Measurement of Water Vapor Levels for Investigating Vitiation Effects on Scramjet Performance

Abstract

C OMBUSTION heated wind-tunnel facilities are required for many different aspects of ground-based testing of hypersonic airbreathing propulsion systems. This is particularly true for largescale scramjet testing in facilities with run times on the order of tens of seconds to minutes at equivalent flight Mach numbers above 4. Unfortunately, hydrogen and hydrocarbon combustion-heated facilities both result in water vapor levels in the test flow that are significantly greater than for atmospheric air. Hydrocarbon heated facilities additionally introduce carbon dioxide into the test flow. The effect of test gas vitiation due to combustion heating on scramjet combustor performance is an area of active research. Several recent studies have shown that the presence of elevated levels ofwater vapor may significantly affect combustor performance in scramjet engine test articles [1–4]. The experimental results reported in [1,2] were obtained in an electrical resistance heated wind tunnel where steam was artificially added to the flow to simulate a combustion-heated facility. Although the amount of steam addition can be calculated, an independent measurement is useful to confirm the presence of water vapor in the scramjet combustor at the reported levels. To that end, this note reports on results of the application of an instream optical measurement technique whereby water vapor levels in the test flow through the scramjet were independently measured and verified using a tunable diode laser absorption diagnostic. This paper first presents and describes the method of steam addition, and the associated makeup oxygen and optional carbon dioxide vitiation systems. Next, the tunable diode laser absorption technique that was applied to the facility is described. Finally, the added and measured water vapor levels are compared to validate the water vapor vitiation technique and to provide confidence in the similarmakeup oxygen and carbon dioxidevitiation techniques. This study represents the first time steam addition levels have been independently verified for the facility of [1,2] and it provides confidence in the claims made by the investigators in [1,2] with respect to the effects of test gas vitiation on scramjet performance and operation.