An ISS testbed approach to passive fluid phase separator

49th International Conference on Environmental Systems ICES-2019-199 7-11 July 2019, Boston, Massachusetts

Copyright © 2019 IRPI LLC

An ISS testbed approach to passive fluid phase separator

device development for life support

Logan Torres1 and Ryan Jenson2 and Mark Weislogel3

IRPI LLC, Portland, OR, 97223

Gravity passively separates gases from liquids on Earth due to buoyancy. This makes it

easy to separate and process 100% gas or liquid streams. But because such buoyancy is

essentially absent aboard spacecraft, nearly all fluid systems aboard them are, or become,

multiphase fluid systems. This outcome presents a plethora of acute fluidics challenges that

are well-known to NASA. The common theme to these issues is a lack of familiarity with large

length scale capillary fluidic phenomena that precludes proper design. If future long-term

missions are to be successful, mundane micro-gravity plumbing must be well-understood. In

response to this need, we are currently developing a testbed for the development and

exhaustive testing of next-generation capillary fluidics solutions for passive and semi-passive

phase separations that are critical to the reliable operation of current and future spacecraft

life support systems, plant and animal habitat systems, propellant/coolant management

systems, bio-fluidics processors, and passive fluids delivery and control systems for physical

sciences experiments. The single component, low profile, low-noise, and low power draw

testbed is developed for quick installation in the open ISS laboratory. Following installation,

it can be controlled from the ground for 24/7 operations for short to long duration phase

separator component testing and qualification. Quick attachment and detachment mounts for

the test devices allow for multiuser options including industry, academy, and government

users. The development and proof of concept of such a system is guided by a breadboard-style

prototype designed for use in a high-rate drop tower. The breadboard design exhibits similar

size and function as the flight ready system, as well as demonstrates the high-fidelity, high-

speed data acquisition capabilities of the system. The current state of the hardware is

presented along with the results of the preliminary low-g tests.

Nomenclature

2ϕ = Two-phase

COTS = Commercial-off-the-shelf

DDT = Dryden Drop Tower

FOV = Field-of-view

F = Flow rate

g = Gravity

go = Terrestrial gravitational constant

GUI = Graphical user interface

LED = Light emitting diode

ls = Length scale

P = Pressure

Q2ϕ = Two-phase flow rate

Qg = Gas flow rate

Ql = Liquid flow rate

T = Temperature

TRL = Technology readiness level

1 Mechanical Engineer, 11535 SW 67th Ave Portland, OR 97223 2 Principle Investigator, 11535 SW 67th Ave Portland, OR 97223 3 Principle Scientist, 11535 SW 67th Ave Portland, OR 97223

International Conference on Environmental Systems

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I. Introduction

e-mixing gas-liquid streams on earth is generally a passive process due to buoyancy. Unfortunately, in orbit

buoyancy is essentially absent causing most fluid systems aboard spacecraft to become multiphase systems. This

situation produces a plethora of acute crosscutting fluidics challenges that are well known to NASA and have

significant impacts on safety, reliability, and operations.1 The challenges are due in part to a lack of familiarity with

the dynamics of large length scale (i.e., ls ~ O(1m)) capillary phenomena2 as a result of limited experimental research

in the relevant low-g environment. Capillarity is often the natural driving force for fluid orientation and flow in such

environments, but is commonly overlooked during the early stages of fluid system design and development. This

oversight can manifest in last minute modifications which may be satisfactory, but are not optimal or, under worse

cases, can lead to complete system failure on orbit. Our intent is to establish solutions to fluidic challenges where

capillary phenomena is included at the forefront of design process.

Greater designer intuition is required that can only be developed by exposure to relevant flow phenomena in the

relevant environment (i.e., low-g). Drop tower facilities provide short duration (≲ 10 s) high quality low-g (≲ 10-4go).

Parabolic flights provide longer durations (≲ 25 s) of reduced-g (≲ 10-2go), but suffer from poorly controlled initial

conditions and unsteady reduced-g levels. Neither of these methods perfectly reproduce the essentially continuous

microgravity environment aboard orbiting or coasting spacecraft. Greater capillary solution awareness and confidence

will only be achieved by direct exposure to the equivalent long duration low-g that is found on-board the ISS (≲106go).

