- Email: [email protected]
Two-phase experiment for the in-orbit demonstration of two-phase heat transport system technology
Adv. Space Res. Vol. 16. No. 7. pp. (7)113-(7)122, 1995 ccpyrisht 0 1995 COSPAR Printed in Great Britain. All ri ts reserved. 0273-l 1771pL?$9.50 + 0.00 0273-l 177(95)00145-X
Pergasnon
TWO-PHASE EXPERIMENT FOR THE IN-ORBIT DEMONSTRATION OF TWO-PHASE HEAT TRANSPORT SYSTEM TECHNOLOGY A. A. M. Deli1 NationalAerospace LaboratoryNL.R,P.0. Box 153, 8300 AD Emmetoord, F%eNetherlands
ABSTRACT As two-phase flow and heat transfer depends on gravity, a Two-Phase experiment has been developed for ESA to demonstrate two-phase heat transport system technology in orbit. TPX, a two-phase ammonia system, has tlown in the 5ft3 nitrogen gas filled Get Away Special canister G5.57, aboard STS60. The system, a reduced-scale capillary pumped two-phase loop with a flat and a cylindrical capillary evaporator and an actively controlled reservoir for loop temperature setpoint control, included downscaled components of a mechanically pumped loop: multichannel condensers, vapour quality sensors, and a controllable 3-way valve for control exercises. The flight configuration of the autonomous experiment (own power supply, data handling, experiment control) is presented. Preliminary results of a comparison between pre-flight, flight and post-tlight data are discussed. BACKGROUND Thermal management systems for future large spacecraft must transport large amounts of waste heat over large distances. Conventional single-phase systems (based on the heat capacity of the working fluid) are simple, we11 understood, easy to test, relatively inexpensive and low risk. However, to realise proper thermal control with small temperature drops from equipment to radiator (to limit radiator size and mass), they require noisy, heavy, high power pumps and large solar arrays. An alternative for single-phase systems is the mechanically pumped two-phase system: a pumped loop which accepts heat by evaporation of the working tluid at heat dissipating stations and release heat by condensation at heat demanding stations and at radiators, for rejection into space. Such a system relies on heat of vaporisation: it operates nearly isothermaily and the pumping power is reduced by orders of magnitude, thus minimising radiator and solar array sizes. The stations can be arranged in series, parallel or hybrid configurations. The series configuration is the simplest, it offers the possibility of heat load sharing between the different stations, with some restrictions with respect to their sequence in the loop. But it has limited growth potential and the higher flow resistance. In the low resistance modular parallel concept, the stations operate relatively independently, thus offering full growth capability. However, the parallel configuration is more complicated, especially when redundancy and heat load sharing (some cold plates operating in reverse mode) is foreseen. A parallel configuration also requires a control system consisting of various sensors, monitoring the loop performance at various spots, control logic and actuators to adjust pump speed, reservoir fluid content and throughputs of valves. Sensors necessary for control are pressure gauges, flow meters, temperature gauges and vapour quality sensors, measuring the relative vapour mass content of the flowing mixture, e.g. at the cold plate exits, as a part of a control system adjusting the liquid fed into a cold plate to prevent evaporator dry-out or to maintain a prescribed quality value at evaporator exits, independent from transient heat sources and heat sinks. ESA supported mechanically-pumped two-phase loop activities have led to the development of components considered to be critical for twophase applications. These critical components, extensively and successfully tested in a R114 technology development two-phase test bed /l/, include amongst others: a newly developed multichannel condenser and a Vapour Quality Sensor, designed as an axial field capacitance meter, based on the differences in dielectric permittivity of vapour and liquid /l & 2/. (7)113
0)114
A. A. M. Deli1
An alternative for a mechanically pumped loop is the capillary pumped system, using surface tension induced pumping of capillary evaporators. Such capillary two-phase systems can be applied in spacecraft not allowing pump induced vibrations. Ammonia is the best working fluid for such loops. As two-phase flow characteristics and heat transfer differ in l-g and low-g environment /3/, the technology of such two-phase heat transport systems and their components was to be demonstrated in orbit. Therefore, the development of a Two-Phase loop experiment started in 1990 within the ESA In-orbit Technology Demonstration Programme TDPl, by NLR (NL, prime contractor), SABCA (B), FSS (NL), BE (NL) and SPE (NL). The flight experiment is a downscaled capillary pumped twophase ammonia system together with scaled-down components of a mechanically pumped loop: multichannel condensers, vapour quality sensors and a controllable three-way valve. The complete experiment, integrated in a Get Away Special canister (5 ft3, gaseous Nitrogen filled), has flown as G557 aboard the US Space Shuttle STS60, early February 1994. TPX has run autonomously, using own power supply, data handling and experiment control, after switch-on by the Shuttle crew. EXPERIMENT BASELINE, OBJECTIVES, PHILOSOPHY The experiment baseline is depicted in Fig. 1. Heat, supplied to the parallel capillary-pumped evaporators (a flat and a cylindrical one), causes evaporation of the working fluid, setting the total mass flow rate and generating pumping pressure to maintain the tluid circulation in the system. The heat, extracted from the fluid in the two condenser sections and the subcooler, is radiated to space via the canister lid.A thermal accumulator (reservoir), which contains liquid and vapour in equilibrium, is used to control the temperature setpoint of the loop. To increase the loop temperature the Peltier element heats the accumulator, hence increases the volume of the existing vapour bubble and thereby pushes liquid into the loop. This liquid blocks part of the condenser, increasing its thermal resistance. If the heat load applied to the loop and the sink conditions do not change, this increase in thermal
APS DPS EC
Absolute Pressure Sensor Differential Pressure Sensor Cylindrical evaporator Flat evaporator Ef EMP Experiment Mounting Plate GAS Get Away Special
LFM A Tc TV
Liquid Flow Meter Mass flow Temperature condenser Temperature vapour Temperature liquid Tt VQS Vapour Qualily Sensor
cylindrical pmllm enclosure
insulating
hltelface equipment We
upper insulating cover (as raquiredl
healout
Y ‘f
510’ahrmlnlum
Fig. 1. TPX Baseline.
L
corGiwy lowerinIulatlnn CoveI
dumlnkedkBpton
Fig. 2. GAS Canister Dimensions.
