This manuscript describes the development of an ultrafast (i.e., femtosecond), mid-infrared, laser-absorption diagnostic and its initial application to measuring temperature, CO, and CH 4 in flames. The diagnostic employs a Ti:Sapphire oscillator emitting 55-fs pulses near 800 nm which were amplified and converted into the mid-infrared (mid-IR) though optical parametric amplification (OPA) at a repetition rate of 5 kHz. The pulses were directed through the test gas and into a high-speed midinfrared spectrograph to image spectra across a ≈30 nm bandwidth with a spectral resolution of ≈0.3 nm. Gas properties were determined by least-squares fitting a spectroscopic model to measured single-shot absorbance spectra. The diagnostic was validated with measurements of temperature, CO, and CH 4 in a static-gas cell with an accuracy of 0.7% to 1.8% of known values. Single-shot, 5 kHz measurements of temperature and CO were acquired near 4.9 µm in a laser-ignited HMX (i.e., 1,3,5,7tetranitro-1,3,5,7-tetrazoctane) flame and exhibited a 1-σ precision of 0.4% at ≈2700 K. Further, CH 4 and temperature measurements were acquired near 3.3 µm in a partially premixed CH 4 -air flame produced by a Hencken burner and exhibited a precision of 0.3% at ≈1000 K. laser-absorption spectroscopy, ultrafast spectroscopy, mid-wave infrared spectroscopy, broadband absorption spectroscopy
This manuscript presents an ultrafast-laser-absorption-spectroscopy (ULAS) diagnostic capable of providing calibration-free, single-shot measurements of temperature and CO at 5 kHz in combustion gases at low and high pressures. Additionally, this diagnostic was extended to provide 1D, single-shot measurements of temperature and CO in a propellant flame. A detailed description of the spectral-fitting routine, data-processing procedures, and determination of the instrument response function are also presented. The accuracy of the diagnostic was validated at 1000 K and pressures up to 40 bar in a heated-gas cell before being applied to characterize the spatiotemporal evolution of temperature and CO in AP-HTPB and AP-HTPB-aluminum propellant flames at pressures between 1 and 40 bar. The results presented here demonstrate that ULAS in the mid-IR can provide high-fidelity, calibration-free measurements of gas properties with sub-nanosecond time resolution in harsh, high-pressure combustion environments representative of rocket motors.
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