Design of a Fiber-Coupled Mid-Infrared Fuel Sensor for Pulse Detonation Engines

A mid-infrared laser absorption sensor is designed and used for time-resolved fuel concentration measurements in pulse detonation engines. A 3.39 μm helium-neon laser is fiber coupled to the engine, enabling optical absorption measurements for engine firing rates ranging from 5 to 40 Hz. Individual components (e.g., fiber-coupling lenses, optical fiber, and detectors) are optimized to minimize the effects of vibration and thermal emission. Fiber-coupling enables delicate components to be isolated from engine vibration and detonation pressure waves. The first-generation design is used to measure ethylene concentration with a single-cycle signal-to-noise ratio of ∼7 for fired tests. The sensor is redesigned and measurements are performed on a different engine at multiple axial locations to extract fuel time of flight and concentration. Ethylene and propane concentration measurements were made, demonstrating the versatility of the sensor for multiple fuels. Single-cycle measurements of ethylene for fired tests have a signal-to-noiseratio of ∼6 whereas measurements of propane have a signal-to-noise ratio of ∼80, reflecting the larger absorption coefficient of propane at this wavelength. Comparison of fired and unfired tests reveals the effects of cycle-to-cycle interactions for the fired tests. Time-resolved fuel concentration measurements are shown to be crucial for understanding engine dynamics in high-repetition-rate pulse detonation engines.

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