Two-PhaseHeatTranspd System
ONI5
resistance will result in an increase in loop operation temperature. To reduce the loop temperature setpoint, the Peltier element is used as cooling device, resulting in a flow of liquid into the accumulator. The loop contains two VQS, a controllable three-way valve and evaporator depthsing heaters. The experiment had to fulfil the GAS requirements and restrictions /4/: allowable volume (I%%.2) and mass (maximum 90 kg), no power and communication links to STS (hence limited battezy energy and internal data storage), Shuttle attitude dependent thermal sink (Fig. 3) and only on/off crew commands. The baseline had to meet the goals for the different experiment constituents: Capillary Pumped Loop, Vapour Quality Sensors and multichannel condensers, each being a scaleddown version of the concept originally developed for high power systems (up to 10 kW).
100
5 FT3 container without insulnted end cap
container temperature (“c)
Fig. 3. GAS Thermal Sink Conditions.
Fig. 4. TPX.
CPL-related objectives were to show in low-g: the capability to transport heat in a smooth and continuous way, proper operation for different heat loads on the parallel evaporators, proper heat load sharing and start-up from low temperatures, the capability to prime an evaporator by a controlled management of the reservoir fluid content, plus the capability to adjust to and maintain a setpoint temperature (under varying heat load/sink conditions) by proper accumulator content control. A cylindrical and a flat evaporator were present to asses low-g transport capabilities, maximum pumping pressures, depriming, repriming capabilities (by controllable accumulator actions), evaporator heat transfer coefficients and the interaction of parallel evaporators, including heat load sharing. An additional objective was to assess the limits of heat transport capability in low-g and on earth and to compare these with ESATANIFHTS thermal modelling predictions. Two VQS were present to prove the concept in space, to demonstrate the proper performance of the sensors accomodating ammonia, to compare the performance of the two sensors in order to assess the influence of the location within the loop and of small construction differences, to perform calibrations and to assess the differences in low and terrestrial performances (important for the design of future space-oriented quality sensors) and to carry out some simple CPL control exercises with the to show the usefulness of vapour quality sensors for system control and to demonstrate the proper performance of the controllable three-way valve. Multichannel condenser objectives can be summa&d by: demonstration and verification of the working principle and determination of the performance limits in a low-gravity environment (aspects of power transported, pressure drop, distribution of the fluid over the different channels and prevention of channel blockage included). Detected condensation front locations can be used to interpret experimental data of the VQS located between the two condenser sections. An additional objective was the use of the low-g and terrestrial TPX test data and the outcomes of testing in the NLR two-phase test loops, in order to verify the NLR approach for the thermalgravitational scaling of two-phase flow and heat transfer /5 & 6/.
(7)116
A. A. M. D&l
In order to fulfil the objectives an experimental flight programme had been carried out, until the battery was exhausted. It included the following individual experiments: Balanced power profile: the same power levels applied to both evaporators. Unbalanced power profile: different power levels fed to the two evaporators, to study interaction. Heat load sharing: heat load applied only to one evaporator, the second one acting as condenser. DeprimingIrepriming: a special depriming heater, shortly powered to induce an evaporator dry-out, followed by a repriming action using the accumulator. Transient response to a step in power applied to an evaporator in a steady system situation. Start-up behaviour and assessment of performance limits. Loop temperature control: maintenance of loop set point and control of setpoint changes. Quality setpoint control: control of bypass valve to control a vapour quality in the VQS. System control: maintain (by accumulator and bypass valve control) the loop saturation temperature setpoint and vapour quality at the mixing point, under changing heat loads and sink temperatures. The predefined test sequence of the above experiments has been based on extensive ground testing. STRUCTURE & LOOP The configuration, shown in the Figs. 4 to 6, consists of a structure of four cohunns and three parallel plates: at one end of the columns a plate accommodating the battery and the electronics hardware at either side, at the other end of the columns the loop plate, attached in a well conductive manner to the canister lid and accommodating most CPL components, and a plate in between, 40 mm from the base plate, thermally decoupled from the others, and accommodating the evaporators. Materials: structure Al 7075, CPL tubing and components Al 6061, liquid flow meters stainless steel.
fi
Fig. 5. Loop & Base Mounting Plate Layout.
Fig. 6. Loop & Evaporators Plate Layout.
Two-Phase Heat Transport System
ON 17
POWER, DATA HANDLING, SAFETY The Data Acquisition and Control system included all electric/electronic hardware and the sofhvwe for testing and operating TPX, storing measured data, includhrg retrieval of experimental data. DAC flight hardware included Battery Pack, Cable Harness, and Payload Measurements and Control Unit. The battery, selected to provide the power for the experiment during the flight, was a modified MAUS battery: 1800 Wb @ 28.5 V DC. The PMCU was the on-board control for experiment execution and safe-guarding, sensor measurements, actuator control, data storage and communication with Electrical Ground Support Equipment. The PMCU contained the System Processor Unit, Sensor Data Acquisition Board and Actuator Control Board, interconnected by a standard VME bus. DC-DC converters provided internally required voltages for analogue and digital circuitry, and for the sensors and actuators. Fuses at appropriate locations ensured functionality and safety of the electrical system. To receive all relevant information about the performance of loop and components - location of condensation front, degree of subcooling, loop set point, evaporator temperature distribution and pressure drop, reservoir and baseplate temperatures - temperatures (38), flow rates (2), vapour qualities (2), absolute pressure, pressure drop across the evaporators, valve position, evaporator heater (2) and depriming heater currents (2), and peltier current were measured. DAC software consisted of on-board and EGSE software. As operational behaviour and details about experiment parameters were test sequence dependent (set by actual in-orbit conditions), the on-board software was split into a fixed programme and a set of experimenter defined tables, without compromising the reliability of the software. Major functions of the embedded software pertained to planning of the experiment, data acquisition from all sensors, execution of specified control algorithms, actuators control, data recording and safeguarding. Because of containment of ammonia the two-phase loop had to be protected against an overheating leading to unacceptable loop pressure. All pressurised components were designed for a maximum pressure of 45 bar. Each component as well as the completely assembled loop has been proof pressure tested with a factor of 1.5 against that value (burst test factor 4). During the whole operation of the experiment the PMCU had to measure the temperature sensors, switching off a heat dissipating unit, when the predefined maximum allowable temperature of this unit was reached. There was also a thermal switch on the vapour line, which had to interrupt power to the evaporator heaters, depriming heaters and Pettier element, when the maximum allowable loop temperature of 80 “C was reached. Also a pressure relief device was installed, designed such that it had to open and release ammonia at a predefined pressure, to follow the approach ‘leak before burst’. Also housekeeping data (6 voltages, PMCU current and 10 temperatures) were to be measured. The measuring/control interval was set to 5 s, as most parameters follow slowly changing temperatures. COMPONENTS The various TPX components are shown in the next, including some test results. 171 contains detailed information on pre-launch testing results. The flat evaporator (Fig. 7) is a (heated) base plate with microchannels for the vapour, a 30 pm porous poly-ethylene wick with an inlet hole for liquid, and a box shell which has been electron beam welded on the base plate, having an inlet tube, tefion insulator and outlet vapour tube. Measured performances & characteristics are: . Mass 0.6 kg. l Heat load 155 W maximum/8.8 W minimum. l Burst pressure 260 Bar. l Capillary static pressure (ammonia) 2933 Pa. l Pumping pressure at 155 W 2200 Pa. The cylindrical evaporator (Fig. 8) is a cylindrical &a&i) shell with inner circular grooves for the vapour, a liquid inlet tube with teflon insulation, an outlet vapour tube and a 30 pm porous polyethylene rod with vapour collecting grooves and an inlet hole for the liquid. Measured performances & characteristics are: . Mass 0.28 kg. 0 Heat load 400 W maximum, 7.7 W minimum. l Pumping presure at 150 W 1800 Pa. l Capillary static pressure (ammonia) 2707 Pa. 0 Burst pressure more than 200 Bar. The control reservoir (Fig. 9) is a cylindrical vessel with an inlet/outlet (liquid) tube, a liquid/vapour separator wick made of 30 pm polyethylene, an acquisition (flower shape) wick and a cover welded on the vessel, equipped with two Peltier elements and a copper braid connected to the inlet. Measured performances & characteristics are: l Mass 0.92 kg. 0 Electric power for Peltier control 4.7 W max. l Burst pressure 275 Bar. 0 Liquid content 0.17 litre. l Temperature control accuracy 0.1 K in range 263 - 323 K.
