Dynamics of Chemi-Ion Driven Flows in an Applied Electric Field
- Author(s): Tinajero, Jesse Alexander
- Advisor(s): Dunn-Rankin, Derek
- et al.
Chemi-ions are produced during combustion of a hydrocarbon fuel. If an external
electric field is present, a charge separation occurs due to the electrical force acting
on the positively and negatively charged species. These ions traverse in the direction
of the electrode of opposite potential. Along their path, they continuously collide with
neutral molecules within the surrounding bulk gas until they are able to recombine and
neutralize at the downstream electrode. During each collision, the charged species
give up their acquired momentum to the neutral molecules. Macroscopically, this
transfer of momentum has been best described mathematically as a body force acting
on the bulk gas. The effect is commonly referred to as an ion wind effect.
Gravity effects make the electric field effects on combustion difficult to study with
earth-based experiments. This is because the gravity-driven buoyancy effects behave
as a body force also acting on the bulk gas. Buoyancy and electrical body forces act on
the same order of magnitude. The two forces are coupled through temperature since
the production of ions is temperature dependent. Between the two, the contribution to
the net momentum of the gas is then difficult to distinguish. On the other hand, micro-
gravity experiments allow for the direct study of electric field effects in the absence of gravity. Micro-gravity experiments on-board the International Space Station through
NASA's Advanced Combustion via Micro-gravity Experiments program, or ACME,
are planned for 2016-17. Nevertheless, preliminary studies are needed in preparation
for the ISS experiments. These studies are described in this thesis.
A replica of the ISS experiment for the electric field effects on laminar diffusion flames
(EFIELD Flames) that is part of ACME was recreated in a ground based laboratory.
A schlieren system was built to visualize the effect an applied electric field has on
the flame's buoyant thermal plume when the electric field is given a step function.
Thermal plumes created by natural convection are well understood and can be related
to parameters such as heat release and local velocity. Thus, when studying thermal
plumes created by forced convection, i.e., ion wind effects, it is thought that similar
relations can be made with electro-hydrodynamic parameters such as ion current and
High-speed videos captured the disturbance in the bulk gas due to a sudden presence
of an applied electric field. These videos reveal a never before seen wave phenomenon
that is created as the bulk gas transitions from a buoyancy driven gas flow to an electrically driven gas flow. Total ion current measurements were taken by using a
shunt resistor in series with the high voltage power supply and the two electrodes.
The experiments performed were focused on transient effects in order to characterize
time-scales related to generation of both an ion current and ion wind. It was seen that
the generation of an ion current was fully established within 10ms. This time-scale,
however, may have been inhibited by a characteristically slow high voltage power
supply system. The first noticeable appearance of an ion wind effect being generated
was also seen to be 10ms. The time-scale related to a volume of \ion wind driven"
gas cloud to travel the length of the electrode space region was seen to be 40ms for
a 3.5cm electrode space length. The time-scale related to the time it takes for the bulk gas to reach a new steady state was seen to be 100ms. Finally, it was also
observed that the time-scale related to the disappearance of visible soot when the flame was compressed by an ion wind was 300ms. These experimental time-scales
were then compared to time-scales developed through simplified electrical aspects of
combustion theory. Both theoretical and experimental time-scales appeared to agree