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Tunable diode laser absorption measurements of gas temperature and water concentration were made at the exit of a model scramjet combustor fueled on JP-7. Multiplexed, fiber-coupled, near-infrared distributed feedback lasers were used to probe three water vapor absorption features in the 1.34-1.47 microm spectral region (2v1 and vl + v3 overtone bands). Ratio thermometry was performed using direct-absorption wavelength scans of isolated features at a 4-kHz repetition rate, as well as 2f wavelength modulation scans at a 2-kHz scan rate. Large signal-to-noise ratios demonstrate the ability of the optimally engineered optical hardware to reject beam steering and vibration noise. Successful measurements were made at full combustion conditions for a variety of fuel/air equivalence ratios and at eight vertical positions in the duct to investigate spatial uniformity. The use of three water vapor absorption features allowed for preliminary estimates of temperature distributions along the line of sight. The improved signal quality afforded by 2f measurements, in the case of weak absorption, demonstrates the utility of a scanned wavelength modulation strategy in such situations.

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS. 1. REPORT DATE (DD-MM-YY) 2. REPORT TYPE Recent results from combustion experiments in a direct-connect supersonic combustor are presented. Successful ignition and sustained combustion of gaseous ethylene have been achieved using an injector/flameholder concept with low-angle, flush-wall fuel injection upstream of a wall cavity. Two interchangeable facility nozzles (Mach 1.8 and 2.2) were used to obtain combustor inlet flow properties that simulate flight conditions between Mach 4 and 6 at a dynamic pressure of 47.9 kPa. Mainstream combustion was achieved at equivalence ratios between 0.25 and 0.75 using only a spark plug and no other external ignition aids. Delta-force levels between 667 and 1779 N were measured, with corresponding combustor pressure ratios between 3.1 and 4.0. Video records of the flame zone show an intensely active combustion zone with rapid flame spreading. One-dimensional performance analysis of the test data indicates a combustion efficiency around 80%with an average combustor skin friction coefficient of 0.0028.

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Sharp leading edge (LE) samples of UHTC (20 vol%SiC–HfB2) and SiC were exposed to simulated hypersonic flight conditions using a direct‐connect scramjet rig and their thermal and oxidation responses measured. The measured back‐wall temperatures and scale thicknesses were significantly smaller than might be expected from stagnation temperatures at the LE. Furthermore, the scale that formed around the LE was more uniform than expected from the steep drop in cold wall heat flux with distance from the tip. These results were interpreted and rationalized using physics‐based models. An aerothermal model in combination with an oxidation model accounted for the observed scale thicknesses at the tip and their slight variation with distance. The scale thicknesses were similar to values reported for exposures in furnaces at temperatures calculated for the tip, but less than those reported in arc jet tests. The formation of hafnon (HfSiO4) and the absence of external glassy layer and of silica in the outer portions of the oxide region are unique to scramjet tested samples, presumably due to the high fluid flow (high shear and evaporation) rates.

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