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John O | March 2017

Scientists observe unusual fluid behavior in heat pipes in microgravity

Scientists aboard the International Space Station (ISS) observed that vapor near a wickless heat pipe condensed into a liquid even when the temperature was 160°K above the normal boiling point of the substance, which demonstrated the affect that microgravity has on the processes of condensation and evaporation, according to an article on Phys.org.



Condensation on a heat pipe on the ISS. (Kundan et al/American Physical Society)


The researchers were from Rensselaer Polytechnic Institute (RPI) and the NASA Glen Research Center and their observations built on previous experiments in space in which liquids accumulated at the heated end of a heat pipe, counter to how that process works on Earth.


The article explained, “The researchers performed a similar heat pipe experiment with pentane and found that, as the heat input to the surface increased, the amount of condensation increased. They observed the effect at temperatures of up to 160°K above the normal boiling point of pentane, the point at which the experiment reached its safety limits.


“In general, liquid above its boiling point is said to be in a ‘superheated’ state. Here, the researchers describe the hot end of the heat pipe as being flooded with superheated liquid.”


The scientists do not yet have a full explanation for this phenomenon, but is partially based on the Marangoni effect, which is derived from the heat pipe’s non-uniform surface and the changes in the three-dimensional temperature gradient it causes. The variances cause surface tension gradients and cooler liquid, which has a higher surface tension, pulls the hotter liquid towards it.


“The effect produces Marangoni-driven flows—one from the heated end to the cooled end, and another from the center of the pipe to its edges,” the article continued. “These flows occur even in the hot ‘evaporation zone’ of the pipe, and they generate an instability in the liquid layer that reinforces the condensation.”


The stronger gravitational forces on Earth reduce the Marangoni effect, but are still occurring on a much smaller scale.


According to the researchers, this observation could have significant impact in the study of closed, rather than open, cooling systems and the importance of interfacial and intermolecular forces. This could be a signal of the limits to heat pipes as cooling solutions for space travel, which has become of increasing interest as talk of manned travel to Mars has heated up.


The research was recently published in Physical Review Letters. The abstract of the report read:


“A wickless heat pipe was operated on the International Space Station to provide a better understanding of how the microgravity environment might alter the physical and interfacial forces driving evaporation and condensation.


“Traditional heat pipes are divided into three zones: evaporation at the heated end, condensation at the cooled end, and intermediate or adiabatic in between. The microgravity experiments reported herein show that the situation may be dramatically more complicated. Beyond a threshold heat input, there was a transition from evaporation at the heated end to large-scale condensation, even as surface temperatures exceeded the boiling point by 160 K.


“The hotter the surface, the more vapor was condensed onto it. The condensation process at the heated end is initiated by thickness and temperature disturbances in the thin liquid film that wet the solid surface.


“Those disturbances effectively leave the vapor ‘superheated’ in that region. Condensation is amplified and sustained by the high Marangoni stresses that exist near the heater and that drive liquid to cooler regions of the device.”

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