Experimental investigation on the characteristic of jet break-up for butanol droplet impacting onto a heated surface in the film boiling regime

Abstract

In view of the importance of atomization, droplet breakup induced by high Weber number and vapor bubble during droplet impacting onto a solid and heated surface has been studied extensively, but the understand on jet break-up phenomenon in the film boiling regime is still lacking. Thus, the work is trying to study the dynamic of jet break-up of n-butanol, a potential alternative biofuel for internal combustion engine, under the influence of Weber number. During experiment, a high-speed camera, set at 512 × 512 pixels, 10,000 fps, and 20 μs exposure time was used to visualize the droplet impacting behavior. The droplet falling height was set from 1 cm to 9 cm with interval of 0.5 cm, with corresponded Weber number around from 6.94 to 102.12. The heated surface temperature was set at 250 °C, to ensure the impact locates at the film-boiling regime. The results show that the jet break-up is dominated by Rayleigh-Plateau instability, the secondary droplet is formed through contraction of symmetric unstable surface waves since the waves are clearly observed on the jet column. Under tested wall conditions, the jet break-up only takes place when the Weber number of butanol droplet is around from 14.34 to 89.13. Within the jet break-up regime, the number of separated droplets first increases then decreases with Weber number increasing, and reaches the maximum at We = 65. The length of jet break-up first increases slowly then decreases rapidly and then increases with Weber number. While, the time of jet break-up first decrease then increase with Weber number and also reaches the minimum at We = 65.20. In addition, the timing of jet break-up is fitting well with the theory of Rayleigh instability.