Such exposure is crucial to gaining industry-level design confidence. Under a recent NASA SBIR Phase I effort,3 such

an effort was initiated through the design, fabrication, and demonstration of a closed loop 2-flow testbed for the

development and exhaustive testing of next-generation capillary fluidics solutions for passive and semi-passive phase

separations. Such a development is critical to gaining precious practical knowledge for the design of current and future

spacecraft fluids systems; i.e., water processing and recovery systems, air purification, plant and animal habitat

systems, propellant/coolant management systems, bio-fluidics sequencers and processors, Lab-On-Chip and Organ-

On-Chip technologies, and others. Our initial successes are currently directing our efforts towards the development of

a flight fidelity version of the testbed with the specific aim to flight qualify specific passive 2𝜙-flow separation

solutions (devices) for critical NASA and commercial aerospace applications. Terrestrial applications are also

pursued.

II. Approach

We employed a drop tower to test and demonstrate a prototype 2-flow testbed for high-TRL passive phase

separation device qualification, with future intentions of developing a flight version for the ISS. The closed-loop flow

schematic of Figure 1 suffices to convey our testbed approach. The idea is to join gas and liquid flows in numerous

ways to deliver 2-flows to any variety of phase-separating devices, conduits, and test cells. The inlet manifold

produces a vast range of laminar to transitional/turbulent flow regimes experienced aboard spacecraft. Sequentially

injecting these flows into the ‘test device’ and monitoring the outlet streams provides a highly applicable quantitative

assessment of performance. It also contributes immediately to designer understanding and intuition. The flow

parameters are varied quickly, transients and steady states are rapidly established, and exhaustive data sets are

promptly collected. Device performance limits are assessed via regime maps cast in terms of the dependent and

independent flow parameters. Closed loop operation is enabled by a novel high-TRL system-level passive phase

separator (Ref. Figure 1) that returns single phase gas and liquid to the pumps. Video and diagnostic telemetry provide

measures of inlet regimes and device performance, including system temperatures and pressures, and overall device

pressure drop. Such a system running remotely and continuously on the ISS could be completely exhausted within a

matter of days (≲10 days). During our Phase I research, our goal was to translate the main features of the Figure 1

schematic into a single remotely controlled planar device. Using primarily COTS components, we designed,

manufactured, assembled, and tested a 2-flow testbed prototype in the high rate Dryden Drop Tower (DDT).4-6 The

DDT, shown in Figure 2, is a 2.1 s dual-capsule drop tower facility. The DDT regularly performs over 1,000 drops a

year at rates of up to 100 drops per day for academic and private sector research and development. The tower provides

high quality low-g levels from 10-6 to 10-4go (depending on system configuration), which is more than adequate for

our 2-flow testbed drop tower rig.

D


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Figure 1. Schematic of highly configurable capillary −flow testbed. T, P, F, and 2 imply temperature,

pressure, flow rate, and 2-flow generator, respectively. The notional test cell shown here is a passive separation

device. Primary and secondary FOVs are indicated by red dashed outlines. The closed loop is completed by

passing 2𝝓 lines through a large verified passive phase separator ensuring single phases are returned to gas

and liquid pumps.

Figure 2. a. Portland State University Engineering Building housing b. the Dryden Drop Tower where the

capillary 2-flow rig of c. is installed and dropped in a drag shield allowing 2.1 s of free fall time.

III. Design and Assembly

Images of our breadboard capillary 2-flow testbed are shown in Figure 3 with key components highlighted. The

testbed unit is compact (56 cm x 42 cm x 13 cm), lightweight (6.4 kg), and low power (< 90W), made from 20 mm

T-slotted aluminum frame with acrylic panels. The top mounting panel consists of a diffuse white acrylic sheet which

is back-illuminated by an LED light panel. The flow circuit components are attached to the top panel by press-fit holes

and bolts. The deck is integrated into the drop tower rig shown in Figure 4, which provides remote power, connection

to an onboard computer, and two overhead cameras. The camera shelf and lower deck are designed for both 1-go and


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low-g experiments. In flight configuration, a similar arrangement is expected where power and a computer interface

are provided by ISS, eliminating the need for most of the components, volume, and mass below the top deck. The two

overhead cameras provide the fields of view (FOV) identified in Figure 1. FOV-1 is the device science camera, while

FOV-2 is the overall system science camera—an excellent view for total visual confirmation and control of the system

as well as extremely rare system level science. Sample images for both testbed FOVs are provided in Figure 5.

