Visualization of the evaporation and condensation phenomena in cryogenic propellants

摘要

Prediction and control of evaporation/condensation of cryogenic propellants is one of the key factors limiting long-term space missions. Modeling propellant behavior and predicting phase change rates require models that need to be calibrated with experimental data. However, no such data is available on controlled phase change of cryogenic propellants. In this work, neutron imaging is employed as a means to visualize the condensed propellant inside opaque metallic containers at temperatures as low as 17 K. By controlling the temperature and pressure, a wide variety of phase change rates could be obtained. An exponential attenuation model is used to accurately determine the liquid-wall interface. Two methods of determining liquid volume as a function of time are described and compared. The interface tracking method uses an adaptive threshold edge detection and fit to the Young-Laplace equation while the optical density method calculates the liquid thickness for every pixel based on the Beer-Lambert law with a beam hardening correction. The former method is applicable only in images that have a fully formed meniscus whereas the latter method can be used on all images despite the shape/location of the liquid in the cell. Uncertainty in volume measurement with the optical density method is 6% lower than with the interface tracking method, and the results are in excellent agreement. In addition to volume, optical density method can be used to measure thickness of the thin liquid film on the wall of the container. For steady states, the interface tracking method will suffice but the optical density method is useful for high-accuracy volume measurements and thin film analysis.