A. A. M. DeliI
Fig. 8. Cylindrical Evaporator.
Fig. 7. Flat Evaporator.
glsssssn& slsmsnt,wmelsctrodes
'Altubs
46. ,
Fig. 10. Vapour Quality Sensor.
Fig. 9. Control Reservoir.
Calibration of the VQS (Fig. IO) has been done in the automated NLR test rig /7/. Typical test results for the - four 3.2 mm ID parallel channels - condenser (horizontally positioned in the aforementioned rig) are depicted in Fig. 11: Steady-state temperature profiles for a power of 68 W and various sink temperatures and in Fig. 12: Thermal transients (at a 30 “C sink temperature) for power steps from 68 W to 30 W, back to 68 W. Experiments with the condenser vertically positioned have been carried out also. 40 h 0*
I
I
401
I
I
I
35-
J30iii $ g 252 20-
150
66W 66 w 50 0
3ow
66W
-4 loo #
150 1
200
250 L
condenser length (mm)
Fig. 11. Steady-State Condenser Temperature Profiles.
300 1
25
/-Gaq, 0
500
1000 time (s)
Fig. 12. Condenser Thermal Transients.
1500
Two-Phsse Heat Trsnspmt System
(7)i 19
PRE-LAUNCH TESTING OF THE SYSTEM Pre-launch testing /7/ consisted of the already mentioned proof pressure test, vibration, modal surveys and random vibration tests according to the GAS specifications 141, thermal performance & acceptance tests and the definition of the flight scenario. Typical results of the performance testing are presented in the Fig. 13. They illustrate the control of the loop vapour temperature setpoint 40 “C for variable heat loads. Fig. 13 depicts the histories of power fed to evaporators and Peltier cell (setpoint control), the corresponding temperature histories for the cylindrical evaporator: surface, out & inlet temperatures (Similar behaviour has been observed for the flat evaporator), and the corresponding temperature history for the long condenser. The figure shows that for low power (50 W) the condenser was completely flooded (reaches loop plate temperature), for high power (200 W) the condenser became fully efficient. After successful testina and flight scenario determination. TPX has been delivered at Kennedy Space Center, to fly on ST!@ laun&ed February 3, 1994. ’
I
I
%
(
I
I
*
c
I
I
[email protected] 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8
&d-
8
1.012 1.4 1.6 1.8 2.0 22 2.4 2.6 28
time (in thousands sets)
time (in thousands
151 1.0 1.2 1.4 1.6 1.8 20
sets)
time (in thousands
22 2.4 2.6 2.8 sea)
Fig. 13. Power histories, corresponding cylindrical evaporator and long condenser temperature histories. PRELIMINARYFLIGHT RESULTS Data retrieved after the flight amount to 1.9 Megabyte, stored before battery exhaust, meaning 40 hours total experiment time, divided into three runs: experimental sequences that start with the very short health check for experiment initialisation. Each run consists of (up to four) passes, containing only those experiments not done before in the same run. Fig. 14 surveys the first 18 hours. Fig. 15 shows the histories of the power and of several important temperatures during the entire 42 hours. From the baseplate temperature curve it can be concluded that TPX has run far hotter than foreseen, due to the fact that the actual in-orbit thermal environment considerably differed from the
i
Pass2
i PasS3:
15 t
; l&l.p*tonn#ommr#p, ;. pi .~ .~ ziidzF-YR
:
0 0 10 -.
0 m -m 0 n
5-
0 .m._ n
=
j
.
i .!!!!!YP~ "‘.‘MT'~ .., .; i .wsunl j ...*.......... *........ .... ..._................ .;,,... .i__._. .,,,... dPf== .....~ ~..."..' :
l
I.
m
i
w
;m 2z$&
Cl
q
md= m.._
” D _m.
O3
RUN2
! Pass4_
experiment number
I 20.000
+
Fig. 14. TPX Flight Experiments: Run 1.
..&
I 4O.wO
run time (s)
..:
j;.
I I 60.000’
. ._._ .. smftup , m.wO
.
