Experimental study of the influence of the model position on the operating conditions of a plasma wind tunnel

Experimental study of the influence of the model position on the operating conditions of a plasma wind tunnel Carlo Purpura, CIRA Capua c.purpura@cira.it, Tel. (+39)0823-623249 - Fax (+39)0823-623947 Federico De Filippis, CIRA Capua Antonio Esposito, DISIS - University of Naples "Federico II" Roberto Renis, DISIS - University of Naples "Federico II" Abstract An experimental campaign was developed at the facility SPES for the investigation of the model insertion effects on the test chamber static pressure increasing. This problem can generate a strong flow recirculation in the test chamber giving rise to the excessive heating of some parts inside it and causing a critical operating condition for the facility. Thus, the target of such an investigation is the identification of a strategy to permits the minimization of the test chamber static pressure increasing due to the model insertion location. In order to make a comparison, here are also reported the results obtained during the development of the test runs on the Hyflex and CRV dummy models carried out in the SCIROCCO facility. Introduction During the operation of the Plasma Wind Tunnel PWT- SCIROCCO facility at CIRA in Capua (Italy), some critical operating conditions could occur. They are actually investigated in the Small Plasma Entry Simulator (SPES) at the University of Naples "FEDERICO II", because of its lower power and scale. As reported in the work of ref. 1, Purpura et al. have observed that because of the size of the models, depending on the facility operating conditions, a flow re-circulation can be generated in the test chamber giving rise to the increasing of the static pressure whose values can became critical for the facility operation. Hanus et al. (ref.?3, verifica) have observed in high enthalpy facilities that models of very large size (high blockage ratios) can affect the diffuser operations (pressure recovery) giving rise to flow blockage effect in the test chamber. Experiments developed by Midden et al. (ref.?, verifica) on blunt models with hemispherical shape, have shown that by means of a properly designed diffuser it is possible to carry out tests on models with blockage ratios up to 18%. In this paper are reported the results of an experimental campaign aimed to investigate the effect of the model insertion in the plasma jet axis on the flow field configuration realised in the Test Chamber: different axial positions are investigated. The presence of the model in the plasma-jet axis could cause a strong recirculation of the flow in the Test Chamber and, as a consequence, the static pressure increasing. In particular, a comparison is reported between results obtained in SPES and in SCIROCCO during the development of the tests on the CRV and HYFLEX Dummy Models. The SPES facility SPES is the acronimous for Small Planetary Entry Simulation. SPES is a blow-down, arc-heated facility based on an industrial-type plasma torch (Sulzer mod. 9 MB, 80 KW max power). It is located at the Space Engineering and Science Department (DISIS) of the University "FEDERICO II" of Naples. The facility can be operated with Nitrogen, Air (Nitrogen-Oxygen mixture), carbon dioxide (Nitrogen-CO 2 mixture) as working gases. Main technical specification for SPES are the following: Total Enthalpy between 3 and 30 MJ/Kg Mass flow between 0. and (g/s) Mach number 2.9 or 4.8 Stagnation-point pressure between 00 10000 (Pa) Stagnation-point heat flux between 100 3000 (KW/m 2 ) Main measurement techniques include: Total enthalpy : energy balance (global measurement) and calorimetric Probe (local measurement) Stagnation-point pressure: water-cooled Pitot probe Stagnation-point heat flux: Gardon Gauges and slug calorimeters Pressures in the test chamber, nozzle and diffuser : precision vacuum transducers Mass flow: thermal mass flowmeters Oxygen percentage in the plasma flow: zirconium oxide analyzer Surface Temperatures : IR Thermography The conical nozzle has an area ratio of A/A*=20, with an exit section area (diameter 0 mm), which permits the flow to reach Mach operation. The test chamber and diffuser are shown in fig. 1. The test chamber it s very large with respect to the nozzle size, cylindrical with a diameter of 800 mm and 900 mm length. 1

