Pentaborane, TEA, HiCal and some blends are listed showing that ignition is a problem under conditions of interest. References 1 and 2 describe studies of the autoignition process for transverse liquid jets in hot, supersonic air streams. The studies of no combustion cases in Refs. 3 and 4 served as motivation. It had been found that inject ant accumulated in a layer on the wall near the injector. It was postulated that residence times near this layer would be long and that ignition might initiate there. No clear evidence of ignition with CS2 or kerosene was found up to conditions where the temperature under the layer was higher than the noflow, ignition temperature for CS2 in the literature. The present study extended the work to higher air temperatures, a wider range of q and other injection angles. Reference 5 showed that the layer and the separation region ahead of the jet increased for upstream angles. Contents The "air" temperature necessary to simulate supersonic flight was produced by a special facility consisting of two components. The first is an Inconel 601 tube heated via electric resistance. The second is an ethylene-fired preburner used to obtain air temperatures higher than 1200 K. The ratio of ethylene to oxygen was determined so that the products maintained the ratio of oxygen to combustion products equal to the ratio of oxygen to nitrogen in air. The temperature profiles leaving the preburner were found to be quite uniform. It is felt that the high static temperature and pressure and the long chamber length combine to insure complete combustion in the preburner. The test nozzle was designed to produce a M = 1.65 at the injection station. At this cross section are two surface thermocouples. One is located adjacent to and slightly downstream of the injection port; the other is located circumferentially 90 deg from the injector. The liquid fuel issued into the airstream via a 0.76 mm diam port. At the same injection pressure and air temperature, the ^(^pjUj/p^U^for water injection is close to that for kerosene and the q for CS2 is higher. The instrumentation included thermocouples, both wall and in-flow, pressure taps and a camera, a video camera and an infrared Thermovision thermographic camera. This camera senses the infrared radiation emitted by a heated surface, processes these images internally and produces 10color, isothermal band images on a color television screen. The exact temperature represented by each isothermal band is a function of emissivity of the object and the "shape factor" of the surface. These values are difficult to estimate for the liquid jet plume and surface layer here. Nonetheless, this method can be used to qualitatively observe the flow. The viewing path for the optical observations was down at an oblique angle from the rear at the emerging jet plume.