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Energetic materials have extremely high volumetric and specific energy densities, making them attractive and important in combustion systems. To improve their combustion performance, reliable temperature acquirement method is highly demanded. A laser sensor was developed for in situ and quantitative measurements of gas temperature. Scanned-wavelength direct absorption spectroscopy was used for line-of-sight temperature measurements. Multiple absorption features in the near-infrared combination band (v1+v3) and mid-infrared fundamental band (v3) were selected to establish four absorption line pairs with good temperature sensitivity. Three infrared distributed feedback lasers (DFB) were used to cover the selected absorption lines. The accuracy and uncertainty of the sensor were first numerically evaluated in a wide temperature range of 1000–3000 K under different Gaussian white noise levels (5–20%). A free-space optical setup was established to experimentally evaluate the sensor performance by measuring benchmark laminar premixed flames, which were compared with additional thermocouple measurements, chemical kinetic modeling and computational fluid dynamics simulations. The good performance of the current sensor indicates the potential of being used in non-intrusive, in-situ and quantitative diagnostics of the energetic materials combustion.

This study investigates the influence of burner configuration on nonpremixed flameless combustion (FLC) of propane in a cylindrical furnace. Two nozzle patterns are adopted, i.e., a central fuel/ox...

Multispectral infrared absorption spectroscopy was developed for in situ, non-intrusive and quantitative measurements of temperature distributions in laminar premixed ethylene/air sooting flames. Tunable distributed feedback (DFB) lasers near 1343 nm, 1392 nm and 2482 nm were used to exploit multiple H2O absorption lines with varied temperature sensitivities. Scanned-wavelength direct absorption spectroscopy combined with the multi-line profile-fitting strategy was conducted for temperature sensing along the line-of-sight. This optical method was first numerically investigated for representative combustion fields to evaluate the measurement accuracy and uncertainty under different noise levels (2–10%). In the experiment, optical measurements were performed at different heights above the burner (3–15 mm) under three representative flame conditions (equivalence ratio Φ = 1.9, 2.1 and 2.3) with different co-flow gases (N2 and air). Our measurements successfully captured the temperature field and were in excellent agreement with the thermocouple data within the high-temperature region. A slight temperature rise in the central flame was quantitatively differentiated when co-flow gas was changed from N2 to air. The proposed method proves to be a promising combustion diagnostic technique for quantitative temperature measurements with the line-of-sight information.

This experimental study investigates the stability and emission characteristics of the nonpremixed MILD combustion from a parallel jet burner firing propane (C3H8) in a cylindrical furnace without preheating and dilution. Forty cases are examined by varying the fuel-air nozzle separation S (= 8 mm–105 mm), global equivalence ratio Φ (= 0.6–1.0) and thermal input Pin (= 10 and 15 kW) to highlight the impact of S. The investigation is carried out based on the measurements of in-furnace temperature and exhaust species concentrations, as well as the visual inspection into the furnace. It is demonstrated that the NO emission (ENO) decreases generally with increasing either S (at S > 8 mm) or Φ (at least for Φ ≤ 1.0). Interestingly, the combustion for S = 8 mm behaves like a premixed counterpart and so is distinct from the other cases: e.g., this combustion is fully flameless or stable in the MILD regime over a wider range of Φ and its ENO has a weaker dependence on the furnace temperature. By contrast, for S = 45 mm, the flameless mode takes place only at Φ = 1.0 due to the small mixing zone under this air-fuel arrangement, which is revealed by aerodynamic analysis. Hence, the corresponding experimental ENO-S relationship appears to have a turning point around S = 45 mm. For the present study, all the measured NO emissions for S = 8–105 mm fall as the overall furnace temperature rises, which seemingly differs from the existing knowledge. To explain this and other results, the numerical analysis from computational fluid dynamics (CFD) is performed. It is found that the variations of NO emissions depends on the maximum temperature within the furnace rather than the average furnace temperature. Finally, the present study approves that the fuel-air jet separation should be taken as a critical operational parameter when designing the parallel multiple jet burner for nonpremixed MILD combustion.

