Researchers: Prof. Owen Smith, Prof. Ann Karagozian Former Researchers: Indy Lee (grad), Caesar Chi-Leung Mak (grad), Lauren Gleason (grad) Research Supported By:   NASA Dryden

Hydrogen leak detection has been a critical issue for many rocket engines in their devlopment and use. Above pictured are some of the engines that have required leak detection testing: the space shuttle, Linear Aerospike SR Experiment (LASRE), and the X-33.

	A schematic of how the silicon wafers are etched (note: holes and overall wafer sizes are not draw to scale).
	Shown above is an closeup photo of a circular orifice 70 micrometers in diameter fabricated and then used in this hydrogen leak detection expirement.

Hydrogen is widely in use in rocket propulsion systems, and as such, leakage of hydrogen from high pressure fuel tanks requires accurate quantification. Safety concerns have led to the practice of conducting leak tests with helium (an inert gas) and to try to infer the hydrogen leak rates from helium data, often employing assumptions of essentially isentropic flow processes and choked leak orifices. The present experimental study seeks to precisely quantify the relationships between hydrogen and helium leak rates for various types of leak, and at a range of pressures and temperatures. Simulated leak sources are fabricated by micromachining leaks or holes of prescribed shapes and cross-sectional areas in silicon wafers, utilizing the processes of photolithography and deep reactive ion etching. Dual thermal conductivity detectors are used to evaluate helium and hydrogen leak rates and to quantify differences in discharge coefficients among the various micro orifices. Based on this quantification, the standard helium signature test (HST) procedure is found to underpredict hydrogen leak rates, in some cases significantly, if the corresponding helium tests are conducted at much lower pressures than those at which hydrogen leak rates are sought.

Recent work on this project seeks to explore the effects of higher tank pressures and lower (cryogenic) temperatures, as well as new deep reactive ion etching (DRIE) techniques for machining smaller orifices.

Publications:

1. Hydrogen-Helium Leak Detection at Elevated Pressures and Low Temperatures, Gleason, L., Mak, C., Smith, O. I., and Karagozian, A. R., AIAA Journal, Vol. 47, No. 5, pp. 1303-1307, 2009.



2. Hydrogen-Helium Leak Rates from Micromachined Orifices, Lee, I., Smith, O. I., and Karagozian, A. R., AIAA Journal, Vol. 41, No.3, pp. 457-464, March, 2003.