Experimental investigation of particle aggregation in a humid and turbulent environment

Abstract

Aggregation of wetted particles in a turbulent flow is experimentally investigated. The particles settle through nearly isotropic turbulence inside a humid air chamber. The particles are pre-chilled so that wetting occurred even at sub-saturated humidity levels. Aggregates that fall out of the turbulent flow are physically sampled and their size distributions quantified to infer bulk sticking efficiencies at various humidity levels. Optical particle tracking techniques are used to detect individual time-resolved collision events, forming an independent measure of the sticking efficiency parameterized by a collision Stokes number. The two methods are contrasted and the limitations of each are discussed. The data are used to develop a new version of the early Costa sticking model, which is utilized by various volcanic ash transport codes. The key outcome of this work is a generalized Costa sticking model which incorporates the effect of the water film thickness coating the particles.