Shock tunnels afford a means of generating hypersonic flow at high stagnation enthalpies, but they have the disadvantage that thermochemical effects make the composition of the test flow different to that of ambient air. The composition can be predicted by numerical calculations of the nozzle flow expansion, using simplified thermochemical models and, in the absence of experimental measurements, it has been necessary to accept the results given by these calculations. This note reports measurements of test gas composition, at stagnation enthalpies up to 12.5 MJ.kg(exp -1), taken with a time-of-flight mass spectrometer. Limited results have been obtained in previous measurements. These were taken at higher stagnation enthalpies, and used a quadruple mass spectrometer. The time-of-flight method was preferred here because it enabled a number of complete mass spectra to be obtained in each test, and because it gives good mass resolution over the range of interest with air (up to 50 a.m.a.)

Skinner, K. A.

Stalker, R. J.

English

NASA Technical Reports Server

i N96-11691
MASS SPECTROMETER MEASUREMENTS OF TEST GAS
COMPOSITION IN A SHOCK TUNNEL Q &
K.A. Skinner* and R.J. Stalked
Department of Mechanical Engineering, University of Queensland ^
INTRODUCTION
Shock tunnels afford a means of generating hypersonic flow at high stagnation enthalpies, but
they have the disadvantage that thermochemical effects make the composition of the test flow-
different to that of ambient air. The composition can be predicted by numerical calculations of
the nozzle flow expansion, using simplified thermochemical models and, in the absence of
experimental measurements, it has been necessary to accept the results given by these calculations.
This note reports measurements of test gas composition, at stagnation enthalpies up to
12.5 MJ.kg'1, taken with a time-of-flight mass spectrometer. Limited results have been obtained
in previous measurements0}. These were taken at higher stagnation enthalpies, and used a
quadruple mass spectrometer. The time-of-flight method was preferred here because it enabled
a number of complete mass spectra to be obtained in each test, and because it gives good mass
resolution over the range of interest with air (up to 50 a.m.a.)
EXPERIMENTS
The experiments were conducted in the free piston shock tunnel T4 at the University of
' Postgraduate Student
f
 Emeritus Professor of Space Engineering, Associate Fellow AIAA
Department of Mechanical Engineering
University of Queensland
Brisbane 4072
AUSTRALIA
https://ntrs.nasa.gov/search.jsp?R=19960001683 2020-06-16T06:54:41+00:00Z
2Queensland*2', using a helium-argon mixture as driver gas. The shock tube, of 10 m length and
75 mm diameter, was operated in the shock reflected mode, and supplied shock heated air to a
contoured hypersonic nozzle with a throat diameter of 25.4 mm, and an exit diameter of 261 mm.
As shown in fig l(a), the mass spectrometer sampled the flow from a point on the nozzle
centreline, well within the test cone of the nozzle flow field.
The stagnation enthalpy was calculated for equilibrium air from the shock speed and initial shock
tube filling pressure, with an isentropic expansion to the measured nozzle reservoir pressure of
14.0±2.5 MPa. The estimated accuracy of the stagnation enthalpy was from +4% to -8%. To
avoid problems with driver gas contamination of the test flow, the test time was confined to a
period from 0.5 to 1.0 milliseconds after initiation of flow in the test section, and the stagnation
enthalpy was limited to 12.5 MJ.kg"1.
The mass spectrometer is described in ref.3. Essentially, it sampled the flow through a series of
three conical skimmers to form a molecular beam. This was bombarded by a 250 e.V. electron
beam for 200 nanoseconds every 55 microseconds, and each time a pulse of ions was produced
which passed into the 1 m long drift tube, and hence to an electron multiplier detector at the end
of the tube. Since the time of arrival of the ions depended on their mass, a mass spectrum of the
type shown in figs l(b) and l(c) was obtained every 55 microseconds, and was recorded by a 50
MHz digital oscilloscope.
Peaks of N2, 02, N and 0 are evident in the spectrum of fig l(b), as well as some residual H20.
The area under each peak is proportional to the number of particles of that particular mass to
reach the detector. There is clearly a problem with overlap of the peaks of N2, NO and 02, and
this particularly effects measurement of the NO peaks. Therefore some tests were made in which
