DINAMICS Project RECI/EMS-SIS/0147/2012
Dynamics of INterfacial Transport PhenomenA in MIcro Scale Energy Conversion Systems
Droplet-Wall Interactions
LIQUID-SURFACE INTERACTION MECHANISMS
Physical models capable of describing the secondary atomization resulting from spray impingement are fundamental tools in many practical applications.
Experiments conducted with single droplets impacting onto cold and heated targets have allowed developing threshold functions, to predict the impact regimes with satisfactory accuracy in most practical applications at ambient temperature. The critical question arises when the same correlations devised from impacts onto cold surfaces are misleadingly extrapolated to the impact onto hot surfaces, even though it is known that the surface temperature influences droplet behaviour.
Experiments make use of high-speed visualization together with image processing technique to characterize the morphology of the impact and to quantify the outcome of secondary atomization in terms of droplet size and number.
The Effects of Surface Topography
SCALING THE EFFECTS OF SURFACE TOPOGRAPHY
The impact of droplets onto micro-structured surfaces has been the focus of numerous recent studies, under the perspective of many different applications. However, much is still to be known about the effects of surface patterning in order to devise realistic physical models to accurately predict interfacial transfer rates. In this context, our work addresses the question of how to scale the effects of the surface topography to find adequate parameters, which can be easily obtained a priori.
The approach is based on the characterization of the hydrodynamic and thermal behaviours of individual droplets impacting onto smooth and micro-structured heated surfaces, with the objective of quantifying the effects of the modified wettability associated with the topography of the surface.
The focus is put on the thermal-induced mechanisms of secondary atomization as these are of particular interest for spray-cooling applications. The analysis suggests that different wetting properties lead to particular characteristics of the thermal-induced atomization, which can be related with the ratio between the roughness amplitude and the fundamental wavelength of the surface topography Ra/kR.
This hypothesis is consistent with the theoretical prediction of the wetting behaviour of the surfaces. The results also show a good correlation between the mean sizes of the secondary droplets generated by thermal induced atomization and the ratio Ra/kR.