PM PM3 Pts P1 P2 P3 P4 P P6 P7 P8 P9 P10 P11 Fig. 1 SPES Test Leg P12 P1 P13 P14 P16 P17 A detail of the test section is shown in fig. 2, and the flow is convoyed in the diffuser which pick-up is 120 mm of diameter, while the diffuser cross-section is 94 mm of diameter. In particular, the diffuser is geometrically the SCIROCCO one sub-scaled. It was realised in order to develop some investigation regarding phenomenology that may occur in the diffuser of SCIROCCO. mm, 10 mm. All the test have been performed at a Total Enthalpy H 0 = 10 MJ/kg. In table 1 is reported for each run the mass flow m&. Let the static pressure in the test chamber be Pts out and Pts in, indicating respectively the static pressure in the test chamber without the model in the jet-axis and with the model inserted in the jet. After the start-up of the facility, and the achievement of the steady operating conditions (H 0 and m& ) of the flow in the test chamber, the value of the static pressure Pts out is measured before the insertion of the model. So, the insertion of the model in the plasmajet starts, and during its travel to the jet-axis position, when the model surface begins to interact with the plasma streamlines some flow particles are deflected from the surface out of the jet. These particles generate a flow re-circulation in the test chamber that makes the Pts to rapidly increase in few seconds and when the model achieves the plasma centreline the static pressure attains its maximum value Pts in for those operating conditions of the facility. In the table 1 are reported the pressure ratios Pts in obtained during of Ptsout each test run, that show the effect of the model size on the test chamber pressure variation. Table 1: facility operating conditions Fig. 2 Test Section detail The test instrumentation consists of many pressure taps distributed at the nozzle exit, in the test chamber, and along the diffuser axis: P M, pressure tap detecting the pressure level at the nozzle exit section (for A/A*=20), measured by a capacitive pressure sensor; Pts, test chamber static pressure, measured by means of a Pirani pressure sensor; Axial Diffuser pressure distribution. Along the diffuser axis many piezo-electric pressure sensors are installed for detecting the pressure level at different sections. During the experiments, the pressure is measured by the following 10 sensors (see fig. 1): P1, P2, P3, P4, P, P9, P10, P12, P1, P18. Model arrangement For the sake of the analysis the investigation was carried out on three spherical models of steel whose diameters were, respectively of, 20-30-40 mm. By defining the model Blockage Ratio (B.R.) as the ratio between the model frontal area A model and the diffuser pick up cross section A Diffuser, the values of B.R. corresponding to the models of 20 mm, 30 mm and 40 mm are, respectively of, 0.028, 0.062 and 0.11. The models are inserted in the test section by means of a manual holder that permits to change the axial position. Each model is inserted in the jet axis at three different locations from the nozzle exit: 10 mm, 62 Test Model diameter mm m& g/s Pts in/pts out x= 10 mm Pts in/pts out x= 62 mm Pts in/pts out x= 10 mm D40-10-0 40 0.. D40-10-1 40 1 6.61 7.8 7.69 D30-10-0 30 0. 4.4. 2.2 D30-10-1 30 1.38 7.14 2.46 D20-10-0 20 0. 2.6 2 1.8 D20-10-1 20 1 2.46 2.28 1.3 Test Results During the development of the test campaign, some interesting changes of the flow field configuration were observed in test chamber, when the model is inserted in the plasma-jet. In fact, when the model begins to interact with the plasma streamlines, these are deflected, and depending on the deflection angle (due to the model geometry), the streamlines can generate a strong re-circulation in the test chamber. Because of the strong re-circulation, the streamlines can go back upstream the detached bow shock wave, increasing the static pressure in the test chamber. This reduces the pressure jump through the shock wave, and if it occurs continuously, these operating conditions correspond to the flow-blockage problem 1. In this case the shock wave begins to extinguish generating a critical operating condition for the facility. The presence of flow re-circulation in the test chamber was observed with models of both large and small sizes. In fig. 3 is reported the test run developed on the 40 mm model diameter (the largest one) located at 10 mm far from the nozzle exit. In this case ( m& = 0. g/s) few streamlines are deflected from the model surface 2

