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
- 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.
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.