Design of a Pressure-fed Gas System Operating at Supercritical Temperatures and Pressures
The purpose of the project is to replicate conditions found inside the reaction chamber of a nuclear thermal propulsion (NTP) rocket engine, thereby evaluating robust materials and construction techniques for future NTPs. The need to test materials exposed to hydrogen under combined high temperatures and pressures is crucial to determine their resistance to hydrogen attack.
The proposed test article is a SiC resistive heating element which would heat the hydrogen gas flowing at 5.6 g/s from 300 to 2400 K at nominal pressure of 1000 psia then cool it to below its auto-ignition temperature before it is vented to the ambient air. The experimental evaluation of the test article should validate the reliability of materials used in the construction of the pressure vessel. The pressure vessel houses a resistive heating element made from open-cell refractory carbide foam which pairs well with hot hydrogen gas due to its resistance to thermal shock. The enclosure to encapsulate the heating element is lined with an oxide coated rhenium tube capable of sustaining high thermal and structural loads, and the outer shell is made from Inconel 718. Rhenium is a robust material with excellent ductility, is non-reactive with hydrogen, and is creep-resistant at high temperatures. Inconel 718 has a high yield strength capable of handling high temperature applications.
Cooling the hydrogen gas requires designing a water-cooled nozzle to transport the gas to a heat exchanger. The design of the nozzle and its mechanical components involved analyzing the heat transfer through materials, predicting their structural integrity, and examining potential failure points. The 1-D steady-state heat transfer analysis is conducted to predict the inner and outer surface temperatures, heat flux, and fluid heat transfer coefficients. These parameters are considered in selecting the best candidate materials, copper and Inconel 718, to make the nozzle. To prevent gas leakage between interfaces of multiple components and joints, a careful selection of sealing techniques are implemented, including the use of bimetallic weldments and pressure-energized metal seals.
Although the proposed test article was never tested due to schedule and budget limitations, the documentation of its design and analysis is complete and the system is ready for manufacturing and testing. The long lead times to manufacture, to inspect, and to validate the vessel were underestimated in the project scheduling. The rental cost of the electrical equipment required to run the test under initial design conditions exceeded budget. As a solution to satisfy the temperature and budget requirements, halving the flow rate and decreasing the delivered electrical power by 48% are proposed.
The success of testing the pressure vessel at operating conditions would provide a physical and quantitative study on potential materials used on future NTP ground tests. The test would run for 5 minutes during which the strength of the materials weaken as a result of the diffusion of free carbon from their surfaces. Upon completion of the test, the performance of these materials would be evaluated for signs of macroscopic and microscopic surface effects on the test article.