Collision of a droplet onto a still spherical particle was experimentally investigated. The effect of droplet impact velocity and wettability of the particle surface on collision outcomes was studied (0.05 < V0 < 5.0 and θ = 70°, 90°, 118°). Compared to the literature, the range of Weber number variations was significantly extended (0.1 < We < 1146), and while focus of the previous works was on impacts in which particle is larger than the droplet (Dr < 1), the drop to particle diameter ratio in this work was larger than one. Therefore, formation of a thin liquid film, i.e., lamella, was observed due to impact of a relatively high velocity droplet onto a hydrophobic particle. Temporal variations of various geometrical parameters of collision outcomes including lamella length and lamella base diameter were investigated during the impact. It was also shown that for hydrophobic targets, the extent of hydrophobicity of the particle does not affect the lamella geometry. A comprehensive map of all the available works in drop impact on a spherical target was also provided.

In this article, the flow pressure drop across a wave‐plate mist eliminator has been studied experimentally by changing both the air velocity and the plate spacing. The measured values for pressure losses have been compared with the results of the available formulas. Despite the literature, this study shows that by decreasing the plate spacing, the pressure loss will not remain constant. Results show that decreasing the plate spacing from 11.5 to 9.5 mm leads to a decrease in pressure loss. However, further decrease in spacing will result in an increased pressure loss. On the basis of dimensional analysis, a dimensionless variable is defined for spacing, and it is shown that the value of 0.24 for this variable gives the minimum pressure coefficient for all cases studied by current experimental setup. Copyright © 2011 Curtin University of Technology and John Wiley & Sons, Ltd.

Shedding of multiple sessile droplets by an airflow in triangle, square, reversed triangle, and diamond arrangements is examined. The interaction of the flow around the sessile droplets is found to be influenced by the type of the arrangement and the spacing of the sessile droplets in each arrangement. Consequently, the minimum airflow velocity required to shed the droplet (Ucr) also changes. Water droplets of 5 and 10 μl were used on both hydrophilic and hydrophobic surfaces in a laminar airflow. In general, at the minimum spacing, the highest increase in Ucr for the upstream droplet(s) (compared with that for a single droplet) was observed for the triangle arrangement (∼40%), followed by the diamond, reversed triangle, and square arrangements. Increasing the spacing resulted in a reduction of the Ucr for all the arrangements, except for the square arrangement where increasing the spacing does not show a substantial change in Ucr. Neither the size of the droplets nor the wettability of the substrate was found to significantly affect the amount of the change in the Ucr.

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Collision of a droplet and a hydrophobic particle in mid-air was investigated. To study the impact outcomes, specifically the lamella formation, a numerical simulation tool was developed and verified with impact experiments (water droplets and glass particles, ρrel = 0.41). The velocity field within the lamella showed that the flow inside the liquid film moves in two opposite directions along the lamella axis of symmetry: one is generated through the momentum transfer from the particle, and the other is due to the droplet initial velocity. This causes the lamella to be stretched in the same direction as the particle moves and forms a rim at the end of the lamella. Although a larger droplet-to-particle diameter ratio (Dr) increased the impact duration, it did not change the collision outcomes and two opposite flows still exist inside a thicker liquid film. However, the liquid viscosity affects impact outcomes; as viscosity increased, a thicker film remained on the particle, the liquid film became shorter, and the lamella formation was hindered accordingly. The pressure of the ambient gas also affects the liquid film formation. Unlike the literature of the drop impact on a flat surface, our results indicate that by increasing the ambient pressure, the lamella formation will be suppressed (hence chance of splashing). The pressure gradient around the liquid film creates a downward force that hinders the stretching of the liquid film. The effect of the ambient pressure on lamella formation is only significant for relatively higher gas pressures (i.e., Pamb > 2 Patm).

A visual study is conducted to determine the effects of operating conditions on the spray cone angle of a two-fluid atomizer. The liquid (water) jets exit from peripheral inclined orifices and are introduced into a high-speed gas (air) stream in the gravitational direction. Using a high-speed imaging system, the spray cone angle is determined for Reynolds numbers ranging from 4×10 4 to 9×10 4 and different Weber numbers up to 140. The droplet sizes (Sauter mean diameter) and their distributions are determined using a Malvern Mastersizer X. The results show that the spray cone angle depends on the operating conditions, especially in lower values of Reynolds and Weber numbers. An empirical correlation is also obtained to predict the spray cone angle in terms of these two parameters.

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