,

Heater

PDMS

The bubble is pinned to the heater surface by capillary forces due to substantial difference in wettability between the heater cover (glass with base aluminum oxide) and the PDMS surface

1, fluid cell; 2, imaging system; 3, data acquisition system; 4, high voltage AC amplifier controlled by a programmable function generator; 5, grounded electrode; 6, two mini-heaters; 7, bubbles; 8, liquid; 9, energized electrode with insulated tip; 10, surface temperature probe.

• The validation of electro-hydrodynamic (EHD) technology for two-phase separation in microgravity. In space technologies, electric fields can be employed to exert a body force on two-phase flows to mitigate the effects of reduced gravity on the performance of thermal management systems [1].

• Gas-liquid phase separation readily occurs in a terrestrial environment as a result of buoyancy forces. However, in a microgravity environment, bubbles do not rise out of a fluid due to the viscosity of the fluid and the absence of buoyant force, leading to bubble coalescence, and the formation of gas pockets during bulk liquid transfers. This leads to failure in storage, analysis and transportation of two-phase physical systems, presenting a problem to life-sustaining systems and biomedical research.

• The main goal of the project is to study bubble dynamics in fluids subjected to alternating-current electric fields, and compare those results with a parabolic flight in which the experiment was recreated in high-g and microgravity.

Heat flux 0.7 MW/m2, heater temperature 150oC, voltage 4kV/20Hz

V = 3mm/s

V = 1mm/s

V = 6mm

/s

V =

5mm

/s

V = 3mm/s

V = 1m

m/s

V = 5mm

/s

V =

6mm

/s

V = 1mm

/s

V = 3mm/s

V = 3mm/s

V =

4mm

/s

V = 3mm/s

V =

2mm

/s

x (mm) x (mm)The bubble trajectories resemble streamlines of the thermocapillary flow observed in subcooled nucleate pool boiling in FC-72 and common refrigerants under microgravity and predicted by computer simulations [5, 6]. Comparison of data with and without voltage pulses demonstrates that the EHD forces suppress velocity fluctuations and stabilize the flow pattern in water under microgravity and normal gravity.

Microgravity Experiments

Payload modules: experimental setup (right) and power supply (left)

As the gravity was reduced, HFE-7100 began to climb along the cuvette walls with some amount eventually escaping through unsealed portions of the cuvette lid. The remaining HFE-7100 was driven back during high gravity.

Bubble Trajectories

Ground-Based Experiments

0 200 400 600 800 1000 1200 14000

0.5

1

1.5

2

2.5

3Bubble Growth

Parabolic Flight

Results on Water

D (

mm

)

Boiling On Small Heaters In Earth and Low GravityDana Qasem, Ian Peczak, Stephanie Stern, Ezinwa Elele and Boris Khusid

Otto H. York Department of Chemical, Biological, and Pharmaceutical EngineeringNew Jersey Institute of Technology, Newark, NJ 07102

• To utilize electrohydrodynamic forces as a substitute for buoyant forces to drive the separation of bubbles in microgravity.

• Previous experiments studied only the dielectrophoretic force, which is exerted by a non-uniform electric field on a particle in a dielectric medium, as a substitute for buoyancy.

• This force manifests when aforementioned medium and the particle have different polarizabilities. If the polarizability of the particle is greater than that of the medium, a positive force on the particle towards the higher electric field gradient is observed. The bubbles in the experiment experience a negative force towards the lower field gradient, because their polarizability is lower than that of the medium.

• The effect of oscillatory fluid flow on the displacement of bubbles in microgravity was also studied; combined with dielectrophoresis, these phenomena enable improved phase gas-liquid phase separation in a low gravity environment.

Objectives

Experimental Procedures

References [1] P. Di Marco, ASME Journal of Heat Transfer 134, 030801-1-15, 2012[2] N. Markarian, M. Yeksel, B. Khusid, A. Kumar, P. Tin, Physics of Fluids 16(5), 1826-1829, 2004 [3] R. Raj, J. Kim, J. McQuillen, ASME Journal of Heat Transfer 134, 011502-1-13, 2012[4] P. Di Marco, R. Raj, J Kim, Journal of Physics: Conference Series 327, 012039-1-14, 2011[5] J. Straub, Ann. N.Y. Acad. Sci. 974, 348–363, 2002[6] J. Wu, V.K. Dhir, ASME Journal of Heat Transfer 133, 041502-1-14, 2011

The heaters operated during zero gravity portions of the flight parabola. As heating was turned on, a large bubble rapidly formed over the heater. It did not detach during the acceleration portion of the parabola and remained on the heater up to 19 consecutive parabolas. Videos indicate that the bubble was pinned to the heater edges

The large bubble pinned to the heater emitted sporadically microbubbles in microgravity which were tracked to compute trajectories and velocities.

Neutrally buoyant blue polyethylene microspheres (75- 90μm) were added to water to study the flow patterns.

In ground experiments, the flow around a large bubble pinned to the heater was driven by the thermocapillary and buoyancy forces. The large bubble did not emit microbubbles. Instead, it produced a hot plume rising to the water surface.

V = 3mm/s

V = 2mm/s V = 1mm/s

V = 6mm/s

V = 4mm/s

x (mm)

y (

mm

)

Zero g 1.8g

The flight tests provided critical data for design future ground and microgravity experiments for the EHD technology development:

• Parabolic flight tests would be sufficient to reveal the effects of wetting forces on EHD phenomena in bubble dispersions in a low surface tension fluid.

• Longer duration microgravity experiments aboard a suborbital reuseable launch vehicle are required for testing the efficiency of the EHD technology.

Future Work

Results on HFE-7100 in Microgravity

D

Heat flux 0.7 MW/m2, heater temperature 150oC, voltage 4kV/20Hz

Pt-temperature sensorsResistivity: 100 Ohm at 0°CT.-Range: -200°C - +400°CDimension: 2.3 x 2 x 1.3 mm (LxWxH)Contacts: Ag-wire, Ø 0.25 mm, 10 mm long

With field: 4kV/20Hz Without field

y (

mm

)

Neutrally Buoyant Particles

Heat flux 0.7 MW/m2, heater temperature 150oC, voltage 4kV/20Hz

The climbing of HFE-7100 was suppressed by the addition of water. In this configuration, spherical bubbles formed on the heater surface without pinning to the heater edges as in water.

water

HFE

0.5min 3.3min 7.5min 12min

The fact that the EHD forces do not influence the heat transfer at a high value of superheat is consistent with measurements of the DC field contribution to the boiling performance of small heaters in FC-72 under microgravity in parabolic flights [4]

Time (s)

0sec 1.1sec 4.0sec3.0sec 4.6sec0.2sec

0sec 6.5sec 13.0sec9.2sec 14.4sec0.2sec

The electric field was supplied by: • a programmable function generator and high-voltage amplifier• Square wave function and AC voltage electric field were used• Tested at field strengths of 3kV and 4kV, and frequencies of 1Hz, 10Hz, 20Hz

The experimental setup consisted of:• 2 quartz cuvettes (containing HFE-7100 and water)• a thermistor powered by DC generators encased in a PDMS insulating layer• Teflon-coated stainless-steel electrodes in each cuvette (did not touch PDMS)• THOR Lab , and LED illumination behind the cuvettes• LabJack U6-Pro Data Logger and uc480 video software Acknowledgements

Sponsors:National Aeronautics and Space Administration (NASA)NASA Goddard Space Flight Center (GSFC)NASA Goddard Institute for Space Studies (GISS)NASA New York City Research Initiative (NYCRI)New Jersey Institute of Technology (NJIT)

Motivation