Figure 6 provides a schematic of the actual testbed flow loop with liquid paths marked by blue arrows and gas

paths by orange arrows. In the current configuration, two variable-speed pumps provide liquid flows up to 6.5 ml/s

and gas flows up to 4.5 ml/s. Eight solenoid valves direct liquid and gas through the left side manifold channels, which

then join to generate single-phase Ql,in and Qg,in and two-phase Q2,in flows upstream of the test cell. These flows are

then directed into the test cell. Figure 7b provides a sample of various inlet flows. All regimes shown are produced

passively using a T-junction approach where phase distributions occur by regulation of water-air flow rate ratios.

Pump bypass lines with control valves provide fine flow rate control (~ 10:1 turn down ratio). Three paths are provided

for outlet connections. All outlets are passed through the low-g certified system phase separator which returns single

phase gas and liquid to the respective pumps. A syringe connected to the liquid flow path and a pinch valve connected

to the gas flow path provides system phase volume ratio control. The flow loop in Figure 6 is predominately monitored

and controlled via the laptop and LabVIEW GUI shown in Figure 8. Figure 8a shows a digital ‘switch-board’ for

cycling solenoid configurations and sending pump speed commands. The user interface also returns pressure and

temperatures for each phase. Automatically following each drop test, or upon command by the user, the data file (e.g.,

Figure 8c) is updated with time, valve configuration, pump speeds, and pressure and temperature telemetry. Once a

desired flow configuration is achieved, test cell performance as well as system wide response is captured on the

overhead cameras (ref. Figure 5). The time stamped video files are hyperlinked from the data log for ease of analysis.

Figure 3. Breadboard capillary 2-flow testbed for drop tower experiments. a. The unit is assembled from

commercial parts sourced for ease of assembly and modularity. b. The back-mounting plate is made of diffuse

white Acrylic which is illuminated from behind by LED lighting, allowing for complete system wide flow

visualization. c. The main structure of the unit is comprised of a 20 mm T-slotted aluminum extrusion frame,

a diffuse white acrylic mounting panel, and an acrylic LED backlight panel. d. The testbed is small and light

enough to be easily hand carried, with external dimensions of 56 cm x 42 cm x 13 cm and mass of 6.4 kg.


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Figure 4. Two-phase drop tower rig. The rig provides a platform with an overhead camera shelf and bottom-

level electronic control and power supply deck with which the mid-level 2-flow testbed integrates. The lower

deck frame provides a stand for benchtop testing as well as integration into the drop tower. The testbed is

bolted to the lower deck and camera shelf with electrical connections between the testbed unit and the control

system including sensor connections, pump motor connections, and valve connections.

Figure 5. a. Two top FOV cameras over 2-flow testbed. b. FOV-2 provides system wide flow visualization to

inlet and outlet manifolds, system phase separator, test cell(s), etc., whereas FOV-1 provides magnified view of

test cell performance. c.-d. FOV-1 also provides high resolution views of device inlets and outlets allowing

accurate regime characterization and flow rate/pump calibration via digital image processing techniques.


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Figure 6. Overhead view of top deck with flow paths marked by colored arrows. Various single-phase and two-

phase inlet conditions are supplied to the test cell from the left side manifold. Multiple outlet connections are

possible to accommodate various test cell designs. All outlets pass through the phase separator before returning

to the loop. Liquid/gas volume ratio is controlled by syringes located middle right. System level temperature

and pressure of both phases are monitored at the pump outlets. A future flight system will provide further

control and automation such as temperature, pressure, and total phase volume ratio control.


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Figure 7. a. Closed-loop capillary 2𝝓-flow schematic as built and tested in 2.1 s drop tower. b. Selection of

highly controlled/highly ordered 2𝝓-flow regimes achieved passively by T-junction approach and delivered as

inlet flows to the test cell to study phase redistribution, stability, and separation. An extremely wide range of

flow patterns are possible including new/rare regimes such as rupturing annular or slug annular flow leading

to intermittent air-driven wall-bound drops and rivulets. Annular flows are generally characterized by a ring

of liquid flow and a core of gas flow, either attached along the tube wall or unstably breaking up. Discrete

volumes of liquid slugs and gas bubbles can vary in volume fraction and flow rate. These flows are ideal as feed

streams to the significantly more complex geometries of the capillary devices to be tested. c. Selection of

resulting regimes during bench top testing.


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Figure 8. a. The 2𝝓-flow testbed is controlled remotely using a LabVIEW GUI. b. User input to the laptop

provides control of pump speeds and valve configurations while returning current gas and liquid phase

temperatures, pressures, and flow rates. c. The LabVIEW program automates data acquisition during

experiments by exporting the valve configuration, pump speeds, and gas and liquid temperature and pressure

to a text file through low-gravity detection or by user command. A two-FOV camera video archive may be

constructed real time with hyperlinks to all diagnostics.

