Effect of near-wall turbulence enhancement on the mechanisms of particle deposition

• Published in: International journal of multiphase flow 2005-08, Vol.31 (8), p.940-956
• Main Authors: Botto, Lorenzo, Narayanan, Chidambaram, Fulgosi, Marco, Lakehal, Djamel
• Format: Article
• Language: English
• Subjects: Chemistry; Colloidal state and disperse state; Deposition; Exact sciences and technology; General and physical chemistry; Particle tracking; Physical and chemical studies. Granulometry. Electrokinetic phenomena; Turbulence
• ISSN: 0301-9322; 1879-3533
• DOI: 10.1016/j.ijmultiphaseflow.2005.05.003

The modification of deposition mechanisms of small particles in wall turbulence due to enhanced near-wall fluctuations is presented. The direct numerical simulation database of turbulent air flow over a water surface populated by gravity-capillary waves of small wave slope was used to mimic the enhancement in fluctuation intensity. Lagrangian tracking of particles is performed under the assumption of one-way coupling between the particles and the flow. Two sets of particles have been considered with inertial response times of 5 and 15, respectively, normalized using the friction velocity at the air–water interface and the kinematic viscosity of air. Compared to wall-bounded flow, the particle deposition rates on the interface were found to be considerably higher; specifically for the low-inertia particles, an eightfold increase was observed. The deposition rate for particles of higher inertia increased by only 60%. The correlation characterizing particle deposition rates for wall-bounded flows, where the deposition rate is proportional to the square of the particle response time, was found to be invalid for the flow with enhanced near-wall turbulence. Comparison with experimental results on particle deposition onto rough walls showed better correlation. Depositing particles were divided into free-flight and diffusional deposition populations. Since the primary effect of the interfacial waves is to increase the turbulence intensity in the near-interface region with high particle concentration, a remarkable increase in diffusional deposition is observed. As in wall-bounded flows, diffusional deposition is seen to be the dominant mechanism of deposition. The free-flight mechanism, where particles acquire velocities high enough to travel directly to the interface, remains unaffected by enhanced near-wall velocity fluctuations.