and the generation of a very low flow re-circulation in the test chamber is observed. The remaining streamlines regularly convey in the diffuser pick-up and the static pressure ratio in the test chamber achieved the maximum value of. in the diffuser and only few streamlines are deflected outside of it generating a slight re-circulation not well observable. This makes the static pressure ratio in the test chamber to increase up to 7.69. Fig. 3 - Test D40-10-0, x = 10 mm By means of fig. 4, it is possible to see the 40 mm model located at 62 mm far from the nozzle exit. In this case the mass flow rate is m& =0. g/s, and by changing only the axial position of the model in the test chamber, it is possible to observe as the shape of the model bow shock wave achieves the same size of the diffuser pick up and the flow re-circulation increases slightly. Here, the static pressure ratio increases too achieving the value of.. Fig. 6 - Test D40-10-1, x = 10 mm In fig. 7 is shown the test case of the 40 mm model diameter, located at 62 mm far from the nozzle exit, during which a strong flow re-circulation was observed. The plasma streamlines do not convey directly in the diffuser pick-up because due to the model location far from the diffuser pick up, the surface of the model deflects partially the flow that goes outside of the diffuser. This makes the flow to recirculate in the test chamber and its static pressure increases up to the value of 7.8, which was the highest value measured during the test campaign. In this case the model bow shock wave is not well distinguishable from the plasma-jet. Fig. 4 - Test D40-10-0, x = 62 mm If the 40 mm model is located close to the nozzle exit (10 mm far from it, see fig. ), it is possible to observe that the shape of the model bow shock wave is larger than the size of the diffuser pick up. Here m& = 0. g/s, and the flow re-circulation in test chamber is not well distinguishable. In this case the static pressure ratio achieves the value of. Fig. 7 D40-10-1, x = 62 mm By reducing the model diameter up to 30 mm, the flow configuration in the test chamber changes strongly depending on the location of the model in the jet axis. In fact, in fig. 8 is shown the 30 mm model located at 10 mm far from the nozzle exit. In this case the m& =0. g/s and due to the location of the model, the deflection of the plasma streamlines caused by the model surface does not affect the flow configuration in the test chamber, and all the streamlines directly convey into the diffuser pick up. Here, due to the model location, the test chamber static pressure ratio attains the value of 2.2. Fig. - Test D40-10-0, x = 10 mm In fig. 6, the 40 mm diameter model is inserted at 10 mm far from the nozzle exit, and the mass flow rate is 1 g/s. In this case, because of the model position close to the diffuser pick up, the streamlines convey directly 3

the flow field around the model when it is located at 10 mm far from the nozzle exit, with a m& = 1 g/s. In this case the flow streamlines directly convey inside the diffuser and no flow re-circulation was observed. The pressure ratio in the test chamber achieved the value of 1.3 which was the lowest measured during all the test campaign. Fig. 8 Test D30-10-0, x = 10 mm By changing the distance of the model from the nozzle exit, and inserting it at 62 mm far from the nozzle, it is possible to observe that the model generates a shock wave which boundary surrounds the diffuser pick up (see fig. 9). Here, the plasma streamlines are partially deflected outside of the diffuser, generating a well distinguishable flow re-circulation in the test chamber. In this conditions, with a m& = 0. g/s, the test chamber static pressure ratio attains the value of.. Fig. 11 - Test D20-10-1, x = 10 mm A totally different flow configuration was observed during the test with the 20 mm model at 10 mm far from the nozzle exit. In fact, as shown in fig. 12, due to the location of the model so far from the diffuser pick up, the flow strongly re-circulates in the test chamber although the model shock wave shape is not large as the diffuser size. In this case the static pressure ratio in the test chamber attains the value of 2.46. Fig. 9 - Test D30-10-0, x = 62 mm If the 30 mm model is inserted in the plasma jet axis at 10 mm far from the nozzle exit, the flow field configuration changes strongly. In fact, by means of fig. 10, it is possible to observe that the model shock wave is well distinguishable in the flow field, and because of the high distance of the model from the diffuser pick up, the flow streamlines deflected by the model surface do not convey in the diffuser, but go outside of it. By this way, a strong flow re-circulation is generated and the static pressure increases. In this operating conditions, with a m& = 0. g/s the pressure ratio attains the value of 4.4. Fig. 10 - Test D30-10-0, x = 10 mm In the case of the 20 mm model diameter (the littlest one), an interesting behaviour of the flow field configuration was observed by varying the location of the model along the jet axis. In fact, in fig. 11 is shown Fig. 12 - Test D20-10-1, x = 10 mm In order to make