Scanned wavelength-modulation absorption spectroscopy with application to hypersonic impulse flow facilities [electronic resource]
Keywords: Mechanical Engineering.

The work presented herein describes several advances in the evolution of combustion diagnostics based upon the technique of tunable diode laser absorption spectroscopy (TDLAS). The primary focus of this work was to provide the theoretical and practical means to perform in-situ TDLAS measurements within the harsh flow environments of hypersonic impulse ground-test facilities. In addition to this primary focus, the development and application of this work led to a comprehensive analytical framework for wavelength-modulation spectroscopy and enhanced the understanding of the complex flow environments within hypersonic flow facilities and hypersonic combustion experiments. Hypersonics is the field of aeronautics concerned with flight speeds that are highly supersonic, typically on the order of Mach 5 and above. Presently, it is the domain of spacecraft during re-entry and a select cadre of rocket-powered and air-breathing test vehicles (e.g., North American X-15, NASA X-43, Boeing X-51, HyShot I-IV, HiFiRE 0-3, etc.). It is also a regime of grand aspirations, including routine inexpensive space access, rapid intercontinental transport, atmospheric entry on other planets, and numerous strategic military applications. The development of hypersonic technology is confounded by the immensity of the problem, especially the complexity of interacting physical phenomena and the shortcomings in the fundamental understanding thereof. Understanding of this complex regime and the development of technologies to utilize this regime for flight require a comprehensive scientific and engineering approach including fundamental theory, computations, ground-testing, and flight testing. Due to the capacity of ground-test facilities to formulate tractable hypersonic experiments and, within those experiments, the capability of specialized diagnostics to extract critical information, ground-testing is the primary approach to cracking the multi-physics puzzle of hypersonic flight. Shock-tube-derived impulse flow facilities are a specific form of hypersonic ground-test facility and fill a critical niche within the field. These facilities are characterized by extremely short test times, often less than a few milliseconds, but excel at producing high-enthalpy flows with realistic flow chemistry. This capability is particularly valuable for the investigation of chemically reacting flows within air-breathing hypersonic vehicles such as the scramjet. Effective use of the short-duration test time is progressively being unlocked by the development of modern high-bandwidth instrumentation. Tunable diode laser absorption spectroscopy (TDLAS) offers the potential to provide quantitative measurement of numerous critical flow parameters within these facilities, including temperature, velocity, and species partial pressure. The violent flow environment of a flow model in an impulse ground-test facility creates a formidable challenge for the implementation of a TDLAS diagnostic. Impulse flow test times on the order of 1 ms necessitate exceptionally high measurement bandwidths to capture relevant transient phenomena. Harsh flow conditions introduce substantial measurement noise through mechanically- and density-gradient-induced beam-steering. Short optical paths within the model limit the magnitude of the optical absorption signal and the confined environment limits the resources available for required optical hardware. Further challenges are introduced through the requirements of generating an academically valuable dataset. Multiple simultaneous quantitative measurement parameters at numerous locations within the flow-field are often desirable in both reacting and non-reacting flows. To address the challenges of TDLAS sensing in impulse flow facilities, the efforts of this work include advances in both diagnostic methodology and optical hardware design. A new analytical framework has been developed for the technique of scanned-wavelength-modulation spectroscopy (scanned-WMS) to reject noise and achieve high-sensitivity while permitting the simultaneous measurement of temperature, partial pressure, and velocity. This new framework has enabled accurate measurements at unprecedented bandwidths in harsh flow environments. Integration of TDLAS optical components into flow models has led to novel approaches for harsh-environment optical alignment hardware, combining precision and sufficient robustness to survive repeated firings of an impulse facility. Furthermore, the intense beam-steering induced by facility vibrations and turbulent density gradients have motivated a comprehensive redesign of conventional beam collection approaches. TDLAS diagnostics leveraging these advances have been implemented in both the Stanford Expansion Tube Flow Facility for the purpose of facility characterization and in the HyShot II scramjet experiment at the High Enthalpy Shock Tunnel in Göttingen, Germany to investigate reacting and non-reacting scramjet flows. Results from each of these campaigns are presented as an illustration of the diagnostic capabilities developed in this work.