Characterisation of drop impact on heated surfaces by optical techniques

The impact of liquid drops on heated surfaces is a phenomenon of relevance in many applied fields, like in direct injection internal combustion engine, where the fuel drops impact on the hot piston wall, or in spray cooling, etc. The first step to a deeper understanding of the phenomenon is to try to investigate the effects produced by the impact of single drops on a heated surface, among these the so called secondary atomisation, i.e. the formation of small droplets after the impact of the primary drop on the wall, is considered of paramount importance for many applications. The use of non-intrusive optical techniques is necessary in this field and the paper describes the application of three of such techniques to the analysis of the secondary atomisation produced by single millimetric drop impacting on a heated aluminium surface under different regimes of phase transition. Phase Doppler anemometry is used to measure the size and velocity of the secondary droplets in a size range between 2μm and 250μm. However, the impact produces droplets having also drops of larger diameter (up to 700 μm) and to extend the range of the measured size an image analysis technique based on a high resolution CCD camera was also used. The present configuration allowed to detect droplets having a size from 30μm on, and a proper analysis of the results from this technique, together with those coming from the PDA measurements, consented to obtain the "extended" secondary droplet size distribution from 2μm to few mm. A PIV (particle image velocimetry) technique was also applied to collect information about the velocity of the secondary droplets acquiring the images produced by a double shot strobolamp and using the cross-correlation method. Results are compared and the advantages and disadvantages of each technique are discussed.