A New Experiment for Determining Evaporation and Condensation Coefficients of Cryogenic Propellants and Development of an Efficient Computational Model of Cryogenic Film Stability in Microgravity

This project addresses two of the three key microgravity challenges in TA-14 Thermal Management Systems; more specifically TA-14.1.2 Active Thermal Control of Cryogenic Systems of Space Technology Roadmap. These two challenges are (i) evaporation and condensation processes and (ii) efficient use of detailed models to simulate cryogenic propellant behavior. To address these challenges, this project will:1.Develop a standard method for measuring evaporation/condensation coefficients for hydrogenated cryogenic propellants using neutron imaging. The experiments addressing this objective will utilize the NIST Neutron Imaging Facility (Gaithersburg, MD). The evaporation/condensation coefficients for liquid hydrogen and liquid methane will be obtained in both a pure vapor environment and a two-component (vapor and gaseous helium) environment. 2.Develop a numerical simulation of liquid films of hydrogen and methane using a modified version of an evolution equation that couples the vapor phase to the liquid film via a kinetic model for evaporation and condensation.
Hydrogen phase-change cryogenic neutron imaging tests were conducted in January 2016 and similar tests with Methane in July and September 2015. A new method of pressure control was established to set a constant saturation pressure while continuously venting. Evaporation and condensation experiments were conducted at various saturation conditions between 15 and 30 psia. Imaging enabled measurement of phase change rates using two methods of data analysis. Two different test cell configurations are shown. During the experiments, temperature of the condensed liquid or the inner wall of the test cell could not be measured. However, discrete temperature measurements were made on the outer wall of the test cell. A CFD thermal model has been built using an axisymmetric approach in ANSYS FLUENT. The model is calibrated against dry thermal cycling test data to accurately capture the heat transfer mechanisms in the sample well of the cryostat. The interior wall temperature distribution can be derived from the discrete exterior wall temperature measurements using the CFD thermal model.
A thin film evaporation model is built based on the modified Schrage equation that accounts for both disjoining pressure and curvature of the liquid vapor interface. The model is built on a thin film evolution equation that evaluates the film profile and the evaporation mass flux at the interface. The model accurately predicts the profile of the thin film. The accommodation coefficient can be determined by comparing the integrated phase change from the thin film model to the measured rate of phase change in the neutron imaging experiments.