Abstract:

Due to the stratification of sludge particles in the ultrasonic field, the acoustic interaction forces the particles to agglomerate on a plane perpendicular to the direction of ultrasonic propagation. Therefore, the thickness of sludge can significantly influence the characteristics of ultrasound-assisted hot air convective drying municipal sewage sludge. In this paper, the stratified aggregation phenomenon of sludge with different thicknesses was observed in the ultrasonic field using the experimental method. It was found that the stratification of the internal structure of sludge became more obvious with the increase of its thickness. The effects of ultrasound on the drying time and the drying rate of sludge with various thicknesses were studied. Meanwhile, the effective moisture diffusivity (D_eff) was analyzed. The experimental results demonstrated that the larger the sludge thickness, the longer the time length of the first falling rate stage and the promotion of the drying rate was worse when the ultrasonic power was less than 135 W. The situation was opposite at the constant rate stage. Among the sludge with the thicknesses of 5, 10, and 15 mm, the total drying time and energy consumption of the sludge with a thickness of 10 mm decreased the most substantially under the condition of ultrasonic power less than 90 W. The smaller the thickness of sludge is, the less obvious the effect of ultrasound on the effective moisture diffusivity, and vice versa.

Key words: sludge, ultrasound, drying, microstructure, aggregation

摘要：

由于在超声波声场中污泥微粒会发生分层现象，声互作用力使得微粒于超声传播方向相垂直的平面上发生凝聚，因此污泥厚度大小对超声波辅助热风干燥污泥特性有着重要的影响。通过实验的方法，对不同厚度污泥在超声波声场中的分层凝聚现象进行观察，发现污泥内部结构的分层现象随其厚度的增加而明显。研究了超声波对不同厚度污泥干燥过程中各时期干燥时长、干燥速率的影响效果，以及分析了湿分有效扩散系数（D_eff）随污泥厚度变化的情况。从实验结果中可以发现，在超声波功率小于135 W范围内，污泥厚度越大，干燥过程中第一降速期时间越长，干燥速率提升效果越差，而对恒速干燥期内干燥速率提升效果更明显；在5、10以及15 mm厚度的污泥中，10 mm厚度的污泥在超声波功率小于90 W的条件下总干燥时长降低幅度最大，干燥速率在各阶段提速也较快；污泥厚度越小，超声波功率对污泥湿分有效扩散系数影响越小，反之影响越大。

关键词: 污泥, 超声波, 干燥, 显微结构, 聚集