IV. Demonstrations and Results

Automated control of the testbed enables a high rate of data acquisition. We demonstrate such capabilities both by

examining bench top experiments as well as drop tower test results. Our bench top tests mimic the expected operations

in the flight system once installed on the ISS. First, desired flow rates and valve configurations are set using the

LabVIEW user interface. The system is then left to reach steady/quasi steady state conditions (usually occurring within

< 60 s). Once in steady state, the cameras are triggered to record for 30 s to capture inlet conditions, test cell

performance, outlet conditions, and overall system response. After the video data is recorded a new flow configuration

is sent and the process is repeated. On our first attempt, 30 data points were acquired within a 45 min period. Drop

tower experiments are conducted in a similar manner, though we note that the 2.1 s of free-fall does not always allow

steady state conditions to be met. However, reorientation of the phases in the test cell and the corresponding

performance response is indictive of what to expect during low-g operation. With actual system control occurring in

a matter of seconds, most of the time consumed during a drop tower experiment is spent for nominal tower functions.

Nonetheless, drop tower experiment turnaround is achieved at a rate of roughly 3.3 min/drop (~ 18 drops/hr), a rate

higher than the highly successful 2-flow Capillary Channel Flow experiment aboard ISS.7

Results for the passive phase separator we present here are depicted in Figure 9. The inline conduit interfaces with

the testbed tubing through 4.8 mm barbed fittings. This device accommodates one 2 inlet and two 1 outlets

(sources). Thousands of data points may be collected for this single device for a wide variety of possible inlet/outlet

flow conditions and device configurations. One data set varying only flow rate ratios using air and partially wetting

contaminated water is shown in the regime map of Figure 9b and Table 1. In Figure 9b, the symbols correspond to

observed regimes and the regime transitions are delineated by black lines. Select images of the phase separator device

performance are shown in Figure 10, where it is clear under what conditions the devices fails or succeeds. Even from

these preliminary tests we identify mechanisms that explain device performance and inspire new directions for device

design. Demonstrations such as presented herein are rare in low-g fluid experiments, where poorly, but typically,

wetting fluids are often looked over for more ideal highly wetting fluids. Furthermore, system wide flow visualization

(Ref. Figure 5b) provides additional insight into system hardware performance and flow behavior at junctions, fittings,

a.

b.

c.


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and valves. We will continue to exploit this drop tower version of the capillary 2-flow testbed to develop and qualify

such passive separating technologies for space applications.

Figure 9. a. Sample test cell employed for drop tower tests. This test cell is pursued more as a conduit segment

than as a device and was selected for its passive phase-separating characteristics. Qualification tests on such

conduits/devices is critical to assess the flow rate ranges that achieve the desired separation function for given

geometry, wetting conditions, exit conditions, etc. b. Sample regime map collected within 45 min period. The

phase separating performance of the device is shown quantitatively in terms of flow rate ratio (correlated from

applied pump voltages Vg and Vl) with images of regime provided, where triangle symbols identify conditions

of 100% liquid flow separation.

Table 1. Sample regime map data collected as displayed in Fig. 9. The two FOV video clips are employed to

create a video archive that can be constructed with hyperlinks to all diagnostics and regime maps.

Vliq Vgas Pgas (psig) Pliq (psig) Tgas (C) Tliq (C) Relative Video path Liq.Ingestion GasIngestion GasOutState LiquidOutState Video Link

3.00 6.50 -0.03 0.21 26.81 29.03 SingleTestCell_RegimeMapTests\6o5\3o0.MP4 Y Y Periodic Slug Periodic Drain video

3.50 6.50 0.05 0.31 26.82 30.51 SingleTestCell_RegimeMapTests\6o5\3o5.MP4 Y Y Periodic Slug Periodic Drain video



6.00 6.50 0.26 1.02 26.91 33.72 SingleTestCell_RegimeMapTests\6o5\6o0.MP4 Y Y Periodic Slug Periodic Bubble Ingestion video

7.00 6.50 0.28 1.38 27.06 34.71 SingleTestCell_RegimeMapTests\6o5\7o0.MP4 Y Y Periodic Slug Periodic Bubble Ingestion video

8.00 6.50 0.36 1.95 27.31 36.10 SingleTestCell_RegimeMapTests\6o5\8o0.MP4 Y N Periodic Slug 1phase video

