Md. Moinul Hossain

Optical Fiber Imaging Based Tomographic Reconstruction of Burner Flames

This paper presents the design, implementation, and evaluation of an optical fiber imaging based tomographic system for the 3-D visualization and characterization of a burner flame. Eight imaging fiber bundles coupled with two RGB charge-coupled device cameras are used to acquire flame images simultaneously from eight different directions around the burner. The fiber bundle has 30k picture elements and an objective lens with a 92° angle of view. The characteristic evaluation of the imaging fiber bundles and the calibration of the system were conducted to ensure the accuracy of the system. A new tomographic algorithm that combines the logical filtered back-projection and the simultaneous algebraic reconstruction technique is proposed to reconstruct the flame sections from the images. A direct comparison between the proposed algorithm and other tomographic approaches is conducted through computer simulation for different test templates and numbers of projections. The 3-D reconstruction of the cross- and longitudinal-sections of a burner flame from image projections obtained from the imaging system was also performed. The effectiveness of the imaging system and computer algorithm is assessed through experimental tests.

Three-dimensional reconstruction of flame temperature and emissivity distribution using optical tomographic and two-colour pyrometric techniques

This paper presents an experimental investigation, visualization and validation in the three-dimensional (3D) reconstruction of flame temperature and emissivity distributions by using optical tomographic and two-colour pyrometric techniques. A multi-camera digital imaging system comprising eight optical imaging fibres and two RGB charged-couple device (CCD) cameras are used to acquire two-dimensional (2D) images of the flame simultaneously from eight equiangular directions. A combined logical filtered back-projection (LFBP) and simultaneous iterative reconstruction and algebraic reconstruction technique (SART) algorithm is utilized to reconstruct the grey-level intensity of the flame for the two primary colour (red and green) images. The temperature distribution of the flame is then determined from the ratio of the reconstructed grey-level intensities and the emissivity is estimated from the ratio of the grey level of a primary colour image to that of a blackbody source at the same temperature. The temperature measurement of the system was calibrated using a blackbody furnace as a standard temperature source. Experimental work was undertaken to validate the flame temperature obtained by the imaging system against that obtained using high-precision thermocouples. The difference between the two measurements is found no greater than ±9%. Experimental results obtained on a laboratory-scale propane fired combustion test rig demonstrate that the imaging system and applied technical approach perform well in the reconstruction of the 3D temperature and emissivity distributions of the sooty flame.

Recent Advances in Flame Tomography

To reduce greenhouse gas emissions from fossil fuel fired power plants, a range of new combustion technologies are being developed or refined, including oxy-fuel combustion, co-firing biomass with coal and fluidized bed combustion. Flame characteristics under such combustion conditions are expected to be different from those in normal air fired combustion processes. Quantified flame characteristics such as temperature distribution, oscillation frequency, and ignition volume play an important part in the optimized design and operation of the environmentally friendly power generation systems. However, it is challenging to obtain such flame characteristics particularly through a three-dimensional and non-intrusive means. Various tomography methods have been proposed to visualize and characterize flames, including passive optical tomography, laser based tomography, and electrical tomography. This paper identifies the challenges in flame tomography and reviews existing techniques for the quantitative characterization of flames. Future trends in flame tomography for industrial applications are discussed.

Three-dimensional temperature field measurement of flame using a single light field camera

Compared with conventional camera, the light field camera takes the advantage of being capable of recording the direction and intensity information of each ray projected onto the CCD (charge couple device) sensor simultaneously. In this paper, a novel method is proposed for reconstructing three-dimensional (3-D) temperature field of a flame based on a single light field camera. A radiative imaging of a single light field camera is also modeled for the flame. In this model, the principal ray represents the beam projected onto the pixel of the CCD sensor. The radiation direction of the ray from the flame outside the camera is obtained according to thin lens equation based on geometrical optics. The intensities of the principal rays recorded by the pixels on the CCD sensor are mathematically modeled based on radiative transfer equation. The temperature distribution of the flame is then reconstructed by solving the mathematical model through the use of least square QR-factorization algorithm (LSQR). The numerical simulations and experiments are carried out to investigate the validity of the proposed method. The results presented in this study show that the proposed method is capable of reconstructing the 3-D temperature field of a flame.

Three-dimensional reconstruction of combustion flames through optical fiber sensing and CCD imaging

This paper presents the design and implementation of an imaging fiber based tomographic system for the three-dimensional (3-D) visualization and characterization of combustion flames. Eight imaging fiber bundles coupled with two RGB CCD cameras are used to acquire flame images simultaneously from eight different directions around the burner. The fiber bundle consists of 30k picture elements and has an objective lens with a 92° angle of view, offering a wide field of view. A series of system calibrations was conducted to ensure system accuracy. Computer simulations were also undertaken to evaluate different tomographic algorithms for 3-D reconstruction of flame cross-sections based on eight image projections obtained from the imaging system. Preliminary experimental results obtained on a gas fired test rig demonstrate that the system is capable of 3-D visualizing and characterizing flames.