A multiline-multiband absorption spectroscopy strategy is proposed and numerically studied in the present work to address the limited detection range of the conventional two-line thermometry. The new method utilizes the information and spectral characteristics of multiple transitions at multiple bands to derive temperature and species concentration. Various combinations of absorption lines are evaluated when measuring the combustion fields of a standard McKenna flame and a jet-in-hot-coflow flame. Results show that the proposed multiline-multiband absorption spectroscopy with the tomographic algorithm is effective in resolving the high-gradient region of the flames. Wide detection range and high accuracy are achieved simultaneously in the tomographic reconstructions of the thermochemical profiles. Using a 3-line scheme, the temperature and species profiles at the temperature range from 300 K to 2000 K are successfully reconstructed. The present study demonstrates the great potential of multiline-multiband absorption spectroscopy for flames with a high gradient region or a steep boundary layer.

We studied the boundary layer effects on flame temperature measurements using line-of-sight (LOS) absorption spectroscopy. The primary objective is to quantify the measurement uncertainty of the LOS-determined temperature, which is influenced by the thermal and species boundary layers existing in standard laminar premixed flames. The boundary layer thickness (δ), central flame temperature (Tc) and species concentration (Xc) are the major factors investigated in this work. Typical absorption lines of H2O in the wavelength range of 1.4–2.9 μm were examined under different boundary layer conditions (δ/L = 0–50%, L is the optical path length above the flame; Tc = 1400–2200 K; and Xc = 14–20%). As a result, the thermal boundary layer is observed to contribute mainly to the temperature deviation of the LOS measurement, which increases with δ and Tc. In comparison, the variation of Xc between 14% and 20% has a negligible effect on the temperature measurement in the presence of both thermal and species boundary layers. A systematical investigation of all the selected line pairs reveals that a properly selected line pair reduces the temperature deviation by a maximum of 16.8% under typical laminar flame conditions. The particular line pair centered at 4029.52 cm−1 and 4030.73 cm−1, which could be detected by a single tunable semiconductor laser, is recommended for the LOS temperature measurement over a temperature range of 1400–2200 K. Finally, we performed a case study of five representative temperature measurements in laminar flames and successfully corrected the LOS-determined temperature by taking into account the boundary layer effects.

We report the theoretical and experimental study of calibration-free heterodyne phase-sensitive dispersion spectroscopy (HPSDS) in the mid-infrared using a direct current modulated mid-infrared quantum cascade laser (QCL). The modulation of QCL current at several hundred MHz or higher generates the synchronous frequency and intensity modulation of the QCL emission. An analytical model of the phase of the beat note signal in HPSDS is derived by considering the absorption and dispersion processes and incorporating the QCL modulation parameters. In the experiment, a 4.5 μm QCL modulated at 350 MHz was used to measure N2O at 200 Torr in a 10 cm gas cell. The N2O concentrations inferred from the analytical model were compared with the nominal values to show good agreement over the concentration range of 189−805 ppm with a standard deviation

We present the first demonstration of heterodyne phase-sensitive dispersion spectroscopy (HPSDS) for in situ, non-intrusive and quantitative CO2 concentration measurements in flames. Dispersion spectroscopy retrieves gas properties by measuring the refractive index in the vicinity of a molecular resonance. The HPSDS scheme features a significant diagnostic advantage of the intrinsic immunity to laser power fluctuations caused by beam steering, thermal radiation and soot scattering in combustion environments, and thus no extra calibration process is required. In this work, we described the spectroscopic fundamentals for measuring heterodyne phase signals in flames. As a proof of principle, we used a mid-infrared interband cascade laser (ICL) near 4183 nm to exploit the strong CO2 transitions in the R-branch of the v3 fundamental band. The HPSDS signals of four CO2 lines, R(76), R(78), R(80) and R(82), were measured in CH4/air flames to obtain CO2 concentrations at different equivalence ratios (Φ = 0.8–1.2), yielding a good agreement with the simultaneous laser absorption measurements using the same ICL. With its immunity to laser power fluctuations verified experimentally, the HPSDS sensor was successfully implemented to measure CO2 concentrations in C2H4/air sooting flames (Φ = 1.78–2.38). Laser dispersion spectroscopy proves to be a promising and alternative diagnostic tool for combustion measurements.

We performed a comprehensive laser absorption/extinction study of temperature and soot volume fraction (SVF) in C2H4/air premixed sooting flames. Laser-absorption two-line thermometry at 2.5 μm provided a temperature uncertainty of 50 K compared with that of 90 K using conventional thermocouples. Laser extinction of soot at 633 nm was first validated against the previous measurements using laser-induced incandescence. All of the measurements were conducted at four representative C2H4 flame conditions (equivalence ratio Φ = 1.78, 1.95, 2.14, and 2.38). In addition, a CFD (computational fluid dynamics) framework coupling a skeletal mechanism (56 species and 428 reactions) with the Moss–Brookes model was developed for interpreting the experimental data. The current CFD simulations well predicted the temperature and SVF distribution along the centerline of flame. It is of interest to observe that the SVF depends on the Reynolds number of reactants by investigating the SVFs at different heights above the burne...