A. A. M. Deli1
OWJ
unobstructed (ideal) thermal radiation environment, being the baseline of the design. Both the presence of the large (energy dissipating and free view obstructing) CAPL experiment and the fact that the TPX position on the GAS bridge was extremely close to the Shuttle bay aft bulk (say less than 0.5 m) are responsible for experiment temperatures substantially higher than the design values (Fig. 3). The consequences of the above are: - As Fig. 14 shows, the long lasting experiment constituents (VQS survey & control, maximum performance, temperature control, balanced/unbalanced heat load) could not be fully completed (within the timeframe allowed) because of the continuously increasing experiment temperature. - The (quasi-) temperature equilibrium, reached after the first 14 hours (Fig. 15), was much higher than expected and the remaining 26 hours consist of switching-on experiments that were shutdown (to cool down) after a limited running time (because a pre-set dry-out criterion was met). Though these experiments (essentially being transient on-off experiments) could never be completed, they are considered to yield valuable information on the transient behaviour of two-phase loops. - As the pre-launch test data pertain to thermal sink conditions far below the in-orbit sink conditions, they are not very adequate to be used for direct comparison with the flight data in order to identify the differences between low-g and one-g two-phase flow and heat transfer. For a correct identification the data to be compared shall pertain to -as close as possible- identical conditions. This meant that TPX had to be re-tested on earth for power and thermal sink conditions recorded during the flight: the full 40 hours flight experiment were to be simulated. This activity also provided calibration data for the liquid flow meter that had to be repaired, in the loop, shortly before launch.
T (“Cl power (Watt * 10)
loo
50 time (s)
Remark:one
t l-lliudulM_ thousands
io
datapoint every 100 seconds is displayed
Fig. 15. 40 hours histories of power fed to evaporators, and evaporators, battery and base plate temperatures. Anticipating a detailed evaluation after the post-flight testing, it already can be concluded that TPX (the overall two-phase loop and all components) functioned without ammonia leakage, without system or component failure. This is illustrated by graphs depicting the time histories of the flat evaporator temperature FET and power Pfe, the loop vapour temperature VAPT, the cylindrical evaporator temperature CET and power Pee, the differences between evaporators and reservoir temperatures FET-ResTemp, CET-ResTemp, the vapour quality measured between the condensers VQSl, the temperature of the inlet node on the second condenser TC2Pl and the swalve position SWALVEPOS. Fig. 16 shows as example the first 2 hours of the experiment, first pass of first run, pertaining to very short health check, start up, balanced/unbalanced heat load and heat load sharing experiments. Both evaporators have functioned, handled power up to or even over the design value, the minimum power behaviour being similar or even better than predicted. The cylindrical evaporator showed high sensitivity to heat load changes, frequently meeting the pre-detined dry-out citerion.
Two-Phase Heat Transport System
17)121
Fig. 16. Flight data of the first 2 hours. FLOW PATTERNS & CONDENSATION LENGTHS A typical example of a measured condenser temperature profile (100 W power, baseplate temperature 26.3 “C, vapour temperature of the loop 40.0 “C) is shown in Fig. 17. Comparison with Fig. 11 yields that for the small diameter condenser fines the condensation length needed in low-g conditions is only slightly larger than in one-g. The ratio of zero-g and terrestrial full condensation lengths for the 3.2 mm ID condenser channels (Fig. 18) clearly fits the curves following from the NLR model developed /5 & 61. Since this model is valid for pure annular flow only, it is worthwhile to investigate the impact of other flow patterns present inside the condenser duct (mist flow at the high quality side, slug and bubbly flow at the low quality side and wavy-annular-mist in between), in other words to investigate whether the pure annular tlow assumption leads towards slightly or substantially overestimated full condensation lengths. A complication is the lower boundary of the anuular-wavymist flow pattern. It is stressed that the transition vapour quality value reported in literature 0.15 /3/ pertains to R114 flowing in a 15.8 mm duct. Analysis of TPX vapour quality sensor flight data indicates that the transition annular-slug occurs, for ammonia flowing in a 5 mm line section, at vapour qualities around 0.3. The above means that flow pattern transitions occur at vapour quality values, that strongly depend on working fluid, operating conditions, and line diameter.
(7)122
A. A. M. Deli1
17
50
83 117 150 183 217 condenser length (mm)
250
283
Fig. 17. TPX second condenser flight temperature profile for 100 W, loop at 40.0 “C, baseplate at 26.3 “C.
pipe diameter (mm)
Fig. 18. Ratio of low-g and terrestrial full condensation lengths.
FINAL REMARKS TPX has successtitlly flown. Preliminary conclusions could be drawn, but final conclusions wait for the detailed comparison of flight data and data resulting from the post-flight tests, using the actual inflight temperature conditions. Based on these outcomes and lessons learned, an improved experiment scenario will be defined for a possible re-flight (in the same or a slightly modified configuration). ACKNOWLEDGEMENT I express my appreciation for all efforts of the TPX team members: W. Supper, R. Aceti (ESTEC); J.F. Heemskerk, A. Monkel, M. Versteeg, G. v. Donk, A., Pauw, C. Slippens (NLR); M. Dubois, S. v. Oost, D. v. Oost (SABCA); R. Voeten (BE); R. Linssen (SPE); M. Coesel (FSS). REFERENCES 1. N. Dunbar & R. Siepmann, European Two-Phase Heat Transport Technology, SAE 901271, 20th Int. Conf. on Environmental Systems, Williamsburg, VA/USA, July 1990. 2. A.A.M. Deli1 & J.F. Heemskerk, Development of a sensor for high-quality two-phase flow, NL& A4P 880.59 U, 3rd European Symposium on Space Thermal Control and Life Support Systems, ES4 SP288, Noordwijk, Netherlands, Oct. 1988, 113. 3. D.G. Hill, K. Hsu, R. Parish & J. Dominick, Reduced gravity and ground testing of a two-phase thermal management system for large spacecraft, SAE 882084, 18th Intersoc. Conf. on Environmental Systems, San Francisco, CA/USA, July 1988. 4. NASA Goddard Space Flight Center, Get Away Special (GAS) small self-contained payloads,
Experimenter Handbook, 1985, last update 199 1. 5. A.A.M. Delil, Thermal gravitational modelling and scaling of two-phase heat transport systems: Similarity considerations and useful equations, predictions versus experimental results, NLR Tp 91477 U, ESA SP3S3, 1st European Symp. on Fluids in Space, Ajaccio, France, Nov. 1991, 579. 6. A.A.M. Delil, Gravity dependence of pressure drop and heat transfer in straight two-phase heat transport system condenser ducts, h%R TP 92167 U/SAE 921168, 22nd Int. Conf. on Environmental Systems, Seattle, WA/USA, July 1992, SAE Trans. J. of Aerospace, Ip1 (1992) 512. 7. A.A.M. Delil, J.F. Heemskerk, M. Dubois, S. van Oost, W. Supper & R. Aceti, In-orbit demonstration of two-phase heat transport technology: TPX/G557 development & pre-launch testing, NL.R 7P 93394 CJ. Sti 932302, 23rd Int. Conf. on Environmental Systems, Colorado Springs, CO/USA, July 1993.
