Comparative Heat Flux Measurements on Standard ...

AIAA/CIRA 13th International Space Planes and Hypersonics Systems and Technologies

AIAA 2005-3324

Comparative Heat Flux Measurements on Standard Models in Plasma Facilities Ali Guelhan * and Burkard Esser † German Aerospace Center (DLR), Linder Höhe, 51147 Cologne, Germany Antonio del Vecchio ‡ Italian Center for Aerospace Research (CIRA), Via Maiorise 81043 - Capua (CE), Italy

Downloaded by ITALIAN CENTER FOR on September 24, 2015 | http://arc.aiaa.org | DOI: 10.2514/6.2005-3324

Stefan Löhle § IRS, University of Stuttgart, Pfaffenwaldring 31, 70550 Stuttgart, Germany Nicolas Sauvage ** EADS ST, Avenue du General Niox - F-33165 Saint-Medard-en-Jalles Cedex, France Olivier Chazot †† and Cem O. Asma ‡‡ VKI, Chaussee de Waterloo 72, 1640 Rhode Saint Genese, Belgium

A comparative experimental study has been performed testing heat flux probes in different European long duration high enthalpy arc heated and inductively heated facilities. Commercial available heat flux sensors, calorimetric probes and SiC probes were used. Tests were carried on three well defined model geometries in stagnation point configuration at two different test conditions defined in terms of specific enthalpy and Pitot pressure. It has been noticed that the reliability of the sensor calibration data is still a major issue. The comparison of measured heat fluxes showed a clear impact of the flow regime (subsonic or supersonic) and enthalpy distribution across the model surface on the results.

Nomenclature CIRA COMETE CP CY DLR EADS ESA FC GG h HFS HFM IRS KHI L3K

= = = = = = = = = = = = = = =

Italian Aerospace Research Center inductively heated facility of EADS calorimeter probe cylinder, model geometry German Aerospace Center European Aeronautic Defence and Space Company European Space Agency flow condition Gardon gauge enthalpy heat flux output signal of HFM heat flux microsensor Institute of Space Systems at the University of Stuttgart Kawasaki Heavy Industries DLR’s 6 MW arc heated facility

*

Department Head, Windtunnel Department, [email protected] Research Scientist, Windtunnel Department, [email protected] ‡ Research Scientist, Plasma Wind Tunnel Unit, [email protected] § Research Scientist, Institute of Space Systems, [email protected] ** Research Scientist, TE374, [email protected] †† Assistant Professor, Aeronautics and Aerospace Department, [email protected] ‡‡ Research Scientist, Aeronautics and Aerospace Department, [email protected] 1 American Institute of Aeronautics and Astronautics †

Copyright © 2005 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

p PLASMATRON PWK-1+2 q&


RTS SCIROCCO SiC SIMOUN T TC TRP TS VKI

ε σ

= = = = = = = = = = = = = = =

pressure VKI’s inductively heated facility plasma wind tunnel of IRS heat flux rate temperature output signal of HFM CIRA’s 70 MW arc heated facility silicon carbide arc heated facility of EADS temperature thermocouple technological research programme tile sensor von Karman Institute surface emissivity Stefan-Boltzmann-constant

I.

Introduction

T

he re-usability, i.e. cost reduction, is one of the major objectives of the future space transportation system development. Because of some shortcomings of all three development tools, ground testing facilities, numerical codes and flight experiments, space vehicles are still designed and manufactured with large safety margins, which increase the costs. In particular, the design of vehicle components with higher thermal loads in a high enthalpy flow is performed conservatively because of the uncertainties of these development tools. Essential parameters for the design margins are the accuracy of the sensors, which are used in ground testing facilities or during flight for the measurement of the aerothermodynamic parameters like heat flux rate and surface temperature in a high enthalpy flow environment, and the physical interpretation of the measured data. In addition the quality of calibration and the repeatability of flow conditions in ground testing facilities play an important role in the assessment of test results and their consideration in the design.

Plasma facilities, mainly arc heated and induction heated facilities are essential for the ground testing of thermal protection materials and structures. Although they do not allow for aerodynamic simulation, they are well suited for long duration tests at high enthalpies and can provide a local aerothermodynamic simulation. Induction heated facilities are mainly operated in subsonic mode. Therefore they are suitable for the characterisation of thermal protection materials. For the design verification and qualification of thermal protection components of re-entry vehicles arc heated facilities with a supersonic flow field are necessary. The measurement of heat fluxes is one of the key measurement techniques in high enthalpy facilities, since heat fluxes are important quantities for judging the quality of a thermal protection system. Furthermore, heat flux measurements are widely used for flow characterisation. The differences in facility operation between induction heated and arc heated facilities may also take influence on heat flux measurements. Characteristic enthalpy profiles in the free stream may be completely different and lead to major differences in the boundary layer flow, which has a direct influence on the heat flux distribution on a model. The flow field is in non-equilibrium for both types of facilities, but due to the subsonic flow the deviation from equilibrium is smaller for inductively heated facilities giving rise to differences in gas surface interaction phenomena and finally in heat flux. In the frame of this study a comparative test campaign was performed on standard models in several European plasma facilities. Cross point tests on well-defined models were carried out using and comparing different heat flux measurement techniques. This comparative experimental study, which was partially supported with numerical simulation, provided additional information on the applicability and comparability of heat flux measurements in different types of plasma facilities.