9.00 6.50 0.46 1.97 27.35 36.57 SingleTestCell_RegimeMapTests\6o5\9o0.MP4 Y Y Periodic Slug Merging Bubble Ingestion video



3.00 12.00 0.36 0.65 28.11 35.21 SingleTestCell_RegimeMapTests\12o0\3o0.MP4 Y N Sparse Slug 1phase video






9.00 12.00 0.55 2.17 28.71 38.82 SingleTestCell_RegimeMapTests\12o0\9o0.MP4 Y N Slug 1phase video















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Figure 10. Sample images for regimes identified in Figure 9b spanning 100% passive liquid/gas separation to

mixed regimes of periodic liquid/gas ingestion. Even outside ideal limits (i.e., 100% passive phase separation),

the quality of 2ϕ flow at the liquid and gas sources is enhanced as the respective volume fraction of liquid or

gas at the sources increases.

V. Transfer to Flight level and concluding remarks

Herein, we have described the design, assembly, and testing of a drop tower 2-flow testbed for phase separation

device qualification in low-g environments. Continuing forward, we are currently transferring our successes into

design concepts for an advanced flight fidelity system. Further optimization of the size, weight, and power is found

through state-of-the-art manufacturing techniques for manifolds and connections and advanced component selection

including considerations for size, shape, performance, and reliability. Use of NASA heritage (high TRL) equipment

ensures streamlined ISS qualification, and compact size and low power characteristics affords versatile integration

with ISS facilities such as the Maintenance Work Area (MWA), Microgravity Science Glovebox (MSG), Fluids

Integrated Rack (FIR), or others. Employing low-toxicity partially wetting water allows for open cabin integration as

well as replicates the poorly wetting behavior of many actual life support and fluid management systems. Automated

control and real-time down link of video and telemetry allows on-the-fly management and reduction of data. The quiet

solid-state device will require less than 1 hour of crew time for setup and deployment upon which it can operate

remotely for hours, days, or weeks without crew interference. Quick attachment and detachment mounts for the test

cell devices allow multiuser opportunities, inspiring other entities to develop such components and expanding ISS

utilization.

Life support systems on-board spacecraft are replete with multiphase flows, further complicated by partially wetting

and contaminating liquids. High reliability for such systems is in need for obvious reasons. However, to our

knowledge, no multiphase investigations aim to provide direct technology solutions that are COTS-marketable across

a broad range of applications. We believe that high-TRL passive capillary devices, exhaustively tested using this

testbed approach in the relevant low-g environment, can meet such needs. The ISS presents an ideal platform for such

device invention and qualification via the 2-flow testbed approach.

Acknowledgments

Support for this research was provided through NASA SBIR Phase I Contract No. 80NSSC18P1991: CoTRs John

McQuillen and Hunt Hawkins, NASA Glenn Research Center.


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References 1National Academies, Congress Appointed NRC Decadal Review of NASA, 8/2009 to 5/2012 (Physical Sciences); National

Research Council, Recapturing a Future for Space Exploration, National Academic Press, Washington DC. (Chpt 9, Applied

Physical Sciences) 2Langbein, D., Capillary Surfaces: Shape-Stability-Dynamics, in Particular under Weightlessness, 1st ed., Springer-Verlag,

New York, 2002.

3Jenson, R., Weislogel, M., “ISS Testbed for Capillary Two-Phase Flow Device Qualification,” NASA SBIR Phase I, Contract

No. 80NSSC18P1991, CoTR John McQuillen, GRC, 2018. 4“Dryden Drop Tower,” Portland State University [public university], URL: http://www.ddt.pdx.edu/ 5Wollman, A., and Weislogel, M., “New investigations in capillary fluidics using a drop tower,” Experiments in fluids, Vol.

54, No. 4, 10 April 2013, p. 1499. 6Wollman, A., “Capillarity-Driven Droplet Ejection,” M.S.M.E. Thesis, Dept. of Mechanical Engineering, Portland State Univ.,

Portland, OR, 2016. 7Conrath, M., Canfield, P. J., Bronowicki, P. M., Dreyer, M. E., Weislogel, M. M., & Grah, A., “Capillary channel flow

experiments aboard the International Space Station,” Physical review E, Vol. 88, No. 6, 10 Dec. 2013, p. 063009.

http://www.ddt.pdx.edu/

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An ISS testbed approach to passive fluid phase separator