声致凝聚行为规律及机理研究

Turbulent mixing exists widely in nature. Acoustic-induced aggregation is effective to artificially intervene turbulent mixing, while the rules and mechanism of the acoustic-induced aggregation in air-liquid droplets two-phase flow needs further study. This project studies three typical physical processes by experiments, including the condensation of liquid droplets on the wall, the dispersion of suspended particles and the turbulent mixing of two-phase particles. The concept and algorithm of Mixing Entropy is innovatively proposed to represent the mixing degree of two-phase particle system. The relationship between acoustic parameters and the growth rate of the wall droplet, the concentration decay rate of the suspending droplet, the adsorption effect of micro-droplet on the wall and the increase process of the Mixing Entropy will be illuminated for systematic study of the rules and mechanisms of acoustic-induced aggregation in air-micro droplet particle flow. As to numerical simulation, the coupling algorithm of computational fluid dynamics (CFD) and discrete element method (DEM) is applied to simulate the acoustic-induced aggregation of the two-phase flow. This project combines the experimental and numerical simulation studies to verify two proposed scientific assumptions: the sound waves can aggravate the air turbulence, as well as the collision and agglomeration of micro-droplets; and there exists a threshold sound intensity and optimal sound frequency for acoustic-induced aggregation. The project helps understanding the law of acoustic-induced aggregation, and will promote the application of acoustic waves in the fog dispersal, rain enhancement, dehaze, airgel precipitation and other industrial or environmental field.

湍流混合现象广泛存在于自然界中。声致凝聚是人为干预湍流混合的有效方法，然而空气-微液滴两相流中的声致凝聚行为规律和机理有待进一步研究。本项目通过实验研究了壁面液滴凝结、悬浮颗粒消散和双相颗粒湍流掺混三个涉及微粒凝聚的典型物理过程；创新地提出代表双颗粒体系掺混程度的掺混熵概念及算法；阐明壁面液滴生长速率、悬浮液滴颗粒浓度衰减速率、壁面微液滴吸附效应及双相颗粒掺混熵熵增过程与声波特性参数的关系，系统性地研究空气-微液滴两相流中的声致凝聚规律及机制。在数值模拟方面，提出采用计算动力学和离散单元方法耦合算法来模拟两相流中的声致凝聚行为。本项目结合实验和数值模拟研究，提出并验证了声波能加剧空气紊流及微液滴群的碰撞聚合、声致凝聚存在阈值声强及最佳频率这两个科学假设。本项目的实施将增加对两相流中声致凝聚行为规律的认识，促进声波增雨技术的进步，同时也对大气消雾、增雨、除霾、气凝胶沉淀等技术的发展大有裨益。

项目的背景：湍流混合现象广泛存在于自然界中。声致凝聚是人为干预湍流混合的有效方法，然而空气-微液滴两相流中的声致凝聚行为规律和机理有待进一步研究。..主要研究内容：本项目通过实验研究了壁面液滴凝结、悬浮颗粒消散等涉及微粒凝聚的典型物理过程；阐明壁面液滴生长速率、悬浮液滴颗粒浓度衰减速率与声波特性参数的关系，系统性地研究空气-微液滴两相流中的声致凝聚规律及机制。..重要结果和关键数据：在数值模拟方面，提出采用计算动力学和离散单元方法耦合算法来模拟两相流中的声致凝聚行为。揭示了如下规律：当声压级不变，改变频率时，低频声波对云滴的凝聚有明显的促进作用；当频率不变，改变声压级时，声波的促凝效果整体随着声压级的增大而增强；随着颗粒初始间距的增大，计算平均粒径增长的速度快速衰减。在实验研究方面发现的现象及规律：通过云雾滴沉降凝结显微实验，直观地展示了云雾滴在声波作用下的沉降凝结生长过程，验证了声波对沉降凝聚的促进作用；通过行波管中微液滴群消散实验，观察到了稳态云滴粒径在声波作用下的跃迁现象，获得了声波促进暖云液滴凝结的证据；两个实验均表明声压级越高、频率越低，云雾滴沉降凝结速度越快，且低频率声波（<200Hz）的效果更显著。.本项目结合实验和数值模拟研究，验证了声波能加剧空气紊流及微液滴群的碰撞聚合、声致凝聚存在阈值声强及最佳频率这两个科学假设。..科学意义：本项目的实施将增加对两相流中声致凝聚行为规律的认识，促进声波增雨技术的进步，同时也对大气消雾、增雨、除霾、气凝胶沉淀等技术的发展大有裨益。

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