by Dr. Mohammad Kassemi, NASA Glenn Research Center, Mohammad.Kassemi@nasa.gov; and Dr. David Chato, ret. NASA Glenn Research Center

Affordable and reliable cryogenic fluid storage for propellant or life support systems is integral to all phases of NASA’s projected space and planetary expeditions. One challenge facing engineers is self-pressurization. It can be caused by the cryogen vaporization that results from heat leaks into a tank from its surroundings and support structure. Engineers can relieve this self-pressurization through venting, but repeated venting of the vapor during long-duration on-orbit or on-surface storage will result in significant propellant loss, rendering the cost of long distance human space expeditions prohibitive [1].

This realization has provided a significant impetus for researchers to develop innovative pressure control designs—based on mixing the bulk tank fluid together with some form of active or passive cooling—to allow storage of cryogenic fluids with zero or reduced boiloff. Complicated dynamic interactions govern both tank pressurization and pressure control, including those between forced mixing, various gravity-dependent transport mechanisms in the vapor and liquid phases, and the condensation-evaporation process at the interface. Consequently, effective implementation and optimization of a dynamic pressure control system for space applications can be difficult to achieve, especially without prior relevant microgravity experimental data.

The Zero Boiloff Tank (ZBOT) experiments are a series of small-scale tank pressurization and pressure control experiments aboard the International Space Station (ISS). ZBOT uses a transparent and volatile simulant fluid in a transparent sealed tank to delineate various fluid flow, heat and mass transport and phase change phenomena that control storage tank pressurization and pressure control in microgravity [2]. Figure 1 shows the hardware for ZBOT-1. It flew to ISS in April 2017, the first of three envisioned hierarchical experiments. Researchers plan to use the setup for more than 90 tests beginning in September. They will study and quantify fluid flow and thermal stratification during self-pressurization, and mixing, thermal destratification, depressurization and jet ullage penetration during the pressure control intervals.

Researchers are also developing a comprehensive two-phase Computational Fluid Dynamics (CFD) model for simulation and prediction of storage tank pressurization and pressure control [3]. The model uses a volume-of-fluid scheme to capture the two phases and tracks velocity, temperature and multi-component species concentrations in both domains under tight coupling—with the evaporative condensing mass transfer and energy-and-force balances at the phase front. At this stage, the CFD model is partially verified and validated with ZBOT ground-based data and existing large-scale 1G LH2 experiments performed by NASA. It will be further validated by the microgravity data provided by the ZBOT experiments for space applications.

Figure 2 shows a schematic of the main components of the ZBOT-1 hardware in the Microgravity Science Glovebox (MSG) and a cross-sectional view of the test cell. The test cell is made of polished optical quality cast acrylic and consists of a cylindrical midsection capped at its two ends by hemispherical domes (4” diameter by 8” height). Engineers mounted a strip heater on the outer surface of the cylindrical acrylic wall, a jet flow nozzle at the bottom that is activated during pressure control tests and a screened liquid acquisition device that ensures only liquid is extracted, thermally conditioned and pumped in through the nozzle during jet flow mixing cycles. The test cell is isolated from the MSG environment via a temperature-controlled vacuum jacket that is, in turn, enclosed within another cooling jacket. The design provides proper containment and definable and controllable thermal boundary conditions for model validation. A Fluid Support Unit (FSU) is located just below the test cell in the MSG. A counter-flow shell-and-tube heat exchanger in the FSU maintains the tank fluid temperature prior to each test and the jet flow temperatures during the mixing cycles.

The simulant phase change fluid used in the experiment is perfluoro-n-pentane (PnP or C5F12), a nonpolar volatile refrigerant with a boiling point of 29°C at 1 atm and a near zero contact angle with the test tank. NASA toxicology and ECLSS (Environmental Control and Life Support System) groups have approved a high purity (99.7 percent straight-chained n-isomer) version of PnP for use on ISS. A bellowed reservoir—shown in the upper left corner of the MSG (Figure 2)—stores the fluid that is degassed on orbit—initially, and before each fill level change—by a radial flow membrane contactor, achieving a high degree of purity (less than 5 torr residual gases), and ensuring that any noncondensable effects are eliminated during the ZBOT-1 tests.

