Radiative Interaction Between Driver and Driven Gases in an Arc-Driven Shock Tube

6 Pages·2001·0.22 MB·English

by  Bogdanoff, David W.

Abstrac: of a paper proposed for _"/'/_9/4_'/
The 23rd International Symposium on Shock Waves, Arlington, TX, July" 2001
Radiative Interaction Between Driver and Driven Gases
in an Arc-Driven Shock Tube
David W. Bogdanoff and Chul Park
ELORET Corporation, Sunnyvale, CA
Summary
An electric-arc driven shock tube was operated with hydrogen as the driven gas and
either hydrogen or helium as the driver gas. Electron density was measured behind the
primary shock wave spectroscopically from the width of the Beta line of hydrogen. The
measured electron density values were many times greater than the values calculated by
the Raz_ne-Hugoniot relations. By accounting for the radiative transfer from the driver
gas to the driven gas, the measured electron density values were numerically recreated.
Introduction
One well known method of producing a strong shock wave in a shock tube is to use
an electrically-heated driver (see, e.g., Ref. 1). At NASA Ames Research Center, one such
shock tube, with a driven-section internal diameter of 10 cm, is in operation. The driver
section, also of 10 cm internal diameter and 75 cm length, is filled either with hydrogen or
helium. An arc is initiated in the driver by exploding a tungsten wire. The mass of the
tungsten wire used is approximately equal to that of the driver gas.
Tests were made in this shock tube with the intention of studying the radiation phe-
nomenon occurring in the shock layer over the Galileo Probe vehicle which entered the
atmosphere of the planet Jupiter in 1995. The tests were motivated by the fact that the
surface recession of its heatshield was different from the prediction. 2 It was hoped that
the shock layer condition for the Galileo Probe would be simulated by the flow behind the
primary shock wave in the shock tube.
Method of Measurement and Analysis
Spectroscopic observation was made through a window in the driven section of the
shock tube. A 0.25 meter focallength McPherson Model 238 grating spectrograph, equip-
ped with a CCD array at itsexitplane, and two narrow band-pass monochromators were
used. Spatially-resolved,time-frozenspectral snap-shots were obtained with the spectro-
graph, and time-resolved monochromatic intensitieswere obtained at 486.13 and 488.69
nm using the two monochromators. Initiallyu,sing the well-known theory of Stark broad-
ening ofhydrogen lines,a chartwas constructed which relatesthe ratioof intensitiesat the
two wavelengths with the width ofthe hydrogen Beta lineat 486.13 nm. By comparing the
measured intensit.v ratio at these two wavelengths with the chart, the width of the Beta
line was deduced. Electron density was then deduced from the width of the line.
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A numerical computation was maid to determine the state of :he driven gas accounting
for the radiative transfer from the driver gas. The driver gas was assumed to emit a black
body radiation at an arbitrarily assigned temperature. Regardless of whether hydrogen
or helium is used as the driver gas, the main radiator in the driver section is likely to
be metallic tungsten. Tungsten has a large number of lines, and therefore the spectrum
of the radiation emitted by the driver gas is believed to be nearly that of a black body.
The one-dimensional radiative transfer phenomenon was calculated assuming the gas to be
inviseid and in equilibrium. The spectrum of hydrogen is represented at 400 wavelength
points.
Thue_ dte_ pems_ d s_tJoe E,mk:_'u_
Results
In Figures l(a) and (b),typicaloutputs from the monochromators axe shown. The
electron density va/ues determined are presented in Figure 2. The figure shows a con-
siderable degree of scatter.The equilibrium temperature corresponding to the measured
electrondensity isin the range between I0,000 and 12,000 K.
Figure 3 shows the wavelength resolutionof thespectrum used in theradiative transfer
calculation.As shown that the spectrum isrepresented by the present method in sufficient
detail.
In the calculationof radiativetransfer,the black-body temperature of the driver gas
was varied arbitrarilyuntilthe calculationagreed with the measurement. In Figure 4, the
electrondensity values so calculatedarecompared with themeasured values. As indicated
in the figure,the calculated values areforthe black body temperature of 18,000 K. The
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Figure 3. Typical spectrum of hydrogen used in radiative transfer calculation.
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Distance behind shock, cm
Figure 4. Comparison between the measured and the calculated electron densities. As-
sumed driver black body ternpeature - 18,000 K.
Ra.nkine-Hugoniot value is shown also for comparison. As seen in the figure, the calculated
values agree with the measured values. However, the Ra_ine-Hugoniot value iq three
orders of rnagnitude smaller th_'_ the measured value. The static enthalpy of the flow
with black-body radiation is 550 MJ/kg While that determined by the Rankine-Hugoniot
relation is only 135 MJ/kg.
The relatively large scatter in Fig. 2 can be attributed to the fluctuatioxt iz_ the
radiation intensity emitted by the driver gas.
The radiation absorbed by the driven gas is calculated to be of the order of 1/100 of
the total radiation emitted by the driver.
Conclusions
When the driver is heated by electric arc discharge, the radiation emitted by the driver
gas can heat the driven gas to a great extent, thereby increasing the entha2py of the driven
gas greatly.
Acknowledgement
The authors wish to acknowledge the support provided by NASA Ames Research
Center through contract NAS2-99092 to ELORET.
References
I. Sharma, S. P., and Park, C., "Operating Characteristics of 60-cm and 10-cm
ElectricArc-Driven Shock Tubes, Part 1: The Driver" Journal of Thermophysics and
Heat Transfer, Vol. 4, No. 3,July 1990,pp. 259-265.
2. Milos, F. S., "GalileoProbe Heat Shield Ablation Experiment," Journal of Ther-
mophysics and Heat Transfer,Vol. 34,No. 6,November-December 1997, pp. 705-713.