only a qualitative comparison with the SCIROCCO facility of CIRA, here are reported the results of test runs carried out on the CRV and HYFLEX dummy models. In fact, it is important to point out that the SCIROCCO operating conditions (H 0 and m& ) are different by the ones of the SPES facility. The CRV model is very large (960 mm diameter), and was tested in the SCIROCCO facility in order to verify the possible occurrence of flow-blockage in the test chamber and the type of criticalities generated by this phenomenon. In fig.s 13a through 13d is shown the sequence of insertion of the CRV in the plasma flow occurred during the development of a test run. Its B.R. is 0.13, thus a little more than the value of the 40 mm model (0.11), tested in the SPES facility. The fig. 13a shows the start of the insertion phase, when the model begins to interact with the plasma-jet boundary, and simultaneously the static pressure in test chamber begins to increase (see fig. 14). In figg. 13b and 13c is shown the flow configuration change around the model surface during its insertion in the test section. In fig. 13d the model achieves the jet axis, and the pressure 4

ratio increases up to 3 (see fig. 14). Here, the model begins to move from the position close to the nozzle exit to a position along the jet axis near the diffuser pick up, but not close to it, and simultaneously the pressure ratio decreases up to 28 (see fig. 14). Because of the high brightness of the plasma, here is impossible to distinguish the model from the flow. P TC Ratio 40 3 30 2 20 1 model in the jet axis 10 0 model insertion start Fig. 13a Fig. 13b 21 216 217 218 219 220 221 222 223 224 22 Time (s) Fig. 14 Test Chamber pressure ratio change during the CRV test run In the case of the HYFLEX dummy model (B.R.= 0.091), there were carried out 4 test runs with the model always located at the same distance from the nozzle exit. The HYFLEX model has a very large size (800 mm diameter) smaller then the CRV one. By means of figg. 1 and 16, referred to two different test runs, it is possible to observe as the model bow shock well enters in the diffuser pick up and during the development of both the test runs the static pressure ratio in the test chamber achieved the value of 6.2. In figg. 17 and 18 the operating conditions of the facility were different and in these cases the model bow shock wave shows a larger size than the first two test runs. Here, depending on the facility operating conditions, during the development of the tests in fig. 17 and 18 the static pressure ratio in the test chamber achieved the values, respectively of,.6 and 6. Fig. 13c Fig. 1 Fig. 13d Fig. 13 - CRV Test Fig. 16 Fig. 17

Fig. 18 HYFLEX Test As shown in fig. 19, the effects of the model location in the test chamber on the static pressure ratios are very important when the model size is smaller then the 40 mm diameter B.R. (0.11). In fact, by means of the figure, it is possible to observe that when the facility operates at H 0 = 10 MJ/kg and m& = 0. g/s, for model diameters of 20 mm and 30 mm, the jet axis location generating the minimum increasing of the static pressure in the test chamber is the one close to the diffuser pick up. About the 40 mm model diameter, it seems that the maximum increasing of the static pressure occurs only when the model is located at 0.38% (x= 62 mm) of the distance (L =162mm) between the nozzle exit and the diffuser pick up. But the values of pressure ratio achieved in the test chamber during the three insertion of the 40 mm model in the operating conditions of the facility do not differ too much. In fact, they change from at the extremities of the distance (x= 10 mm and x = 10 mm) to. at the centre (x= 62 mm). This is probably due to the very large size of that model, that generates a flow field in the test chamber with a re-circulation which intensity does not strongly depend on the model location in the jet-axis. In particular, it is interesting to point out that both the models of 30 and 40 mm show the same static pressure ratio increasing of. in the test chamber when located at 62 mm far from the nozzle exit. Here are also indicated the results of the tests developed in the SCIROCCO facility with the Hyflex dummy model. In fact, it is possible to observe that the pressure ratios obtained with the Hyflex model are over the ones measured during the tests on the 40 mm model. Ratio P TC 10 9 8 7 6 4 3 2 B.R. = 0.11 B.R. = 0.062 B.R. = 0.028 B.R. Hyfle x = 0.091 When the mass flow rate is increased up to 1 g/s (see fig. 20), there is a change on the flow field configuration generated by the models. Generally, the static pressure ratio in the test chamber