II.

Experimental tools

The SCIROCCO Plasma Wind Tunnel is a major hypersonic arc heated facility, capable of producing a lowpressure high-enthalpy plasma flow of large dimension. I can be used for testing and validating the thermal protection system (TPS) materials on samples partially in scale 1:1 with a test duration up to 30 minutes [1,2]. Conical Nozzles with exit diameters of 0.9, 1.15, 1.35, and 1.95 m are in use. The arc heater of SCIROCCO is one of the biggest in the world in the family of constricted, low-pressure arc heaters. It can be configured in different lengths by modifying the number of modular packs of constrictor segments. Multi-gas testing or dilution of the primary air 2 American Institute of Aeronautics and Astronautics


for low enthalpy test conditions is provided by a mixing plenum chamber at the arc heater’s outlet. In order to prevent damages very advanced and non-conventional sensors such as voltage fluctuations and water vapour sensors are used in addition to the standard safety fields instruments. The test chamber has a cylindrical shape with an overall height of 9.217 m and an inner diameter of 5.17 m. It is installed vertically and the structure provides openings for personnel access as well as top and side windows for plasma flow monitoring and diagnostics. A purging fan for re-pressurisation and an analyser device for O2, O3 and H provide a correct restore of the air component concentrations inside the test chamber before access is permitted to the personnel. The plasma flow from the test chamber is collected and decelerated by the diffuser, which is essentially constituted by a horizontal cylinder, jacket cooled. The diffuser exit is directly connected to the plasma heat exchanger. The total length results to be 49.8 m and the inner diameter of the throat section is 2.12 m. The pressure rise inside in the axial direction is monitored by several pressure taps. The arc heated facilities LBK consists of two test legs L2K and L3K, which have been playing an important role in the qualification and testing of TPS components and materials in the frame of different European programmes [3,4]. Additional activities like tests on HYFLEX tile model and ablator material of the USERS capsule, etc. allowed the LBK team to gain more experience in the qualification of TPS components of space vehicles. The L3K facility is equipped with a segmented arc heater of 6 MW maximum electrical power, which allows to achieve total enthalpies up to 20 MJ/kg at reservoir pressures between 0.15 MPa and 1.8 MPa. Modular conical nozzles with exit diameters of 100 mm, 200 mm, 300 mm and 400 mm and a half angle of 12° are installed. In combination with the two throat diameters of 14 mm and 29 mm the nozzles provide free stream Mach numbers between 5 and 10 at Reynolds numbers up to 10000/m. Models with a size of 280 mm (W) x 350 mm (L) x 70 mm (H) can be tested in the homogeneous hypersonic flow field of this facility. In stagnation point configuration cold wall heat flux rates up to 4 MW/m2 at pressures up to 350 hPa can be set on models with a diameter up to 150 mm. The plasma wind tunnels at IRS have a continuous stream of plasma of high specific enthalpy, which is produced with the help of thermal or magnetoplasmadynamic generators (TPG or MPG) as well as with inductively heated generators [5,6]. The facilities equipped with a thermal arc generator are suitable for studies of aerodynamic loads of high enthalpy flows. High impact pressures and fairly high Mach numbers and specific enthalpies can be generated. However, the exhaust velocity is limited to several km/s and low impact pressures cannot be achieved. The erosion rate of heat protection material is especially high at low pressure levels and high enthalpies. These conditions can be simulated with the help of MPGs. The inductively heated generator in PWK 3 is used for basic research on thermal protection materials, especially regarding their catalytic behaviour, because no plasma impurities occur from electrode erosion. In addition, tests in a pure oxygen or in a mainly CO2 atmosphere, for example for entry into Mars' (incl. dust simulation) or Venus' atmosphere, can be performed. Tests were performed in the facilities PWK1 and PWK2 (both facilities are named PWK4 in the case of the thermal arc generator installation). The arc heated facility SIMOUN has a Huels type arc heater with an electrical power of about 5 MW [7]. Other main components are a supersonic nozzle, a test chamber and a convergent-divergent diffuser tube. Two nozzles are used to cover all of the operating domains: an axisymmetric nozzle for "stagnation point" tests and a super-elliptical nozzle (with flat surface extended by the plate of material), for "flat plate" tests. The flow - homogeneous and at the required conditions - enters the test chamber as a "free jet". The test chamber is cylindrical with a length and a diameter of 1 m. It includes four doors, three of which are equipped with windows for optical access (temperature measurements and visualizations), and one in its lower part that supports the test set-ups. Downstream the chamber, a convergent-divergent diffuser tube picks up the flow and provides a first stage of recompression. Two different diffusers are used, depending on the nozzle being used. All the facility components and the test set-ups are cooled by water circulation under pressure. The vacuum system includes nine steam ejectors arranged in three stages, providing the successive compressions. The working steam generated by the first two stages, which create vacuum in the chamber, is eliminated in two water spray condensers that are connected by barometric columns to the cooling water reserves, which are conditioned by a set of air coolers. The circulation is achieved by pump units. Upstream the extractor, a gas cooler lowers the flow temperature from 2,500 K to 400 K to reduce the volume per unit mass and the thermal stresses. COMETE is a high enthalpy facility with an induction heater, which was developed since 1965 by IPM (Institute for Problems in Mechanics). COMETE provides a high enthalpy, low pressure, subsonic flow around a flat faced cylindrical measurement arm [8]. High enthalpy, as the result of an electrodeless high frequency discharge, is obtained without any pollution. Low pressure in the test chamber is supplied by a vacuum system. COMETE allows reaching specific enthalpies up to 40 MJ/kg at total pressure levels of 30 hPa to 1000 hPa. The Mach number varies between 0.02 and 0.5. Maximum available Reynolds number is 140. Typical probe diameter is 50 mm. 3 American Institute of Aeronautics and Astronautics


