Pulsed Mid-Infrared Upconversion Imaging in Rotating Detonation Combustors
White, Logan
2020
Abstract
This work examines the application of pulsed mid-IR UpConversion Imaging (UCI) to study processes in unsteady combustion environments. UCI is a method for mid-IR light detection that leverages a nonlinear optical process known as Sum-Frequency Generation (SFG) to shift the wavelength of IR light carrying a target image to shorter, more easily detected wavelengths. This can be used to detect images in the mid-IR wavelength band (2 to 5 micrometers) that is populated with spectral signatures from many important combustion species with a silicon-based CCD camera that only responds to wavelengths below 1100 nanometers. Not only does this extend the operational range of silicon-based cameras, but UCI also exhibits unique detection properties that make it useful for unsteady combustion applications. These include a narrowband spectral response that can be tuned in real-time to isolate spectral signals from selected target species. Pulsed pump implementations of UCI also allow for precise time-gating of mid-IR imaging measurements, providing the temporal resolution that is critical for highly transient unsteady combustion environments. Despite these potential advantages of pulsed UCI compared to direct detection mid-IR cameras, there have been no prior reported examples of UCI used in practical combustion systems. As part of this work, a pulsed mid-IR UCI system was designed and developed that was suitable for species-specific imaging measurements in unsteady combustion systems. This system was applied to perform spatiotemporally resolved mid-IR imaging measurements in a laboratory-scale Rotating Detonation Combustor (RDC) to demonstrate and test the capabilities of pulsed UCI. RDCs are an emerging combustion technology where chemical energy release is achieved primarily by a detonation wave that continuously rotates in an annular combustion channel; a configuration that may enable system-level gains in both fuel and size efficiency over conventional alternatives. Many research questions remain open related to RDC operation including the conditions under which stable detonation wave propagation can be maintained and how to maximize pressure-gain benefits to efficiency. Examples of imaging diagnostics within these systems have been limited due to the challenges associated with implementing optical diagnostics in them including vibrations, high background luminosities, and the requirements for high spatial and temporal resolution to observe features near the detonation wave front. Addressing these issues by leveraging the unique properties of pulsed mid-IR UCI is a goal of this work. The pulsed mid-IR UCI system developed in this work was used to detect thermal radiation from ch{CO2} within the combustion channel of an RDC. These measurements were sensitive to the local pressure and temperature in the high-temperature regions of the RDC flow field. The results demonstrate species-specific imaging at the leading edge of detonation waves with sub-millimeter spatial resolution (an object plane Gaussian point-spread width of 400 micrometers) and sub-microsecond temporal resolution (an effective exposure time of 240 nanoseconds). These measurements are novel in that they are the first application of mid-IR UCI to a practical combustion system and they are the first examples of fully spatiotemporally resolved mid-IR imaging in RDCs. The UCI system was also used to design a mid-IR absorption diagnostic that enables tracer-free detection of hydrocarbon fuel species distributions within RDCs. The results of this work demonstrate that pulsed mid-IR UCI is a valuable tool for studying unsteady combustion processes.Subjects
upconversion infrared imaging rotating detonation combustor
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