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Dynamics of Chemi-Ion Driven Flows in an Applied Electric Field

  • Author(s): Tinajero, Jesse Alexander
  • Advisor(s): Dunn-Rankin, Derek
  • et al.
Abstract

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

electric potential.

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

within reason.

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