In complex environments, the measurement of trace gases is inevitably greatly affected by broadband absorption (such as water absorption in combustion exhaust). Based on the study of high-order harmonics in linear wavelength modulation spectroscopy, a method for detecting target gases (trace, weak absorption) in the presence of broadband absorption interference is proposed. Theoretical analysis and numerical simulation work give the applicable conditions of this method. The effectiveness of this method is verified by NO2 measurement of combustion exhaust gas. This study proves the application potential of this method for the measurement of trace gases under broadband absorption interference.
Further development of hybrid propulsion systems requires a deeper understanding of the complex physicochemical mechanisms governing its combustion performance. A tunable diode laser absorption tomography (TDLAT) method was developed for investigating the thermochemical processes at the nozzle exit of an oxygen/Poly Methyl MethAcrylate (PMMA) hybrid rocket motor. Firing tests were conducted for different oxidizer mass fluxes ranging from 2.73 to 3.51 g/ (cm2·s). A distributed feedback (DFB) laser was tuned to cover three H2O absorption lines near 2.5 μm, using scanned-wavelength direct absorption (DA) mode with 2.0 kHz repetition rate. Under an assumption of cylindrical symmetry, a Radon transformation was applied to yield radially- and time- resolved absorption coefficient, from which the radial distribution of temperature and H2O partial pressure were reconstructed. Based on the Taylor series method (TSM), measurement uncertainty was analyzed in detail considering line-strength uncertainty, Voigt fitting residuals and Radon transformation. Finally, the radial distribution and dynamic variations of both temperature and H2O partial pressure were obtained in all firing tests, both the constructed results show measurement sensitivity to chemical kinetic progress and oxidizer mass flux changes. Our experimental results highlight the capability of TDLAT to characterize combustion processes of hybrid rocket motors.
A mid-infrared TDLAS sensor near 2.5μm was designed for time-resolved measurements of temperature and water vapor partial pressure at the nozzle exit of a laboratory-scale hybrid rocket motor. Several previously used H2O transitions within 2.4-2.9μm were thoroughly investigated, and a line-pair containing three transitions (4029.52 cm-1 , 4030.51 cm-1 and 4030.73 cm-1 ) was selected for the optimal overall properties like strong absorbance, sufficient temperature sensitivity, single laser scan, high immunity from the ambient H2O transitions and low measurement uncertainty affected by temperature over the range of 1500K-2500K. Firing tests were conducted on an oxygen/paraffin-fueled hybrid rocket motor operating at oxygen/fuel ratios (O/Fs) of 3.10, 2.77 and 2.88, corresponding to average combustion pressures of 1.91MPa, 2.09MPa and 2.38MPa. A distributed feedback (DFB) laser tuned repetitively at 2kHz was used as the light source, and simultaneously the transmitted spectra were detected at a 2MHz sampling rate. Finally, a 4.5ms time-scale variations of temperature and H2O partial pressure were captured by TDLAS sensor. Uncertainty analysis was made in detail based on average temperature (1929.8K, 1926.5K, and 1990.7K) and average H2O partial pressure (0.237MPa, 0.253MPa, and 0.285MPa), leading to temperature uncertainty of around 2.24% and partial pressure uncertainties of around 3.80%, 3.79% and 4.04% respectively. The time-resolved measurement results and small measurement uncertaintiesindicate that TDLAS has the potential to evaluate the combustion performance of hybrid rocket motor
Tunable Diode Laser Absorption Spectroscopy (TDLAS) has been one of the most powerful techniques for combustion diagnostics in different kinds of burners. Combined with Hyperspectroscopy Tomography (HT), TDLAS can improve its spatial solution. This study reports a TDLAS-tomography system and its application in a swirl burner. The diagnostics system composed of sixteen beams (13X13, 13 parallel beams and 13 vertical beams). Four water vapor absorption lines, 7185.6 cm-1, 7444.3 cm-1, 7466.3 cm-1, and 6807.8 cm-1, were utilized in each beam using time-division-multiplexed (TDM) method at total measuring frequency of 2.5 kHz. A reconstruction routine based on simulated-annealing algorithm was used to deduce distributions of temperature T and water partial pressure PX. Dynamic data was obtained during the ignition of hydrogen fuel at the exit of the scramjet combustor. T and PX distribution of cross section indicate the flame location and its intensity. Successful experiments show great performance of this diagnostic method.
