Spectroscopic study of high enthalpy flow around a blunt body in arc-heated wind tunnel

Energy Convers. Mgmt Vol. 38, No. 10-13, pp. 1177-1186, 1997

Pergamon PII: S0196-8904(96)00147-1

© 1997 Elsevier Science Ltd All fights reserved. Printed in Great Britain 0196-8904/97 $17.00 + 0.00

SPECTROSCOPIC STUDY OF HIGH ENTHALPY FLOW AROUND A BLUNT BODY IN ARC-HEATED WIND TUNNEL N. HAMAMOTO,

I H . K A W A Z O E , 2. Y. N A K A M U R A , 3 N . A R A P K. K I T A G A W A 4

and

1School of Engineering, 3Department of Aerospace Engineering, 4Research Center Advanced Energy Conversion, Nagoya University, Furoh-cho, Chikusa-ka, Nagoya 464-01, Japan, and 2Department of Mechanical Engineering, Tottori University, 4-101 Koyama-Minami, Tottori 680, Japan

Abstract--High enthalpy argon gas flow was made by an arc-heated wind tunnel to analyze physical phenomena around a blunt body. The flow fields with and without an axisymmetric blunt body were studied by a spectroscopic approach. Number density of excited argon was measured spatially. It is found that the highest number density of excited argon atom was obtained at the stagnation point at the body, which was 2.4 times as high as the uniform flow without body for the atom with 4p' [1/2] level. Furthermore, it was confirmed that the dark region exists just in front of the shock wave, where self-luminosity of the high enthalpy flow is reduced. In the region called "dark space", the number density of atom with higher energy level 4d and 5p was found to relatively decrease compared with that of lower level 4p'. © 1997 Elsevier Science Ltd. Arc-heated wind tunnel

Reentry

High enthalpy flow

Spectroscopic measurement

NOMENCLATURE kb = kf ---N= No = N, = Tc = T~0=

Backward reaction rate constant (m3/s) Forward reaction rate constant (m6/s) Total number of argon atom (l/era 3) Number of argon atom at stagnation point (1/cm3) Number of argon atom with m'th energy level (1/cm3) Electron temperature (K) The characteristic temperature of the first ionization of argon (K)

I. INTRODUCTION

An arc-heated wind tunnel has been employed to develop new thermal protection shield materials for aerospace vehicle at reentry and to investigate catalysis on the body surface due to nonequilibrium high enthalpy flow around the vehicle [1]. On the other hand, communications between spaceplane and ground facilities are cut off by ionized gas in a shock layer between shock wave and body, so called "black out" and aerodynamic forces acting on the vehicle under such a condition are also not adequately known. Furthermore, electro-magnetic thruster is considered to be one of promising engines for future spaceplane, and its components are quite similar to those of arc-heated wind tunnel [2]. Therefore, it is interesting and useful to examine the phenomena of high enthalpy flow produced by arc-heated wind tunnel [3]. In arc-heated wind tunnel, high enthalpy flow can be easily obtained, which is purer than that of chemical combustion type. Furthermore it can produce steady flow for a relatively long time compared with shock tunnel. However, it needs a high electrical power supply to make such a *Corresponding author. 1177

1178

HAMAMOTO et al.:

ENTHALPY FLOW AROUND A BLUNT BODY

Test

~

Diffuser

Vacuum Tank

/indow

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l

'Lens

-

Valve

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I

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I

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condition as that in reentry. Most of the arc-heated wind tunnels in the world have been used mainly for developing heat shield materials, and only a few have been utilized to study this kind of complex high enthalpy flow [4]. Therefore, the flow characteristics with high enthalpy around a space vehicle are not adequately made clear. We designed and made an arc-heated wind tunnel at our laboratory and the flow field, especially inside the conical nozzle, was investigated by measuring pressure distributions and electron number density with Langmuir probe [5]. In this study, characteristics of high enthalpy argon flow produced by an arc-heated wind tunnel was experimentally studied with a spectrometer. The number density distribution of argon atom with 4p', 4d and 5p electron energy levels was measured along the stagnation line of a blunt body which was located along the centerline of the wind tunnel and was cooled from inside by water. The flow characteristics without body was also measured. It has been pointed out that a dark area is formed in front of the shock wave in plasma flow, which is called "dark space" [6, 7]. A previous study [8] explained the reason why the dark space takes place as follows. Production of a neutral atom with high electron energy level plays an important role in self-luminosity. Therefore, this high energy neutral atom must be produced by the excitation due to heating up or the recombination of free electron with ionized atom. However, when the electron temperature increases, the rate of recombination decreases because the ionization reaction becomes predominant. In this research, our objectives is to investigate the behavior of the neutral argon atom and ionized one.