Pergasnon
TWO-PHASE EXPERIMENT FOR THE IN-ORBIT DEMONSTRATION OF TWO-PHASE HEAT TRANSPORT SYSTEM TECHNOLOGY A. A. M. Deli1 NationalAerospace LaboratoryNL.R,P.0. Box 153, 8300 AD Emmetoord, F%eNetherlands
ABSTRACT As two-phase flow and heat transfer depends on gravity, a Two-Phase experiment has been developed for ESA to demonstrate two-phase heat transport system technology in orbit. TPX, a two-phase ammonia system, has tlown in the 5ft3 nitrogen gas filled Get Away Special canister G5.57, aboard STS60. The system, a reduced-scale capillary pumped two-phase loop with a flat and a cylindrical capillary evaporator and an actively controlled reservoir for loop temperature setpoint control, included downscaled components of a mechanically pumped loop: multichannel condensers, vapour quality sensors, and a controllable 3-way valve for control exercises. The flight configuration of the autonomous experiment (own power supply, data handling, experiment control) is presented. Preliminary results of a comparison between pre-flight, flight and post-tlight data are discussed. BACKGROUND Thermal management systems for future large spacecraft must transport large amounts of waste heat over large distances. Conventional single-phase systems (based on the heat capacity of the working fluid) are simple, we11 understood, easy to test, relatively inexpensive and low risk. However, to realise proper thermal control with small temperature drops from equipment to radiator (to limit radiator size and mass), they require noisy, heavy, high power pumps and large solar arrays. An alternative for single-phase systems is the mechanically pumped two-phase system: a pumped loop which accepts heat by evaporation of the working tluid at heat dissipating stations and release heat by condensation at heat demanding stations and at radiators, for rejection into space. Such a system relies on heat of vaporisation: it operates nearly isothermaily and the pumping power is reduced by orders of magnitude, thus minimising radiator and solar array sizes. The stations can be arranged in series, parallel or hybrid configurations. The series configuration is the simplest, it offers the possibility of heat load sharing between the different stations, with some restrictions with respect to their sequence in the loop. But it has limited growth potential and the higher flow resistance. In the low resistance modular parallel concept, the stations operate relatively independently, thus offering full growth capability. However, the parallel configuration is more complicated, especially when redundancy and heat load sharing (some cold plates operating in reverse mode) is foreseen. A parallel configuration also requires a control system consisting of various sensors, monitoring the loop performance at various spots, control logic and actuators to adjust pump speed, reservoir fluid content and throughputs of valves. Sensors necessary for control are pressure gauges, flow meters, temperature gauges and vapour quality sensors, measuring the relative vapour mass content of the flowing mixture, e.g. at the cold plate exits, as a part of a control system adjusting the liquid fed into a cold plate to prevent evaporator dry-out or to maintain a prescribed quality value at evaporator exits, independent from transient heat sources and heat sinks. ESA supported mechanically-pumped two-phase loop activities have led to the development of components considered to be critical for twophase applications. These critical components, extensively and successfully tested in a R114 technology development two-phase test bed /l/, include amongst others: a newly developed multichannel condenser and a Vapour Quality Sensor, designed as an axial field capacitance meter, based on the differences in dielectric permittivity of vapour and liquid /l & 2/. (7)113
0)114
A. A. M. Deli1
An alternative for a mechanically pumped loop is the capillary pumped system, using surface tension induced pumping of capillary evaporators. Such capillary two-phase systems can be applied in spacecraft not allowing pump induced vibrations. Ammonia is the best working fluid for such loops. As two-phase flow characteristics and heat transfer differ in l-g and low-g environment /3/, the technology of such two-phase heat transport systems and their components was to be demonstrated in orbit. Therefore, the development of a Two-Phase loop experiment started in 1990 within the ESA In-orbit Technology Demonstration Programme TDPl, by NLR (NL, prime contractor), SABCA (B), FSS (NL), BE (NL) and SPE (NL). The flight experiment is a downscaled capillary pumped twophase ammonia system together with scaled-down components of a mechanically pumped loop: multichannel condensers, vapour quality sensors and a controllable three-way valve. The complete experiment, integrated in a Get Away Special canister (5 ft3, gaseous Nitrogen filled), has flown as G557 aboard the US Space Shuttle STS60, early February 1994. TPX has run autonomously, using own power supply, data handling and experiment control, after switch-on by the Shuttle crew. EXPERIMENT BASELINE, OBJECTIVES, PHILOSOPHY The experiment baseline is depicted in Fig. 1. Heat, supplied to the parallel capillary-pumped evaporators (a flat and a cylindrical one), causes evaporation of the working fluid, setting the total mass flow rate and generating pumping pressure to maintain the tluid circulation in the system. The heat, extracted from the fluid in the two condenser sections and the subcooler, is radiated to space via the canister lid.A thermal accumulator (reservoir), which contains liquid and vapour in equilibrium, is used to control the temperature setpoint of the loop. To increase the loop temperature the Peltier element heats the accumulator, hence increases the volume of the existing vapour bubble and thereby pushes liquid into the loop. This liquid blocks part of the condenser, increasing its thermal resistance. If the heat load applied to the loop and the sink conditions do not change, this increase in thermal
APS DPS EC
Absolute Pressure Sensor Differential Pressure Sensor Cylindrical evaporator Flat evaporator Ef EMP Experiment Mounting Plate GAS Get Away Special
LFM A Tc TV
Liquid Flow Meter Mass flow Temperature condenser Temperature vapour Temperature liquid Tt VQS Vapour Qualily Sensor
cylindrical pmllm enclosure
insulating
hltelface equipment We
upper insulating cover (as raquiredl
healout
Y ‘f
510’ahrmlnlum
Fig. 1. TPX Baseline.
L
corGiwy lowerinIulatlnn CoveI
dumlnkedkBpton
Fig. 2. GAS Canister Dimensions.