Temperature-controlled windows on the vacuum jacket accommodate camera and white light/laser sheet illumination packages for both image capture of the ullage and flow visualization and Particle Imaging Velocimetry (PIV) in the liquid region. Tank pressure and jet flow rate are measured using a high accuracy pressure transducer and a Coriolis flow meter. Engineers embedded 43 RTDs in the system—on a rack in the fluid, on the inside and outside surfaces of the tank wall, on the VC and in the jet flow line—to measure fluid and wall temperatures with the precision and accuracy required for computational model validation.

ZBOT-1 will conduct self-pressurization tests under three modes—vacuum jacket heating, strip heating and simultaneous vacuum jacket and strip heating—in an attempt to simulate heat leaks from the environment, the support structure and both together. In order to have consistent data conducive for model validation, researchers will perform all self-pressurization tests under the same quantifiable conditions established during the pre-test tank preparation by intervals of constant temperature mixing and hold.

Researchers will also conduct pressure control studies, performed either from an elevated uniform temperature condition or from thermally stratified conditions following a self-pressurization run. Liquid will be drawn from the tank for the studies, passed through the heat exchanger in the FSU and then injected into the tank at a given flow rate, either at the average tank fluid temperature for mixing only and ullage penetration studies or at a prescribed subcooled level for active cooling pressure control tests.

Jet flow rates are varied from 2-25 cm/s spanning a range of jet Re numbers in laminar, transitional and turbulent regimes; and a range of Weber numbers covering no ullage penetration, partial penetration and complete ullage penetration and break-up. Figure 3 shows sample microgravity jet mixing and ullage penetration CFD simulations that were used to guide the design of the microgravity experimental test matrix.

Since particle injection and PIV will be carried out for the first time with this class of nonpolar fluids in microgravity, ZBOT-1 tests will be performed with and without particle injection to ensure the integrity of the primary temperature and pressure data is not compromised. The ZBOT-1 microgravity test matrix will thus consist of an array of 67 tests performed first without particle injection and PIV. Researchers will vary fill level, heating mode, vacuum jacket temperature and heater power during the self-pressurization runs, and vary fill level, jet speed and jet temperature during the pressure control and ullage penetration tests. After the completion of the first test array, researchers will inject particles and conduct 25 additional self-pressurization and jet mixing tests with PIV measurements.

The digital imaging and machine readable textual/numerical data from the experiments will be continuously downloaded. Both the raw experimental data and a set of reduced data analyzed and processed by the ZBOT science team will be stored together with CFD simulation results on the ISS Physical Sciences Open System Repository Server that is operated and maintained by NASA. Approximately one year after the end of ZBOT-1’s microgravity operations, NASA plans to release its data to the entire scientific and engineering community.

NASA is also planning two follow-on ZBOT microgravity experiments. ZBOT-2 is designated to fly to ISS in 2022. It will focus on studying the transport and kinetics effects of noncondensable pressurant gases on interfacial evaporation and condensation mass transfer rates during pressurization and pressure control. ZBOT-3 will follow at a still undetermined time. It will examine other active pressure control mechanisms such as the spray-bar droplet pressure control, with special attention devoted to delineating the details of droplet transport in microgravity and heat and mass transfer interactions between the droplets and the ullage during the spray mixing cycles.

References
1. J. Salzman, “Fluid management in space-based systems,” in Proceedings of the Engineering, Construction, and Operations in Space, 5th International Conference on Space, Vol. 1, 1996.
2. S. Barsi and M. Kassemi, “Investigation of Tank Pressurization and Pressure Control-Part I: Experimental Study,” in ASME Journal of Thermal Science and Engineering Applications, Vol. 5, No 2, 2013.
3. C. Panzarella and M. Kassemi, “On the validity of purely thermodynamic descriptions of two-phase cryogenic fluid storage,” in Journal of Fluid Mechanics 484, 2003.

Acknowledgement
Funding from the ISS Microgravity Physical Sciences Program, NASA HQ, tireless efforts by William Sheredy, ZBOT Project Manager, and John McQuillen, ZBOT Project Scientist, and CFD work by Sonya Hylton and Dr. Olga Kartuzova are gratefully acknowledged. ■