ZBOT-1

Status

Complete

Summary

ZBOT-1 – the first in a series of three investigations – studies the self-pressurization and mixing destratification of cryogenic storage tanks in microgravity using a small-scale simulant-fluid experimental system.

The ZBOT flight hardware was delivered to the ISS aboard Orbital Cygnus flight OA-7, which launched on April 18, 2017 and was installed in the Microgravity Science Glovebox (MSG) facility by Astronaut Joe Acaba on September 19 and 20. System thermal and fluid characterizations started on September 24 and actual test runs began on October 1. All data and images were downloaded to the ground prior to hardware removal from the MSG and stowage, which occurred on December 1, 2017.

The ZBOT flight hardware was returned to Earth on SpaceX flight SpX-13 on January 18, 2018 and ultimately delivered to NASA Glenn Research Center (GRC) to be reconfigured for follow-on investigations.

Key Findings and Science Deliverables

• Provided first data on microgravity self-pressurization rate that was used to validate CFD models
• Showed that classic self-pressurization can be easily disrupted by nucleate boiling in micro-g as compared to 1g, due to thermal stratification in the absence of natural convection, changing heat transfer at the tank wall
• Revealed a non-intuitive and unexpected Jet-Ullage interaction, and ullage deformation and movement in microgravity defying previous CFD prediction that form the basis of current tank design
• Established that tank fluid flow, heat transfer, ullage movement and self-pressurization rate are all insensitive to high-frequency vibrational accelerations but significantly impacted by low-frequency engine thrust acceleration
• Demonstrated for the first time an unexpected and inadequately understood intense microgravity cavitation during subcooled jet mixing with significant implications for tank pressure control design for microgravity operations

Implication of ZBOT Findings for Propellant Tank Design

• CFD two-phase model validated by over 30 model validation test case studies
• Thermodynamic model predictions and ground-based data can be used as conservative estimates of the stationary (long term, undisturbed) tank self-pressurization rate needed for sizing tank insulation system
• Transient self-pressurization rates and pressure levels under different mission scenarios can only be provided to designers with high fidelity, validated ZBOT-CFD models
• Boiling will be prevalent in storage tanks in microgravity under heat fluxes that don’t lead to boiling in 1g
  • Use of metallic (steel and aluminum) tanks with perforated inner surfaces is advantageous where boiling will occur sooner and with minimal intensity and is predictable by Boiling Incipience models
  • Composite tanks with smooth inner surfaces can result in delayed explosive boiling that is hard to predict with pressure spikes that may be detrimental to the tank structural integrity
• Envisioned use of thruster accelerations to position the ullage for liquid-free venting or vapor-free liquid extraction is effective but during self-pressurization can lead to undesirably large pressure spikes
• Ullage-jet interaction is non-intuitive and the envisioned use of liquid jet to split the ullage and cool the tank wall may not be possible as envisioned by designers especially at higher fill levels
• Mixing without cooling causes destratification but cannot be effective for reducing the tank pressure
• ZBOT demonstrated that ZBO pressure control is feasible and effective in microgravity using subcooled jet mixing
• Unexpected ZBOT results demonstrated intense microgravity cavitation during subcooled jet mixing that is an important strike against this pressure control design option for Space applications

Strip Heater Self Pressurization Test 230 (0.11W, FL 82%) : Model Validation

Strip Heater Self Pressurization Test 230 (0.11W, FL 82%) : Model Validation

Model Validation Comparison of 2D & 3D Model Prediction to Microgravity SH & VJ Self Pressurization Results

Microgravity Strip Heating Self Pressurization – Localized Microgravity Boiling at higher Heat Fluxes

3D VOF Model Simulation of Jet Mixing & Ullage Penetration in Microgravity

Case 254, 90.62% fill ratio, 10 cm/s jet speed, T0 1 jet temperature

	TVJ = TT Vjet = 6 cm/s	FL = 70% T0 = 38 C
ZBOT Results – Microgravity Cavitation at the LAD during Pressure Control