The Plasmatron at VKI is an inductively heated facility that is equipped with an ICP torch of 160 mm diameter suited for large samples as well as for full TPS tiles [9,10]. The torch is mounted inside a support enclosure, which is fixed on one side of the test chamber. The test chamber is a 2.5 m long, 1.4 m diameter vessel equipped with multiple portholes and windows to allow maximal flexibility and unrestrained optical access for plasma diagnostic techniques. Inside the enclosure, the samples and probes are mounted on a fast-injection system. The plasma jet is collected at the outlet of the enclosure and cooled in the heat exchanger to a maximum temperature of 50°C to protect the vacuum plant from overheating damage. The vacuum plant consists of three volumetric vacuum pumps, which allow operating pressures between 1 hPa and atmospheric pressure with a maximum flow rate of 3000 m3/h. A Roots pump can be inserted in the circuit to achieve lower operating pressures. Exhaust gases are then vented to the atmosphere through a stack. The Plasmatron is equipped with a 1.2 MW, 400 kHz, high-frequency generator of relatively new solid-state technology, using thyristors and MOS inverters instead of vacuum tubes. An extensive closed circuit cooling system using de-ionised water protects all facility parts from melting due to the plasma heat, which is dissipated through three dry air coolers located on the roof of the installation. The full facility is computer-controlled from a remote cabin. The operator can check and control the main parameters through two computers in the cabin. It is also possible to prepare a small program to be introduced to the PLC of the facility, for performing an automated test.

III.

Models and sensors

The models used for testing are standard models. In the past, i.e. starting with the European HERMES project, a flat faced cylinder geometry with a diameter of 50 mm and an edge radius of 11.5 mm had been defined as standard geometry for material qualification tests. Due to this fact this geometry was chosen to be one of the reference geometries, denoted by CY-III. When defining the model geometries it had to be considered that the models have to be tested in facilities of very different size. The definition of CY-III was done based on the set of facilities, which were operational when HERMES was running. All these facilities were small-size or mid-size plasma facilities. Therefore the geometry CY-III is well suited for small-size facilities and applicable for mid-size facilities. The SCIROCCO facility, however, is a large-size facility. Some of its characteristic dimensions, e.g. the nozzle exit diameter, differ for almost an order of magnitude from small-size facilities. Considering the individual facilities’ model size capabilities it was found that it was impossible to test one geometry in all facilities, which are involved in the project. E.g., CY-III is too small to be handled by SCIROCCO’s model mounting system. Therefore, a second geometry was defined, which is manageable in SCIROCCO, but still can be investigated in a mid-size facility, as it is L3K. This geometry (CY-II) was chosen to be a flat-faced cylinder with a diameter of 100 mm and an edge radius of 11.5 mm. As a third standard model geometry (CY-I) a hemisphere with a diameter of 100 mm was defined. There were three main reasons to choose this geometry: first, it is very simple and therefore qualified to become a standard geometry. Second, there is extensive experience on testing this shape at SCIROCCO and L3K. In the end, this geometry is well suited for numerical treatment. The complete set of model geometries is listed in Table 1. Figure 1 shows all three CY-I, CY-II and CY-III cylinders with an integrated Gardon gauge sensor. Table 1. Model geometries type

diameter

edge radius

CY-I

hemisphere

100 mm

CY-II

flat-faced cylinder

100 mm

11.5 mm

CY-III

flat-faced cylinder

50 mm

11.5 mm

Figure 1. Geometries used for the comparative test campaign. 4 American Institute of Aeronautics and Astronautics

