Investigating the Inequality of Phase Change Coefficients Using ISS Experimental Data

Document Type

Article

Publication Date

8-20-2025

Department

Department of Mechanical and Aerospace Engineering

Abstract

Kinetic theory is a popular approach to model liquid-vapor phase change but accurate determination of evaporation and condensation coefficients remains a challenge. Reported values of coefficients vary by several orders of magnitude. For simplicity and convenience evaporation and condensation coefficients are assumed to be equal though there is little physical evidence to support this. This study presents a novel methodology to test this assumption using data from Constrained Vapor Bubble (CVB) experiments conducted on the International Space Station (ISS). The experiments consist of a quartz cuvette that is partially filled with n-pentane; heated and cooled at opposite ends to induce simultaneous evaporation and condensation around a central bubble. Data obtained from the NASA Physical Sciences Informatics (PSI) database enabled a three-dimensional reconstruction of the liquid–vapor interface. The net mass flux over the vapor bubble surface is zero at steady operation, providing a closure relationship for simultaneous and independent calculation of both evaporation and condensation coefficients. The resulting coefficient values are within 1% of each other but are not equal. The two coefficients are also within 2% of those predicted using transition state theory. When the evaporation and condensation coefficients are forced to be equal, the deviation from transition state theory is approximately 60%. This deviation monotonically increases with increasing rates of evaporation/condensation due to a systemic under-prediction of the bubble surface area. The agreement between derived coefficients and those predicted by transition state theory is maintained when the bubble surface area is corrected to account for Marangoni-induced interfacial instabilities.

Publication Title

Nanoscale and Microscale Thermophysical Engineering

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