Vapour cooling of poorly conducting hot substrates increases the dynamic Leidenfrost temperature

Michiel A.J Van Limbeek,Minori Shirota,Pascal Sleutel,Chao Sun,Andrea Prosperetti,Detlef Lohse

doi:10.1016/j.ijheatmasstransfer.2016.01.080

Michiel A.J Van Limbeek, Minori Shirota + Show 4 more

Open Access

https://doi.org/10.1016/j.ijheatmasstransfer.2016.01.080

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Abstract

A drop impacting a smooth solid surface heated above the saturation temperature can either touch it (contact boiling) or not (film boiling), depending on the surface temperature. The heat transfer is greatly reduced in the latter case by the insulating vapour layer under the drop. In contrast to previous studies, here we use a relatively poor thermally conducting glass surface. Using a total internal reflection method, we visualise the wetting dynamics of the drop on the surface. We discover a new touch-down process, in which liquid–solid contact occurs a few hundred microseconds after the initial impact. This phenomenon is due to the cooling of the solid surface by the generation of vapour. We propose a model to account for this cooling effect, and validate it experimentally with our observations. The model leads to the determination of a thermal time scale (about 0.3ms for glass) for the cooling of the solid. We conclude that when the impact time scale of the drop on the substrate (drop diameter/impact velocity) is of the order of the thermal time scale or larger, the cooling effect cannot be neglected and the drop will make contact in this manner. If the impact time scale however is much smaller than the thermal time scale, the surface remains essentially isothermal and the impact dynamics is not affected.

Highlights

In spray cooling applications and others, it is necessary to maintain contact between the droplets and the heated surface
For impacts from room temperature (Fig. 2a) up to 186 C we observe the spreading of the drops on the solid surface to proceed at velocities similar to those reported in previous studies of the impact on cold surfaces [26,27,28]
Heating the substrate further changes this correspondence: at 210 C we see that the growth of the wetted area is suppressed: the drop liquid is partially levitating above the solid surface, forming a lamella which hovers over its own vapour (Fig. 2b)

Summary

Introduction

In spray cooling applications and others, it is necessary to maintain contact between the droplets and the heated surface. As has been known for centuries [1,2], contact is abruptly lost once the surface of the hot solid exceeds a rather well defined value, called the Leidenfrost temperature TL. This temperature depends on the thermo-physical properties of the liquid and the solid [3,4], the micro-structure of the surface [5,6] and the impact velocity U [7]. The dynamic Leidenfrost temperature is found to depend on the drop size, impact velocity, liquid properties, surface roughness [11,13] and solid vibrations [12]. By using microdroplets instead of millimetre sized drops, we recover an isothermal impact, as simp 1⁄4 D0=U is decreased by the decrease of the drop size

Experimental setup and procedure

Observations

Subcooling effect causes touch-down

Conclusions

Micro droplets show isothermal behaviour