fewer spectra were taken, but the sampling rate of the oscilloscope was increased. The resolution
was therefore improved, as shown in fig l(c), and these spectra were used for measurements of
the NO peak.
RESULTS AND DISCUSSION
The ratio of the measurements of peak sizes is presented in fig. 2 for Oj/N2, NO/N2 and 0/02. Each
of the points plotted is the mean of a number of readings during a test, and therefore may be
regarded as the mean of a statistical sample. The error bars indicate the standard deviation of that
mean.
The measurements are compared with theoretical curves for the relative peak sizes. These were
obtained by first performing numerical calculations*4' of the inviscid nonequilibrium steady
expansion through the shock tunnel nozzle to yield the freestream species concentrations. These
were then used to obtain values for the relative size of the mass spectra peaks. The effect of mass
separation in the molecular beam was assessed by testing in the shock tunnel with known
mixtures of nitrogen and helium, and nitrogen and argon. This indicated that the relative
enhancement of the molecular species considered was less than 10%. The relative number of ions
produced was determined from the ionization cross-sections given in table 1, and the relative
efficiency with which ions were collected was taken as unity. This was checked by operating the
instrument "ex runnel" (ie. outside the shock tunnel), with the first of the three skimmers replaced
by a solenoid valve, which delivered a short pulse of room air to the mass spectrometer. The
measured ratio of the 02 peak area to N2 peak area was 0.30 ± 0.02 which, allowing for the
uncertainty in ionization cross sections, compared satisfactorily with the expected value of 0.25.
Table 1. Electron Impact Ionization Cross-Sections at 250 e.V.
Process
02 + e -> 02+ + 2e
0 + e -> 0+ + 2e
N2 + e -> N2* + 2e
NO + e -»• N0+ + 2e
Cross- Section (cm2)
1.57 x lO'16
1.08 x 10-16
1.68 x 1006
1.89 x 10'16
Error (%)
13
5
8
20
Reference
5
6
7
8
4To calculate theoretical values for the peaks of atomic oxygen, the combined effect of mass
separation and ion collection efficiency was obtained from the helium/nitrogen and nitrogen/argon
results by assuming that these effects varied linearly with the mass ratio of the two species,
yielding a factor of 2.0±0.4 for enhancement of 0 with respect to 02. This was applied to the
predicted numerical values for relative concentrations of 0 and 02 and, taking account of the
ionization cross-sections, the theoretical curve in fig. 2(c) was obtained. Because of uncertainties
in mass separation and ion collection efficiencies, as well as in ionization cross-sections, a
possible error attaches to all the theoretical curves. The limits of this are indicated by the broken
lines on either side of each curve.
It will be noted that the effect of dissociative ionization of the 02 molecule in the mass
spectrometer, with production of ionized 0 atoms, has not been taken into account in obtaining
the theoretical curve of fig. 2(c). This is because the experimental results in the tunnel show the
ratio of peak sizes increasing with enthalpy from a value near zero, and this is not consistent with
the presence of a substantial number of 0 ions due to dissociative ionization. On the other hand,
it must be observed that results of the "ex tunnel11 tests shown in fig. 2(c) demonstrate that
substantial dissociative ionization does indeed take place in air which is supplied from a room
temperature source. It may be speculated that this difference is due to thermal excitation of the
02 molecules in the tunnel flow, leading to an increased velocity spread and consequent lowered
collection efficiency of the 0 ions resulting from dissociative ionization. However, until this
apparent anomaly is resolved, some doubt must attach to the experimental results of fig. 2(c).
Therefore, not withstanding the remark made below concerning their validity, their worth is
mainly in indicating the stagnation enthalpy at which the rise in 0 atom concentration due to free
stream dissociation takes place.
The overall ratio of the number of atoms of oxygen in any form to nitrogen in any form can be
obtained from the results in fig. 2. If the "ex tunnel11 result in fig. 2(a) is used to generate a
calibration factor for the Oj/Nj ratio, values of 0.33±0.04, 0.27±0.03 and 0.30±0.04 are obtained
5for the overall ratio at stagnation enthalpies of 8.3 MJ kg'1, 10 MJ kg"1 and 12.5 MJ kg'1
respectively. The expected value is 0.27±0.05, the quoted error limits resulting from residual
