Final Report Summary - COOLING TECHNIQUE (Elaboration of an evaporative cooling technique with shear-driven liquid films)
Thin and ultra-thin shear-driven liquid films in a narrow channel are a promising candidate for the thermal management of advanced semiconductor devices in earth and space applications. Such flows experience complex, and as yet poorly understood, two-phase flow phenomena requiring significant advances in fundamental research before they could be broadly applied.
One of the goals of the project was experimental study of locally heated shear-driven liquid films in a flat mini-channel. A detailed map of the flow sub-regimes in a shear-driven liquid film have been obtained for 1-2 mm channels operating at room temperature. It was found, that when heated the shear-driven liquid films are less likely to rupture than gravity-driven liquid films. For shear-driven water films the critical heat flux was found of up to ten times higher than that for a falling film, exceeding 400 W/cm2 for moderate liquid flow rates. This fact makes use of shear driven films (annular or stratified two-phase flows) more suitable for cooling applications than falling liquid films.
It is revealed that maps of isothermal flow regimes as well as rupture and critical heat fluxes (CHF) are weakly affected by the channel inclination angle in the plane parallel to the flow. On the contrary, the inclination angle in the plane perpendicular to the flow has an important effect on the film flow patterns and crisis phenomena. However, there exists a critical gas velocity value after which there is no effect of the inclination angle. This shows that such a cooling system can be potentially used in moving devices and vehicles.
A series of experiments was performed on microstructured heaters. The critical heat flux for microstructured heaters was found to be up to two times higher than that for the smooth heater. Procedures to organise a gas shear-driven liquid film flow under variable gravity conditions (parabolic flights) have been verified. It was found that the flow dynamics in normal gravity differs significantly from that in microgravity. The film is wavier under low gravity conditions.
Rupture of a subcooled liquid film flowing under the action of gravity over an inclined heated plate was studied for a wide range of liquid viscosity and plate inclination angle with respect to the horizon (0-90 deg). Traditional criterion, which is used in the literature to predict thermocapillary film rupture, was modified by taking into account characteristic critical film thickness for film rupture under isothermal conditions. The modified criterion allowed to successfully generalise the whole range of data obtained.
Liquid drop is a common phenomenon in nature, with which we encounter in our everyday life. Evaporation of drops is an efficient way to cool a heated surface by latent heat of evaporation when the drops change phase. On the other hand, liquid drop is a simplest object to study the triple liquid-gas-solid contact line. The problem of the contact line is very important for a number of phenomena in the domain of fluid mechanics and heat transfer. Although there are a lot of investigations on contact line in the literature, there are only a few performed under different gravity conditions. In the project the dynamics of contact line in a growing sessile liquid drop was studied under gravity level from 0 to 20 g. The microgravity experiments were conducted during three European space agency parabolic flight campaigns (ESA PFCs) in 2010-11. The hypergravity experiment was carried out in the ESA large diameter centrifuge. eleven different smooth and rough surfaces were used, with different contact angles (CA) and contact angle hysteresis (CAH). For the first time the spreading of a sessile drop under the effect of gravity has been observed on surfaces with low CAH. For surfaces with high CAH the contact line was pinned while CA adjusted for different gravity. Good agreement was obtained between the experiment and theoretical modeling. The dynamic advancing CA was found to increase with the gravity level.
One of the goals of the project was experimental study of locally heated shear-driven liquid films in a flat mini-channel. A detailed map of the flow sub-regimes in a shear-driven liquid film have been obtained for 1-2 mm channels operating at room temperature. It was found, that when heated the shear-driven liquid films are less likely to rupture than gravity-driven liquid films. For shear-driven water films the critical heat flux was found of up to ten times higher than that for a falling film, exceeding 400 W/cm2 for moderate liquid flow rates. This fact makes use of shear driven films (annular or stratified two-phase flows) more suitable for cooling applications than falling liquid films.
It is revealed that maps of isothermal flow regimes as well as rupture and critical heat fluxes (CHF) are weakly affected by the channel inclination angle in the plane parallel to the flow. On the contrary, the inclination angle in the plane perpendicular to the flow has an important effect on the film flow patterns and crisis phenomena. However, there exists a critical gas velocity value after which there is no effect of the inclination angle. This shows that such a cooling system can be potentially used in moving devices and vehicles.
A series of experiments was performed on microstructured heaters. The critical heat flux for microstructured heaters was found to be up to two times higher than that for the smooth heater. Procedures to organise a gas shear-driven liquid film flow under variable gravity conditions (parabolic flights) have been verified. It was found that the flow dynamics in normal gravity differs significantly from that in microgravity. The film is wavier under low gravity conditions.
Rupture of a subcooled liquid film flowing under the action of gravity over an inclined heated plate was studied for a wide range of liquid viscosity and plate inclination angle with respect to the horizon (0-90 deg). Traditional criterion, which is used in the literature to predict thermocapillary film rupture, was modified by taking into account characteristic critical film thickness for film rupture under isothermal conditions. The modified criterion allowed to successfully generalise the whole range of data obtained.
Liquid drop is a common phenomenon in nature, with which we encounter in our everyday life. Evaporation of drops is an efficient way to cool a heated surface by latent heat of evaporation when the drops change phase. On the other hand, liquid drop is a simplest object to study the triple liquid-gas-solid contact line. The problem of the contact line is very important for a number of phenomena in the domain of fluid mechanics and heat transfer. Although there are a lot of investigations on contact line in the literature, there are only a few performed under different gravity conditions. In the project the dynamics of contact line in a growing sessile liquid drop was studied under gravity level from 0 to 20 g. The microgravity experiments were conducted during three European space agency parabolic flight campaigns (ESA PFCs) in 2010-11. The hypergravity experiment was carried out in the ESA large diameter centrifuge. eleven different smooth and rough surfaces were used, with different contact angles (CA) and contact angle hysteresis (CAH). For the first time the spreading of a sessile drop under the effect of gravity has been observed on surfaces with low CAH. For surfaces with high CAH the contact line was pinned while CA adjusted for different gravity. Good agreement was obtained between the experiment and theoretical modeling. The dynamic advancing CA was found to increase with the gravity level.