Analyse de la dynamique du film liquide dans un caloduc oscillant

Abstract : We experimentally study the behavior of liquid films - so called Landau-Levich films - when they evaporate in their pure vapor atmosphere. This film is deposited by a meniscus receding inside a capillary. The applied heat flux causes the film evaporation. The film thins with time and remains nearly flat during its whole lifetime (its slope is smaller than 0.1°). Both the thinning and the slope are caused by evaporation; the film drainage is negligible. Under heating conditions, the film gradually recedes. Once evaporation starts, a ridge is formed near the triple contact line. Such a ridge commonly appears at capillary dewetting under non-wetting conditions. This is surprising as our liquid (ethanol) wets completely the (sapphire) substrate at equilibrium. At the nanometric scale the contact angle between the liquid and the solid wall is thus low. However, we measure a large apparent contact angle (visible at the millimetric scale) which increases with the wall superheating. Once this angle becomes large, the dewetting ridge is formed and the film recedes. The large apparent contact angle is explained by evaporation in the microscopic vicinity of the contact line. The measured apparent contact angle value agrees quantitatively with theoretical results obtained by other researchers. Our analysis also shows the relationship between triple line velocity and apparent contact angle. The conclusions are broad and are applicable to any situation involving a triple line such as films evaporation or boiling phenomena. In particular, we explain how to estimate with a good precision the triple line receding velocity when it is caused by evaporation. The dynamics of this film is a key parameter that rules out the functioning of Pulsating Heat Pipes (PHPs). PHPs are highly conductive thermal links. Their heat transfer capability is known to be extremely high. For this reason they are promising for numerous industrial applications. Their geometry is simple. It is a capillary tube bent in several branches that meander between a hot part (called evaporator) and a cold part (called condenser), and filled up with a pure two-phase fluid. When the temperature difference between evaporator and condenser exceeds a certain threshold, gas bubbles and liquid plugs begin to oscillate spontaneously back and forth inside the tube – depositing liquid films at each passage - and PHP starts transferring the heat. From our conclusions and knowing the capillary superheating with respect to the saturation temperature, one is able to determine the film thickness, as well as the triple line velocity - and then, the film length and profile. As a consequence, the evaporation/condensation rate inside a PHP can be assessed. This will lead to improvement of the PHP modeling. Our experimental setup features the simplest, single branch PHP. A liquid/vapor interface oscillates in a tube. It deposits a liquid film at each passage. In addition to the film behavior analysis obtained thanks to an original combination of optical measurement techniques, we focus on the mechanism which makes possible self-sustained interface oscillations and defines its frequency. The obtained motion equation accounts for the viscous dissipation in oscillatory flow. In existing PHP models, a Poiseuille flow is supposed. Yet, our analysis shows that the assumption of weakly inertial flow works better. For our case, it leads to a dissipation rate twice larger that the Poiseuille value.
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Contributor : Laura Fourgeaud <>
Submitted on : Monday, December 5, 2016 - 10:23:09 PM
Last modification on : Thursday, April 4, 2019 - 5:10:42 PM
Long-term archiving on : Tuesday, March 21, 2017 - 4:06:18 AM


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  • HAL Id : tel-01409530, version 1


Laura Fourgeaud. Analyse de la dynamique du film liquide dans un caloduc oscillant. Physique [physics]. Université Grenoble - Alpes, 2016. Français. ⟨tel-01409530v1⟩



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