Study Of Laser-Based Measurement Techniques For Particle Motion Characteristics In Multi-Phase Flows

Multi-phase flows have wide applications in the fields of Energy, Chemical and Environmental engineerings. Examples are pneumatic conveying, fluidized beds, sprays, vertical risers, particle separation in cyclones, mixing devices, and others. Due to the complexity of themselves, however, the present measurement techniques have not been developed so far to meet all the demands in the dynamical researches of multi-phase flows and on-line and portable measurements. Concerning with some difficulties encountered in this field, this paper aims mainly to carry out studies of several laser-based measurement techniques and their applications in the multi-phase flows.First, a high-speed digital imaging system was used to visualize in a micro-scale level the motion behavior of spherical glass beads with an average diameter of about 0.5 mm in the upper dilute zone of a cold CFB riser with a dimension of 200 mm×200 mm×4 m(H). The particle rotation and collision motions were investigated. The high-speed digital imaging system is mainly composed of a high-speed digital camera, a high power laser and corresponding digital imaging processing procedures. Four kinds of judgement methods for particle rotation speed were presented. Three of them are manual determinations and the other is digital image processing method based on correlation analysis. Among them, the method based on reconstruction of rotation axis provides one to obtain simultaneously the direction of rotation axis and the relatively correct particle rotation speed. The method based on correltion analysis is only aimed to those particles whose rotation axes are perpendicular or nearly perpendicular to the object plane. The maximal possible rotation speeds for particles in CFB with typical operation conditions are theoretically predicted based on particle collision. The reliability of obtained particle rotation speeds was also verified by a validation method based on particle's trajectory. Meanwhile, a dual-frequency imaging method was presented, which has been shown to be effective to enlarge the measurable range of rotation speed under finite recording rates of the camera.Combined with statistical analysis, the characteristics of particle rotation and collision were carried out and the results are as follows: 1) the effects on particle rotation speed were studied. It is shown that the average rotation speed of small particles is higher than that of large ones in the same testing area at the same operation condition; for the particles within the same size range, some of which with relative higher horizontal translational velocity components may be found to be with higher rotation speeds statistically, but the particle's vertical translational velocity component may have a little effect on its rotation speed; the average rotation speed of irregular particles is much higher than that of spherical particles; and the collision rate in the testing area may take great effect on particle rotation; 2) the spatial distribution of average rotation speeds in the upper dilute phase zone of the operating CFB riser has been analyzed. It is shown that in the same working condition, the average rotation speed for particles near wall area is higher than that in the center area. And there is a decrease in the average rotation speed when the height of testing area increases, i.e. in the vertical direction. The effects of operation parameters on the spatial distribution of average rotation speeds have also been investigated. The results show that the increasing superficial gas velocity impoves the aveage rotation speed of particles near wall area but takes nearly no effects on that in the center area. The external solids mass flux, however, takes the opposite effect. And the average rotation speeds of particles in both areas are found to increase as the total amount of bed material increases; 3) the collision rate as well as collision properties is experimentally measured. The collision rate is found to be proportional to the square of particle number density, which coincides with the collision theory derived according to the analogy of kinetic theory of gases. It is also shown that, however, this theoretical model totally overestimates the real collision rates at all particle number densities investigated in the experiment. The author suggests that this discrepancy may be corrected by a coefficient a with values of 0.42-0.68. The measurement results of collision properties based on about 17 particle collision events agrees well with the Walton's hard-sphere collision model and the three collision parameters, i.e. the average coefficient of frictionμ, the normal and tangential coefficients of restitution e andβ₀, for the glass beads are measured.Then, to be capable of simultaneously measureing the velocity, size distribution, and density for irregular particles embedded in flows, a Backscatter Laser Doppler measurement system (BLDMS) based on near backward scattering mode was presented. The particle velocity is obtained by the well known Doppler Effect, while the near backward scattered powers are used to unfold particle size distribution. The feasibility and performance of particle size distribution measured by near backward scattering mode are intensively investigated both theoretically and experimentally. In the theoretical simulation, the scattering response character of irregular particles is described by using a response band with a floating coefficient K rather than a response curve. The simulation work is done for both typical unimodal and multi-modal size distributions, and the results verify the feasibility of the method. In the experiemental study, a measurement system of BLDMS with near backward scattering mode was established, using which preliminary experiments on both spherical glass beads and irregular quartz sands particulate flows were conducted. The experiments include the calibration experiement which provides the inversion matrix and the average scattered response function for particles and the verification experiment which measures the size distribution of the pre-known particulate flow. The results show that BLDMS is very suitable to conduct size distributuion measurements for irregular particles and therefore BLDMS is attractive as a portable and on-line measurement method.At last, an imaging-based Laser Correlation Velocimetry (ILCV) is presented to conduct velocity measurements of high density gas-liquid two-phase flow at the first break up zone of a pulsed spray. The imaging system is analyzed by using a rigorous generalized Lorenz-Mie theory (GLMT) algorithm and the light signal as well as its frequency spectrum obtained by ILCV system when a spherical particle passes through the measurement volumes is modeled. And also the effective length of the measurement volumes is studied. Signal processing methods concerning with ILCV are intensively investigated with several parameters in considerations. An ILCV experimental setup is established and with which a low-velcoity high-density gas-liquid flow very near to the pulsed injector exit is tested. A series of experiments show that ILCV can be well applied to the velocity measurements of high-density gas-liquid flows, especially for sprays.