Thermocapillary instability as a mechanism for film boiling collapse

Abstract

We construct a model to investigate the interfacial stability of film boiling, and discover that instability of very thin vapour films and subsequent large interface superheating is only possible if thermocapillary instabilities are present. The model concerns horizontal saturated film boiling, and includes novel features such as non-equilibrium evaporation based on kinetic theory, thermocapillary and vapour thrust stresses and van der Waals interactions. From linear stability analysis applied to this model, we are led to suggest that vapour film collapse depends on a balance between thermocapillary instabilities and vapour thrust stabilization. This yields a purely theoretical prediction of the Leidenfrost temperature. Given that the evaporation coefficient is in the range 0.7–1.0, this model is consistent with the average Leidenfrost temperature of every fluid for which data could be found. With an evaporation coefficient of 0.85, the model can predict the Leidenfrost point within 10 % error for every fluid, including cryogens and liquid metals where existing models and correlations fail.

Bouncing off the Walls: The Influence of Gas-Kinetic and van der Waals Effects in Drop Impact

“…Splashing can be tackled, where the main challenges are numerical, but many other recent experimental results also reveal unexpected flow structures that their authors claim are driven by GKE. Examples ripe for theoretical investigation include sawtooth contact line instabilities [77], double contacts upon impact [78], "extreme wetting" [79], and the Leidenfrost effect [80][81][82]. This work has been financially supported by EPSRC (UK) (Grants No.…”

A two-dimensional numerical study of film boiling over an elliptical cylinder in the mixed regime under aiding and orthogonal saturated liquid flow configurations

“…Additionally, low values of are employed as discussed in the beginning of this section. Furthermore, several studies (Panzarella, Davis & Bankoff 2000; Aursand, Davis & Ytrehus 2018) have relied on two-dimensional models to obtain significant insights pertaining to nonlinear dynamics and thermocapillary effects in horizontal pool film boiling, where the physics is inherently three-dimensional. Based on the aforementioned considerations, it is expected that the observations from the present study are not just limited to two-dimensional scenarios, and the physical insights would very well apply to real problems.…”

The thermo-wetting instability driving Leidenfrost film collapse

“…There are many approaches to examine the stability of a vapor film adjacent to a superheated wall in two dimensions. Models have been developed with a base solution imposing static equilibrium, where the interface is at the saturation temperature corresponding to the imposed, far-field liquid pressure (12,16). Here, we consider the thickness of the vapor film to be in dynamic equilibrium, as in a vertical plate configuration (17)…”

“…Many theoretical frameworks have been used to characterize the Leidenfrost effect and estimate the LFP, including hydrodynamic instability (6,7), superheat spinodal limits (8,9), and the change of liquid wettability on the heated surface with temperature (10,11). A thermocapillary model has also been proposed that attributes the film instability to fluctuations at micrometer length scales; however, the analysis posits that the thermocapillary effect is the dominant destabilizing term, which does not explain the significant change in LFP on surfaces with different wettabilities (12). For example, the LFP of water can vary from 300 • C for hydrophilic surfaces to 145 • C on hydrophobic surfaces (13,14).…”