One of the main objectives of the test campaign was to compare different heat flux measurement techniques and obtain more information on their individual reliability. The set of measurement techniques includes sensor-based measurements of cold wall heat fluxes as well as the non-intrusive determination of hot wall heat fluxes. The following sensors were used:


• • • •

measurement of cold wall heat fluxes by a Gardon gauge with a fully catalytic surface, measurement of cold wall heat fluxes by a heat flux microsensor (HFM) with a highly catalytic surface, measurement of cold wall heat fluxes by a calorimeter probe with a fully catalytic copper surface, measurement of hot wall heat fluxes to a SiC probe having a low catalycity.

Gardon gauges basically consist of a thin metallic foil suspended over a cavity in a heat sink. It is thermally and electrically attached to the heat sink at the periphery of the cavity using metallurgical bonding techniques. The metallic foil (mostly constantan) and the material of the periphery (copper) form a thermocouple. By attaching a wire of the periphery’s material to the center of the foil’s rear surface a differential thermocouple is obtained, which measures the temperature difference between the center and the edge of the foil. This difference is directly proportional to the heating rate. The influence of the heat sink’s temperature should be considered by the specification of the sensor. Copper-Constantan combination meets the requirements with respect to the linearity of the sensor’s signal. The response time of these sensors is about 1 second. Data reduction is very easy, since the heat flux rate is directly proportional to the temperature difference. Low sensitivity and a limited surface temperature are the main disadvantages. When using the Gardon gauge in a non-cooled environment within a high enthalpy flow field the limitation to the surface temperature restricts the maximal time that the sensor can be exposed to high temperature conditions. In general, the exposure time should be kept as short as possible. Depending on the conditions, typical times are 1-2 seconds. For the comparative test campaign Gardon gauges with a diameter of 1/8” were selected. The very small sensitive area allows a nearly spot-wise measurement in the model’s stagnation point. The heat flux microsensor HFM consists of two elements: a thermopile heat flux sensor HFS and a resistance temperature sensing element RTS [11]. The RTS is composed of a pure platinum thin film, which is deposited in a loop pattern around the outer edge of the sensor face. The HFS consists of several thin-film layers forming a differential thermopile across a thermal resistance layer of aluminum nitride. Since the thermal resistance layer is very thin (δ ≈ 1 μm), the temperature difference across the layer is extremely small, even at high heat fluxes. Therefore, many thermocouple pairs are put in series to form a differential thermopile, which gives a measurable signal even at low heat flux rates. Platinum and Nichrome are used as thermocouple material, since they have a large thermoelectric potential and can withstand temperatures up to 1000 K. The response time of the heat flux sensor (HFS) is about 6 μs. HFS allows to measure both, transient and steady components of the heat flux rate. The RTS signal is used for the correction of the HFS signal with respect to the sensor’s surface temperature. The surface of the HFM sensor is coated with a thin Zynolyte layer to reach a high emittance of 0.94 at a spectral range around 2 μm. For measuring a cold wall heat flux in a high enthalpy flow with HFM the surface temperature has to be kept low during the measurement. If the sensor is embedded in a non-cooled environment, cold wall heat flux measurements are usually short duration measurements, where the sensor is exposed to the high temperature conditions for about 2 seconds only. The dimensions of the HFM sensor are comparable to those of the Gardon gauge. The calorimeter probe mainly consists of a thin metallic disc, which is exposed to the flow on one side and cooled by water on the other. At steady state the incident heat flux is completely transferred to the cooling water, and can be determined by measuring the water’s mass flow rate and temperature difference between the water supply and return lines. Besides a precise measurement of these quantities the elimination of lateral heat conduction to the probe holder is essential for achieving a high accuracy in the heat flux measurement. The calorimeter provides cold wall heat flux rates to a catalytic surface. Although the calorimetric probes of DLR, IRS and VKI are based on the same principle, there are differences in the design. SiC probes were used as standard for hot wall heat flux determination. During the test the probe’s front surface temperature is measured with pyrometers. The probe will be exposed to the high enthalpy flow until radiative equilibrium is achieved. At this condition the heat flux to the probe is equal to the radiation, which is radiated from the probe’s surface. The radiated heat flux can be determined from the measured surface temperature by applying the Stefan-Boltzmann relation.