Three-dimensional reconstruction of flame temperature and emissivity through tomographic imaging and pyrometric measurement

This paper presents the reconstruction of 3-D (three-dimensional) flame temperature and emissivity distributions using tomographic imaging and pyrometric techniques. An imaging system comprising eight optical imaging fiber bundles and two RGB CCD (Charged-couple Device) cameras is used to acquire flame images simultaneously from eight different directions. A combined LFBP (Logical Filtered Back-Projection) and SART (Simultaneous Iterative Reconstruction and Algebraic Reconstruction Technique) algorithm is applied to the 3-D reconstruction of gray-level intensity for the two primary color (red and green) images. The temperature distribution of the flame is determined from the ratios of the reconstructed gray-levels intensity and then the emissivity is estimated from the ratio of the gray-level of a primary color of the images to that of a blackbody source at the same temperature. The system calibration was conducted on a blackbody furnace. Experimental results on a gas fired combustion test rig demonstrate that the proposed technical approach performs well in the measurement of the 3-D temperature and emissivity distributions of the flame.

Geometric calibration of focused light field camera for 3-D flame temperature measurement

Focused light field camera can be used to measure three-dimensional (3-D) temperature field of a flame because of its ability to record intensity and direction information of each ray from flame simultaneously. This work aims to develop a suitable geometric calibration method of focused light field camera for 3-D flame temperature measurement. A modified method based on Zhangs camera calibration is developed to calibrate the camera and the measurement system. A single focused light-field camera is used to capture images of bespoke calibration board for calibration in this study. Geometric parameters including intrinsic (i.e., camera parameters) and extrinsic (i.e., camera connecting with the calibration board) of the focused light field camera are calibrated to trace the ray projecting onto each pixel on CCD (charge-coupled device) sensor. Instead of using line features, corner point features are directly utilized for the calibration. The characteristics of focused light field camera including one 3-D point corresponding to several image points and matching main lens and microlens f-numbers, are used for calibration. Results with a focused light field camera are presented and discussed. Preliminary 3-D temperature distribution of a flame is also investigated and presented.

Measurement of flame temperature distribution using optical tomographic and two-color pyrometric techniques

This paper presents the technique which combines optical tomographic and two-color parametric techniques for the three-dimensional (3-D) reconstruction of temperature distribution of a burner flame. Flame images are acquired using eight optical fiber bundles and two RGB CCD (charge-coupled device) cameras from eight different directions on one side of the burner. The new tomographic algorithm which combines the LFBP (Logical Filtered Back-Projection) and SART (Simultaneous Algebraic Reconstruction Technique) is proposed for the 3-D reconstruction of the gray-levels for two primary color (Red and Green) images. The temperature distribution of the flame is then determined from the ratios of the reconstructed gray-levels of the two primary color images based on the two-color principle. Experimental results on a gas fired combustion test rig are also presented and discussed. The results demonstrate that the proposed technique is effective in measuring the 3-D temperature distribution of the flame.

Temperature measurement of gas turbine swirling flames using tomographic imaging techniques

This paper presents the 3-D (three-dimensional) temperature measurement of swirling flames of a well-characterized tangential swirl burner using a RGB (red, green and blue) CMOS (Complementary metal-oxide-semiconductor) camera associated with four flexible imaging fiber bundles for flame image acquisition. Optical tomographic algorithms were used to reconstruct the 3-D model of grey-level intensity of the flame and the two-color pyrometric technique was applied for computing the flame temperature based on the reconstructed 3-D model. Three R-type thermocouples were also employed to measure the flame temperature which was then used as a reference for validating the temperature derived from the flame images. Experimental results obtained show that the proposed technique is capable of determining flame temperature profiles, and consequently can be an effective means of characterizing the 3-D swirling flame behaviors, including stability limits such as flame blow-off/flashback, thus reducing the event probability by changing inlet conditions.

Tomographic imaging based measurement of three-dimensional geometric parameters of a burner flame

This paper presents the measurement of 3-D (three-dimensional) flame geometric parameters based on optical fiber imaging and tomographic techniques. Two identical CCD (Charge-coupled Device) cameras coupled with eight imaging fiber bundles are used to capture the 2-D (two-dimensional) images of a burner flame concurrently from eight different directions around the burner. An optical tomographic algorithm LFBP-SART is utilized to reconstruct the cross-sections and generate a complete 3-D model of the flame. A set of geometric parameters, including length, volume, surface area and circularity, are then determined from the model generated and used for characterizing the flame. The proposed technical approach is firstly evaluated using an LED (light emitting diode) tube with known dimensions, and then on a gas-fired combustion rig. The results obtained demonstrate that the proposed algorithms are effective for measuring the 3-D geometric parameters of a burner flame over a range of combustion conditions.