Combined use of TDLAS and LIBS for reconstruction of temperature and concentration fields

Temporally modulating laser pulses to stabilize LIBS measurement locations under large gas temperature gradients

In combustion research, laser-induced breakdown spectroscopy (LIBS) has been widely employed in local equivalence ratio measurement. However, the potential temperature gradients in the probe volume can significantly affect the shape of induced plasmas, resulting in unstable measurement locations. In this work, we improved the stability of measurement locations by modulating the laser pulse duration. In a hot-cold gas flow interface with large temperature gradients, when using the original laser pulse with a full width at half maximum (FWHM) of 4 ns, the locations of initial plasma core were insensitive to gradient variations; however, the plasma expansion behaviors differed significantly after 3 ns. The hot spots of plasmas diverged bi-directionally under high temperature, resulting in two-lobe structures and unstable measurement locations. After the laser pulse was modulated to a shorter duration using a pressure chamber, the plasma expansion was suppressed which constrained the plasma volume. Specifically, using a modulated pulse with a FWHM of 1.9 ns, the two-lobe structure was eliminated across the interface, and the standard deviation of measurement locations was reduced to 0.27 mm. The measured equivalence ratios across the interface showed favorable agreement with the simulation.

Pressure sensing with two-color laser absorption spectroscopy for combustion diagnostics

Pressure is an important parameter in assessing combustion performance that is typically measured using contact sensors. However, contact sensors usually disturb combustion flows and suffer from the temperature tolerance limit of sensor materials. In this Letter, an innovative noncontact two-color pressure sensing method based on tunable diode laser absorption spectroscopy (TDLAS) is proposed. This makes it possible to measure pressure at high temperature environments for combustion diagnostics. The proposed method uses the linear combination of the collision-broadened linewidths of two H2O absorption lines near 1343 and 1392 nm to measure the pressure. The feasibility and performance of such method have been demonstrated by measuring pressures from 1 to 5 bars at temperatures up to 1300 K with a laser wavelength scanning rate of 20 kHz. Measurement errors were found to be within 3%. Compared to previously reported TDLAS pressure sensors, this method is free from the influence of concentration and can also be combined with the existing two-color TDLAS thermometry to realize a fast, on line, and multi-parameter measurement in combustion diagnostics.

Carbon Dioxide Concentration Estimation in Nonuniform Temperature Fields Based on Single-Pass Tunable Diode Laser Absorption Spectroscopy

We propose a novel technique to accurately predict carbon dioxide (CO2) concentrations even in flow fields with temperature gradients based on a single laser path absorption spectrum measurement and machine learning. Concentration measurements in typical tunable diode laser absorption spectroscopy are based on a ratio of two integrated absorbances, each from a spectral line with different temperature dependence. However, the inferred concentrations can deviate significantly from the actual concentrations in the presence of temperature gradients. Furthermore, it is also difficult to find an analytical expression to compensate for the effect of nonuniform temperature profiles on concentration measurements. In this study, the entire absorption feature was considered since its shape and peak intensities vary with temperature and concentration. Specifically, a predictive model is obtained in a data-driven manner that can identify and compensate for the effect of a nonuniform temperature field on the spectrum. Despite a very detailed understanding of the CO2 absorption spectrum, it is nearly impossible to collect sufficient spectra for model acquisition by varying all temperature gradient conditions. Therefore, the model was obtained using only simulated data, much like the concept of a "digital twin". Finally, the predictive performance of the acquired model was verified using experimental data. In all test cases, the predictive performance of the model was superior to that of the two-line method. Additionally, a gradient-weighted regression activation mapping analysis confirmed that the model utilizes both the peak intensities as well as the change in the shape of absorption lines for prediction.

