Effects of wall effusivity on secondary droplet atomisation from single and multiple drop impact onto heated surfaces

The paper reports on experimental investigations of single and multiple drop impacts onto solid heated surfaces. Three different materials were used: aluminium alloy, bronze and stainless steel with effusivity of 20.7•103 kg/Ks5/2 14.1•103 kg/Ks5/2 and 7.19•103 kg/Ks5/2 respectively. Experiments were performed using distilled and degassed water and repeated under identical conditions with a single drop and a line array made by three drops. Millimetric drops were used at moderate Weber number (247) and two different surface temperatures (145°C and 260°C) were analysed to encompass the two main boiling regimes (bubble and film boiling). Quantitative results about secondary drop size obtained by PDA and by image analysis techniques are reported and the effect of the wall effusivity is analysed and discussed. Impact morphology appears to be strongly influenced by wall effusivity, by changing the characteristic times of the phenomenon and the intensity of the heat transfer.