This paper reports the combustion and emission characteristics of the premixed MILD combustion of propane established by a single jet burner in a laboratory-scale cylindrical furnace. Measurements ...

This paper reports a comprehensive investigation into the occurrence and identification of nonpremixed MILD combustion of C3H8/air in a cylindrical furnace. The combustion mode is altered by varying burner arrangement, air-fuel nozzle separation, equivalence ratio and thermal input. Based on the experimental observations, flow analysis and chemical calculations, a new criterion is proposed to identify the occurrence of nonpremixed MILD combustion using local heat release rate (HRR) particularly for CFD simulations. Importantly, unlike the case of CH4, the nonpremixed MILD combustion of C3H8 exhibits a negative HRR (HRR−) region that departs distantly from the positive HRR (HRR+) region, due to complex pyrolysis. This is distinct from the conventional flame where the HRR− region is adjecent to the HRR+ one. The new criterion notably improves the mode identification of nonpremixed combustion operating in various furnaces at different conditions. Moreover, the current study highlights the importance of local HRR in linking the experimental observation, CFD simulation and chemical calculation of MILD combustion.

We performed a combined experimental and numerical study of the flame uniformity for a standard McKenna burner by investigating the spatially-resolved temperature and species concentrations in laminar CH4/air premixed flames. In the experiment, multi-wavelength laser absorption spectroscopy (LAS) was employed for in situ, non-intrusive, and quantitative measurements of temperature, H2O and CO2. In the simulation, computational fluid dynamics (CFD) simulations were performed to model the thermochemical structures of the flames. The LAS measurements and CFD simulations were compared by varying the co-flow rate, equivalence ratio (Φ = 0.8–1.2), and the Reynolds number of the reactant flow. In general, all the simulations were in relatively good agreement with the experimental data. We observed that the flame temperature and H2O/CO2 concentrations have almost the identical radial distributions with a certain non-uniformity, which should be carefully considered when using the standard flame for combustion research. In particular, the flame uniformity could be improved by increasing the Reynolds number of reactant flow or reducing the co-flow rate.

We report the influence of absorption line selection on the tomographic results for high-temperature flames by numerical and experimental methods. Different combinations of infrared H2O absorption transitions are utilized with the Tikhonov-regularized Abel inversion to reconstruct the radial distribution of temperature and H2O concentration in a flat flame. It is shown that besides using the mathematical algorithm such as regularization, selecting a line pair with a large Δ E″ (>1390 cm−1) also reduces the reconstruction uncertainty at 300–2000 K. In this study, a proper selection of absorption line pairs reduces the reconstruction uncertainty by 25% at the same level of noise. The line pair of H2O transitions at 4029.524 cm−1 and 4030.729 cm−1 is recommended for the tomography of high-temperature flames at 1000–3000 K, whereas the line pair of 7185.597 cm−1 and 7444.352 cm−1 can be used at 300–1000 K.

We measured CO 2 concentrations in laminar premixed flames using heterodyne phase sensitive dispersion spectroscopy by directly modulating the injection current of an interband cascade laser.

We performed time-resolved characterization of plasma-photocatalytic removal of nitric oxide (NO) in a ceramic foam packing double dielectric barrier discharge (DBD) reactor using mid-infrared laser absorption spectroscopy (LAS). Non-thermal plasma (NTP) was generated in the porous ceramic foam coated with mesoporous TiO2 thin film. Real-time NO monitoring was performed using a quantum cascade laser (QCL) at 5.26 µm combined with a custom-designed multipass gas cell, leading to a sub-ppm detection limit at 0.1-s measurement time. By analyzing the feed gas of 90 ppm NO/1% O2 balanced in N2, the plasma treatment without TiO2 photocatalyst had no effect on NO removal when the discharge power was below 45 W. In comparison, the NTP-TiO2 system achieved an efficiency of 49% for NO removal even at the relatively low discharge power of 18 W. The synergy between NTP and TiO2 photocatalyst effectively promotes the NO removal at low plasma discharge powers. However, we also observed a negative effect of TiO2 photocatalyst on NO removal at a relatively high discharge power of 135 W.