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This paper presents the development and application of a broadband ultrafast-laser-absorption-spectroscopy (ULAS) technique operating in the mid-infrared for simultaneous measurements of temperature, methane (CH₄), and propane (C₃H₈) mole fractions. Single-shot measurements targeting the C-H stretch fundamental vibration bands of CH₄and C₃H₈near 3.3 µm were acquired in both a heated gas cell up to ≈650K and laminar diffusion flames at 5 kHz. The average temperature error is 0.6%. The average species mole fraction errors are 5.4% for CH₄and 9.9% for C₃H₈. This demonstrates that ULAS is capable of providing high-fidelity hydrocarbon-based thermometry and simultaneous measurements of both large and small hydrocarbons in combustion gases.

 

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.

 

This manuscript describes the first application of ultrafast-laser-absorption spectroscopy (ULAS) to characterizing high-pressure (up to 40 bar), multi-phase combustion gases. Single-shot measurements of temperature and CO were acquired at 5 kHz in AP-HTPB propellant flames with and without aluminum. An ultrafast light source was used to produce broadband pulses of light near 4.96 𝜇m at a repetition rate of 5 kHz and a high-speed mid-infrared imaging spectrometer was used to image the pulses across an 86 nm bandwidth with a spectral resolution of 0.7 nm. Measurements of temperature and CO concentration were obtained by least-squares fitting simulated absorbance spectra of CO to measured spectra. A system of corrective optics was used to diminish the e˙ect of beam steering during high-pressure experiments, greatly increasing the pressure capabilities of the diagnostic. The diagnostic was used to characterize AP-HTPB propellant flames in an argon bath gas at pressures of 1, 10, 20, and 40 bar. An aluminized AP-HTPB propellant was also characterized at 10 and 20 bar to demonstrate that ULAS can provide high-fidelity measurements in particulate-laden flames. The results demonstrate that ULAS is capable of providing single-shot temperature and species measurements at high pressures with 1-𝜎 precisions less than 1.1% and 3% for temperature and species respectively, despite non-absorbing transmission losses in excess of 90%. 

This paper presents a data-processing technique that improves the accuracy and precision of absorption-spectroscopy measurements by isolating the molecular absorbance signal from errors in the baseline light intensity ($I_o$) using cepstral analysis. Recently, cepstral analysis has been used with traditional absorption spectrometers to create a modified form of the time-domain molecular free-induction decay (m-FID) signal, which can be analyzed independently from$I_o$. However, independent analysis of the molecular signature is not possible when the baseline intensity and molecular response do not separate well in the time domain, which is typical when using injection-current-tuned lasers [e.g., tunable diode and quantum cascade lasers (QCLs)] and other light sources with pronounced intensity tuning. In contrast, the method presented here is applicable to virtually all light sources since it determines gas properties by least-squares fitting a simulated m-FID signal (comprising an estimated$I_o$and simulated absorbance spectrum) to the measured m-FID signal in the time domain. This method is insensitive to errors in the estimated$I_o$, which vary slowly with optical frequency and, therefore, decay rapidly in the time domain. The benefits provided by this method are demonstrated via scanned-wavelength direct-absorption-spectroscopy measurements acquired with a distributed-feedback (DFB) QCL. The wavelength of a DFB QCL was scanned across the CO P(0,20) and P(1,14) absorption transitions at 1 kHz to measure the gas temperature and concentration of CO. Measurements were acquired in a gas cell and in a laminar ethylene–air diffusion flame at 1 atm. The measured spectra were processed using the new m-FID-based method and two traditional methods, which rely on inferring (instead of rejecting) the baseline error within the spectral-fitting routine. The m-FID-based method demonstrated superior accuracy in all cases and a measurement precision that was$\approx <\#comment/>1.5$to 10 times smaller than that provided using traditional methods.