increases. In particular, with the 40 mm model the static pressure achieves the highest value when it is located at 62 mm far from the nozzle exit (x/l =0.38), while the lowest pressure is measured with the model located at 10 mm (x/l =0.062) far from the nozzle. By using the 30 mm model, the position that generates the lowest static pressure increasing is the one close to the diffuser pick up, and the same result is obtained with the 20 mm model. Thus, for the given facility operating conditions, the location of the model that generates the most critical static pressure increasing in the test chamber is the one corresponding to 62 mm (x/l =0.38) far from the nozzle exit, as already observed by means of the test runs developed at m& = 0. g/s. Here are also indicated the results obtained with the Hyflex model. It is possible to see that in this case the pressure ratios increasing obtained with Hyflex seem to be comparable to the ones obtained with the 30 mm model. Ratio P TC 10 9 8 7 6 4 3 2 1 0,0 0,1 0,2 0,3 0,4 0, 0,6 0,7 0,8 0,9 1,0 X / L B.R. = 0.11 B.R. = 0.062 B.R. = 0.028 B.R. H yflex = 0.091 Fig. 20 - Model position effect on thetest chamber pressure ratio. H 0 =10 MJ/kg, Mass Flow =1 g/s OBSERVED PHENOMENOLOGY During the development of the test campaign, some interesting phenomenology were observed. - Free-jet diameter restriction During the development of a test run, it is possible to observe the change of the free-jet diameter occurring during the model insertion phase in the plasma flow. In fact, as well observable in the case of fig. 21, the free-jet is wide as the size of the diffuser pick up. 1 0 0,0 0,1 0,2 0,3 0,4 0, 0,6 0,7 0,8 0,9 1,0 X / L Fig. 19 Model position effect on thetest chamber pressure ratio. H 0 =10 MJ/kg, Mass Flow =0. g/s 6

Fig. 21 When the model is inserted in the flow (in this case the 40 mm model, see fig. 22) it is possible to see that upstream the model bow shock wave the diameter of the free-jet decreases. This is due to the flow recirculation in the test chamber that makes the static pressure to increase. By this way, the nozzle operating mode tends to change from under-expanded to over-expanded and these are the bases for the occurrence of the flowblockage phenomenon. Because of the size of the models and the facility performances, this critical operating condition did not occur. Fig. 23 - Influence of the model trim. When the model is inserted in the plasma flow, it is very important to verify if it is well aligned with the jet axis or it has an angle of attack. In fact, as well represented in fig. 24, due to the presence of an angle of attack of the model, part of the plasma flow is deflected from the body surface. The flow deflected could interact with parts of the test chamber heating them. Depending of these parts, the presence of such a deflection can become critical for the operations of the facility, thus it is necessary to verify before the model insertion that its axis is well aligned with the jet axis. Fig. 22 - Model shock wave and jet boundary layer interaction Usually, by using models which sizes are compatible with the test section diameter of the facility, the model shock wave should be completely inside the flow field. If the model shape is large as the test section size, then the possibility that the weak part of its shock wave interacts with the boundary of the flow field. This fact is shown in fig. 23, where the interaction zone has an o-ring shape surrounding the flow downstream of the model. Of course, the presence of this interaction zone gives the indication that the model size is large respect to the test section diameter, thus the size of such models can be considered as a limit of the facility. Moreover, the flow re-circulation generated by models of such a size could affect the results of the test run. Fig. 24 - Drop shape of the model shock wave During the development of the test campaign, depending on the model location on the jet axis and the facility operating conditions, it is possible to observe the whole shape of the model shock wave. In fact, in fig. 2 is shown the test run developed on the model of 20 mm at m& =0. g/s when located at 10 mm far from the nozzle exit. In this case the flow recirculation in the test chamber is well distinguishable as well as the shape of the model shock wave, which geometry is similar to a drop shape and its size is (in this case) however smaller then the diffuser pick up diameter. 7

