Abstract
The impact of single drops on a thin liquid film was studied to understand the mechanism of secondary atomisation of sprays colliding on a wetted, cold, solid surface. To span a wide range of conditions various mixtures of water and glycerol were used. The use of Weber number, Ohnesorge number and non-dimensional film thickness to describe the peculiarities of the phenomenon allowed to carry out the experiments under appropriate similarity conditions. The impact of millimetric drops was analysed in detail by photographic means, using both still photography to study impact morphology, and laser sheet visualisation to investigate secondary droplet formation. Two mechanisms of splash were identified, depending essentially on the liquid viscosity (Ohnesorge number), a parameter which appears to play an important role also in defining the splash morphology. A photographic documentation is annexed. The characteristic times of the crown formation, the non-linear evolution of cusps (jet formation) and the surface roughness influence are further discussed. The experimental results allow to propose an empirical correlation for the splashing/deposition limit, for a wide range of conditions, and a comparison to available previous works is presented. The influence of the film thickness and liquid viscosity on the splash is confirmed and quantified.
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Abbreviations
- Bo:
-
Bond number (= ρgh2/σ)
- Ca:
-
capillary number (= µV/σ)
- D:
-
nozzle diameter
- f:
-
impact frequency
- fnd :
-
non-dimensional impact frequency (=f φ/V)
- Fr:
-
Froude number (= V2φ/g)
- h:
-
film thickness
- H:
-
crown height
- K:
-
We Oh- 0.4
- KL :
-
splashing/deposition limit
- N:
-
number of secondary droplets
- Njet :
-
number of jets detaching from the crown
- Oh:
-
Ohnesorge number (= µ/ φσρ 1/2)
- Ra :
-
roughness
- Rc, nd :
-
nondimensional crown radius (= Rc/φ)
- Rc :
-
crown radius
- Re:
-
Reynolds number (= σVφ/µ)
- Rnd :
-
non-dimensional roughness (= Ra/φ )
- t:
-
time
- ts :
-
splash beginning time
- Ug :
-
gravitation potential energy
- Us :
-
surface potential energy
- V:
-
terminal drop velocity
- We:
-
Weber number (= ρV2φ/σ)
- Wecr :
-
critical Weber number
- Wed :
-
deposition Weber number
- Wes :
-
splash Weber number
- Y:
-
splashing/deposition parameter
- α:
-
drop impact angle
- δ:
-
non-dimensional film thickness (= h/φ)
- φ:
-
drop diameter
- Φ:
-
coalescence parameter
- φjet :
-
jet diameter
- λ:
-
viscosity length ( = (µ/αf)1/2σρ/µ2)
- µ:
-
liquid viscosity
- ρ:
-
liquid density
- σ:
-
surface tension
- τ:
-
non-dimensional time (= t φ/V)
- τ0 :
-
splash time scale ( = φ/V)
- τs :
-
nondimensional splash beginning time (= ts/τ0)
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Milan. The authors would like to thank Mr. Valentino Michelotti and Mr. Stefano Milza for their work during the experiments. Mr Gianni Brunello for his assistance, Ing. Bollina for the surface treatment and roughness measurement of the aluminium disk. We are also indebted to Prof. Cam Tropea and Prof. A.L. Yarin for many helpful and stimulating discussions.
The experiments were performed at CNPM-CNR laboratories in Milan. The authors would like to thank Mr. Valentino Michelotti and Mr. Stefano Milza for their work during the experiments. Mr Gianni Brunello for his assistance, Ing. Bollina for the surface treatment and roughness measurement of the aluminium disk. We are also indebted to Prof. Cam Tropea and Prof. A.L. Yarin for many helpful and stimulating discussions.
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Cossali, G.E., Coghe, A. & Marengo, M. The impact of a single drop on a wetted solid surface. Experiments in Fluids 22, 463–472 (1997). https://doi.org/10.1007/s003480050073
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DOI: https://doi.org/10.1007/s003480050073