Regimes of coalescence and separation in droplet collision

Abstract

An experimental investigation of the binary droplet collision dynamics was conducted, with emphasis on the transition between different collision outcomes. A series of timeresolved photographic images which map all the collision regimes in terms of the collision Weber number and the impact parameter were used to identify the controlling factors for different outcomes. The effects of liquid and gas properties were studied by conducting experiments with both water and hydrocarbon droplets in environments of different gases (air, nitrogen, helium and ethylene) and pressures, the latter ranging from 0n6 to 12 atm. It is shown that, by varying the density of the gas through its pressure and molecular weight, water and hydrocarbon droplets both exhibit five distinct regimes of collision outcomes, namely (I) coalescence after minor deformation, (II) bouncing, (III) coalescence after substantial deformation, (IV) coalescence followed by separation for near head-on collisions, and (V) coalescence followed by separation for off-centre collisions. The present result therefore extends and unifies previous experimental observations, obtained at one atmosphere air, that regimes II and III do not exist for water droplets. Furthermore, it was found that coalescence of the hydrocarbon droplets is promoted in the presence of gaseous hydrocarbons in the environment, suggesting that coalescence is facilitated when the environment contains vapour of the liquid mass. Collision at high-impact inertia was also studied, and the mechanisms for separation after coalescence are discussed based on time-resolved collision images. A coalescence\separation criterion defining the transition between regimes III and IV for the head-on collisions was derived and found to agree well with the experimental data.

Droplet Collision Simulation by a Multi-Speed Lattice Boltzmann Method

“…6. The approximate line dividing coalescence and separation is qualitatively similar to that observed by Qian and Law [9]. It is possible that the transition from coalescence to head on separation is being observed at low impact parameters, however this requires a further investigation.…”

“…Many comprehensive experimental studies of binary droplet collisions are available [7,8], including one by Qian and Law [9]. Various regimes of droplet collisions are identified by two dimensionless parameters, the Weber number We,…”

Hypergolic ignition by head-on collision of N,N,N′,N′ −tetramethylethylenediamine and white fuming nitric acid droplets

“…The ambient pressure influences the collision outcomes [20,23] . Specifically, droplets tend to bounce back with increasing the ambient pressure because the increased inertia of the gas film separating the droplets becomes more resistant to be drained out to effect the interface merging [24] .…”

Numerical analysis of sprays with an advanced collision model

“…In some studies and for some liquids also a coalescence region for very low We over a wide range of impact parameters was found (Qian and Law 1997, Chen 2007and Huang and Pan 2015, which however will not considered here for the moment. For collisions of identical sized droplets the critical Weber-number WeC has been quite often identified to shift to the right with increasing Ohnesorge-number and Qian and Law (1997) found a good correlation with minor scatter considering also various literature data (e.g. Jiang et al 1991):…”

“…Now we turn to the low Oh-regime wherefore numerous data are available (Figure 7 b). Here only the initial correlation proposed by Qian and Law (1997) is shown for comparison. Most of the data for the critical Webernumber are lying around and along this correlation.…”