Reconstruction Of Three-dimensional Soot Temperature Field And Species Concentration Field Of Soot Flame Based On Active And Passive Tomography

At present,fossil fuel is still the main way of energy supply in China.The theoretical and experimental research on combustion helps to analyze and understand the nature and law inside combustion,and provides reference for further improvement of combustion system,and optimization of equipment operation.Accurate measurement of three-dimensional(3D)temperature distribution of flame not only helps to understand the heat and mass transfer process inside flame and the generation mechanism of combustion pollutants,and also provides reliable data support for the safe operation of production equipment,full utilization of fuel heat energy,and the effective control of combustion pollutants.The concentration of typical combustion products like H₂O and CO₂ also reflects the situation and efficiency inside the combustion chamber of the equipment.Thereby,developing an effective and accurate combustion measurement diagnosis technology,to retrieve multiple parameters of the flame like temperature,radiative properties and species concentration and to achieve simultaneous reconstruction of the above parameters inside the flame,contributes a lot on the combustion equipment optimization design and pollutant emission control.At present,the existing optical tomography combustion diagnostic measurement techniques,whether active or passive measurement,are not available to achieve simultaneous measurement of the 3D temperature and combustion species concentration distributions inside the high-temperature soot flame with high efficiency and accuracy.Given the situation,the combination measurement method of active and passive optical tomography is proposed in this paper.By combining active laser absorption spectroscopy with passive radiation imaging tomography technique,a collaborative reconstruction model to achieve simultaneous measurement of 3D temperature,radiative properties and species concentration distributions inside the soot flame are established in this study.Firstly,the acquisition model of active and passive photothermal information of the high temperature soot flame is introduced.The calculation method of radiative transmission of dispersive medium in absorption flame and absorption scattered flame is introduced,which is available to realize fast and accurate calculation of the radiative intensity distribution emitting from the soot flame in any direction.On this basis,the basic theory and principle of active tomographic absorption spectroscopy and passive light-field radiation imaging tomography are introduced respectively,which provides the forward problem calculation model for the future research.Based on the laser absorption spectroscopy and computational tomography theory,the simultaneous measurement of two-dimensional temperature and H₂O species concentration distributions of high temperature flame was investigated.The linear and nonlinear tomography absorption spectroscopy measurement models are adopted in this study,respectively.Both symmetrical and asymmetric flame distributions are investigated in this study with numerical simulation.The Algebraic reconstruction technique and Evolution Strategy based on Covariance Matrix Adaption Algorithm are adopted to solve severe ill-posed and rank-deficient problems of tomography absorption spectroscopy and nonlinear tomography absorption spectroscopy model.The simulative reconstruction is conducted on both symmetric and asymmetric flame distributions,proving the widely feasibility of the measurement model and method.Due to the limitations of optical access and optical path,the active laser detection method based on laser absorption spectroscopy is generally used for two-dimensional combustion diagnosis,while the flame presents 3D distribution actually.Therefore,the 3D temperature field measurement of high temperature flame based on passive radiation imaging of light field is investigated.Based on the Fresnel diffraction theory of wave optics and the radiation transfer equation of participating medium,a light field convolution imaging model suitable for soot flame is established.The image of flame light field is adopted as the measurement signal.The numerical simulative 3D temperature reconstruction of different types of flames and multi-modal flames with multi-peaks distribution are carried out.And the influence factors and uncertainty of the measurement method on the reconstructed quality are analyzed.Based on researches above,a new method of simultaneous measurement of temperature,radiative properties and species concentration inside the soot flame based on active and passive optical tomography is proposed,which combines laser absorption spectroscopy technique and light-field radiation imaging tomography together.Utilizing the multi-spectral laser transmission signals and the light field radiation distribution signal emitted from soot flame itself to achieve simultaneous reconstruction multis parameter distributions of soot flame.The model of simultaneous reconstruction based on the combination of active and passive tomography is established.According the model,the simulative reconstructions of 3D distributions of temperature,radiative properties,soot volume fraction and H₂O species concentrations are conducted.The error propagations of different measurement signals are analyzed.Based on the simultaneous reconstruction model established before,the verification experiment is conducted.The experiment is implemented on the typical soot flame,the ethylene diffusion flame,in which the radiative properties,soot temperature,soot volume fraction and H₂O species concentration are reconstructed simultaneously.To verify the accuracy of the measurement method,the thermal couple is adopted to measurement the temperature inside the soot flame,which is treated as the reference temperature to compare the reconstructed temperature based on the measurement method proposed.The maximum temperature deviation is less than 50 K,verifying the accuracy of the measurement method.Compare the reconstructed soot volume fraction and H₂O species concentration distributions with reference results in another similar research on ethylene diffusion flame measurement,corresponding distribution trend and order of magnitude were consistent,which proved the effectiveness of the measurement method.