Practical applications of laser absorption spectroscopy for aeroengine testing [electronic resource]
Keywords: Mechanical Engineering.

Reliable air-breathing hypersonic propulsion systems offer the potential to revolutionize aircraft performance in a variety of high-speed aerospace applications through substantial efficiency gains and hardware cost savings. Supersonic combustion ramjet (scramjet) engines are one such device that promise propulsion capabilities up to about Mach 10. However, before these devices are ever practically realized, considerable technical challenges must be overcome in combustor-inlet interaction, fuel-air mixing, and coupled turbulent flow/combustion modeling. The growing power of computational tools have accelerated the pace of solving these problems, but the accuracy of computational approaches can only be validated by rigorous experimental testing. Thus, there is a need for both facilities capable of creating conditions experienced during hypersonic flight, as well as diagnostics that can characterize the operation of those facilities and provide experimental data for the validation of computational models. Optical diagnostics such as laser absorption spectroscopy are capable of providing non-intrusive, in situ measurements of important flow-field parameters such as temperature, velocity, species concentrations, which makes them an invaluable resource to hypersonic aeroengine researchers. Absorption spectroscopy, in particular, has benefited from recent advances in laser and optics technology, allowing access to a variety of wavelengths corresponding to absorption transitions of important combustion species such as O2, H2O, and CO2. Moreover, these sensors only require compact, low-power laser sources and light can be delivered via fiber-optics, which enables the sensor to more easily integrate with test facility hardware. As a result, laser absorption spectroscopy has become a workhorse in experimental scramjet research, and has been applied in test facilities around the world. Building upon this prior work, here the design and results of several different spectroscopic sensors for facility characterization and distinct scramjet operation modes are presented. Both hydrogen-fueled and hydrocarbon-fueled scramjets are investigated in a variety of geometric configurations. These results comprise the largest data set of laser absorption spectroscopy measurements within scramjet combustors published to date, and are a valuable resource for computational researchers who wish to compare their models with experimental data. A primary drawback of laser absorption spectroscopy is that some techniques are sensitive to nonuniformity along the measurement line-of-sight. In highly three-dimensional flows such as within a scramjet combustor, this can prove to be a considerable hindrance. However, in the work here particular care has been taken to account for nonuniformity along the measurement path, and new techniques, including a new approach to wavelength-modulation spectroscopy data reduction, have been developed and applied to provide quantitatively accurate path-integrated measurements in the presence of nonuniformities. Additionally, novel applications of laser absorption spectroscopy are presented, including the use of absorption data to place an upper bound on the cavity residence time within a scramjet combustor, and a new sensor design for measuring air temperature in high-enthalpy facilities by tracking the formation of nitric oxide.