Thermocapillary Instabilities

 

On phase change in Marangoni-driven flows and its effects on the hydrothermal-wave instabilities

This paper investigates the effects of phase change on the stability of a laterally heatedliquid layer for the first time. The interface is open to the atmosphere and vapordiffusion is the rate-limiting mechanism for evaporation. In this configuration, the planar layer is naturally vulnerable to the formation of travelling thermal instabilities, i.e., hydrothermal waves (HTWs), due to the presence of temperature gradients along the gas-liquid interface.Recent work carried out for deformable interfaces and negligible evaporation indicates that the HTWs additionally give rise to interface deformations of similar features, i.e., physical waves. The study presented here reveals that phase change plays a dual role through its effect on these instabilities: the latent energy required during the evaporation process tends to inhibit the HTWs while the accompanying level reduction enhances the physical waves by minimizing the role of gravity. The dynamics of the gas phase are also discussed. The HTW-induced convective patterns in the gas along with the travelling nature of the instabilities have a significant impact on the local evaporation flux and the vapor distribution above theinterface. Interestingly, high (low) concentrations of vapor are found above cold (hot) spots. The phase-change mechanism for stable layers is also investigated. The Marangoni effect plays a major role in the vapor distribution generating a vacuum effect in the warm region and vapor accumulations at the cold boundary capable of inverting the phase change, i.e., the capillary flow can lead to local condensation. This work also demonstrates the inefficiencies of the traditional phase change models based on pure vapor diffusion to capture the dynamics ofthermocapillary flows.

 

Paper:

On phase change in Marangoni-driven flows and its effects on the hydrothermal-wave instabilities.
P. J. Sáenz, P. Valluri, K. Sefiane, G. Karapetsas, O. K. Matar
Physics of Fluids26, p 024114
 (2014) [PDF]

 

Linear and nonlinear stability of hydrothermal waves in planar liquid layers driven by thermocapillarity

A shallow planar layer of liquid bounded from above by gas is set into motion via the thermocapillary effect resulting from a thermal gradient applied along its interface. Depending on the physical properties of the liquid and the strength of the gradient, the system is prone to departure from its equilibrium state and to the consequent development of an oscillatory regime. This problem is numerically investigated for the first time by means of two-phase direct numerical simulations fully taking into account the presence of a deformable interface. Obliquely travelling hydrothermal waves (HTWs), similar to those first described by Smith and Davis [J. Fluid Mech. 132, 119–144 (1983)], are reported presenting good agreement with linear stability theory and experiments. The nonlinear spatiotemporal growth of the instabilities is discussed extensively along with the final bulk flow for both the liquid and gas phases. Our study reveals the presence of interface deformations which accompany the HTWs pattern with a certain time-delay. The local interface heat fluxes are found to be significantly affected by the transient nature of the HTWs, contradicting the results of previous single-phase studies.

 

Paper:

Linear and nonlinear stability of hydrothermal waves in planar liquid layers driven by thermocapillarity
P. J. Sáenz, P. Valluri, K. Sefiane, G. Karapetsas, O. K. Matar
Physics of Fluids25, p 094101 
(2013) [PDF]

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