(358b) Condensation on Highly Superheated Surfaces: Unstable Thin Films in a Wickless Heat Pipe

The Constrained Vapor Bubble (CVB) heat pipe is a wickless heat pipe that is designed to produce a simple, light, and reliable heat transfer system that can be used for cooling critical components of spacecraft. The CVB system consists of a relatively simple setup - a quartz cuvette with sharp corners partially filled with pure pentane as the working fluid. A full-scale fluid experiment was conducted 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. Along with temperature and pressure measurements, the Light Microscopy Module (LMM) was used to determine the two-dimensional thickness profile of the menisci formed on the wall surfaces as well as at the corners of the cuvette. Interfacial forces dominate in these extremely small Bond number systems. The transport processes were found to be complex despite being conceptually simple.

Traditional heat pipes are divided into three zones: evaporation at the heated end, condensation at the cooled end, and intermediate/adiabatic in between. The microgravity experiments reported herein show 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 end that drive liquid to cooler regions of the device. This led to flooding of the heater end of the device and the forming of a large liquid drop on the wall surface. This condensation phenomenon at the heater end also occurs in a 1-g environment. However, since the return flow of liquid is meager in 1-g, there is little vapor to condense at the heater end and the phenomenon appears as if pentane were a partially wetting fluid. Thus, it can be easily mistaken for the effects of surface contamination.

This material is based on the work supported by the National Aeronautics and Space Administration (NASA) under Grant No. NNX13AQ78G and the National Science Foundation under Grant No. CBET-1603318.