Study Of Light Field Sectioning Pyrometry For Three-dimensional Flame Temperature Measurement

Combustion widely exists in the field of power generation,aerospace,metallurgy,chemical industry,which involves coal-fired boilers,internal combustion engines,gas turbines,and other combustion devices.In the combustion process,the accurate flame temperature measurement is essential because it directly governs the pollutant emissions,combustion efficiency,and operation safety of furnaces and other combustion devices.Meanwhile,the accurate measurement of the three-dimensional(3D)flame temperature distribution is of importance for the in-depth insight of combustion mechanism and the design of combustion devices.The 3D temperature field measurement method based on flame radiation light field(LF)imaging has attracted wide attention because it has the potential advantages of non-intrusive,fast response,and low system complexity.In this thesis,the three-dimensional flame temperature measurement based on LF imaging and sectioning techniques have been studied to develop a reliable measuring instrument for flame temperature.Firstly,the process of flame radiation transmission and LF imaging is introduced,and a reverse raytracing model of flame radiation is proposed.Quantitative indices are proposed to evaluate the direction and position characteristics of the LF sampling for the flame radiation.The influence of position and optical parameters on the LF sampling characteristics is analyzed.An optimized scheme of LF sampling is then proposed to collect the radiation information of axisymmetric and non-symmetric flames.The 3D flame temperature distribution is reconstructed based on the NNLS algorithm.The results show that the LF sampling angle of the optimized scheme is increased by 23 times,and the reconstruction error of the nonsymmetric flame is less than 3%.To improve the spatial resolution of the reconstructed flame temperature field,the reconstruction is performed by combining the LF refocusing technique and optical sectioning technique(OST).For the issue of low reconstruction accuracy of the OST,the effect of flame emissivity on the temperature reconstruction accuracy is analyzed,and the modified light field sectioning pyrometry(LFSP)is then proposed.The model of the point spread function of the LF refocus image is studied,and the relationship between the light field camera(LFC)parameters and the spatial resolution is analyzed.Then the spatial resolutions of the LFSP reconstruction are quantitatively evaluated,and the LFC parameters are optimized.The results show that the spatial resolution of the LFSP reconstruction is up to 100μm and 10 mm in lateral and depth direction,respectively,which is significantly higher than the NNLS algorithm.According to the optimized LFC parameters,a cage-typed LFC was designed and implemented,which can adjust parameters and optical components flexibly.The imaging performance of the camera was systematically tested and analyzed,including the assembly accuracy,imaging vignetting and distortion effect,sensor noise and linearity.Furthermore,the information processing software based on the cage-typed LFC has been developed,which mainly realizes the functions of camera parameter control,LF raw image acquisition and decoding,LF imaging display and record.Subsequently,the parameter calibration and performance evaluation of the measurement system is carried out.The results show that the assembly accuracy,imaging characteristics and image sensor performance of the cage-typed LFC meet the experimental requirements.The calibration results show that the relative error of temperature measurement is less than 3%.To study the feasibility of LFSP,experiments are performed for ethylene diffusion flames and hightemperature multi-phase flow.For the ethylene diffusion flame experiment,the 3D flame temperature field under different combustion cases such as laminar,double-peak,and turbulent flames are reconstructed,then a comparative study is carried out with thermocouple measurements.The results show that the LFSP measurement result is consistent with the thermocouple temperature measurement.The LFSP has good temperature measurement accuracy,higher spatial and temporal resolutions.For the high-temperature gassolid two-phase flow experiment,the temperature measurement algorithm is firstly modified according to the characteristics of the object and then combined with the particle tracking velocimetry.The temperature and velocity of the high-temperature metal particle sprays are measured,which is then compared with the theoretical model and experimental phenomena.The results show that temperature measurement is consistent with the theoretical model.It is also observed that the temperature of a large number of particles is higher than the melting point of Fe and Fe O,which is consistent with the morphological observation that the metal particles are molten.In general,the experimental results demonstrate that the LFSP has wide application prospects.