Self-Propelled Leidenfrost Droplets

Abstract

Droplets of liquid in the transition and film boiling regimes can be driven in a direction by viscous drag forces from their own vapour that has been directed by a ratchet toothed surface. To move towards engineering applications of this phenomenon the effects of tooth shape on the dynamics of these droplets must be well understood.
In evaluating the dynamics of droplets, the velocity, reliability, operational temperature range of droplet propulsion and forces acting on the droplet all need to be considered. To accomplish this droplet dynamics have been evaluated experimentally to take data on velocity, reliability and operational range and the simplified case of sublimating pucks have been evaluated in a CFD (Computational Fluid Dynamics) simulation to take data on the forces acting on pucks due to vapour flow.
Previously, to perform experiments on multiple different tooth shapes, many different ratcheted blocks would have to be produced. To remove this problem, a variable ratchet block was designed which uses many thin stacked cards to create a changeable ratchet surface. This allowed for many ratchet shapes to be tested. The most striking result from the experiments performed is the effect of overhang on droplet dynamics. Increasing the overhang angle of teeth is seen to increase the operational range and reliability of propelled droplets.
The CFD model of a dry ice puck above ratchet teeth is set up so that the ratchets can be changed in the same way as the variable block, and thus experimental data points can be matched to modelled ones. The simulation is also set to be periodic in the direction of puck motion, to reduce computational cost. Comparing blocks with overhangs to blocks without overhangs in CFD visualisations of vapour flow reduces the extent of vapour flow in the negative direction, as well as the peak velocity of this flow as compared to the peak velocity of the forward flow. From the CFD simulations it can also be seen that driving force acting on the puck increases as teeth get deeper. As tooth depth increases the puck sits at a lower height, and the gap beneath the puck and the teeth is constricted, increasing the interaction of the flow with the teeth.
Finally, preliminary work on a possible application for self-propelling droplets of a Leidenfrost pipe is described. Such a pipe would have ratchets coating its inner surface. These pipes were found to propel liquid at Leidenfrost temperature in an open system and alter the boiling characteristics of liquid introduced to them in a closed pressurised system. This suggests they may be useful in a cooling application.

Date of Award	19 Nov 2019
Original language	English
Awarding Institution	University of Bath
Supervisor	Andrew Rhead (Supervisor), Kei Takashina (Supervisor) & David Cleaver (Supervisor)