Two-PhaseHeatTranspd System
ONI5
resistance will result in an increase in loop operation temperature. To reduce the loop temperature setpoint, the Peltier element is used as cooling device, resulting in a flow of liquid into the accumulator. The loop contains two VQS, a controllable three-way valve and evaporator depthsing heaters. The experiment had to fulfil the GAS requirements and restrictions /4/: allowable volume (I%%.2) and mass (maximum 90 kg), no power and communication links to STS (hence limited battezy energy and internal data storage), Shuttle attitude dependent thermal sink (Fig. 3) and only on/off crew commands. The baseline had to meet the goals for the different experiment constituents: Capillary Pumped Loop, Vapour Quality Sensors and multichannel condensers, each being a scaleddown version of the concept originally developed for high power systems (up to 10 kW).
100
5 FT3 container without insulnted end cap
container temperature (“c)
Fig. 3. GAS Thermal Sink Conditions.
Fig. 4. TPX.
CPL-related objectives were to show in low-g: the capability to transport heat in a smooth and continuous way, proper operation for different heat loads on the parallel evaporators, proper heat load sharing and start-up from low temperatures, the capability to prime an evaporator by a controlled management of the reservoir fluid content, plus the capability to adjust to and maintain a setpoint temperature (under varying heat load/sink conditions) by proper accumulator content control. A cylindrical and a flat evaporator were present to asses low-g transport capabilities, maximum pumping pressures, depriming, repriming capabilities (by controllable accumulator actions), evaporator heat transfer coefficients and the interaction of parallel evaporators, including heat load sharing. An additional objective was to assess the limits of heat transport capability in low-g and on earth and to compare these with ESATANIFHTS thermal modelling predictions. Two VQS were present to prove the concept in space, to demonstrate the proper performance of the sensors accomodating ammonia, to compare the performance of the two sensors in order to assess the influence of the location within the loop and of small construction differences, to perform calibrations and to assess the differences in low and terrestrial performances (important for the design of future space-oriented quality sensors) and to carry out some simple CPL control exercises with the to show the usefulness of vapour quality sensors for system control and to demonstrate the proper performance of the controllable three-way valve. Multichannel condenser objectives can be summa&d by: demonstration and verification of the working principle and determination of the performance limits in a low-gravity environment (aspects of power transported, pressure drop, distribution of the fluid over the different channels and prevention of channel blockage included). Detected condensation front locations can be used to interpret experimental data of the VQS located between the two condenser sections. An additional objective was the use of the low-g and terrestrial TPX test data and the outcomes of testing in the NLR two-phase test loops, in order to verify the NLR approach for the thermalgravitational scaling of two-phase flow and heat transfer /5 & 6/.
(7)116
A. A. M. D&l
In order to fulfil the objectives an experimental flight programme had been carried out, until the battery was exhausted. It included the following individual experiments: Balanced power profile: the same power levels applied to both evaporators. Unbalanced power profile: different power levels fed to the two evaporators, to study interaction. Heat load sharing: heat load applied only to one evaporator, the second one acting as condenser. DeprimingIrepriming: a special depriming heater, shortly powered to induce an evaporator dry-out, followed by a repriming action using the accumulator. Transient response to a step in power applied to an evaporator in a steady system situation. Start-up behaviour and assessment of performance limits. Loop temperature control: maintenance of loop set point and control of setpoint changes. Quality setpoint control: control of bypass valve to control a vapour quality in the VQS. System control: maintain (by accumulator and bypass valve control) the loop saturation temperature setpoint and vapour quality at the mixing point, under changing heat loads and sink temperatures. The predefined test sequence of the above experiments has been based on extensive ground testing. STRUCTURE & LOOP The configuration, shown in the Figs. 4 to 6, consists of a structure of four cohunns and three parallel plates: at one end of the columns a plate accommodating the battery and the electronics hardware at either side, at the other end of the columns the loop plate, attached in a well conductive manner to the canister lid and accommodating most CPL components, and a plate in between, 40 mm from the base plate, thermally decoupled from the others, and accommodating the evaporators. Materials: structure Al 7075, CPL tubing and components Al 6061, liquid flow meters stainless steel.
fi
Fig. 5. Loop & Base Mounting Plate Layout.
Fig. 6. Loop & Evaporators Plate Layout.
Two-Phase Heat Transport System
ON 17
POWER, DATA HANDLING, SAFETY The Data Acquisition and Control system included all electric/electronic hardware and the sofhvwe for testing and operating TPX, storing measured data, includhrg retrieval of experimental data. DAC flight hardware included Battery Pack, Cable Harness, and Payload Measurements and Control Unit. The battery, selected to provide the power for the experiment during the flight, was a modified MAUS battery: 1800 Wb @ 28.5 V DC. The PMCU was the on-board control for experiment execution and safe-guarding, sensor measurements, actuator control, data storage and communication with Electrical Ground Support Equipment. The PMCU contained the System Processor Unit, Sensor Data Acquisition Board and Actuator Control Board, interconnected by a standard VME bus. DC-DC converters provided internally required voltages for analogue and digital circuitry, and for the sensors and actuators. Fuses at appropriate locations ensured functionality and safety of the electrical system. To receive all relevant information about the performance of loop and components - location of condensation front, degree of subcooling, loop set point, evaporator temperature distribution and pressure drop, reservoir and baseplate temperatures - temperatures (38), flow rates (2), vapour qualities (2), absolute pressure, pressure drop across the evaporators, valve position, evaporator heater (2) and depriming heater currents (2), and peltier current were measured. DAC software consisted of on-board and EGSE software. As operational behaviour and details about experiment parameters were test sequence dependent (set by actual in-orbit conditions), the on-board software was split into a fixed programme and a set of experimenter defined tables, without compromising the reliability of the software. Major functions of the embedded software pertained to planning of the experiment, data acquisition from all sensors, execution of specified control algorithms, actuators control, data recording and safeguarding. Because of containment of ammonia the two-phase loop had to be protected against an overheating leading to unacceptable loop pressure. All pressurised components were designed for a maximum pressure of 45 bar. Each component as well as the completely assembled loop has been proof pressure tested with a factor of 1.5 against that value (burst test factor 4). During the whole operation of the experiment the PMCU had to measure the temperature sensors, switching off a heat dissipating unit, when the predefined maximum allowable temperature of this unit was reached. There was also a thermal switch on the vapour line, which had to interrupt power to the evaporator heaters, depriming heaters and Pettier element, when the maximum allowable loop temperature of 80 “C was reached. Also a pressure relief device was installed, designed such that it had to open and release ammonia at a predefined pressure, to follow the approach ‘leak before burst’. Also housekeeping data (6 voltages, PMCU current and 10 temperatures) were to be measured. The measuring/control interval was set to 5 s, as most parameters follow slowly changing temperatures. COMPONENTS The various TPX components are shown in the next, including some test results. 171 contains detailed information on pre-launch testing results. The flat evaporator (Fig. 7) is a (heated) base plate with microchannels for the vapour, a 30 pm porous poly-ethylene wick with an inlet hole for liquid, and a box shell which has been electron beam welded on the base plate, having an inlet tube, tefion insulator and outlet vapour tube. Measured performances & characteristics are: . Mass 0.6 kg. l Heat load 155 W maximum/8.8 W minimum. l Burst pressure 260 Bar. l Capillary static pressure (ammonia) 2933 Pa. l Pumping pressure at 155 W 2200 Pa. The cylindrical evaporator (Fig. 8) is a cylindrical &a&i) shell with inner circular grooves for the vapour, a liquid inlet tube with teflon insulation, an outlet vapour tube and a 30 pm porous polyethylene rod with vapour collecting grooves and an inlet hole for the liquid. Measured performances & characteristics are: . Mass 0.28 kg. 0 Heat load 400 W maximum, 7.7 W minimum. l Pumping presure at 150 W 1800 Pa. l Capillary static pressure (ammonia) 2707 Pa. 0 Burst pressure more than 200 Bar. The control reservoir (Fig. 9) is a cylindrical vessel with an inlet/outlet (liquid) tube, a liquid/vapour separator wick made of 30 pm polyethylene, an acquisition (flower shape) wick and a cover welded on the vessel, equipped with two Peltier elements and a copper braid connected to the inlet. Measured performances & characteristics are: l Mass 0.92 kg. 0 Electric power for Peltier control 4.7 W max. l Burst pressure 275 Bar. 0 Liquid content 0.17 litre. l Temperature control accuracy 0.1 K in range 263 - 323 K.
