Session: 11-08-01: Fundamentals of Phase-Change Including Micro/Nanoscale Effects-Boiling, Evaporation, Freezing and Condensation

Paper Number: 95834

95834 - Self-Ejection of Boiling Droplets on Thin Heated Oil Films 

The Leidenfrost effect, in which a droplet levitates over a heated surface, has attracted significant interest for the removal of fouling droplets from heated surfaces. In the Liedenfrost regime, heat transfer from surface to droplet is severely limited due to an insulative vapor film. As such, in situations where increased heat transfer from surface to liquid is desired, research focus has been on techniques to increase the Leidenfrost point.

A liquid droplet in contact with a surface above the boiling point is typically thought to have two regimes of behavior. As surface temperature increases, the droplet transitions from boiling in contact with the surface to Leidenfrost levitation with a vapor film beneath the droplet. Control of the transition temperature between boiling and Leidenfrost levitation has important industrial applications. Increasing the transition temperature can allow for a higher surface critical heat flux, improving heat transfer performance and safety. Lowering the transition temperature can improve the mobility of droplets on surfaces for droplet transport. Previous studies on the transition temperature have focused on the interaction between liquid droplets and solid surfaces. Studies have also focused on the Leidenfrost effect on thick pools.

Here, we focus on the case of liquid droplets on thin, immiscible liquid films, where we observe a novel phenomenon of droplet self-ejection observed below the Leidenfrost onset temperature and above the boiling temperature. The oil film is observed to delay the onset of Leidenfrost to higher temperatures than on the same surface without oil film. The droplets, in contact with the viscous oil films, are observed to propel on the oily surfaces at velocities comparable to frictionless, levitating Leidenfrost droplets on ratchets (ca. 10cm/s). In a previous study, the droplet ejection velocity was found to be the result of the interplay between momentum and thermal effects in the oil film beneath the droplet. Rapid bubbling at 100us timescales disturb the momentum and thermal boundary layers beneath the ejecting droplets, reducing friction and increasing heat transfer.

The aforementioned previous study focused on the specific liquid combination of a water droplet and silicone oil film. In the present study, the implications of changing the droplet-film liquid combinations on the self-ejection phenomenon is explored. Furthermore, the scale of heat transfer into the droplet is analyzed and compared to literature values of critical heat flux. Thus, in this presentation, we present experiments elucidating which combinations of liquids promote self-ejection and estimate the increase in heat transfer caused by the interaction between oil film and self-ejecting droplet.

Presenting Author: Victor Leon MIT

Presenting Author Biography: MIT Mechanical Engineering PhD student. Graduating December 2022.

Authors:

Victor Leon MIT
Fabian Dickhardt MIT
Kripa Varanasi MIT