Structure Analysis Of Random Signals In Multi-phase Flow

The analysis of random signals sampled by different probes is important in the investigation of multi-phase reactors. How to select the information most relative to the process is the frontal and full of challenge in chemical engineering research. Signal analysis and multi-scale resolution were combined in this paper in order to establish the criterion of dividing the random signals to different scales, and the corresponding relationship between signal structures and physical scales were discussed. Based on this structure analysis, the mechanism of acoustic emission (AE) signals in gas-solid fluidized bed was revealed and a novel method to measure the particle size distribution by AE probes was presented. Also based on the structure analysis of AE signals, AE technique was applied to determine the critical speed for complete solid suspension and slurry suspension height in liquid-solid stirred tank was developed. The innovative results can be summarized as following:1. The criterion of dividing the random signals sampled in multi-phase reactors to three characteristic scales was established by Hurst analysis of the wavelet decomposed signals: micro-scale signals with all Hurst exponents less than 0.5; meso-scale with two characteristic Hurst exponents (one less than 0.5, and the other more than 0.5); macro-scale with all Hurst exponents more than 0.5.2. Compare the data structures of pressure fluctuation signals and AE signals both sampled in gas-solid fluidized bed, and the difference of data structures among AE signals sampled in gas-solid fluidized bed, pneumatic transport tube, slurry bubble column, liquid-solid loop reactor, and liquid-solid stirred tank. Based on the compare, the corresponding relation between signal structure and physical multi-scales was established. In the multi-phase systems with solid particles, the micro-scale signals represent the particles' random movement relative to the dominant movement of main body; meso-scale signals represent the dominant movement relative to the whole system; macro-scale signals represent to the movement of the whole system which in most cases was steady.3. The mechanism of AE signals emitted from the collision between particles and the wall, a frequency model was developed to reveal the relationship between the main-frequency of AE signals and characteristics of particles. The Hou-Yang equation which presents the relationship between the structure of AE signals and particle size distribution in fluidized bed was developed, and some errors in the original equations presented by Hou Linxi were corrected. A method to calibrate the AE signals by mixed particles was discussed based on the experimental results from the lab apparatus with the diameter of 100 mm.4. The minimum agitation speed in stirred tank was experimentally investigated by AE technique. The criterion of determination of minimum agitation speed in stirred tank that the impeller speed in which the characteristic value representing the collision between solid particles and the wall decreases quickly and begins to be constant was presented by analysis of the relationship between R\ and impeller speed. A satisfactory agreement was obtained between measurement by AE technique and visual method for beading-water system, and the AARD were 3.51%, which indicated AE technique would measure the minimum agitation speed fast, non-intrusively and with good accuracy.5. The slurry suspension height was experimentally investigated by using energy analysis, wavelet transform and R/S analysis. It was found that the characteristic RE which is the energy ratio between measurement point and the bulk region would decreases quickly twice. So, the criterion of the slurry suspension height that the location is the slurry suspension height and liquid layer height in which the RE attend the mutation was present. The slurry suspension height and the liquid layer height were that the location is the in which the RE =0.62 for water-beading system respectively. Experiments carried out in the stirred tank showed that the results measured by AE technique agreed quite well with that directly observed for water-beading system, and the AARD was less than 11.8 %. So the AE approach to measuring the suspension height in the stirred tank can be developed to judge the tank level precisely with good accuracy.