Conclusions Fig. 2 Experiments were developed in the SPES facility by using spherical steel models of different diameters and located in the plasma jet axis at different distances far from the nozzle exit. These models have diameters of 20 mm, 30 mm and 40 mm, which B.R. are, respectively of, 0.028, 0.062 and 0.11. The static pressure effects in the test chamber were investigated by changing the location of the models, in order to identify the location of the model which static pressure increasing is the lowest. The models were inserted at 10 mm, 62 mm and 10 mm far from the nozzle exit. By means of the experiments it is possible to point out that the static pressure increasing is due both to the model size and its location in the jet axis. At m& =0. g/s, for model diameters of 20 mm and 30 mm the location that makes the static pressure in the test chamber to shortly increase is the one at 10 mm far from the nozzle exit, and corresponding to the position close to the diffuser pick up. In the case of the 40 mm model diameter, its shape is such that the change of the plasma flow configuration change due to the insertion generates a static pressure increasing in the test chamber that seems to be independent by the location of the model in the jet axis. In fact, as shown in fig. 22, the static pressure ratio increasing changes from at 10 mm and 10 mm to. at 62 mm. In the experiments carried out at m& =1 g/s, it was observed that for models of 20 mm and 30 mm, the location for which the test chamber static pressure increasing is the lowest is also the one at x= 10 mm far from the nozzle exit, which corresponds to the location close to the diffuser pick up. In fact, at this location the pressure ratio increasing obtained on the 20 mm model was 1.3, while on the 30 mm model was obtained a value of 2.46. Here, the model of 40 mm shows that its insertion in the plasma jet axis generates always a strong change in the flow configuration in the test chamber, which static pressure increasing seems to be always independent by the location of the model in the jet axis. In order to make a comparison between the SPES and the SCIROCCO facilities, by analysing the test runs developed on the Hyflex dummy model, which B.R. is 0.03, it is possible to observe that the maximum static pressure increasing due to the model insertion achieves the value of 6.2 at the location in the SCIROCCO facility that in the SPES one corresponds to x= 30 mm far from the nozzle exit. In the experiments developed at SPES with m& =0. g/s this result is comparable to the ones obtained on the 40 mm model. While during the development of the test runs at m& =1 g/s, the pressure increasing achieved with the Hyflex model is comparable to the ones corresponding to the 30 mm model of the SPES facility. A particular discussion must be done on the test run carried out at SCIROCCO on the CRV dummy model. In fact, as already observed by means of fig. 17, the pressure increasing obtained on such a model was 3, a very high value, that is not possible to compare to the tests carried out at SPES. But, from a qualitative point of view, the analysis of the time evolution of the test chamber static pressure can be done. In fact, when the CRV model is inserted in the plasma jet, its location is just in front of the nozzle exit, where the highest value of the pressure ratio of 3 is measured. After, when the displacement of the model starts in direction of the diffuser pick up, the static pressure in the test chamber decreases, achieving its minimum value of 28 when the model is located near the diffuser pick up. Thus, such a behaviour seems to indicate that in the case of very large models, the best location of the model in the test section of the facility where the pressure increasing is the minimum, is the one close to the diffuser pick up. In conclusion, the flow re-circulation in the test chamber does not depend on the shape of the model only, but from its position too. Because of the possible generation of critical operating conditions of the facility, the preliminary study of the model location influence on the static pressure increasing in the test section is very important, and by means of this strategy it is possible to carry out tests on very large models, by inserting them in such a position that minimizes the static pressure. Bibliography 1 C. Purpura, F. De Filippis, A. Esposito, R. Renis, Model Shape Influence On The Facility Pressure Distribution, 4 th ESA European Workshop on Hot Structures and Thermal Protection Systemls for Space Vehicles, Palermo, Italy, 26-29 Nov. 2002. 2 A. Del Vecchio,F. De Filippis, F. Ferrigno, G. Palumbo, Analysis and Discussion about the Plasma Radiation at Different Efficiency Yields of a Hypersonic Diffuser in an Arcjet Wind Tunnel Wind Tunnel, AIAA 99-3494,33 rd Thermophysics 8

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