Liuhao Ma

Optical thermometry based on spontaneous flame emission is an efficient temperature monitoring method with wide applications in industrial combustion devices. However, traditional methods relying on spectral soot emission become irrelevant in the combustion of zero-carbon fuels such as hydrogen and ammonia. Here, we report a solution for ammonia flame thermometry based on spontaneous infrared H₂O emission using a single photodetector (PD). To our knowledge, this is the first time such a handy yet precise temperature monitoring technique has been developed and demonstrated for the thermometry of ammonia (NH₃) flames. Specifically, a PD equipped with interchangeable bandpass filters was used to collect the infrared flame emission in two carefully selected spectral bands. Detailed spectroscopic analysis was performed to derive the quantitative correlation between the measured emission and the flame temperature. Extensive experiments were conducted to validate the method for spatially resolved measurements in laminar premixed ammonia flames. The proposed method boasts the significant advantage of being economically scalable to the monitoring of flames produced by industrial zero-carbon fuels due to its low cost and fast data processing.

The present numerical study proposes a technique to extend the sensing range of tunable diode laser absorption spectroscopy (TDLAS) for flame measurement by involving physics constraints on both gas condition and spectroscopic parameters in the interpretation of spectra from multiple bands. A total of 24 major spectral lines for 2 spectral segments 4029–4031 cm⁻¹ and 7185–7186 cm⁻¹ are determined by specially designed detection function and contribution filtering. Numerical tests on uniform and complicated combustion fields prove the high accuracy, strong robustness to noise, wide sensing range, and good compatibility with tomography. The present study provides a strong technique for future complex combustion detection with advanced laser sources of broad spectrum.

Laser absorption spectroscopy (LAS) is a well-established measurement technique for quantitative chemical speciation in a combustion environment. However, in-situ LAS measurement of nitric oxide (NO) in ammonia flames has never been reported in the literature. This is despite the community’s recent strong interest in carbon-neutral ammonia combustion and the associated NO formation problem. In this work, we demonstrate the development and validation of a mid-infrared laser absorption sensor for in-situ measurements of NO formation and evolution in premixed ammonia and ammonia-methane cofired flames. To achieve calibration-free and interference-free measurements, the sensor exploits the NO absorption feature near 1900.07 cm^-1 using the techniques of both direct absorption spectroscopy and wavelength modulation spectroscopy. Special efforts were given to address the thermochemical non-uniformity along the light path which was shown to have notable effects on measurement accuracy. Detailed computational fluid dynamics modeling on the flame structure was performed along with theoretical spectral simulation to assist in the treatment of the non-uniformity effects. Comprehensive measurements were then performed in flames with different ammonia proportions and equivalence ratios, with results compared to data from probe sampling methods and kinetic modeling. The present work is the first demonstration of an in-situ mid-infrared LAS sensor for quantitative and spatially resolved NO measurement in ammonia flames.

Heterodyne phase-sensitive dispersion spectroscopy (HPSDS) is a quantitative non-intrusive gas sensing technique based on the determination of the refractive index of the target gas in the vicinity of an absorption transition. Since the phase instead of the intensity of the probing laser light is targeted, the technique boasts the advantage of being normalization-free. It is thus largely immune to laser intensity fluctuations due to either system instability or ambient interferences. Previous HPSDS-based sensors typically require calibration using standard mixtures to establish a look-up table between the measured phase signal and gas concentrations, which is both cumbersome and problematic when there are significant compositional variations between the calibration standards and the target gas. In this work, we present a robust and generic technique that addresses this issue with a successful realization of fully calibration-free measurements. Spectral-fitting to the entire dispersion spectra with free variables related to transition linecenter, broadening width, and integrated absorbance were used to eliminate the effects of unknown spectral broadening coefficients. What we believe to be a novel analytical model was proposed to unify both direct injection-current dithering-based HPSDS that includes simultaneous frequency/intensity modulation, and the external electro-optic modulator (EOM) modulation-based HPSDS with a non-ideal linear response of EOM. The proposed technique was first validated via numerical experiment to determine the gas concentration and the recovery of the absorption profiles. Actual experiments were subsequently performed for the measurement of CH₄ near 1.65 µm, N₂O near 4.46 µm, and NO near 5.26 µm, collectively demonstrating the capability of the technique for both near- and mid-infrared lasers with diverse modulation characteristics. Further demonstrations were performed to measure NH₃ concentrations at elevated temperatures through the fitting of the multiple dispersion spectra near 9.06 µm. The robust iterative spectral-fitting strategy and the measurement accuracies confirm the robustness of the proposed calibration-free (CF) HPSDS technique for quantitative gas sensing.