Download The Case for EHD Control in Low Gravity Systems and the Need for a Long-duration, Low-gravity Test Environment

The case for EHD control in low gravity systems and the
need for a long-duration, low-gravity test environment
Jeffrey S. Allen1 and Subramanian Sankaran2
1
Michigan Technological University, Michigan USA
Abstract
A fluid-fluid interface in an electric field is subject to a
force normal to the interface due to the sharp change
in the dielectric properties. Generally, this force is relatively weak in comparison to gravitational forces. However, this phenomenon offers the ability to dynamically
control liquid-gas interfaces in a low-gravity environment. There are a number of space technologies that
could benefit from a more thorough understanding of
electrohydrodynamic (EHD) forces on gas-liquid interfaces. Some of these technologies and applications include high heat transfer through boiling using EHD to
suppress a critical heat flux condition, separation of helium from cryogenic oxygen, and selectively moving liquid water in low-temperature fuel cells. EHD could be
utilized to improve the efficiency, reliability, and capacity of these technologies for extraterrestrial applications
as well as terrestrial purposes. In particular, application
to fuel cell operation would assist with the development
of “portable and economical energy on demand,” which
is one of the key Space Technology Grand Challenges.
Though the science of EHD is well understood, the
engineering of low-gravity systems is not well developed, particularly electrode design in multiphase flows.
A long-duration, low-gravity environment necessary to
understand the interaction between electric fields and
fluid interfaces in an environment where inertial forces
are also important.
Enhanced Boiling Heat Transfer
When the rate of vapor production from boiling exceeds the rate of vapor removal at a heater surface,
film boiling and the associated high surface temperatures occur; a condition known as critical heat flux.
The result may be a run-away temperature and subsequent material and device failure. Critical heat flux happens very easily in microgravity because of the absence
of buoyancy-induced bubble motion. Similarly, for microscale heaters, the vapor produced covers the heater
surface quickly. Thus, both in terrestrial and in space
environments, it becomes important to understand the
ways to destabilize the vapor microlayer associated with
film boiling and reach nucleate boiling for various heat
exchanger applications. The vapor microlayer, or vapor
2
MEI Technologies/NASA JSC, Texas USA
film, is the thin layer of vapor covering the heater surface during film boiling. Electric fields provide a powerful force to destabilize the microlayer and maintain
nucleate boiling.
There are several variables that affect the boiling
phenomena, such as the thermophysical properties of
the solids and liquids, the fluid subcooling level, the
fluid flow velocity, geometry, gravity, system pressure,
surface roughness, oxidation levels and contact angle.
The maximum heat flux chievable in the nucleate boiling
mode beyond which film boiling occurs, and the minimum heat flux below which the film boiling can revert
back to nucleate boiling are of theoretical and practical
interest. Electric fields are known to cause an order of
magnitude increase in heat transfer efficiency, eliminate
the boiling hysteresis at the incipience of boiling, and
revert film boiling into nucleate boiling.
The central idea of enhancing the boiling heat transfer can be viewed either as removing the vapor that is
produced at the heater surface or equivalently as driving
the liquid towards the heater and increasing the liquidsolid contact area. In either view the microlayer of vapor
is destabilized. The combined effect of liquid subcooling, liquid velocity, and applied electric fields on boiling heat transfer is very important in determining how
quickly and efficiently recovery from film boiling to nucleate boiling can occur. Such a study does not currently exist in the literature. There is little information
on the combined effect of liquid subcooling and applied
electric fields without liquid flow as well which is the
subject of the current study. Figure 1 shows preliminary
experimental results on the effect of radial dc electric
fields on nucleate and film boiling.
Enhanced heat transfer and stabilization of boiling
heat transfer in low gravity is could enable more robust,
compact thermal management systems. However, there
is insufficient understanding of the interaction between
electric fields and the vapor film in boiling. This is lack
of engineering knowledge is particularly acute in a lowgravity environment.
Other Technologies
A variety of other technologies necessary for human
space travel could also benefit from a more thorough
understanding of the interaction between electric fields
and liquid-vapor/gas surfaces. These technologies in-
clude bubble control in electrolyzers, separation of he- force that we intuitively utilize in the design of fluid syslium from cyrogenic oxygen, and water management in tems for phase separation. Electric fields can provide a
fuel cells. The reduction in gravity eliminates the body controlled alternative ‘body force’ for these systems.
(a)
(b)
(c)
(d)
(e)
Figure 1: Use of radial DC electric field to destabilize microlayer of vapor and revert back to nucleate boiling
and enhance heat transfer from a 127 µm diameter resistance wire in PF-5060 fluid. The image sequence
shows the transition from nucleate to film boiling and back to nucleate boiling after application of the electric
field. (a) Nucleate boiling in quiescent fluid and no electric field. (b) Transition to film boiling progressing
along the wire from the left. (c) Film boiling without the electric field. (d) Transition to enhanced nucleate
boiling with application of electric field. (e) Enhanced nucleate boiling with electric field.
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