Internal temperature monitoring of high speed propulsion system is of great significance for engine performance evaluation and life prediction. As a passive optical measurement method without external light source and flow field interference, emission spectrum measurement technology has a good application prospect in harsh measurement environment. As the main combustion product, high temperature water vapor has a high emission intensity in the near infrared band, which is very suitable for temperature measurement applications. The frequency band integral ratio is proposed to eliminate the high resolution measurement requirement of spectrum acquisition system, and the temperature distribution of swirl flame is measured successfully.
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
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.
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.
Nonintrusive temperature measurements for a real ammonium dinitramide (ADN)-based thruster by using tunable diode laser absorption spectroscopy and monochromatic radiation thermometry are proposed. The ADN-based thruster represents a promising future space propulsion employing green, nontoxic propellant. Temperature measurements in the chamber enable quantitative thermal analysis for the thruster, providing access to evaluate thermal properties of the thruster and optimize thruster design. A laser-based sensor measures temperature of combustion gas in the chamber, while a monochromatic thermometry system based on thermal radiation is utilized to monitor inner wall temperature in the chamber. Additional temperature measurements of the outer wall temperature are conducted on the injector, catalyst bed, and combustion chamber of the thruster by using thermocouple, respectively. An experimental ADN thruster is redesigned with optimizing catalyst bed length of 14 mm and steady-state firing tests are conducted under various feed pressures over the range from 5 to 12 bar at a typical ignition temperature of 200°C. A threshold of feed pressure higher than 8 bar is required for the thruster’s normal operation and upstream movement of the heat release zone is revealed in the combustion chamber out of temperature evolution in the chamber.
Instantaneous measurement of flame spatial structure has been long desired for complicated combustion condition (gas turbine, ramjet et.). Three dimensional computed tomography of chemiluminescence (3D-CTC) is a potential testing technology for its simplicity, low cost, high temporal and spatial resolution. In most former studies, multi-lens and multi-CCD are used to capture projects from different view angles. In order to improve adaptability, only one CCD was utilized to build 3D-CTC system combined with customized fiber-based endoscopes (FBEs). It makes this technique more economic and simple. Validate experiments were made using 10 small CH4 diffusion flame arranging in a ring structure. Based on one instantaneous image, computed tomography can be conducted using Algebraic Reconstruction Technique (ART) algorithm. The reconstructed results, including the flame number, ring shape of the flames, the inner and outer diameter of ring, all well match the physical structure. It indicates that 3D combustion chemiluminescence could be well reconstructed using single camera.
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