title: Thermocapillary phenomena and performance limitations of a wickless heat pipe in microgravity.
abstract: A counterintuitive, thermocapillary-induced limit to heat- pipe performance was observed that is not predicted by current thermal-fluid models. Heat pipes operate under a number of physical constraints including the capillary, boiling, sonic, and entrainment limits that fundamentally affect their performance. Temperature gradients near the heated end may be high enough to generate significant Marangoni forces that oppose the return flow of liquid from the cold end. These forces are believed to exacerbate dry out conditions and force the capillary limit to be reached prematurely. Using a combination of image and thermal data from experiments conducted on the International Space Station with a transparent heat pipe, we show that in the presence of significant Marangoni forces, dry out is not the initial mechanism limiting performance, but that the physical cause is exactly the opposite behavior: flooding of the hot end with liquid. The observed effect is a consequence of the competition between capillary and Marangoni-induced forces. The temperature signature of flooding is virtually identical to dry out, making diagnosis difficult without direct visual observation of the vapor-liquid interface.
authors.affiliation: The Howard P. Isermann Department of Chemical and Biological Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180, USA.
authors.forename: Akshay
authors.initials: A
authors.lastname: Kundan
authors.affiliation: The Howard P. Isermann Department of Chemical and Biological Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180, USA.
authors.forename: Joel L
authors.initials: JL
authors.lastname: Plawsky
authors.affiliation: The Howard P. Isermann Department of Chemical and Biological Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180, USA.
authors.forename: Peter C
authors.initials: PC
authors.lastname: Wayner
authors.affiliation: NASA Glenn Research Center, Cleveland, Ohio 44135, USA.
authors.forename: David F
authors.initials: DF
authors.lastname: Chao
authors.affiliation: NASA Glenn Research Center, Cleveland, Ohio 44135, USA.
authors.forename: Ronald J
authors.initials: RJ
authors.lastname: Sicker
authors.affiliation: NASA Glenn Research Center, Cleveland, Ohio 44135, USA.
authors.forename: Brian J
authors.initials: BJ
authors.lastname: Motil
authors.affiliation: Zin Technologies, Cleveland, Ohio 44130, USA.
authors.forename: Tibor
authors.initials: T
authors.lastname: Lorik
authors.affiliation: Zin Technologies, Cleveland, Ohio 44130, USA.
authors.forename: Louis
authors.initials: L
authors.lastname: Chestney
authors.affiliation: Zin Technologies, Cleveland, Ohio 44130, USA.
authors.forename: John
authors.initials: J
authors.lastname: Eustace
authors.affiliation: Zin Technologies, Cleveland, Ohio 44130, USA.
authors.forename: John
authors.initials: J
authors.lastname: Zoldak