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

Chemi-ions are produced during combustion of a hydrocarbon fuel. If an externalelectric field is present, a charge separation occurs due to the electrical force actingon the positively and negatively charged species. These ions traverse in the directionof the electrode of opposite potential. Along their path, they continuously collide withneutral molecules within the surrounding bulk gas until they are able to recombine andneutralize at the downstream electrode. During each collision, the charged speciesgive up their acquired momentum to the neutral molecules. Macroscopically, thistransfer of momentum has been best described mathematically as a body force actingon 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 withearth-based experiments. This is because the gravity-driven buoyancy effects behaveas a body force also acting on the bulk gas. Buoyancy and electrical body forces act onthe same order of magnitude. The two forces are coupled through temperature sincethe production of ions is temperature dependent. Between the two, the contribution tothe 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 throughNASA's Advanced Combustion via Micro-gravity Experiments program, or ACME,are planned for 2016-17. Nevertheless, preliminary studies are needed in preparationfor 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 onthe 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 relatedto parameters such as heat release and local velocity. Thus, when studying thermalplumes created by forced convection, i.e., ion wind effects, it is thought that similarrelations can be made with electro-hydrodynamic parameters such as ion current andelectric potential.High-speed videos captured the disturbance in the bulk gas due to a sudden presenceof an applied electric field. These videos reveal a never before seen wave phenomenonthat 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 ashunt resistor in series with the high voltage power supply and the two electrodes.The experiments performed were focused on transient effects in order to characterizetime-scales related to generation of both an ion current and ion wind. It was seen thatthe generation of an ion current was fully established within 10ms. This time-scale,however, may have been inhibited by a characteristically slow high voltage powersupply system. The first noticeable appearance of an ion wind effect being generatedwas 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 fora 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 alsoobserved 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-scaleswere then compared to time-scales developed through simplified electrical aspects ofcombustion theory. Both theoretical and experimental time-scales appeared to agreewithin reason