uncertainties in the ionization cross-sections, together with the mass separation and ion collection
efficiencies, after the calibration for the 02/N2 ratio has been taken into account. Thus, the values
for the overall oxygen/nitrogen ratio fall within the quoted limits of error, and provide
confirmation of the experimental measurements.
It is worth remarking that, at 12.5 MJ.kg"1, the measured 0/02 value in fig. 2(c) contributes a
substantial 0.08 of the total oxygen/nitrogen ratio of 0.30, thereby providing an indirect
confirmation of the 0/02 measurement.
It can be seen that the experimental measurements in fig. 2 generally fall outside the theoretical
limits indicated by the broken lines. In fig. 2(a) the proportion of molecular oxygen exceeds
theoretical limits as the stagnation enthalpy is increased. This is consistent with the results in fig.
2(c), which shows the proportion of atomic oxygen remaining at low levels for much higher
enthalpies than predicted. The proportion of nitric oxide is shown in fig. 2(b), and is seen to be
in excess of predicted values, at least for the range of stagnation enthalpies covered by the results.
The numerical model(4) on which the theoretical curves are based gives free stream compositions
which are consistent with those given by other numerical models(9). Therefore, even when
allowance is made for the experimental uncertainties
 t there are clear discrepancies between the
theory of non-equilibrium nozzle flow and the results of these experiments, indicating a need for
further experimental and theoretical work. Until these discrepancies are resolved predictions of
the composition ofxtest section flows in high enthalpy facilities should be treated with caution.
Acknowledgments
The authors gratefully acknowledge the support received from the Australian Research Council
and through NASA grant NAGW-674.
6REFERENCES
1. Crane, K.C.A., and Stalker, R.J, "Mass-spectrometric Analysis of Hypersonic Flows"
J. Phys. D: Appl. Phys., Vol 10, 1977, pp 679-695.
2. Stalker, R.J., "Recent Developments with Free Piston Drivers". Proceedings of the 17th
Int. Symp. on Shock Waves & Shock Tubes. AIP Conference Proceedings 208, New
York, 1990, pp 96-105.
3. Skinner, K.A., and Stalker, R.J. "A Time-of-Flight Mass Spectrometer for Impulse
Facilities" AIAA Journal, Vol 32, No 11, 1994, pp 2325-2328.
4. Lordi, J.A., Mates, R.E., & Moselle, J.R., "Computer Program for the Numerical Solution
of Non-Equilibrium Expansions of Reacting Gas Mixtures", NASA CR-472, May 1966.
5. Krishnakumar, E., & Srivastara, S.K., "Cross-Sections for Electron Impact lonization of
O2", Int. J. Mass. Spec. Ion Proc., Vol 113, 1992, pp 1-12.
6. Bell, K.L., Gilbody, H.B., Hughes, J.G., Kingston, A.E. & Smith, F.J. "Recommended
Data on the Electron Impact lonization of Light Atoms and Ions". J. Phys. Chem. Ref.
Data Vol 12, 1983, pp 891-916.
7. Krisnakumar, E., & Srivastava, S.K., "Cross-Sections for the Production of N+2, N+, N22+
and N2 by Electron Impact on N2", J. Phys. B: At. mol. Opt. Phys., Vol 23, 1990, pp 1893-
1903.
8. Rapp, D. & Englander-Golden, P., "Total Cross-Sections for lonization and Attachment
in Gases by Electron Impact. I. Positive lonization"., J. Chem. Phys., Vol 43, 1965,
pp 1464-1479.
9. Sagnier, P., & Marraffa, L., "Parametric Study of Thermal and Chemical Nonequilibrium
Nozzle Flow" AIAA Journal, Vol 29, No. 3, 1991, pp 334-343.
LIST OF FIGURES
Fig I. Mass Spectrometer. (a) Experimental arrangement
(b) Typical mass spectrum
(c) Improved resolution mass spectrum.
Fig 2. Relative Size of Mass Peaks. (a) Molecular oxygen Vs molecular nitrogen
(b) Nitric oxide Vs molecular nitrogen
(c) Atomic oxygen Vs molecular oxygen.
(a)
3 conical
skimmers
A
Electron gun and Ion
Ion acceleration drift
electrodes tube
\
Mach 6 contoured
nozzle
T4 test
section
Dump
volume
1 metre
(b)
,N2
(c) N2
N
O2
MffJ
—^
02
IS 20 25 30 25 30
Time after Electron Gun pulse (microsec)
Fig 1. Mass Spectrometer. (a) Experimental arrangement
(b) Typical mass spectrum
(c) Improved resolution mass spectrum.
NO/N,
COd)
N
•Hin
C^O
<1>
CM
U-t
O
0.3 -
0.2
0.1
(b)
o
0/0,
0
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
(c)
, Dissoc,
loniz.
0 5 10 15
Stagnation Enthalpy (MJ/kg)
6 Experiment o ex tunnel Theory
Fig 2. Relative Size of Mass Peaks. (a) Molecular oxygen Vs molecular nitrogen
(b) Nitric oxide Vs molecular nitrogen
(c) Atomic oxygen Vs molecular oxygen.


Mass spectrometer measurements of test gas composition in a shock tunnel

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