Table 1. Specifications of arc-heated wind tunnel Heater type Cathode Anode Gas Electrical power Nozzle type Entrance diameter Exit diameter Half angle Mach number Vacuum tank

Constrictor tungsten with thorium copper argon max. 12 kW conical 10 mm 62 mm 15° 8.3 50 m 3

HAMAMOTO et al.: ENTHALPY FLOW AROUND A BLUNT BODY

r t

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Fig. 2. Detailed arc-heated part. 2. E X P E R I M E N T A L APPARATUS

The arc-heated wind tunnel at our laboratory was used for this study, which is shown in Fig. 1 and the specifications are listed in Table 1. Figure 2 shows the arc-heated part in detail. The arc-heated part is a constrictor type and the tnngsten-thorium bar with a diameter of 10 mm is used as cathode, while the anode has a shape of ring which is composed of copper. Argon is employed as a working fluid gas and supplied with swirling motion into the upward plenum chamber to get stable uniform flow in thearc-heated portion. The test section is attached to the conical nozzle and the vacuum tank with the volume of 50 m ~ was set downstream. The present arc-heated wind tunnel has an electrical power supply of 12 kW, where argon gas was heated by arc in a constrictor with 50 mm in length and 10 ram in internal diameter. Argon plasma flow was accelerated up to Mach number of 8.3 via a conical nozzle, whose diameter ratio at inlet and exit is 10/62 and its length is 97 ram. At the test section a sphere-cylinder model made of copper with a diameter of 10 mm is placed along the center line of the wind tunnel. The radiation from the gas around the body was measured through an observation window via a spectrometer (Jarrel Ash). This spectrometer can be arbitrarily traversed in three-dimensional space. Experimental conditions are listed in Table 2. The total pressure in the plenum chamber and the initial ambient pressure in the vacuum tank are 255 and 0.2 torr, respectively. The mass flow rate of argon gas was 2.7 g/s. Therefore, total temperature and total enthalpy become 2450 K and 1.3 MJ/kg by assuming isentropic condition with a very small degree of ionization. 3. RESULTS

3.1. Reproducibility of arc-heated flow In our previous study, the present arc-heated wind tunnel has a flow separation in the conical nozzle after the high enthalpy flow is established, that is, about one and a half minutes after the Table 2. Experimental conditions Electric current 300 A Voltage 30V Ar mass flow rate 2.7 g/s Pressure in plenum chamber 255 ton" Initial ambient pressure 0.2 torr Total temperature 2450 K Total enthaipy 1.3 MJ/kg Heat-up efficiency 40%

1180

HAMAMOTO et al.:

ENTHALPY FLOW AROUND A BLUNT BODY

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Vacuum Tank Pressure (torr) Fig. 3. Time variations of radiation intensity by excited argon atom.

Fig. 4. Luminous argon gas flow around the blunt body.

flow starts and this separation moves upwards in the nozzle [5]. Therefore, the flow changes with time and its reproducibility have to be checked for producing the results from several measurement runs. The reproducibility of the arc-heated flow was examined by the spectrometer, where the blunt body was removed from the flow. Time variations of radiation intensity from excited neutral argon atom with the wave length of 515.14 nm are shown in Fig. 3 for four cases. The horizontal axis is the ambient pressure in the vacuum tank. The reproducibility is found to be sufficient and steady flow can be assumed until the vacuum tank pressure becomes 1.2 torr. After that, the radiation intensity suddenly increases because a flow separation occurs in the conical nozzle and a barrel shock with high density plasma moves upstream. Therefore, the period before the occurrence of the separation is proper to sample spectroscopic data, which corresponds to 1 min at least.

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1182

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ENTHALPYFLOW AROUND A BLUNT BODY

al.:

Table 3. Measured neutral and ionized argon emission spectral lines Statistical Wavelength (.~) Transition Energy (cm-t) weight Neutral argon atom 6965.43 4s[3/2]°-4p'[l/2] 107,496 3 6752.84 4p[1/2]-4d[3/2]° 118,907 5 4702.32 4s'[l/2]°-5p[1/2] 116,660 3 Ionized argon atom 4609.56 4s'[5/2]-4p'[7/2] 170,530 8

Transition probability 0.067 0.0201 0.00113 0.91

Photograph of high enthalpy flow heated by arc around the blunt body is shown in Fig. 4, which is luminous owing to excited argon atom. The argon gas flows from left to right as shown in Fig. 5, where the top of the blunt body was located 60 mm downstream from the nozzle exit in the test section and the body is axisymmetric with a diameter of 10 mm. Figure 5 indicates the coordinate system; the horizontal x-axis represents the distance from the stagnation point towards the downstream along the center line of the wind tunnel and the y-axis is perpendicular to the x-axis from the top of the blunt body, that is the stagnation point. In the Fig. 4, the shock wave is clearly observed in front of the blunt body and a slightly dark layer can be seen along ahead of the shock wave. This is the dark space. Figure 6 shows the spectral intensity of excited argon plasma flow without a blunt body. It indicates that all spectra being in some magnitude come from neutral argon atom with the energy level ranging from 4d to 9d. Among them the excited argon with an energy level of 5p is dominant. Therefore, these excited neutral argon atoms are considered to cause the flow luminosity. The excited argon emission spectral lines measured by the spectrometer are listed in Table 3. The flow luminescence in the Fig. 4 is essentially caused by the transition from high energy levels, that is, from 5p and 4d. Therefore, the spectral lines of argon corresponding to 4p'[1/2], 4d[3/2] °

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HAMAMOTO et al.:

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and 5p[1/2] were selected for the present measurement, which have wave lengths of 6965.43, 6752.84 and 4702.32/~, respectively. An ionized argon line was also listed to discuss about the phenomena near a shock wave later. The variations of neutral argon number density, which is normalized by the number density at the stagnation point, are shown in Figs 7-9 for excited levels of 4p'[1/2], 4d[3/2] and 5p[1/2]. In Figs 7 and 8, the results without a blunt body are also presented for comparison. The argon number density with a low energy level of 4p' gradually increases up to x = - 1 5 mm and then becomes constant up to x = - 3 mm. However, it suddenly increases from thispoint because of the existence of the shock wave, the location of which corresponds to Fig. 4. On the other hand, the variation without a blunt body slightly decreases from x = - 7 mm. Although the variation of argon number density for high energy levels of 4d and 5p is roughly similar to that of 4p', the different behavior can be found just before the shock wave. The number density decreases definitely in the region between x--- - 1 0 mm and - 3 mm. Figure 10 indicates the ratio of argon number density at the energy level of 5p to that of the 4p' level, which was calculated from the results of Figs 7 and 9. The ratio begins to decrease at x = - 1 0 mm, which indicates that the number of atom with excited electron to 5p[1/2] relatively decreases compared with 4p'[1/2] in this region. Since the radiation from argon atom which is predominant in the luminosity of the high enthalpy flow is mainly due to the transition from 5p, this region produces a dark region and coincides with the dark space observed in Fig. 4. The number ratio for 5p and 4p' energy levels has discontinuity between x = - 3 mm and - 2 mm, as shown in Fig. 10. Therefore, the excited atom is affected by passing through the shock wave near the body. Small change related with this can also be seen in Fig. 9. The electronic excitation temperature can be considered to increase through the shock wave as the translation temperature. Furthermore, these temperatures will rise toward the stagnation point. The number density of neutral excited argon atom with high electron energy level increases consequently as shown in Figs 7-9. However, the increase rate of argon with lower energy is much higher than 25 20 @@ • O

5 0 -30

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0 of 5p and that of the 4p'.

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HAMAMOTO et

ENTHALPY FLOW AROUND A BLUNT BODY

al.:

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y/D L5

1.0

0.5

0

r./7 =45 Fig. 11. Electron temperature counters [9]. Electron temperature is normalized by the electron temperature at the uniform flow Te~ --- 181 K. those at high energy levels like 5p and 4d. Owing to this, the number ratio of Fig. 10 decreases towards the stagnation point. 4. D I S C U S S I O N S The dark space appears just in front of the shock wave, because the number density of the excited argon with 5p-level energy relatively decreases here. The reason for this decrease might be to promote the backward reaction in the following chemical reaction when neutral, ionized argon and free electron exist, k, Ar + + e- + e - ~ A r + e-. kb

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HAMAMOTO et al.: ENTHALPYFLOW AROUND A BLUNT BODY

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A

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4650

Wavelength (A) Fig. 13. Spectral line profile of excited argon at x = -0.5 ram. Since electron temperature increases in the shock layer and electron has high heat conductivity, heat is conducted by electron in the upstream direction beyond shock wave, leading to increase in electron temperature upstream of shock wave. This was confirmed by our previous numerical study as shown in Fig. 11 [9]. In the dark space region, the backwards reaction in the above chemical reaction is promoted owing to high electron temperature. The reason is that the reaction rate constants kf and kb are expressed as follows [10]. kr-

4

x

10 -9

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(1)

4 . 8 x 10i~_,,)3/: exp (~_L~ x lO_6[m3/s]

(2)

kb = 7.3 (5.556 x

where T~o= 1.831 x 105 K is the characteristic temperature of the first ionization of argon, kf, kb are functions of electron temperature To and their profiles are shown in Fig. 12. When electron temperature increases, ionization is promoted, so that more neutral argon atoms with 5p-level 1 0.9 0.8

0.7 o0.6

.!