Detection of molecular oxygen using nanosecond-laser-induced plasma

Molecular oxygen (O2) concentration is measured by employing nanosecond laser-induced plasmas (ns-LIP) over a broad temperature spectrum ranging from 300 K to 1000 K, in the presence of an additional oxygen-containing molecule, CO2. Typically, emission spectra emanating from ns-LIP are devoid of molecular information, as the ns-LIP causes the dissociation of molecular species within the plasma. However, atomic oxygen absorption lines that momentarily appear at 777 nm in the broadband emission from the early-stage plasma are determined to be highly sensitive to the O2 mole fraction but negligibly affected by the CO2 mole fraction. The atomic O absorbing the plasma emission originates from the O2 adjacent to the plasma: robust UV radiation from the early-stage plasma selectively dissociates adjacent O2, exhibiting a relatively low photodissociation threshold, thus generating the specific meta-stable oxygen capable of absorbing photons at 777 nm. A theoretical model is introduced, explicating the formation of the meta-stable O atom from adjacent O2. To sustain the UV radiation from the plasma under high-temperature and low-density ambient conditions, a preceding breakdown is triggered by a split laser pulse (532 nm). This breakdown acts as a precursor, seeding electrons to intensify the inverse-Bremsstrahlung photon absorption of the subsequent laser pulse (1064 nm). Techniques such as proper orthogonal decomposition (POD) and support vector regression (SVR) are employed to precisely evaluate the O2 mole fraction (<1% uncertainty), by analyzing the short-lived (<10 ns) O2-indicator depicted in the early-stage plasma.

Machine learning in analytical spectroscopy for nuclear diagnostics [Invited]

Analytical spectroscopy methods have shown many possible uses for nuclear material diagnostics and measurements in recent studies. In particular, the application potential for various atomic spectroscopy techniques is uniquely diverse and generates interest across a wide range of nuclear science areas. Over the last decade, techniques such as laser-induced breakdown spectroscopy, Raman spectroscopy, and x-ray fluorescence spectroscopy have yielded considerable improvements in the diagnostic analysis of nuclear materials, especially with machine learning implementations. These techniques have been applied for analytical solutions to problems concerning nuclear forensics, nuclear fuel manufacturing, nuclear fuel quality control, and general diagnostic analysis of nuclear materials. The data yielded from atomic spectroscopy methods provide innovative solutions to problems surrounding the characterization of nuclear materials, particularly for compounds with complex chemistry. Implementing these optical spectroscopy techniques can provide comprehensive new insights into the chemical analysis of nuclear materials. In particular, recent advances coupling machine learning methods to the processing of atomic emission spectra have yielded novel, robust solutions for nuclear material characterization. This review paper will provide a summation of several of these recent advances and will discuss key experimental studies that have advanced the use of analytical atomic spectroscopy techniques as active tools for nuclear diagnostic measurements.

Flame Temperature Measurement Based on Laser-Induced Breakdown Spectroscopy and Element Doping

In this study, based on the existing high-temperature measurement and calibration equipment, calibration experiments using the spectral emissivity of intrinsic element particles in the field were designed to achieve the accurate measurement of a temperature field. Laser-induced breakdown spectroscopy was used to select the corresponding elements, and the element doping method was used to approximate the real temperature field. After calibrating the camera, the temperature distribution and spectral emissivity distribution of the flame were calculated. The range of calculated values was determined to be well-consistent with data collected using an infrared thermal imager, which verified the accuracy of the experiment.

Measurement of Temperature and H2O Concentration in Premixed CH4/Air Flame Using Two Partially Overlapped H2O Absorption Signals in the Near Infrared Region

It is important to monitor the temperature and H2O concentration in a large combustion environment in order to improve combustion (and thermal) efficiency and reduce harmful combustion emissions. However, it is difficult to simultaneously measure both internal temperature and gas concentration in a large combustion system because of the harsh environment with rapid flow. In regard, tunable diode laser absorption spectroscopy, which has the advantages of non-intrusive, high-speed response, and in situ measurement, is highly attractive for measuring the concentration of a specific gas species in the combustion environment. In this study, two partially overlapped H2O absorption signals were used in the tunable diode laser absorption spectroscopy (TDLAS) to measure the temperature and H2O concentration in a premixed CH4/air flame due to the wide selection of wavelengths with high temperature sensitivity and advantages where high frequency modulation can be applied. The wavelength regions of the two partially overlapped H2O absorptions were 1.3492 and 1.34927 μm. The measured signals separated the multi-peak Voigt fitting. As a result, the temperature measured by TDLAS based on multi-peak Voigt fitting in the premixed CH4/air flame was the highest at 1385.80 K for an equivalence ratio of 1.00. It also showed a similarity to those tendencies to the temperature measured by the corrected R-type T/C. In addition, the H2O concentrations measured by TDLAS based on the total integrated absorbance area for various equivalent ratios were consistent with those calculated by the chemical equilibrium simulation. Additionally, the H2O concentration measured at an equivalence ratio of 1.15 was the highest at 18.92%.