5 American Institute of Aeronautics and Astronautics

IV.

Flow conditions and test matrix

Due to the different operating principles and sizes the operating regimes of the facilities vary in a broad range. Nevertheless, flow conditions had to be found, which can be realised in the majority of the facilities, and which are in addition representative for the experimental work in plasma facilities. Of course, not the complete flow field around a model can identically be duplicated in each facility. E.g., plasmatrons are operated in subsonic mode, while in arc heated facilities the models are exposed to supersonic or hypersonic flow. Therefore, the flow conditions were defined in such a way that the measured heat fluxes were comparable, i.e. that the parameters which define the heat flux rate were duplicated. All model geometries defined above are blunt bodies and the stagnation point heat flux of a blunt body mainly depends on the flow’s total enthalpy h0 and Pitot pressure pt2. Consequently, the flow conditions for the test campaign are defined in these two quantities. Two different flow conditions were defined:


flow condition I (FC-I): flow condition II (FC-II):

pt2 = 35 hPa, h0 = 9 MJ / kg, pt2 = 35 hPa, h0 = 13 MJ / kg.

The Pitot pressure was intentionally defined identical for both conditions. By that it is achieved that the mechanical contribution to the measurements is the same and a comparison of the two flow conditions can concentrate on purely thermal considerations due to the different enthalpy levels. For arc heated facilities equipped with a segmented arc heater FC-I is a condition that is located near the low enthalpy boundary of the operating regimes, while FC-II is located near to the centre of the operating regime. Considering the differences in the operating regimes of the participating facilities it was not possible to define two flow conditions that can be run in each facility. But the flow conditions are defined so that they can be run in the majority of the facilities and there are only very few combinations which cannot be realised. E.g., FC-II is outside the operating range of the SIMOUN facility and FC-I cannot be run in PWK-1+2. There is one aspect, which has to be discussed in more detail. The radial enthalpy profile at the model location is significantly different in plasmatrons compared to arc heated facilities. In an arc heated facility the profile is flat in the core part of the flow field, i.e. the total enthalpy is almost constant along the models’ front surface. In a plasmatron facility the enthalpy has a clear maximum near the axis and decreases outwards. Here, the enthalpy of the free stream varies along the models’ cross section. For this case, the enthalpy has to be averaged across the model and this averaged value is important for the setting of the facility’s parameters. As already mentioned above, there were also restrictions with regard to the model geometries. Not all geometries could be investigated in each facility and there also are some restrictions with regard to the measurement techniques. The different capabilities are accounted for in the test matrices. These were defined based on a preliminary comparative analysis of the facilities’ performance data using the following considerations: • • • • • •

One of the cold wall heat flux measurement techniques was defined as reference technique and was applied in all participating facilities. The reference technique for cold wall heat flux measurements should be a sensor-based technique in order to reduce the dependencies on facilities’ supply systems and interfaces. At least one facility should perform tests on all model geometries to allow for a direct comparison of the model geometries. At least one facility should run tests using all measurement techniques on the same model geometry to allow for a direct comparison of the measurement techniques. In each facility at least two of the cold wall heat flux measurement techniques should be applied. Whenever possible, each combination of flow condition, model geometry and measurement techniques should be investigated in at least two facilities.

The Gardon gauge and the SiC probe were chosen as reference techniques for cold wall and hot wall heat flux measurement, resp. In almost all facilities these techniques had been widely used during the past and the necessary equipment was available. In order to maximize the comparability of test results, identical sensors (HFM and Gardon) were used in the different facilities. In addition, also the SiC were provided centrally to ensure that all probes are made of exactly the same material belonging to a single charge of production. During a previous comparative test campaign in the L3K facility of DLR it had been observed that the heat flux output of different commercial heat flux sensors differ significantly not only when applied to high-enthalpy flow 6 American Institute of Aeronautics and Astronautics


