Independent Verification of Cryogenic Propellant Feedline Cooling Model Using Thermodynamic Vent System
West Virginia University, NASA Glenn Space Academy 2011
NASA Glenn Research Center
The Thermodynamic Vent System (TVS) is a passive cooling method that efficiently vents propellant vapor while cooling thruster feed lines, thus taking advantage of the thermal capacity of the fluid. To improve the performance of the TVS, a tool was developed to predict the fluid properties of this cooling loop which is coupled with the propellant. However, when this tool was applied to test data, it predicted a pressure drop much smaller than what was experimentally observed. For this reason, it was requested that the theoretical calculations be independently verified. The approach used in this verification was the Two Phase Separated Flow model, a different model than used in the tool. The magnitudes of the determined pressure drops were similar to what was computed in the original tool, thus this investigation verified the original calculations, but concluded that the experimental data was of poor quality. It was also determined that the resulting pressure drop was very sensitive to varying temperature inputs, indicating the importance of obtaining accurate fluid properties. Driving factors for the poor data were the uncertainty of the fluid state due to poor sensor readings and potential problems at the orifice preceding the TVS line.
A major component of a spacecraft using chemical propulsion is the cryogenic propellant storage system. Due to low boiling points of the liquid propellants used, storage tanks require cryogenic cooling. However, maintaining low temperatures is difficult, as there are many opportunities for heat transfer from launch ascent through orbit. This heat transfer ultimately causes the propellant to boil, produce vapors, and thus pressurize the tank.
There exist numerous methods to reduce the pressure of the storage tank; however, the most basic is to simply vent the vapor to an appropriate level. Rather than allowing the vented gas to go to waste, systems have been designed to make use of it. For example, it is sometimes routed such that its exhaust performs small attitude adjustments while in orbit. The Thermodynamic Venting System (TVS) also can be used prior to exhaust, utilizing its low temperature to cool the motor feed lines as a counter-flow heat exchanger, improving motor efficiency.
The Cryogenic Fluid Management (CFM) Project currently has an analytical tool with the capability of predicting heat leak in propellant lines using TVS. This tool, built in Microsoft Excel employs a graphical user interface that calls functions from the NIST REFPROP program. The tool can also calculate the TVS temperature distribution and differential pressure from inputs such as: insulation type, technique, line supports, penetrations, and instrument additions. This tool is very beneficial in designing TVS systems because results for many different systems can be found quickly.
Also concerning tank pressurization is the efficiency improving technique; propellant scavenging. Often times, a spacecraft cannot utilize the last percentages of the propellant in its storage tank. To do so, the helium that is used to pressurize the feed lines is pumped into the tank, causing the propellant to boil and turn to vapor, which is then usable. Because the system is multi-phase and multi-element, it is difficult to model and predictions for the final pressure are generally inaccurate.
As a Research Associate (RA), I have been given multiple tasks and problem statements. The foremost of which is to resolve a pressure discrepancy between experimental and theoretical results in a test of the TVS. Calculations using the Lochart-Martinelli correlation predict a very small differential pressure ( < 1 psid); however the test results show a larger differential ( ~19 psid). To accomplish this, I will review the calculations performed, identify the problem, and then address the issue causing this discrepancy.
The second objective of this summer is to find lightweight, high performing cryogenic feed line thermal control concepts. This will be accomplished by utilizing the current tool to compare thermal control concepts. Parametric variables will be identified then studied using the tool so that an optimized solution can be found.
The final objective is to apply a new tank pressurization model to what is currently used on the propellant scavenging project. This model predicts a resulting pressure that is less than three orders of magnitude than the experimental value. The new calculations will employ Barsi’s Homogenous Three Lump Model and if they produce more accurate results, this model will be incorporated into the current tool.
Expected Outcomes and Deliverables:
- Modifications to the current tool to resolve pressure discrepancy in TVS lines.
- Results of thermal control concept optimization with documentation.
- Comparison of results in propellant scavenging project and potential integration of new code.
- Poster presentation at the end of internship summarizing summer work.