Abstract
The impact of liquid droplets onto spherical stationary solid particles under isothermal conditions is simulated. The CFD model solves the Navier-Stokes equations in three dimensions and employs the Volume of Fluid Method (VOF) coupled with an adaptive local grid refinement technique able to track the liquid-gas interface. A fast-marching algorithm suitable for the quick computation of distance functions required during the grid refinement in large 3-D computational domains is proposed. The numerical model is validated against experimental data for the case of a water droplet impact onto a spherical particle at low We number and room temperature conditions. Following that, a parametric study is undertaken examining (a) the effect of Weber number (= ρu2Do/σ) in the range of 8 to 80 and (b) the droplet to particle size ratio ranging in-between 0.31 and 1.24, on the impact outcome. This has resulted to the identification of two distinct regimes that form during droplet-particle collisions: the partial/full rebound and the coating regimes; the latter results to the disintegration of secondary satellite droplets from elongated expanding liquid ligaments forming behind the particle. Additionally, the temporal evolution of variables of interest, such as the maximum dimensionless liquid film thickness and the average wetting coverage of the solid particle by the liquid, have been quantified. The present study assists the understanding of the physical processes governing the impact of liquids onto solid spherical surfaces occurring in industrial applications, including fluid catalytic cracking (FCC) reactors.
Highlights
The dynamics of droplet impingement onto solid surfaces is realised in many engineering applications, as for example, spray cooling, spray coating, fuel injection in internal combustion engines, fire suppression, inkjet printing, metallurgy and electronic circuits cooling among other
A parametric study on the effect of droplet-particle size ratio for a wider range of Weber number of impact has not been presented, it is needed for establishing collision outcome maps
After the validation of the numerical model for the case of droplet impingement onto a spherical particle, a parametric study was performed in order to investigate the dynamics of the droplet-particle collisions under different impact conditions
Summary
The dynamics of droplet impingement onto solid surfaces is realised in many engineering applications, as for example, spray cooling, spray coating, fuel injection in internal combustion engines, fire suppression, inkjet printing, metallurgy and electronic circuits cooling among other. The effect of droplet to particle size ratio was not explicitly investigated, as the authors differentiated collision outcomes based only on the impact Weber number. In 2013, Mitra et al [15] presented simulations and experiments on droplet impingement onto a spherical particle, under isothermal and non-isothermal conditions In their work, they focus more on the effect of particle temperature on the solid-liquid contact and not on quantifying the collision outcomes. The authors investigated the effect of We and droplet-particle size ratio parameters on the outcome; they only studied low Weber numbers (up to 26.14) which are below the threshold for different impact regimes to be realised. A parametric study on the effect of droplet-particle size ratio for a wider range of Weber number of impact has not been presented, it is needed for establishing collision outcome maps. In the present work, the assumptions of stationary particles and the inclusion of gravity, which may not be directly relevant to FCC conditions, were mainly adopted for the validation of the proposed methodology against experimental measurements and relevant numerical works
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