fields, but even in quasi-standardized environments, as e.g. a black body radiation source. To account for this experience and to avoid the manufacturers’ calibration procedure to become important for the comparative tests all commercial sensors, i.e. Gardon gauges and HFM sensors, were procured centrally and they were subjected to a special calibration procedure before and after testing. For the calibration the sensors were mounted to a sensor socket and then fixed to the end of a cylindrical tube. Afterwards this tube was introduced into the orifice of a black body radiator. The radiator had been pre-heated to a given temperature level and the temperature inside was checked by a calibrated pyrometer. Three different nominal temperature levels, i.e. 1000°C, 1300°C, and 1500°C were used, which were chosen in order to achieve almost identical steps in nominal radiative heat flux up to the maximum temperature of 1500°C, which can be operated by the black body radiator. It has to be mentioned that neither the calibration setup nor the calibration procedure are suitable for absolute sensor calibration. But it is suitable for a relative assessment of the manufacturers’ calibration. Comparison of the heat flux rates measured by Gardon gauges and HFM sensors before the test campaigns shows a systematic difference. While the HFM measurements were in the range of the nominal heat flux rates, the Gardon gauges provided substantially lower heat fluxes, which differ by about 20% from the nominal radiative heat flux rates. These differences are related to the manufacturer’s calibration procedure only. The situation becomes even more precarious by the fact that both manufacturers refer to the same calibration standard. The calibration measurements were repeated after the individual test campaigns in order to obtain information about possible sensor aging. Comparison shows that for most of the sensors the post-campaign measurements differ by not more than 4% from the values obtained before testing.

V.

Experimental results

The test procedure for the tests on SiC probes defined a testing time of 300 s, which is appropriate for the heating process to reach steady state condition. Therefore, the hot wall heat fluxes to the probe can be computed from the measured steady state surface temperatures. The test results obtained on SiC probes are directly comparable. No further uncertainties apply, since the surface properties of SiC are well-known and pyrometry, which was used for temperature measurement, is a qualified standard measurement technique. Two geometries were investigated: geometry CY-II with a SiC probe of 70 mm diameter was tested in L3K and SCIROCCO, and geometry CY-III with a SiC probe of about 30 mm diameter in all facilities except SCIROCCO. The values listed in Table 2 show an excellent agreement of the heat flux rates obtained for CY-II. At both flow conditions the measured hot wall heat flux rates are almost identical in L3K and SCIROCCO. In addition to the values of total enthalpy and Pitot pressure the hot wall heat flux rate measured on SiC probes are influenced by the chemical composition of the free stream and the homogeneity of all free stream parameter across the area that is occupied by the model. Therefore, the agreement of the results from L3K and SCIROCCO, which both are driven by a segmented arc heater, confirm the homogeneity of the flow fields which were proven during the flow characterization by Pitot pressure and heat flux profile measurements. Table 2. Measured hot wall heat fluxes on SiC probes measured surface facility geometry temperature [°C] FC-I FC-II

heat flux rate [kW/m2] FC-I FC-II

L3K SCIROCCO

CY-II CY-II

1261 1260

1410 1410

267 266

387 387

L3K PWK-1 + 2 SIMOUN/COMETE PLASMATRON

CY-III CY-III CY-III CY-III

1309

1447 1654 1030 1330

302

422 665 139 318

1320 1150

310 198

For geometry CY-III the results differ significantly at each flow condition. At flow condition FC-I only the results from SIMOUN and L3K are comparable. Again, these are the two arc heated facilities. At the PLASMATRON the surface temperature is lower by 170 K, which corresponds to a 35% lower heat flux rate. It has been pointed out before that the flow field of arc heated and induction heated facilities differ significantly with respect to flow mode and homogeneity. While an arc heated facility generates a hypersonic free stream with a homogeneous core, the flow field inside an induction heated facility is subsonic with an inhomogeneous enthalpy distribution with a peak at the axis. Although the flow condition in the PLASMATRON had been chosen in such a 7 American Institute of Aeronautics and Astronautics

way that the average enthalpy across the model surface corresponds to the prescribed conditions, the differences in both, surface temperature and heat flux rate, are significant. This might be an indication that the distribution of energy and chemical composition in addition to the pure energy content of the flow strongly influences the probe heating.


The scattering of results is larger at flow condition FC-II. Even the results obtained in the two inductively heated facilities (PLASMATRON and COMETE) differ by far. One reason for this difference is concerned to the setting of the flow conditions. In COMETE the peak enthalpy is matched to the prescribed condition. Due the strong inhomogeneity of the flow field, the measured values are even lower than those which had been obtained in SIMOUN at the significantly lower enthalpy level of FC-I, but with a homogeneous enthalpy profile. As mentioned above the measurements in the PLASMATRON were performed using an averaged enthalpy condition, which leads to significantly higher heat flux rate compared to the peak enthalpy condition in COMETE, but there is again almost the same absolute difference between L3K and PLASMATRON that had been found for FC-I. The by far highest surface temperatures and heat flux rates were obtained in PWK-1+2. Compared to the L3K measurements the surface temperature is by about 200 K higher. This difference can hardly be explained from the way the facility is operated. More obviously it is related to a basically different way of enthalpy determination. This assumption is confirmed by the results of cold wall heat flux measurements using calorimetric probes which are listed in Table 3. Again the highest heat fluxes are measured in PWK-1+2. Table 3. Measured cold wall heat fluxes on calorimetric probes facility geometry measured heat flux rate [kW/m2] FC-I FC-II L3K PWK-1 + 2 SIMOUN/COMETE PLASMATRON

