Breakup of suspended aggregates is a ubiquitous phenomenon in particle systems and plays an important role in many industrial and environmental processes. Breakup of suspended aggregates is driven by two mechanisms: (1) In impact breakup, aggregate breakup is caused by energetic collisions with solid objects such as walls, moving equipment (e.g. impeller blades) or other particles. Impact breakup is the dominant mechanism for large and strong agglomerates such as found in powder processing or industrial crystallization. (2) In hydrodynamic breakup, aggregate breakup is caused by viscous stress acting on the aggregate. Hydrodynamic breakup is the dominant mechanism for small and weak aggregates where the constituting particles are hold together by Van der Waals or electrostatic forces [1,2]. In this work the breakup of small inertial aggregates due to hydrodynamic stress in homogeneous isotropic turbulence is investigated by means of numerical simulations. Hydrodynamic breakup in turbulence is a challenging problem as the viscous stress acting on the aggregate is subject of strong fluctuations, and situations where the hydrodynamic stress overcomes the cohesive strength of the aggregate occur only intermittently and with timescales controlled by turbulent fluid and particle motions. The aim of our study is to determine the frequency at which the hydrodynamic stress acting on an aggregate suspended in a turbulent flow overcomes a predefined threshold value representing the aggregate strength. The resulting frequency is referred to as aggregate breakup rate [3,4] which in this work is determined as a function of the aggregate strength and aggregate inertia. For fixed aggregate inertia, characterized through an aggregate Stokes number, the breakup rate is decreasing with increasing aggregate strength. This implies that vigorous turbulent events that are violent enough to breakup an aggregate become rarer the stronger the aggregate. For small aggregate strength, the decrease of the breakup rate shows power law behavior while for large aggregate strength a sharp drop-off of the breakup rate is observed. With increasing aggregate breakup the contribution of drag acting on the aggregate is increasing. This leads to an increase of the breakup rate. To the authors knowledge this is the first systematic study on the role of drag stress on the breakup of aggregates in turbulence.
NUMERICAL SIMULATION OF BREAKUP OF SMALL INERTIAL AGGREGATES IN HOMOGENEOUS TURBULENCE
Alessandra S Lanotte
2015
Abstract
Breakup of suspended aggregates is a ubiquitous phenomenon in particle systems and plays an important role in many industrial and environmental processes. Breakup of suspended aggregates is driven by two mechanisms: (1) In impact breakup, aggregate breakup is caused by energetic collisions with solid objects such as walls, moving equipment (e.g. impeller blades) or other particles. Impact breakup is the dominant mechanism for large and strong agglomerates such as found in powder processing or industrial crystallization. (2) In hydrodynamic breakup, aggregate breakup is caused by viscous stress acting on the aggregate. Hydrodynamic breakup is the dominant mechanism for small and weak aggregates where the constituting particles are hold together by Van der Waals or electrostatic forces [1,2]. In this work the breakup of small inertial aggregates due to hydrodynamic stress in homogeneous isotropic turbulence is investigated by means of numerical simulations. Hydrodynamic breakup in turbulence is a challenging problem as the viscous stress acting on the aggregate is subject of strong fluctuations, and situations where the hydrodynamic stress overcomes the cohesive strength of the aggregate occur only intermittently and with timescales controlled by turbulent fluid and particle motions. The aim of our study is to determine the frequency at which the hydrodynamic stress acting on an aggregate suspended in a turbulent flow overcomes a predefined threshold value representing the aggregate strength. The resulting frequency is referred to as aggregate breakup rate [3,4] which in this work is determined as a function of the aggregate strength and aggregate inertia. For fixed aggregate inertia, characterized through an aggregate Stokes number, the breakup rate is decreasing with increasing aggregate strength. This implies that vigorous turbulent events that are violent enough to breakup an aggregate become rarer the stronger the aggregate. For small aggregate strength, the decrease of the breakup rate shows power law behavior while for large aggregate strength a sharp drop-off of the breakup rate is observed. With increasing aggregate breakup the contribution of drag acting on the aggregate is increasing. This leads to an increase of the breakup rate. To the authors knowledge this is the first systematic study on the role of drag stress on the breakup of aggregates in turbulence.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
