Abstract:
This article highlights the role of non-Newtonian
(elastic) effects on the droplet impact phenomenon
at temperatures considerably higher than the boiling
point, especially at or above the Leidenfrost regime.
The Leidenfrost point (LFP) was found to decrease
with an increase in the impact Weber number
(based on the velocity just before the impact) for
fixed polymer (polyacrylamide) concentrations.
Water droplets fragmented at very low Weber
numbers (approx. 22), whereas the polymer droplets
resisted fragmentation at much higher Weber
numbers (approx. 155). We also varied the polymer
concentration and observed that, up to 1000 ppm, the
LFP was higher than that for water. This signifies
that the effect can be delayed by the use of elastic
fluids. We have shown the possible role of elastic
effects (manifested by the formation of long lasting
filaments) during retraction in the increase of the
LFP. However, for 1500 ppm, the LFP was lower
than that for water, but had a similar residence time
during the initial impact. In addition, we studied
the role of the Weber number and viscoelastic effects
on the rebound behaviour at 405°C. We observed that
the critical Weber number up to the point at which
the droplet resisted fragmentation at 405°C increased
with the polymer concentration. In addition, for a
fixed Weber number, the droplet rebound height
and the hovering time period increased up to
500 ppm, and then decreased. Similarly, for fixed
polymer concentrations like 1000 and 1500 ppm,
the rebound height showed an increasing trend
up to certain a certain Weber number and then
decreased. This non-monotonic behaviour of rebound
heights was attributed to the observed diversion of the rebound kinetic energy to rotational energy during the hovering phase. Finally, a
relationship between the non-dimensional Leidenfrost temperature and the associated Weber
and Weissenberg numbers is developed, and a scaling relation is proposed.
