Study On Trajectory And Velocity Measurement Method Of Particle Field By Digital Holography

The testing and diagnosis of particle field are important in science and engineering and the like fields. A method for the trajectory and the velocity measurement of particle field by digital holography is studied, and hologram of the particle field is recorded directly by CCD digitally, and then reconstructed numerically in computer. And intensity as well as phase information of 3D particle field can be achieved. The parameter information of the size and positions of particle can be obtained through analysis of the numerically reconstructed image. Combined with the technique of particle tracking velocimtry (PTV), trajectory of particle can be extracted from a time sequence reconstructed image through particle tracking algorithm,and velocity can be obtained and the velocity measurement of particle filed is then completed. The subject, based on the reseach of the basic theory of digital holography, centers about the method of digital holography for the trajectory and the velocity measurement of particle field and its application in practical spray particle field. The work has been done in this paper mainly as follows:1. The method for the trajectory and the velocity measurement of particle field by digital holography is studied.2. The method for particle size and position measurement of digital holography by Hough transform is improved and a method of top cap transform filter is proposed and a method of automatic threshold extraction is also presented. And two focus judging criteria, namely, the gray level focus judging criteria and complex amplitude judging criteria are also presented to act as the rule by which to judge if the particle is in focus or out of focus. Computer simulations of the 2D and 3D particle fields are performed and the experiment is conducted for diameter of 51.1μm and diameter of 119.4μm standard particle fields. Measurement results are as follows:for the standard particle field with diameter of 51.1μm,the particle diameter is 52.5μm,and the relative error is 5.40%;for the standard particle field with diameter of 119.4μm,the particle diameter is 120.48μm,and the relative error is 2.30%。The measurement uncertainties of particle size and position are investigated. Furthermore , the relations between signal-to-noise(SNR) of hologram, percentage of extracted particles E p and depth of particle field L , particle number density ns are also discussed. 3. The 3-frame particle tracking algorithm is proposed. Measurement uncertainties of the particle velocity as well as variation under different particle number density or different velocity are investigated. Computer simulations of 2D and 3D dynamic particle fields are performed. Velocity measurement results are as follows:for 2D particle field measurement,the relative error of velocity measurement is 0.17%,which becomes 5.6% for 3D particle field. The research results show that the method presented in this paper is feasible.4. Experiments of digitally recording dynamic particle fields with CW laser and high-speed CCD have been carried out. The method for trajectory and velocity measurement of particle field presented in the paper is used to analyze trajectory and velocity of standard particle field,diesel spray and alcohol spray. 2D and 3D trajectory vector and velocity distribution are obtained. The velocity variation curve of the spray field with different pulse width of the injector along X axis and Y axis are given, and particle sizing, trajectory and three-dimensional three-component (3Dt-3C) velocity distribution are completed. The experiment results show that the method presented in this paper is feasible in trajectory and velocity measurement of particle field.