CY-III CY-III CY-III CY-III

1043 / 1032 439 (336)

1351 / 1294 2300 362 796 (655)

As well as the measurements on SiC probes the calorimetric heat flux measurements provide integral information and depend on the free stream conditions and the variation of flow parameters across the area that is occupied by the model. When ordering the calorimeter measurements according the amount of measured heat flux rates the sequence of the facilities is identical to the measurements on SiC probes, e.g. for FC-II the highest heat flux was measured in PWK-1+2 followed by L3K, PLASMATRON and COMETE. For the L3K and PLASMATRON two values are listed in Table 3. For L3K the values refer to two consecutive measurements at identical flow conditions. For the PLASMATRON the main value refers to the flow condition which matches the averaged enthalpy, while the value in brackets refers to the peak enthalpy flow condition. From the energetic point of view the peak enthalpy flow condition at FC-II should correspond to flow condition FC-II in COMETE. Nevertheless, the measured heat flux rates differ remarkably, the COMETE measurement being only 55% of the value measured in the PLASMATRON. Therefore, the results of non-local heat flux measurement techniques, as calorimeter and SiC probe, which are obtained in different induction heated facilities, do not necessarily agree, even if the flow condition and the procedure of setting the enthalpy level (average vs. peak) coincide. Due to the subsonic flow field design details of the plasma torch have a direct influence on the radial profiles of the flow parameters, and as a consequence also on the measurements. For the measurements with the Gardon gauges the measured values as well as the corrected heat fluxes are listed in Table 4. Here the amount of the corrections is about 20 percent, which had been found as a typical deviation to the reference heat fluxes during calibration measurements. For the comparison of cold wall heat fluxes Gardon gauge measurements served as reference configuration, which therefore was applied in all facilities as well as on all geometries and flow conditions. When comparing the arc heated facilities L3K and SCIROCCO, it becomes obvious that the agreement is not as good as it had been found for hot wall heat fluxes measured on SiC probes, irrespective of the geometry. In general, in L3K higher heat fluxes are obtained. With regard to the flow condition the differences are higher at FC-I and regarding geometry they are higher for geometry CY-II. In SCIROCCO the Gardon gauge measurements on geometry CY-I were compared to measurements using another water-cooled Gardon gauge, that is commonly used 8 American Institute of Aeronautics and Astronautics

in the facility. The comparison shows that both measurements are in good agreement as long as the corrections are considered. A similar difference is observed between L3K and SIMOUN (CY-III, FC-I). Here also, the SiC measurements were close to each other, while the values obtained from Gardon gauges are apparently higher in L3K. Unfortunately, due to the restricted operating conditions of SIMOUN this comparison could only be performed for a single flow condition and a single geometry. In particular, comparative results were more substantial if verified at another flow condition and/or geometry.


Table 4. Measured cold wall heat fluxes on Gardon gauges using correction from facility geom. measurement pre-test calibration FC-I FC-II FC-I FC-II

using correction from post-test calibration FC-I FC-II

L3K1 SCIROCCO2 PLASMATRON3

CY-I CY-I CY-I

1129 930 (1160) 332 (228)

L3K1 SCIROCCO2 PWK-1 + 2 PLASMATRON3

CY-II CY-II CY-II CY-II

678 / 690 480 190 (135)

733 / 822 620 765 336 (260)

848 / 863 608 227 (161)

916 / 1028 785 956 402 (311)

832 / 847 590 229 (163)

899 / 1009 763 966 405 (314)

CY-III 1024 CY-III CY-III 770 CY-III 328 (243)

1057 / 1214 1175 290 484 (401)

1280 952 392 (290)

1321 / 1518 1469 358 578 (479)

1256 942 396 (293)

1297 / 1490 1484 355 584 (484)

L3K1 PWK-1 + 2 SIMOUN/COMETE PLASMATRON3

1325 / 1437 1411 1656 / 1796 1385 1626 / 1763 1250 (1510) 1177 (1160) 1583 (1510) 1144 (1160) 1538 (1510) 525 (460) 397 (272) 628 (550) 400 (275) 633 (555)

1

If two values are listed, they refer to two individual measurements at identical conditions. Numbers in brackets refer to measurements using a different Gardon gauge (SCIROCCO standard gauge). 3 Numbers in brackets refer to peak enthalpy condition. 2

