Design of Spray Cooling Systems for Chilldown of Propellant Tanks

In-orbit cryogenic propellant transfer is a key enabling technology for future long duration space exploration missions. Tank chilldown will be one of the primary challenges to be overcome to achieve refueling in space. It is envisioned that tank cooling will be facilitated by the use of spray injection nozzles, achieving high heat removal rates through phase change. Tank filling protocols such as charge-vent-hold and vented-chill-non-vented-fill are being investigated to improve the probability of successful refueling while minimizing propellant boil-off. In this collaborative effort between the Univ of Connecticut and CRAFT Tech, experimental visualization and diagnostic measurements of a sub-scale tank are being used to understand the complex heat-transfer interaction modes between the spray and the ullage as well as the spray and the tank walls. Validation datasets are being collected and used in the development of a specialized spray cooling models within a comprehensive high-fidelity Multiphysics simulation framework. The simulation framework can be used for design support, analyzing tank filling protocols and prediction of chilldown times and propellant loss as part of the refueling process in a microgravity environment. In-space cryogenic propellant transfer is a key technology for future long duration space exploration missions.  Successfully refueling cryogenic tanks in space, in the absence of a cryocooler, presents challenges related to chilldown of the transfer line/receiving tank. Challenges include identifying cooling and filling protocols, minimizing propellant loss due to venting and maintaining a threshold pressure in the tank. Tank cooling can be effectively achieved with the use of optimal spray injection nozzles that remove thermal energy rapidly, minimizing propellant loss and chilldown time. In this program it is proposed that the heat transfer modes between the spray and the tank will be studied through extensive experimental visualization/diagnostics. The measurements will help refine the heat transfer CFD models with emphasis on the different interactional regimes of the spray with the wall/ullage. The program will be used to carry out parametric studies of spray cooling with different thermodynamic conditions in the tank, injector mass flow rates, pressure ratios and sub-cooling.      The goals for the program are a well-validated comprehensive cryogenic spray modeling framework for application to tank chilldown and fill procedures in terrestrial and microgravity environments. The goals include both numerical modeling and experimental objectives that will aid in the development of a predictive capability for refueling cryogenic tanks from depots/supply dewars.  Main objectives are: (i) Design and fabricate a scaled tank facility with advanced visualization to study chilldown; (ii)  Study the heat transfer interaction modes between spray, ullage and tank walls; (iii) Improve the droplet-wall interaction modeling framework; (iv) demonstrate the comprehensive tank modeling framework for tank chilldown applications. Proposed deliverables are: (i) License for the comprehensive spray modeling and tank chilldown software; (ii) Model details of the spray modeling framework for integration with NASA’s Cryogenic Flow Modeling codes (iii) experimental validation datasets of spray characterization, spray interaction with tank ullage and tank walls including thermocouple measurements, PIV data and shadowgraph/schlieren visualizations. 

Cryogenic propellant storage and transfer is critical to nearly all NASA’s future human exploration missions such as the Moon Gateway Mission and the more distant Mars Exploration Campaign. Successful propellant transfer in space is the cornerstone of NASA’s Reduced Gravity Cryogenic Transfer program and the technology in this program will impact it by improving our understanding of the physical processes, proving validation datasets and high-fidelity predictive tools. The technology will be valuable in decarbonization efforts and the transition to a hydrogen economy since storage and transfer of hydrogen remains a significant challenge. The technology can also be used for design of cryogenic spray nozzles for advanced propulsion concepts, improving life support systems in space as well as cryogenic preservation techniques in medical applications.

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