A. A. M. DeliI
Fig. 8. Cylindrical Evaporator.
Fig. 7. Flat Evaporator.
glsssssn& slsmsnt,wmelsctrodes
'Altubs
46. ,
Fig. 10. Vapour Quality Sensor.
Fig. 9. Control Reservoir.
Calibration of the VQS (Fig. IO) has been done in the automated NLR test rig /7/. Typical test results for the - four 3.2 mm ID parallel channels - condenser (horizontally positioned in the aforementioned rig) are depicted in Fig. 11: Steady-state temperature profiles for a power of 68 W and various sink temperatures and in Fig. 12: Thermal transients (at a 30 “C sink temperature) for power steps from 68 W to 30 W, back to 68 W. Experiments with the condenser vertically positioned have been carried out also. 40 h 0*
I
I
401
I
I
I
35-
J30iii $ g 252 20-
150
66W 66 w 50 0
3ow
66W
-4 loo #
150 1
200
250 L
condenser length (mm)
Fig. 11. Steady-State Condenser Temperature Profiles.
300 1
25
/-Gaq, 0
500
1000 time (s)
Fig. 12. Condenser Thermal Transients.
1500
Two-Phsse Heat Trsnspmt System
(7)i 19
PRE-LAUNCH TESTING OF THE SYSTEM Pre-launch testing /7/ consisted of the already mentioned proof pressure test, vibration, modal surveys and random vibration tests according to the GAS specifications 141, thermal performance & acceptance tests and the definition of the flight scenario. Typical results of the performance testing are presented in the Fig. 13. They illustrate the control of the loop vapour temperature setpoint 40 “C for variable heat loads. Fig. 13 depicts the histories of power fed to evaporators and Peltier cell (setpoint control), the corresponding temperature histories for the cylindrical evaporator: surface, out & inlet temperatures (Similar behaviour has been observed for the flat evaporator), and the corresponding temperature history for the long condenser. The figure shows that for low power (50 W) the condenser was completely flooded (reaches loop plate temperature), for high power (200 W) the condenser became fully efficient. After successful testina and flight scenario determination. TPX has been delivered at Kennedy Space Center, to fly on ST!@ laun&ed February 3, 1994. ’
I
I
%
(
I
I
*
c
I
I
[email protected] 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8
&d-
8
1.012 1.4 1.6 1.8 2.0 22 2.4 2.6 28
time (in thousands sets)
time (in thousands
151 1.0 1.2 1.4 1.6 1.8 20
sets)
time (in thousands
22 2.4 2.6 2.8 sea)
Fig. 13. Power histories, corresponding cylindrical evaporator and long condenser temperature histories. PRELIMINARYFLIGHT RESULTS Data retrieved after the flight amount to 1.9 Megabyte, stored before battery exhaust, meaning 40 hours total experiment time, divided into three runs: experimental sequences that start with the very short health check for experiment initialisation. Each run consists of (up to four) passes, containing only those experiments not done before in the same run. Fig. 14 surveys the first 18 hours. Fig. 15 shows the histories of the power and of several important temperatures during the entire 42 hours. From the baseplate temperature curve it can be concluded that TPX has run far hotter than foreseen, due to the fact that the actual in-orbit thermal environment considerably differed from the
i
Pass2
i PasS3:
15 t
; l&l.p*tonn#ommr#p, ;. pi .~ .~ ziidzF-YR
:
0 0 10 -.
0 m -m 0 n
5-
0 .m._ n
=
j
.
i .!!!!!YP~ "‘.‘MT'~ .., .; i .wsunl j ...*.......... *........ .... ..._................ .;,,... .i__._. .,,,... dPf== .....~ ~..."..' :
l
I.
m
i
w
;m 2z$&
Cl
q
md= m.._
” D _m.
O3
RUN2
! Pass4_
experiment number
I 20.000
+
Fig. 14. TPX Flight Experiments: Run 1.
..&
I 4O.wO
run time (s)
..:
j;.
I I 60.000’
. ._._ .. smftup , m.wO
.
