Most existing spacecraft thermal subsystems rely on single-phase heat transfer, but the drive towards lighter, smaller, higher power subsystems to make future missions possible will require use of…

The National Aeronautics and Space Administration (NASA) awarded a $256,550.33 grant to the University of Maryland, College Park to develop a heat transfer database and reliable models for two-phase thermal systems in variable gravity environments. This research aims to address the key challenge of predicting the performance of two-phase heat exchangers on spacecraft, which is currently hampered by the lack of sufficient data and models for flow boiling under different gravity conditions. The grant does not have a set-aside designation. The research will involve obtaining high-resolution measurements of wall heat transfer coefficients using temperature-sensitive paints, and studying various flow regimes relevant to advanced spacecraft heat exchangers. At the end of the test campaign, the project is expected to reach a Technology Readiness Level of 4, laying the groundwork for a future experiment on the International Space Station.

Most existing spacecraft thermal subsystems rely on single-phase heat transfer, but the drive towards lighter, smaller, higher power subsystems to make future missions possible will require use of two-phase thermal systems. The key challenge in developing two-phase thermal systems is the development of a heat transfer database and reliable models for flow boiling in variable gravity environments from which the performance of two-phase heat exchangers in spacecraft can be confidently predicted. The use of two-phase thermal systems on spacecraft has been greatly hampered by the inability to predict with sufficient confidence their performance at various gravity levels (earth, mars, and lunar gravity and low-g). The performance prediction of two-phase systems under these conditions requires a sufficient heat transfer database and reliable models, both of which are not currently available. Although some research in low-gravity adiabatic flows has been performed, very little heat transfer data relevant to advanced space heat exchangers is available and the mechanisms by which heat is removed from the surfaces under varying gravity environments are unclear. We will develop the data and models needed to predict the behavior of two-phase flows in geometries relevant to advanced heat exchangers in variable gravity environments throughout the boiling curve in low-g, 1-g, and high-g (1.6G). Unlike previous work where only time- or spaceaveraged heat transfer data were measured, we will obtain local measurements of the wall heat transfer coefficient with high temporal and spatial resolution using temperature sensitive paints (TSP). Test conditions include bubbly flow, slug flow, churn flow, annular flow, inverse annular flow, and ultimately dryout. We have performed measurements with infrared thermography in the past, but have recently transitioned to using tsps since the data interpretation is much easier and the uncertainties are much smaller. Examples of data recently obtained using tsps are shown along with flow visualization. At the end of the test campaign, we expect to have completed a preliminary overview of the test matrix for a future ISS experiment. The TRL level is currently 4, and is expected to exit at 4 as well.