1. The NASALangley Research Center has a number of facilities for studying high speed propulsion and combustion. 24,92,196,219 A summary of the major facilities and the operating parameter range is shown in Table I. The facilities can be divided into two classes: those used primarily for fundamental combustion research ·(Fig. 6a) and those used for engine and component testing (Fig. 6b and Table I). The facilities include equipment ranging from the discharge-flow massspectrometer to the 20 mega-watt, arc heated scramjet test facility. (A flight scale propulsion facility will be available in 1989 when upgrading of the 8-FT High Temperature TUnnel to provide oxygen replenishment and alternate nozzles to allow testing at Mach numbers of 4, 5 and 7 is completed.)
2. Anumber of direct connect test configurations have been constructed to investigate various aspects of mixing controlled supersonic combustion related to the design and development of practical engines. Most of these configurations have been operated in several tunnel entries with successive modifications and variations to improve performance and determine the key features affecting performance. Since direct combustion efficiency measurement by conventional means is difficult, if not impossible (Ref. 148), extensive use was made of wall pressure measurements and one dimensional analysis to infer combustion efficiency. In addition, gas sample surveys were made in a variety of configurations at different axial stations to determine fuel distribution and to calculate mixing and combustion efficiency. Usually, inexpensive heat-sink hardware was used. Fifteen to twenty direct connect tests of 10-20 second duration can be conducted on a single day. Tests have been conducted at conditions simulating flight conditions from below Mach 3 to Mach 8. Table II lists a summary of the classes of direct connect test hardware that have been used in these investigations.
3. Anumber of ignition and piloting concepts have also been explored. These include external preburnersf SF6-seeded pilot burners,66plasma torches, 16 prevaporization186andtheuseofpyrophorics. 39 Autoignition of hydrogen does not occur in the scale of the engine module tests conducted at Langley at Mach 4 enthalpy (total temperature 1600R). A pyrophoric hydrogen/silane (SiH4) mixture has been used as an ignitor. 24 (A more complete discussion of silane as an aid in subscale scramjet research is included in a later section on "Combustion Kinetics" •) Flameholding has also been investigated using the series of struts shown in Figure 10.149The struts were mounted in semi-free jet Mach 3.3 flow. By definition, flameholding is a characteristic of the combustor that can only be determined after combustion is established. This fact makes it difficult to model the flame holding limit by consideration of the combustor inlet conditions alone. It is hypothesized that flameholding is related to the flame velocity through the shear layer associated with the recirculation zone. 106 In the engine concepts being developed at Langley, flame holding occurs either at the inlet isolation step (with injection 3 to 5 step height downstream), just ahead of the wall perpendicular injectors, or in the base region of the struts. Figure 19 illustrates the lean flameholding limits for three of the struts shown in the previous figure.