A. A. M. Deli1
OWJ
unobstructed (ideal) thermal radiation environment, being the baseline of the design. Both the presence of the large (energy dissipating and free view obstructing) CAPL experiment and the fact that the TPX position on the GAS bridge was extremely close to the Shuttle bay aft bulk (say less than 0.5 m) are responsible for experiment temperatures substantially higher than the design values (Fig. 3). The consequences of the above are: - As Fig. 14 shows, the long lasting experiment constituents (VQS survey & control, maximum performance, temperature control, balanced/unbalanced heat load) could not be fully completed (within the timeframe allowed) because of the continuously increasing experiment temperature. - The (quasi-) temperature equilibrium, reached after the first 14 hours (Fig. 15), was much higher than expected and the remaining 26 hours consist of switching-on experiments that were shutdown (to cool down) after a limited running time (because a pre-set dry-out criterion was met). Though these experiments (essentially being transient on-off experiments) could never be completed, they are considered to yield valuable information on the transient behaviour of two-phase loops. - As the pre-launch test data pertain to thermal sink conditions far below the in-orbit sink conditions, they are not very adequate to be used for direct comparison with the flight data in order to identify the differences between low-g and one-g two-phase flow and heat transfer. For a correct identification the data to be compared shall pertain to -as close as possible- identical conditions. This meant that TPX had to be re-tested on earth for power and thermal sink conditions recorded during the flight: the full 40 hours flight experiment were to be simulated. This activity also provided calibration data for the liquid flow meter that had to be repaired, in the loop, shortly before launch.
T (“Cl power (Watt * 10)
loo
50 time (s)
Remark:one
t l-lliudulM_ thousands
io
datapoint every 100 seconds is displayed
Fig. 15. 40 hours histories of power fed to evaporators, and evaporators, battery and base plate temperatures. Anticipating a detailed evaluation after the post-flight testing, it already can be concluded that TPX (the overall two-phase loop and all components) functioned without ammonia leakage, without system or component failure. This is illustrated by graphs depicting the time histories of the flat evaporator temperature FET and power Pfe, the loop vapour temperature VAPT, the cylindrical evaporator temperature CET and power Pee, the differences between evaporators and reservoir temperatures FET-ResTemp, CET-ResTemp, the vapour quality measured between the condensers VQSl, the temperature of the inlet node on the second condenser TC2Pl and the swalve position SWALVEPOS. Fig. 16 shows as example the first 2 hours of the experiment, first pass of first run, pertaining to very short health check, start up, balanced/unbalanced heat load and heat load sharing experiments. Both evaporators have functioned, handled power up to or even over the design value, the minimum power behaviour being similar or even better than predicted. The cylindrical evaporator showed high sensitivity to heat load changes, frequently meeting the pre-detined dry-out citerion.
Two-Phase Heat Transport System
17)121
Fig. 16. Flight data of the first 2 hours. FLOW PATTERNS & CONDENSATION LENGTHS A typical example of a measured condenser temperature profile (100 W power, baseplate temperature 26.3 “C, vapour temperature of the loop 40.0 “C) is shown in Fig. 17. Comparison with Fig. 11 yields that for the small diameter condenser fines the condensation length needed in low-g conditions is only slightly larger than in one-g. The ratio of zero-g and terrestrial full condensation lengths for the 3.2 mm ID condenser channels (Fig. 18) clearly fits the curves following from the NLR model developed /5 & 61. Since this model is valid for pure annular flow only, it is worthwhile to investigate the impact of other flow patterns present inside the condenser duct (mist flow at the high quality side, slug and bubbly flow at the low quality side and wavy-annular-mist in between), in other words to investigate whether the pure annular tlow assumption leads towards slightly or substantially overestimated full condensation lengths. A complication is the lower boundary of the anuular-wavymist flow pattern. It is stressed that the transition vapour quality value reported in literature 0.15 /3/ pertains to R114 flowing in a 15.8 mm duct. Analysis of TPX vapour quality sensor flight data indicates that the transition annular-slug occurs, for ammonia flowing in a 5 mm line section, at vapour qualities around 0.3. The above means that flow pattern transitions occur at vapour quality values, that strongly depend on working fluid, operating conditions, and line diameter.
(7)122
A. A. M. Deli1
17
50
83 117 150 183 217 condenser length (mm)
250
283
Fig. 17. TPX second condenser flight temperature profile for 100 W, loop at 40.0 “C, baseplate at 26.3 “C.
pipe diameter (mm)
Fig. 18. Ratio of low-g and terrestrial full condensation lengths.
FINAL REMARKS TPX has successtitlly flown. Preliminary conclusions could be drawn, but final conclusions wait for the detailed comparison of flight data and data resulting from the post-flight tests, using the actual inflight temperature conditions. Based on these outcomes and lessons learned, an improved experiment scenario will be defined for a possible re-flight (in the same or a slightly modified configuration). ACKNOWLEDGEMENT I express my appreciation for all efforts of the TPX team members: W. Supper, R. Aceti (ESTEC); J.F. Heemskerk, A. Monkel, M. Versteeg, G. v. Donk, A., Pauw, C. Slippens (NLR); M. Dubois, S. v. Oost, D. v. Oost (SABCA); R. Voeten (BE); R. Linssen (SPE); M. Coesel (FSS). REFERENCES 1. N. Dunbar & R. Siepmann, European Two-Phase Heat Transport Technology, SAE 901271, 20th Int. Conf. on Environmental Systems, Williamsburg, VA/USA, July 1990. 2. A.A.M. Deli1 & J.F. Heemskerk, Development of a sensor for high-quality two-phase flow, NL& A4P 880.59 U, 3rd European Symposium on Space Thermal Control and Life Support Systems, ES4 SP288, Noordwijk, Netherlands, Oct. 1988, 113. 3. D.G. Hill, K. Hsu, R. Parish & J. Dominick, Reduced gravity and ground testing of a two-phase thermal management system for large spacecraft, SAE 882084, 18th Intersoc. Conf. on Environmental Systems, San Francisco, CA/USA, July 1988. 4. NASA Goddard Space Flight Center, Get Away Special (GAS) small self-contained payloads,
Experimenter Handbook, 1985, last update 199 1. 5. A.A.M. Delil, Thermal gravitational modelling and scaling of two-phase heat transport systems: Similarity considerations and useful equations, predictions versus experimental results, NLR Tp 91477 U, ESA SP3S3, 1st European Symp. on Fluids in Space, Ajaccio, France, Nov. 1991, 579. 6. A.A.M. Delil, Gravity dependence of pressure drop and heat transfer in straight two-phase heat transport system condenser ducts, h%R TP 92167 U/SAE 921168, 22nd Int. Conf. on Environmental Systems, Seattle, WA/USA, July 1992, SAE Trans. J. of Aerospace, Ip1 (1992) 512. 7. A.A.M. Delil, J.F. Heemskerk, M. Dubois, S. van Oost, W. Supper & R. Aceti, In-orbit demonstration of two-phase heat transport technology: TPX/G557 development & pre-launch testing, NL.R 7P 93394 CJ. Sti 932302, 23rd Int. Conf. on Environmental Systems, Colorado Springs, CO/USA, July 1993.