Study On Moire Tomography Based On Spatial Phase-shiftted Method

With the rapid development of aviation, aerospace and energy engineering, the measurement and diagnose methods of complex flow fields are widely used in the 3D physical parameters measurements and visualizations for exhaust plume of rocket engine, jet flow fields of engine, supersonic wind tunnel and the furnace combustion field. Optical computerized tomography technology is a combination of optical measurement methods and computerized tomography methods; it has been considered an effective tool for quantitatively measuring the various 3-D physical parameters of the flow fields with its unique advantages such as non-contact, real time, and whole field measurement characteristics. Among the techniques, moire tomography has the advantages of simple optical structure, strong anti-disturbing capacity, and wide measurement range, which is more suitable for diagnosing the complex flow field in noisy environments with strong motion in actual.Extracting the projection information from moire fringes is crucial to the reconstruction quality by moire CT. In order to extract the projection distribution, in this thesis, the phase information retrieve method of the moire CT projection based on the spatial phase shifting method is studied via analyzing the phase-shifting characteristics between interferograms with different diffractive orders. Based on these, the projection optical structures of spatial phase shifting moire CT are constructed and according experimental verifications are performed.The main works are summarized as follows:1. The forming process of moire fringe by double linear gratings is analyzed via the scalar diffraction theory, and the phase-shifting characteristics between the interferograms are studied. Based on it, a very simple spatial phase shifting moire CT optical structure which only consists a crossed grating and a linear grating is designed. Then, the explicit analytical expressions of intensity distribution of the interferograms with different diffractive orders are derived via the scalar diffraction theory, and find the spatial phase shifting characteristics existing between the interferograms. Base on these, corresponding four-step and six-step phase shifting method is designed and its feasibility is proved respectively through measuring the phase projection of propane flame. Therefore, the phase information retrieve of the moire CT projection based on the spatial phase shifting method is performed.2. In order to overcome the problems encountered in above system, such as too long optical path, low diffraction efficiency and difficulty to ignore the high-order partial derivative terms, a two-step spatial phase-shifting moire CT optical configuration based on angular spectrum filtering is proposed. After filtering in the angular spectrum domain using 4-f system, the new CT projection optical path not only is shorted but also the influence of the high-order partial derivative terms can be eliminated. According to the two phase-shifted interferograms obtained in the new optical structure, a corresponding two-step phase-shifting method is proposed. Bsed on these, through measuring the phase information of spherical wave, the phase retrieve error is analyzed and the phase information of the propane flame is measured by this proposed method.3. In order to reconstruction the real 3-D temperature distribution, we integrate moire CT and ECT (Emission CT) method to diagnose the combustion flame. The 3-D refractive index distribution of a combustion flame can be reconstructed by the proposed spatial phase-shifted moire CT method. The combustion area and component distribution of the flame can be determined by ECT via reconstructing its intensity distribution. Based on these, the real 3-D temperature distribution of flame is accurately calculated from the 3-D refractive index distribution.