This work aims to develop a fan-beam tomographic sensor using tunable diode lasers that can simultaneously image temperature and gas concentration with both high spatial and temporal resolutions. The sensor features three key advantages. First, the sensor bases on a stationary fan-beam arrangement, by which a high spatial resolution is guaranteed because the distance between two neighboring detectors in a view is approximately reduced to the size of a photodiode. Second, fan-beam illumination from five views is simultaneously generated instead of rotating either the fanned beams or the target, which significantly enhances the temporal resolution. Third, a novel set of optics with the combination of anamorphic prism pair and cylindrical lens is designed, which greatly improves the uniformity of the planar beams, and hence improves the reconstruction fidelity. This paper reports the tomographic model, optics design, numerical simulation and experimental validation of the sensor. The sensor exhibits good applicability for flame monitoring and combustion diagnosis.

The purpose of this paper is to monitor the dynamic cross-sectional behavior of swirling flames using the tunable diode laser absorption spectroscopy (TDLAS) tomographic system. The newly developed online and highly spatially resolved imaging system based on TDLAS tomography was employed to monitor and reveal directly the reaction process in the swirling flame by reconstructing the 2-D distributions of temperature and H₂O concentration over a cross section of the flame. The system was demonstrated to be capable of capturing the temperature distribution accurately and inferring the thermal expansion over the cross section of interest in the swirling flame generated by a model swirl injector operating in partially premixed combustion mode. As the equivalence ratio was decreased, thermal oscillations extracted from the real-time 2-D reconstructions were used to infer the instability of the swirling flame. Furthermore, the developed system was applied to capture dynamically the process of blowout of the swirling flame, illustrating that the system can provide firsthand and reliable visual data to help prevent the flame from lean blowout (LBO). This paper reports the first experimental observation of the dynamic cross-sectional behavior of the swirling flame, enabled by the high spatial and temporal resolutions provided by the TDLAS tomographic imaging system. The developed system can help better understand the LBO mechanism so as to improve the performance of low-emission gas turbine combustors.

Laser absorption spectroscopy combined with spectral analysis has been extensively investigated in detection and measurements of gas samples because of their broad applicability for measuring more than one hundred species, such as O₂, CO₂, H₂O, HCl, NH₃, NO_x, and hydrocarbons. Recently, real-time detection and measurements of gas samples based on laser absorption spectroscopy have attracted considerable interest from various fields owing to their performance of robustness, fast response speed, high sensitivity, and good precision. Here, we review the commonly used infrared laser absorption spectroscopy techniques for real-time measurements and the significant optical equipment utilized in measuring systems. Then, we discuss practical applications based on infrared laser absorption spectroscopy which require real-time determination of gas component concentrations, including atmospheric environment monitoring, breath analysis, combustion diagnostics, and industrial applications. Finally, perspectives of real-time determination of gas component concentrations based on infrared laser absorption spectroscopy are presented.

The Wavelength Modulation Spectroscopy (WMS) method is typically used in the Tunable Diode Laser Absorption Spectroscopy (TDLAS) for spectrum extraction in noisy background. Usually, two entire absorption spectra for target species are scanned in a cascade way in the WMS method, and it limits the temporal response of the TDLAS sensor. In this paper, the frequency division multiplexing and main peak scanning method were introduced to the WMS method and enabled a high frame rate six times of that in the classical TDLAS sensor at the same scanning speed. For the first time, two wavelengths modulated at different frequencies were used to extract the absorptions in parallel along various laser paths and implemented successfully on a TDLAS tomography hardware, to reconstruct the distributions of both temperature and specie concentrations, at a frame rate upto 10kHz. A five-view fan-beam tomographic sensor was utilized in the tomographic imaging. In each view, lasers at two wavelengths were selected and modulated at different sinusoidal frequencies on the same sawtooth signal. Numerical simulations varied the high-quality tomographic imaging of both temperature and H2O molar concentration distributions for different phantoms. Experiments were also carried out on dynamic flames, which were generated by exciting a Bunsen burner acoustically. The acoustic excitation frequency varied from 40 Hz to 750 Hz and results of the tomographic images agreed well with the flame evolutions.

Temperature distributions of dynamical flames were measured by using a proposed calibration-free wavelength modulated lateral shearing interferometry. A distributed feedback (DFB) laser was modulated in the wavelength as a sawtooth profile. The wrapped flame phase in the images was obtained pixel by pixel by calculating the phase shift between the interference light intensity profiles before and after flame ignition. Phase unwrapping and recovery were adopted to solve the flame phase distribution. The refractive index distribution was reconstructed with the help of Inverse Abel transform. The temperature imaging was further achieved by the ideal gas law and Gladstone-Dale model. Numerical simulations were conducted for performance evaluations. The flame phase, refractive index and temperature distributions were reconstructed with errors of 0.0094, 0.0154 and 0.0958, respectively. Standard deviations were in the magnitude of 10 ^-4 at the presence of Gaussian random noises and showed the noise immunity of the proposed method. In actual experiments, reconstructed temperature distribution of a steady flame generated on a Bunsen burner agreed well with the thermocouple measurements. A dynamical process of flame extinguishment was portrayed at one kilo frames per second. Temporal variation of the flame temperature distribution well reflected the extinguishing process. The entire measurement process did not require prior calibration of the modulated wavelength, and the imaging speed was more than ten times faster than the existing phase modulated interferometer. The proposed method can be used for on-line evaluation of dynamic flames.