Shock tube experiments are carried out to study the physical and chemical processes during a vehicle entry into the Mars atmosphere using tunable diode laser absorption spectroscopy (TDLAS) and optical emission spectroscopy (OES). CO concentration distributions are diagnosed behind a shock wave in a CO2-N2 mixture with three different conditions of initial pressure and velocity. The strong shock wave is established in a shock tube driven by combustion of hydrogen and oxygen. Time-resolved spectra of the Δv = 0 sequence of the B2Σ+ →X2Σ+ electronic transition of CN have been observed through OES. A precise analysis of the CN violet spectra is performed and used to determine rotational and vibrational temperatures. Two absorption lines in the first overtone band of CO near 2.33 μm, are selected from a HITRAN simulation to calibrate laser wavelength and detect the CO concentration. Combined with these temperature results using OES, CO concentrations in the thermal equilibrium region are derived, which are 2.91 × 1017 cm-3, 7.46 × 1017 cm-3 and 1.01 × 1018 cm-3, corresponding to equilibrium temperatures equal to 7000 ± 400 K, 7400 ± 300 K, 6000 ± 300 K in the low, medium and high pressure conditions, respectively.
This paper reports the laser absorption measurements of atomic oxygen in the FD04 arc-heater at China Academy of Aerospace Aerodynamics (CAAA). An atomic oxygen absorption line at 777.19 nm is utilizied for detecting the population of electronically excited oxygen atom in an air plasma flow. A scanned-wavelength direct absorption mode is used in this study. The laser is scanned in wavelength across the absorption feature at a rate of 200 Hz. Under the assumption of thermal equilibrium, time-resolved temperature measurements are obtained on one line-of-sight in the arc-heater. The good agreement of the temperature inferred from the sonic throat method suggests the equilibrium assumption is valid. These results illustrate the feasibility of the diode laser sensors for flow parameters in high enthalpy arc-heated facilities.
Shock tube experiments are carried out to study the physical and chemical processes during a vehicle entry into the Mars atmosphere using optical emission spectroscopy (OES) and tunable diode laser absorption spectroscopy (TDLAS). Gas temperature and CO concentration distribution are diagnosed behind a shock wave in a CO2-N2 mixture with two different conditions of initial pressure and velocity. The strong shock wave is established in a shock tube driven by combustion of hydrogen and oxygen. Time-resolved spectra of the Δv = 0 sequence of the B2Σ+→X2Σ+ electronic transition of CN have been observed through OES. A precise analysis of the CN violet spectra is performed and used to determine rotational and vibrational temperatures. Two absorption lines in the first overtone band of CO near 2.33 μm, are selected from a HITRAN simulation to calibrate laser wavelength and detect the CO concentration. Combined with these temperature results using OES, CO concentrations in the thermal equilibrium region are derived, which are 2.91 × 1012 cm-3 and 1.01 × 1013 cm-3, corresponding to equilibrium temperatures equal to 7000 ± 400 K and 6000 ± 300 K in low and high pressure conditions, respectively.
This paper describes the CO concentration and gas temperature distribution measurements behind a strong shock wave in the simulated Martian atmosphere by an optical diagnostic system. The strong shock wave (6.31 ± 0.11 km/s) is established in a shock tube driven by combustion of hydrogen and oxygen. The optical diagnostic system consists of two parts: the optical emission spectroscopy (OES) system and the tunable diode laser absorption spectroscopy (TDLAS) system. For OES system, high temporal and spatial resolution experimental spectra of CN violet system (B2Σ+→X2Σ+, v = 0 sequence) have been observed. Rotational and vibrational temperature distribution along the shock wave is inferred through a precise analysis of high-resolution experimental spectra. For TDLAS system, a CO absorption line near 2335.778 nm is utilized for detecting the CO concentration using scanned-wavelength direct absorption mode. Combined with these experimental results using OES, CO concentration in the thermal equilibrium region is derived. The detected average CO concentration is 7.46 × 1012 cm-3 with the average temperature of 7400 K ± 300 K, which corresponds to the center fractional absorption of 2.7%.
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