| Diagnosing the distribution of physical parameters of complex flow fields is a key technology in modern aerospace technology,energy engineering,instrument manufacturing and so on.However,single-point,single-line or single-sided diagnosis of physical parameters is far from satisfactory.The three-dimensional spatial distribution of physical parameters in complex flow fields is an urgent requirement for industrial development at this stage.Spectral intensity is one of the important physical parameters of a complex flow field.If the three-dimensional distribution of the spectral intensity of the flame field is known,combined with the blackbody furnace or thermocouple calibration,the three-dimensional temperature field distribution of the flame field can be diagnosed;If the three-dimensional spectral intensity distribution of plasma jet is known and calibrated by electrostatic probe or microwave interference,the three-dimensional electron density field distribution of dynamic plasma jet can be diagnosed.Three-dimensional reconstruction methods of spectral intensity are divided into two categories: emission spectroscopy and transmission spectroscopy.The emission spectral tomography method uses the projection data of the flow field in multiple directions to reconstruct the three-dimensional distribution of the spectral intensity of the flow field,which has the advantages of non-contact,undisturbed,transient and three-dimensional full field measurement.Compared with the transmission spectroscopy method,it has a simple structure,does not require complicated optical equipment,and has low requirements on the experimental environment.In recent years,with the development of camera technology,emission spectral tomography has become a research hotspot,but there are still problems such as parallel projection hypothesis,no correlation between layers,inaccurate spatial positioning,and approximate calculation.In order to solve these problems,this paper has done an in-depth study of the emission spectral tomography algorithm:1.The emission spectrum tomography technique is introduced,and the principle of reconstructing the three-dimensional spectral intensity by the traditional surface tomography method is expounded.Through the analysis of experimental data,the performance of surface tomography reconstruction algorithm is analyzed.It is pointed out that transformation reconstruction algorithm needs more projection directions,and has higher requirements for the completeness and uniformity of projection data.Iterative reconstruction algorithm can get better reconstruction results when the number of projection directions is small.2.The limitations of surface tomography method are analyzed,and a new method of volume tomography suitable for incomplete data-body-based tomographic reconstruction based on projective geometry model is proposed.The camera projective geometry model is used to determine the correspondence between the two-dimensional image plane and the three-dimensional space,and the multi-camera calibration technology is used to accurately locate the three-dimensional space.The method of projecting convex polygon and sub-pixel segmentation is proposed to calculate the projection contribution rate of the spatial voxel lattice to the pixels.According to the law of conservation of mass and the law of conservation of energy,a method of local normalization of the projection weight matrix is proposed.Finally,the volume tomography optimization objective function is established and the optimal solution of the model is solved iteratively.3.In the numerical simulation experiments,symmetrical and asymmetrical flow fields are used to verify the correctness of the proposed reconstruction method.And the effects of image resolution,projection directions(the number of camera views)and noise level on the accuracy of three-dimensional reconstruction of flow field are discussed.In addition,with the experimental results,the rationality of using the error side of the verification direction to reflect the accuracy of the three-dimensional flow field reconstruction is analyzed.4.The three-dimensional spectral intensity of real flame field was reconstructed.Among them,because different cameras have different gray-scale responses to the same intensity of light,this paper proposes a multi-segment linear transformation method to uniformly correct the gray-scale response curves of different cameras.Because different cameras have different gray-scale responses to the same intensity of light,In order to reduce the computational complexity,it also proposes to restrain the reconstruction of spatial regions by using multi-visible shell technology.In the real experiment,eight cameras were used to shoot three-dimensional flame from eight different angles of view,and eight two-dimensional projection images were obtained.Seven of them were used to participate in the three-dimensional reconstruction.The remaining one was used as the verification image.The three-dimensional reconstruction results were re-projected onto the verification image,and the re-projection errors(average relative error,maximum relative error,root mean square error)were obtained.The square root errors were 1.46%,14.72% and 9.40%respectively. |
