Development of nanosecond-gated laser induced breakdown spectroscopy for fuel mole fraction measurements in supersonic combustion
Hypersonic air-breathing propulsion systems such as supersonic combusting ramjets or scramjets are of great interest due to the high flight velocities and increased specific impulse offered over solid or liquid fuel rockets. Scramjets operate on the same Brayton cycle as typical jet turbine engines; however, there is no rotating compressor. Rather, the scramjet takes advantage of the high dynamic pressure of incoming air and all shockwaves formed by the vehicle forebody or engine inlet geometry compressing the incoming air. Ramjets and scramjets operate on the same principal, but there is one key difference. In a ramjet, the air is decelerated to subsonic speeds in the combustor, while in a scramjet the core flow remains supersonic. Both engines require an external or secondary propulsion system to accelerate the engine to the appropriate speed prior to operation. A dual mode scramjet is a design which allows for operation in both ram-mode and scram-mode and allows for scramjet operation with a lower required takeover velocity from the secondary propulsion system. Efficient and robust scramjet takeover is one of the most important aspects of scramjet development. Scramjet takeover refers to the transition from the secondary propulsion system to scramjet propulsion. An important aspect of this takeover is the ignition of the scramjet combustor. Scramjet engines operating at hypersonic velocities undergo autoignition, which occurs when the total temperature of the compressed flow is high enough to directly ignite the injected fuel. For dual-mode scramjets, the total temperature at takeover velocity is insufficient to ignite the injected fuel. In such engines, an ignition device is necessary. In order to understand the ignition process and ignition probability in a scramjet combustor, an accurate understanding of fuel and air mixing and the ignition transient process is required. A model scramjet combustor presents a challenging environment for measurements as the supersonic nature of the flow prohibits intrusive measurements, as well as high temperatures from combustion. Laser-induced breakdown spectroscopy (LIBS) is a measurement technique utilizing the emissions from a plasma-generated from a focused laser pulse to determine quantitative parameters of the medium, specifically atomic composition and number density. As LIBS is non-intrusive prior to the generation of the plasma, it can be implemented in both the supersonic and sub-sonic portions of the scramjet flowpath. The LIBS plasma can be generated in both combusting and non-reacting environments and can be used to quantify the fuel mole fraction within the plasma volume. LIBS can be used with either gated or ungated emission collection. For the use of LIBS within scramjet combustors, nanosecond-gating is required as the plasma will be transported by the high flow velocities over longer time periods. The presented work evaluates and outlines the use of nanosecond-gated LIBS (n-LIBS) for fuel mole fraction measurements in a model dual-mode scramjet. The entire n-LIBS process, including matching database construction, the quantification process, and the implementation within a combustor is outlined. Measurements taken in model scramjet combustors at different inlet Mach numbers, in both reacting and non-reacting conditions are compared to each other and to different simulation techniques. The effects of inlet distortion on fuel distribution in scramjet cavity flameholders was evaluated using n-LIBS. The limitations of n-LIBS are explored in terms of resolution and lower detection limits. The use of carbon dioxide as a fuel surrogate for improved statistics at locations where the n-LIBS probe would ignite fuel air mixtures is evaluated. Finally, the current state and areas under continued development for n-LIBS with direct spectrum matching are reviewed.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign