EXPERIMENTAL STUDY TO INCREASE THE SHOCK SPEED IN A FREE PISTON DOUBLE DIAPHRAGM SHOCK TUBE Hiroyuki Ishida (1), Kyosuke ARAI (2), Yusuke SHIMAZU (3), Asei TEZUKA (4), Yasunori NAGATA (5), Kazuhiko YAMADA (6) and Takashi ABE (7). (1) Waseda University, 3-4-1 Okubo, Shinjuku-ku, 169-855, Japan,Email:hiro1115@fuji.waseda.jp (2) Tokai University, 4-1-1 Kitakaname, Hiratukashi-shi, Kanagawa, 259-1292, Japan, Email: kyoo7013@gmail.com (3) Waseda University, 3-4-1 Okubo, Shinjuku-ku, 169-855, Japan, Email: y.shimazu@suou.waseda.jp (4) Waseda University, 3-4-1 Okubo, Shinjuku-ku, 169-855, Japan, Email: mechaero.tezuka@cls.waseda.jp (5) Okayama University, 3-1-1 Tsushimanaka, Kita-ku, Okayama-shi, Okayama 700-8530, Japan, Email:ynagata@okayama-u.ac.jp (6) Japan Aerospace Exploration Agency, Institute of Space and Aeronautical Science, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara-shi, Kanagawa, 252-5210, Japan, Email: yamada.kazuhiko@jaxa.jp (7) Japan Aerospace Exploration Agency, Institute of Space and Aeronautical Science, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara-shi, Kanagawa, 252-5210, Japan, Email: tabe@gd.isas.jaxa.jp ABSTRACT In future planetary explorations, sample return missions from deeper space than Itokawa where the "Hayabusa" traveled from 2003 to 2010 have been proposed. In this mission, reentry speed is up to 15km/sec. It is important to predict aerodynamic heating environment exactly to design the heat shield of reentry capsule. Therefore the ground experimental facility which can produce airflow of 15km/sec is necessary in order to succeed in this mission. While there are various experimental facilities, we focus on a free piston double diaphragm shock tube located in ISAS/JAXA. The free piston double diaphragm shock tube can produce a high speed shock wave. Its characteristics were investigated for various operation parameters; such as 1st diaphragm rupturing pressure, the initial pressure in the medium pressure tube, the initial pressure in the low pressure tube. Under the set-up that the 1st diaphragm rupturing pressure (65.0MPa), the initial pressure in the medium pressure tube (1.0kPa), and the initial pressure in the low pressure tube (0.6Pa), a shock speed reached to 14.04 km/sec in our facility. 1. Introduction On June 2010, the HAYABUSA re-entry capsule successfully returned to the Earth. With this success, the demand for sample return missions has been increasing for future planetary explorations. Sample return missions from deeper space than Itokawa where the "Hayabusa" traveled is proposed. For example, the Jovian Trojan mission with solar power sail technology proposed by the Japan Aerospace Exploration Agency and CSSR (Comet Surface Sample Return) proposed by NASA [1,2]. While the re-entry speed of the HAYABUSA sample return capsule is about 12 km/sec, the re-entry speed reached more than 15 km/sec in these sample return missions from deep space. To develop a re-entry capsule, which can survive the severe aerodynamic heating environment in these missions, it is important to precisely predict the aerodynamic heating environment. Under high speed environment, radiation heating, in particular ultraviolet and vacuum ultraviolet region, is critical contributing factors. Therefore, we focus on a free piston double diaphragm shock tube to produce high speed shock wave. In this research, we used a free piston double diaphragm shock tube in JAXA/ISAS. It has been confirmed that this facility can produce a shock wave to about 12 km/sec, which contributes to improving SRADIAN that is the radiation analysis code developed by Fujita [3,4]. However, it has not been verified under high speed environment. Considering these, it is essential to experimentally validate the radiation analysis code under the high speed environment. In this paper, the experimental investigation related to increasing of the shock speed at this facility is described. 2. Overview of facility and measurement system In this research, we used a free piston double diaphragm shock tube in JAXA/ISAS. Figure 1 is the overview of this facility. Figure 2 shows x-t Diagram of the free piston double diaphragm shock tube. This facility consists of the high pressure chamber, compression tube, medium pressure tube (MPT) and low pressure tube (LPT). The length of compression tube and MPT is 530cm and 300cm, respectively. The cross section of LPT is a square 35cm on a side. Shock speed of shock wave propagating in LPT can be varied by adjusting the initial pressure in each section. Overview of the measurement system of our facility is shown in Figure 3. The 1 st diaphragm made of steel is located between compression tube and MPT. The second diaphragm made of lumirror film is located between the MPT and LPT. Four pressure sensor (PCB PIEZOTRONIC, INC: H113B28)are installed in each section as shown
in Figure 3. Sensor A is installed to measure the 1st diaphragm rupturing pressure. Sensor B is installed to sense the timing of the rapture of 2 nd diaphragm. Sensor C and D are installed to estimate the shock speed propagating in LPT from the time difference of output rising edges of these two sensors. Additionally, the shock wave propagating in LPT is observed using the High-speed camera (HPV-1: SHIMADZU CORPORATION) in this experiment. The location of its observation is in the vicinity of Sensor C. 3. Results 3.1 Overview of the experimental results Through experiments, the shock speed is considered to be controlled by the following parameters. 1. 1 st Diaphragm rupturing pressure. 2. the initial pressure in the medium pressure tube. 3. the initial pressure in the low pressure tube. The main objective of this experiment is to increase the shock speed by adjusting these parameters. Furthermore, in order to understand the qualitative trend about effect of these parameters on the shock speed, the experimental results are compared with calculated prediction under the assumption on the equilibrium flow [5]. 3.2 Nominal case Figure 1 Free-piston double-diaphragm shock tube located in ISAS/JAXA used in this research The initial set-up that can produce shock wave of about 12 km/sec was already established by previous research. The set-up to produce the shock wave with shock speed of 12km/sec is listed in Table 1. This set-up is defined as nominal case in this research. Table 1 Operational condition in nominal case. The initial pressure (Filling gas) High pressure chamber (Air) 2.77 [MPa] Compression tube (He) 101.3 [kpa] Medium pressure tube (Air) 1 [kpa] Low pressure tube (Air) 4 [Pa] 1st Diaphragm rupturing pressure 50 [MPa] 1st Diaphragm (Steel) 1.8t-0.4d [mm] 2nd Diaphragm (Lumirror) 12 [μm] Figure 2 x-t diagram of the free-piston double-diaphragm shock tube. Figure 3 Schematic view of measurement system of our facility in this experiment. The predicted shock speed under this initial set-up is shown in Table 2. The measured shock speed in the actual operation is also listed. The data in the Table 2 shows that the shock speed of experiments is 1 km/sec faster than the prediction calculated on the equilibrium flow assumption. According to CFD analysis, the shock speed calculated on the non-equilibrium flow assumption is faster than one calculated on the equilibrium flow assumption. Therefore, the difference of the experimental results and the calculated prediction may be caused due to not only the viscous effect by the boundary layer, but also the effect of the heat and chemical non-equilibrium. The reproducibility on this facility was investigated in actual operation under the nominal initial set-up. The reproducibility in term of the shock speed is within the range of 0.5 km/sec.
Table 2 Experiment and calculation result in nominal case. Calculation (equilibrium)[km/sec] 13.12 average[km/sec] 11.88 Experiment U middle max[km/sec] 11.98 min[km/sec] 11.55 3.3 The effect of the 1st diaphragm rupturing pressure At first, the effect of the 1 st diaphragm rupturing pressure on the shock speed was investigated experimentally in the actual operation in the case that only 1 st diaphragm rupturing pressure was varied from the nominal condition. To change the 1st diaphragm rupturing pressure, the high pressure chamber pressure and groove depth of 1 st diaphragm have to be adjusted. The relation between high pressure chamber pressure, groove depth of 1 st diaphragm and 1 st diaphragm rupturing pressure is listed in Table 3. Table 3 Relation between high pressure chamber pressures, groove depth of 1 st diaphragm and 1 st diaphragm rupturing pressure. High pressure chamber pressure [MPa] Groove depth of 1 st diaphragm [mm] 1 st diaphragm rupturing pressure [MPa] 2.77 0.40d 50~53 2.77 0.36d 53~56 2.90 0.32d 56~59 2.95 0.28d 59~62 3.00 0.24d 62~64 The relation between 1st diaphragm rupturing pressure and the shock speed propagating in LPT is shown in Figure 4. The blue symbols are the experimental results, and the red line is calculation results. This result shows that the shock speed increased as rising of the 1st diaphragm rupturing pressure. This trend is also predicted by calculation. The increasing of 1st diaphragm rupturing pressure to 65.0 MPa make the shock speed faster 1km/sec then shock speed in nominal case. This facility can be operated under the higher 1st diaphragm rupture pressure where the rupturing pressure is up to 85MPa, according to the previous research [5]. Therefore, the shock wave with higher shock speed may be produced in this facility by increasing the 1 st diaphragm rupturing pressure. Figure 4 Relation between 1 st diaphragm rupturing pressure and shock speed. 3.4 The effect of initial pressure in the medium pressure tube The effect of initial pressure in the medium pressure tube on the shock speed was investigated experimentally based on the results of actual operation in the case that only initial pressure of the medium pressure tube was varied from the nominal condition. The initial pressure in the medium pressure tube is varied in the range as shown in Table 4 in this research. Table 4 Operational condition of Initial medium pressure tube. Medium pressure tube (Air) 0.5~2.0 [kpa] The Figure 5 shows the relation between the initial pressure in the MPT and the shock speed in the LPT. Both the experimental results and calculated prediction are plotted in Figure 5. The shock speed discrepancy between the experimental results and calculated prediction is 1km/sec at any condition of the initial pressure in MPT. Its trend is similar to nominal condition. According to calculation results in Figure 3, the variation of shock speed does not change appreciable even if the initial pressure in MPT is varied from 0.5 to 2.0 kpa in the condition of initial pressure in LPT of 4.0kPa. The experimental results indicate the same tendency of the predication. It is confirmed experimentally that the increase in shock speed in LPT cannot be expected by adjusting pressure in MPT in our facility.
Figure. 5 Relation between the initial pressure of MPT and the shock speed. (Initial pressure of LPT: 4.0[Pa]) 3.5 The effect of initial pressure in the low pressure tube The effect of initial pressure in the low pressure tube on the shock speed was investigated experimentally on the basic from the results of actual operation in the case that only initial pressure of the low pressure tube was varied from the nominal condition. The initial pressure in the low pressure tube is varied in the range shown in Table 5 in this research. Table.5 Operational condition of initial low pressure tube. Low pressure tube (Air) 0.15~40.0 [Pa]. Figure 6 is the relation between the initial pressure in the LPT and the shock speed in the LPT. In Figure 6, blue symbols show the experimental results and red line show the calculated prediction on the assumption of equilibrium flow. Both prediction and experimental results indicate that the shock speed increased by decreasing the initial pressure in the LPT. The shock speed is 13.45km/sec in the case that the initial pressure in the LPT is 0.6Pa. Shock speed become faster in around 1.5km/sec than the shock speed in nominal case. However, the quantitative discrepancy between the prediction and experimental results is 1km/sec in the case. One of the reasons is influence from the boundary layer. CFD analysis in past research indicated that the boundary layer becomes thick when the initial pressure in the LPT [6]. Furthermore, the shock speed measured in actual operation reached the plateau when the initial pressure in the LPT is less than 1.0Pa. Its phenomenon was not predicted by the calculation as shown in Figure 6. Figure 6 Relation between the initial pressure of LPT and the shock speed. The reason of this saturation may be not only the boundary layer but also the rarefaction effect. It is revealed that a shock wave is decelerated and curved by relatively large mean free path due to the high vacuumed condition [7]. The schematic of the deformed shock wave by the effect of boundary layer and rarefaction is shown in Figure 7. The mean free path of the air in LPT was estimated in accordance with Eq. (1) in the case that the pressure in LPT is 0.1, 1.0 and 10Pa. In the estimation, we assumed that T (temperature) is 300 [K], σ (aerial molecules diameter) is 0.372[m]. The result is listed in Table 6. 1 λ = 2πσ 2 n = kt 2πσ 2 p Table.6 Relation between the initial pressure of LPT and mean free path Initial pressure of LPT[Pa] 0.1 1.0 10 Mean free path [mm] 67 6.7 0.67 The cross section in the low pressure tube is a square of 35 [mm] 35 [mm]. Therefore, mean free path is longer than the distance between the walls when the initial pressure in the low pressure tube is less than 1.0Pa. The deceleration of a shock wave might become more conspicuous by the rarefaction effect in high vacuumed condition in LPT. To confirm that a shock wave is curved in the experiments, a shock wave propagating through the low pressure tube is captured using the High-speed camera (HPV-1). Figure 8 and 9 indicate that a shock wave propagating through the low pressure tube is curved. Its deformation becomes significant as the initial pressure in the LPT decreases. The reason might be influence of the rarefaction effect. This is one of the reasons why the shock speed reaches its plateau when the initial pressure in the low pressure tube at the pressure less than 1.0Pa. (1)
Figure 7 The schematic of deformed shock structure propagating in LPT Figure 8 The emission of shock wave propagating in LPT captured by HPV-1 (4.0Pa). Table.7 Operational condition to produce the highest shock speed in our facility Initial pressure (Filling gas) High pressure chamber (Air) 3.00 [MPa] Compression tube (He) 101.3 [kpa] Medium pressure tube (Air) 1.0 [kpa] Low pressure tube (Air) 0.6 [Pa] 1st Diaphragm rupturing pressure 65.0 [MPa] 1st Diaphragm (Steel) 1.8t-0.4d~0.24d [mm] 2nd Diaphragm (Lumirror) 12 [μm] 5. Summary The experimental investigations related to increasing the shock speed produced by our facility located in ISAS/JAXA are reported in this paper. 1) The shock speed measured in actual operation is 1 km/sec faster than calculated prediction on the assumption of the equilibrium flow. 2) The shock speed increases by raising the 1st diaphragm rupturing pressure. 3) The shock speed is not affected by changing of the initial pressure in the medium pressure tube. 4) The shock speed increases by decreasing the initial pressure in the low pressure tube. However, the shock speed reaches its plateau by effect of the boundary layer and rarefaction when the initial pressure in the low pressure tube is less than 1.0Pa. According to above knowledge, the shock tube was operated to produce the strong shock wave with highest shock speed. The 13.88km/sec shock wave was produced in our facility. For the future work, spectroscopic measurement of the airflow with such a high speed shock wave will be conducted, especially focusing on the in UV and VUV wavelength region. Figure 9 The emission shock wave propagating in LPT captured by HPV-1 (0.7Pa). 4. Highest shock speed operation The shock tube was operated to produce the strong shock wave with the highest shock speed based on the above results. The initial set-up is listed in Table 7 in this operation the shock speed in this operating condition is 13.88km/sec 6. Reference 1. Yano. H, Mori, O. Funase, R. Matsuura, S.Fujimoto, M., Takashima, T., Solar Power Sail/Jovian Trojan Working Group. (2012). Solar Power Sail, the Jovian Trojan Explorer and Deep Space Astronomical Platform. Asteroids, Comets, Meteors 2012, LPI Contribution No. 1667, id.6251, Niigata, Japan. 2. Glen H. Fountain, Robert D, Strain, Hal Weaver, Mike A' Hearn. (2008). Comet Surface Sample Mission Study. NASA's Planetary Science Division SDO-11998. 3. Ogura, E., Funabiki, K., Sato, S., and Abe, T. (1997). Free Piston Double Diaphragm Shock Tube. Inst. of Space and Astronautical Science, Rept. 96. Kanagawa, Japan.
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