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多相反应器在工业中有着十分广泛的应用，多相反应器中的颗粒尺寸分布、相含率等非均相特性（多相流体在时间和空间分布的不均匀性）是反应器优化设计和放大的重要依据，也可为多相反应器数值模拟验证和模型关键参数确定提供基础数据。工业多相反应器中分散相的相含率往往比较高，理论上侵入式原位图像法由于短光程的特征可测量分散相浓度很高的体系，然而实际可测量的最大相含率还与颗粒粒径密切有关。另外，对于高相含率体系颗粒图像重叠现象很普遍，因此影响图像法测量准确性的一个关键步骤——目标颗粒的自动、准确选取也异常重要。本文利用课题组研制的远心照相测量仪，对液固搅拌槽中的固体颗粒（近球形的氧化铝和镀银玻璃珠，不规则石英砂）进行了原位图像测量。首先基于不同的颗粒图像（石英砂和氧化铝），发展了不同特征颗粒图像的选取策略，并通过与作为基准的手动圈选方法的比较，对颗粒选取方法进行了优化。研究发现：（1）在规则球形颗粒图像的识别中，直接对原图进行圆检测会导致较大的误差，使用 Canny算子边缘增强后，目标颗粒容易被检出，但也会增强非目标颗粒的边缘，从而导致错误识别的颗粒增多。使用阈值分割后再进行圆检测的误差最小，并且最佳阈值为最大类间方差法（Otsu）计算得到阈值的60%左右；（2）在不规则颗粒图像的识别中，使用椭圆拟合误差较大，基于阈值分割后的轮廓检测会导致索特（Sauter）直径偏大。使用分水岭分割的检测结果对相含率和索特直径的相对误差均较小，最佳阈值同样为Otsu法计算得到阈值的60%左右。使用优化的图像处理策略，将氧化铝测量结果与游标卡尺测量进行了比较，同时将石英砂和氧化铝的粒度测量结果与激光粒度分析仪的测量结果进行了对比，对镀银玻璃珠进行了高相含率测量的验证并校准了景深（DOF）变化。实验所用颗粒均通过筛分得到了不同粒径分布范围。通过比较发现：（1）较规则颗粒测量结果与游标卡尺测定结果比较接近；（2）远心照相测量仪和激光粒度分析仪两种测量手段得到的粒径分布结果存在较大的差异，用远心照相测量仪对不规则颗粒粒径的测量相比激光粒度仪测量结果更加集中在筛分区间；（3）高相含率的镀银玻璃珠测量中，发现实际计算景深（DOFC）随体系相含率的增大呈线性减小的趋势。;Multiphase reactors are widely used in process industries. The heterogeneous characteristics (the inhomogeneity of the distribution of multiphase fluids in time and space) such as particle size distribution and phase holdup in multiphase reactors is an important information for reactor optimization and scaling-up design. It can also provide basic data for the numerical simulation verification and the model key parameters determination of multiphase reactors. The dispersed phase holdup of industrial multiphase reactors is generally higher than negligible. In theory, the intrusive in-line imaging technique can measure the system with a high concentration of the dispersed phase due to the feature of a short optical path, but in practice, the maximum phase holdup is also closely influenced by the particle size. In addition, the phenomenon of particle image overlapping in high phase holdup systems is very common, so the automatic and accurate selection of target particles is very important for the accuracy of image-based measurement.In this thesis, the in-situ particle images of solid particles (subsphaeroidal alumina and silver-plated glass beads, irregular quartz sand) were acquired in a liquid-solid stirred tank by using the telecentric photographic probe developed by our group. Firstly, the selection strategy for particle images of different characteristic was developed based on quartz sand and alumina particle images, and the particle selection method was optimized by comparing with the manual marking. The conclusions are as follows: (1) For regular particles, the circle detection on original particle images causes a large error. The target particles can be easily detected after edge enhanced using the Canny operator, but the edges of non-target particles will be also possibly enhanced, leading to more particles being misidentified. The error of circle detection after the threshold segmentation is the smallest, and the optimal threshold is about 60% of the threshold calculated by the Maximum Between-Class Variance Method (Otsu). (2) For irregular particles, the ellipse fitting method suffered to a large error. Contour detection based on the threshold segmentation will lead to a larger Sauter diameter. The relative errors of the detection results by using the watershed segmentation are small on the phase holdup and the Sauter diameter measurements. The optimal threshold is also about 60% of the threshold calculated by Otsu.By utilizing the optimal image processing strategy, the measurement results of alumina were compared with vernier calipers, and the measured particle sizes of quartz sand and alumina were compared with the measurements by a laser particle size analyzer. The change of depth of field (DOF) of silver-plated glass bead images was also verified and calibrated by measurement at high concentration. All the particles used in the experiment were obtained through sieving. The analysis suggested conclusions are as follows: (1) The measurement result of regular particle size by the present imaging method is close to that measured by the vernier caliper. (2) The results of particle size distribution obtained by the telecentric photographic probe and the laser particle size analyzer are quite different. The measurement results of irregular particle size by the telecentric photographic probe are more concentrated in the sieve partition. (3) The calculated depth of field (DOFC) decreases linearly with the increase of phase holdup in the measurement of high phase holdup of silver-plated glass bead.
|任世龙. 测量多相反应器颗粒性质分布的原位图像法研究[D]. 中国科学院大学,2020.|
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