The measurement of temperature and species concentration in combustion fields is very significant for the development of highly efficient combustion technologies for energy conservation and emission reduction. There are various measurement technologies, including contact and non-contact measurement. Tunable diode laser absorption spectroscopy (TDLAS) technology is a proven non-contact method for detection of the temperature and species concentration by absorption measurement. To enable two-dimensional (2D) representation of temperature and species concentration in combustion fields, the TDLAS technology is usually combined with computed tomography (CT). The latter is, however, considerably new in combustion research, especially in the solid fuel reaction environment. In this paper, a 32-path 2D CT-TDLAS system for temperature measurement in a pilot-scale coal-fired furnace was developed. The accuracy of the CT algorithm in the reconstruction of 2D temperature distributions in different laser-path arrangements was first analyzed using the sum of squared difference (SSD) and zero-mean normalized cross-correlation (ZNCC) by comparing it to the 2D temperature distribution of a full-scale coal-fired furnace simulated using computational fluid dynamics (CFD). The accuracy was improved by 32-path reconstruction. The study was then progressed to investigate its measurement accuracy in a simple CH 4 -air burner configuration with rounded and rectangular cells as well as sensitivity for flame shift detection, whereby the reconstructed temperature distribution was compared to the temperature measured using a thermocouple. It is verified that this CT reconstruction was feasible for various measurement areas, even if the center of the flame was shifted. Finally, a 32-path 2D CT-TDLAS system with a rectangular-structured cell was developed and applied for temperature measurement in a Tenaga Nasional Berhad (TNB) Research's pilot-scale coal-fired furnace. The 2D temperature distribution in the coal-fired furnace was reconstructed according to the experimental results. The potential of the CT-TDLAS for online 2D temperature measurement for actual applications is demonstrated.
Tunable diode laser absorption spectroscopy (TDLAS) technology is a developing method for temperature and species concentration measurements with the features of non-contact, high precision, high sensitivity, etc. The difficulty of two-dimensional (2D) temperature measurement in actual combustors has not yet been solved because of pressure broadening of absorption spectra, optical accessibility, etc. In this study, the combination of computed tomography (CT) and TDLAS with a wide scanning laser at 1335–1375 nm has been applied to a combustor for 2D temperature measurement in high temperature of 300–2000 K and high pressure of 0.1–2.5 MPa condition. An external cavity type laser diode with wide wavelength range scanning at 1335–1375 nm was used to evaluate the broadened H2O absorption spectra due to the high-temperature and high-pressure effect. The spectroscopic database in high temperature of 300–2000 K and high pressure of 0.1–5.0 MPa condition has been revised to improve the accuracy for temperature quantitative analysis. CT reconstruction accuracy was also evaluated in different cases, which presented the consistent temperature distribution between CT reconstruction and assumed distributions. The spatial and temporal distributions of temperature in the high-temperature and high-pressure combustor were measured successfully by CT-TDLAS using the revised spectroscopic database.
Computed tomography-tunable diode laser absorption spectroscopy (CT-TDLAS) has been widely used in the diagnosis of the combustion flow field. Several optimized CT reconstruction algorithms such as iteration methods, transformation methods, and nonlinear least squares were applied. Considering the industrial application background, the performances of algebraic iteration reconstruction with the simultaneous algebra reconstruction technique (SART), Tikhonov regularization, and least squares with the polynomial fitting method were discussed in this study. For the mentioned algorithm, identical simulated reconstruction parameters that contained 32-path laser structures, assumed temperature distribution, and absorption databases were adopted to evaluate the reconstruction performance including accuracy, efficiency, and measurement of environment applicability. In this study, different CT reconstruction algorithms were also used to calculate the temperature distribution of the Bunsen burner flame. The different reconstruction results were compared with thermocouple detection data. With the theoretically simulated and experimental analysis, the least squares with the polynomial fitting technique has advantages in reconstruction accuracy, calculation efficiency, and laser path applicability for the measurement condition. It will be helpful in enhancing CT-TDLAS technique development.
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