Light Field Optical Sectioning Tomography For 3D Flame Temperature Measurement

Combustion is a luminous and exothermic phenomenon caused by rapid chemical reaction between fuel and oxidant,and it is widely used in many industries such as metallurgy,aerospace,power boilers and gas turbines.The flame is the visible form of the combustion.The temperature measurement of flame is one of the most important research areas in combustion diagnostic field.In order to gain insight into the combustion mechanism,the measurement of the flame temperature field is becoming increasingly desirable for the researchers in the field of combustion diagnostics.Existing flame-temperature measurement methods and systems are complex in terms of system installation and operation.In this study,an optical sectioning tomographic technique for threedimensional flame temperature measurement combined with light field imaging(light field OST)is proposed and investigated.First,the fundamental principle of light field imaging is introduced in details.Based on light field imaging,the light field camera and digital refocusing technology are presented.Then the digital refocusing is combined with the optical sectioning tomographic technique(OST)to obtain the image sequences at different depths in a single exposure.Numerical simulations of light field imaging and refocusing are carried out.Then the Van Citter iteration method is used to reconstruct the original radiation distribution of different sections,and reconstructed results verify the feasibility of the light field OST system.Secondly,a series of calibration works are carried out to calibrate the light field OST system.To ensure that the refocused depth of the OST system is located at the corresponding section of the flame,the relationship between the refocused parameter and refocused depth is determined according to the image clarity.After locating the depth of each section of the flame,the PSF of each section is calibrated by edge method.Then,a novel microlens geometric parameter calibration method based on Hough transform is proposed.Finally,the relationships between the gray values from Red(R),G(Green)and B(Blue)channels and the radiation intensity are fitted based on imaging processing.Finally,the light field OST temperature measurement system and software is developed.A candle flame is used to evaluate the performance of the measurement system.The flame is divided into four sections.The radiation distribution at each section of the flame is reconstructed.The reconstructed radiation distribution at each section of the flame is in accordance with the contour structure of the real candle flame.The reconstructed temperature has a good agreement with the temperature measured by thermocouple,which validates the feasibility of the light field OST for flame temperature measurement.Further,the light field OST is used to measure three-dimensional temperature distribution of the transient flame of a single burning biomass particle under oxy-combustion conditions.The reconstructed results demonstrated that the light field OST system can be utilized in practical application.