Boundary-layer ingestion in airframe-integrated scramjet engines causes engine stall (engine unstart hereafter) and restricts engine performance. To improve the unstart characteristics in engines, boundary-layer bleed anda two-staged injection of fuel were examined in Mach 4 and Mach 6 engines. The bleed system consisting of a porous plate, an air cooler, a metering orifice, and an on/off valve was designed for each of the engines. First, a method to determine bleed rates was developed. Porous plates were designed to extract air from the Mach 4 engine at a rate of 200-g/s and from the Mach 6 engine at a rate of 30 g/s. Air coolers were then optimized based on the bleed rates. The exhaust air could be cooled below 600 K in the porous plates and the compact air coolers. The Mach 4 engine tests showed that the 200-g/s bleed (3% of the captured air) doubled an engine operating range (limit fuel rate) and thrust. With two-staged injection of H 2 , the engine operating range was extended from Φ0.3 to Φ0.95, and the maximum thrust was increased from 1200 to 2560 N. The Mach 6 tests showed that the 30-g/s bleed (0.6% of the captured air) extended the start limit from Φ0.48 to Φ1 and increased the maximum thrust from 1620 to 2460 N.
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Mitani et al. (2005) studied this question.