The results of the Gardon gauge measurements in PWK-1+2 are comparable to the L3K measurements for both geometries that were investigated. A result like that could not be expected, since at the same flow condition the measurements on SiC and calorimetric probes provided significantly higher heat fluxes in PWK-1+2 compared to all other facilities. In combination with the differences that were observed in the Gardon gauge measurements of SCIROCCO and L3K this observation indicates that there are parameters that Gardon gauge measurements are sensitive to, while the other techniques are not. In the induction heated facilities PLASMATRON and COMETE the Gardon gauges provide substantially lower heat fluxes, as it already had been observed for the SiC and calorimetric probes. At FC-I the PLASMATRON heat fluxes reach between 27 and 31 percent of the values measured in L3K, at FC-II that percentage is between 38 and 43. Here, L3K has been taken as reference not because its measurements are regarded as more exact than others, but only because measured values are available for all geometries. When comparing the two induction heated facilities the Gardon gauges provide lower heat fluxes in COMETE (for the PLASMATRON the values in brackets must be considered), but the differences are not as large as found for the calorimetric measurements. The heat fluxes that were measured with HFM sensors are listed in Table 5. Since the HFM sensor was not chosen for cross checking the number of measurements available for comparison is reduced compared to Gardon gauge. In general, the measured values confirm most of the comparative results that were deduced from the Gardon gauge measurements. Again, deviations between L3K and SCIROCCO are observed, but here the differences are more prominent for flow condition FC-II, while the values are closer to each other for FC-I. The agreement between SIMOUN and L3K is better. The HFM measurements differ by 10%, while for the Gardon gauge a difference of 34% was obtained. The HFM measurements in the PLASMATRON provided similar heat flux values as the Gardon gauge measurements, the level being again much lower than the results obtained in the arc heated facilities. In COMETE a higher heat flux is measured by the HFM sensor. The value, however, is still lower than for the corresponding peak enthalpy condition in the PLASMATRON.


Table 5. Measured cold wall heat fluxes on HFM sensors using correction from facility geometry measurement pre-test calibration FC-I FC-II FC-I FC-II L3K

CY-I

L3K SCIROCCO PLASMATRON1

CY-II CY-II CY-II

L3K SIMOUN/COMETE PLASMATRON1

CY-III CY-III CY-III

using correction from post-test calibration FC-I FC-II

1204

1828

1200

1823

1241

1885

658 560 216 (100)

1120 780 427 (353)

656 580 210 (97)

1117 808 415 (343)

678 571 221 (103)

1155 795 438 (362)

1039 1100 388

1580 420 612

1036 1141 377

1575 436 594

1071 1124 398

1629 429 627


1

Numbers in brackets refer to peak enthalpy condition.

VI.

Conclusion

In the frame of this study the following main results were achieved: • The measured so-called hot wall heat flux rates to a SiC probe in the arc heated facilities SCIROCCO and L3K show an excellent agreement. Both facilities use the same type of heater, a segmented arc heater. The data of the SIMOUN facility equipped with a Huels type arc heater is also comparable to these data. The PWK-1+2 facility of IRS with a magnetoplasmadynamic plasma generator provides a significantly higher heat flux rate. The two inductively heated facilities PLASMATRON and COMETE measure remarkably lower heat fluxes compared to the arc heated facilities. Even the data of these two facilities, which operate with a fully subsonic flow field and the same type of heater, differ strongly. It has to be mentioned that the enthalpy profile in these facilities has a strong peak on the flow axis. The measurements at VKI’s PLASMATRON showed that enthalpy averaging across the model surface is not sufficient for obtaining heat flux rates that are comparable to arc heated facilities. • The agreement of cold wall heat fluxes measured with a HFM sensor in SCIROCCO and L3K is not as perfect as in the hot wall case. The deviation varies from 11% to 27%, while the SIMOUN results are close to the L3K data. Again, significantly lower heat fluxes have been measured in the inductively heated facilities PLASMATRON and SIMOUN. • Gardon gauges provide cold wall heat fluxes that are comparable to HFM sensors. The same tendency has been noticed with respect to the comparison of the different facilities. The PWK-1+2 facility, where no measurements with HFM sensors were performed, provided heat flux rates that were comparable to L3K. This result was not expected, because with all other measurement techniques the by far highest heat flux rates were obtained in PWK-1+2. • The cold wall heat flux rates measured with calorimeters in L3K, PWK-1+2, COMETE and the PLASMATRON differ significantly. The highest value was obtained in PWK-1+2, the lowest in COMETE. As for the hot wall heat fluxes there are substantial differences between the two inductively heated facilities. In order to understand the differences in the results of the different facilities on the same model configuration, a better flow characterisation is essential, which also includes an experimental characterisation of all free stream parameters including chemical composition and influence of thermal non-equilibrium effects using sophisticated spectroscopic measurement techniques.

Acknowledgments This study has been performed in the frame of an ESA TRP. The financial and scientific support of ESA colleagues in particular Mr. Jean Muylaert is acknowledged. .


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