~0.5 Z 0.4 0.3 0.2 0.I 0

-1

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1186

HAMAMOTO et al.: ENTHALPY FLOW AROUND A BLUNT BODY

become argon ion and electron. Thus, the number density of neutral argon atom causing flow luminosity is reduced upstream of the shock wave and produces the dark space. Because the number density of neutral argon atoms excited to higher energy level increases in a high temperature region behind a shock wave and in front of the stagnation, dark space is not observed here. Ionized argon atom could not be observed in the dark space region by the spectroscopic method. This means that ionized argon atoms are not remarkably increased by this reaction. However, we consider that a slight decrease in neutral atom is enough to produce the dark space. Figure 13 shows spectral line profiles from 4550 to 4650/~ at x = - 0 . 5 mm for two cases with and without a body. Only neutral argon spectra can be seen in the case without body, whereas the spectra produced by ionized argon becomes greatly dominant compared with the neutral ones in the case with a blunt body. Specifically this is a spectrum of 4609.60 A. The variation of this ionized argon along the stagnation line is shown in Fig. 14. The number density, which is normalized by the value at the stagnation point, increases towards the stagnation point. From these results, it can be considered that free electrons are generated just behind the shock wave and in front of the blunt body because of high temperature. Therefore, electron temperature increase due to high heat conductivity of electron not only in the shock layer, but also in front of the shock wave. This high electron temperature field promotes ionization and reduces number density of neutral argon atom, which consequently forms dark space. On the other hand, Nishida reported that the excited argon atoms with various energy levels gradually decrease from the upstream [11], which is contrary to the present result. The reason for this is that equilibrium state could not be reached in the hypersonic flow as in the present study. 5. S U M M A R Y

It was confirmed that the region called dark space exists just in front of the shock wave, where self-luminositY of the high enthalpy flow is reduced. In this region the number density of neutral argon atom with a higher energy level such as 5p, which mainly produces the bright luminescence, is found to be relatively decreased compared with that of lower energy level of 4p'. The reason for the decrease of neutral argon with high energy has been experimentally examined; electron temperature increases behind the shock wave due to high temperature, then electron temperature is conducted to upstream through the shock wave, because of its high heat conductivity. And it promotes the chemical reaction which is Ar + e---,Ar + + e- + e-. On the other hand, the dark space does not take place in the region behind the shock wave, because the neutral argon atom with 5p-level energy essentially increases owing to high temperature. Finally, the present high enthalpy flow is considered to be nonequilibrium in the test section because of very high Mach number. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

Auweter-Kurtz, M., Habiger, H., Kurtz, H., Laure, S., R6ck, W. and Tubanos, N., ZFW, 1993, 17, 1-15. Boyd, I. D., Beattie, D. R. and Cappelli, M. A., J. Fluid Mech., 1994, 280, 41-67. Tahara, H., Uda, N., Ono¢ K., Tsubakishita Y. and Yoshikawa, T., IEPC-93, 1993, 133, 1-10. Babikan, D. S., Park, C. and Raiche, G. A., AIAA Paper, no. 95-0712, 1995. Ishiguro, M., Nakamura, Y., Shimizu, M. and Kawazoe, H., AIAA Paper, no. 94-2594, 1994. Grewal, M. S. and Talbot, L., J. Fluid Mech., 1963, 16, 573-594. Kamimoto, G. and Sirai, H. J. Japan Soc. Aero. Space Sci., 1968, 16, 378-385 (in Japanese). Kamimoto, G. and Sirai, H., J. Japan Soc. Aero. Space Sci., 1969, 17, 267-273 (in Japanese). Urita, A., Studies on aerodynamic properties of re-entry body. Doctor thesis, Nagoya University, 1995 (in Japanese). Hoffert, M. I. and Lien, H., Phys. Fluids, 1967, 10, 1769-1777. Nishida, M. and Nakajima, A., Z. Naturforsch., 1983, 38a, 802-807.