Electric Field-Driven Flame Dynamics

Hydrocarbon flames naturally produce charged intermediate species during the chemical-to-thermal energy conversion process. Although low in concentration, chemi-ions can dramatically influence flame behavior due to their electrical nature. The body force exerted on these species by an electric field is lost through mean free path collisions to neighboring neutral molecules. The frequent collisions of the ions help quickly establish a drift velocity governed by their mobility, along with a small, but measurable, net velocity of neutral molecules in the same direction. This ion-driven wind, analogous to buoyancy, can alter the flow environment, which produces a complex coupling that can affect flame shape, luminosity, and stability limits. This ability to globally influence flame behavior can potentially create a powerful sensing and actuation platform. This dissertation describes a fundamental study on the complex interaction between chemi-ionization in flames and any physical changes in a flame which are created by an electric field-driven convective flow. A theoretical and historical background is included to give the reader the necessary foundation to appreciate the tests performed and their implications. Because natural convection and the ion wind have a similar influence, in addition to testing in a 1g environment, this dissertation discusses results from a number of tests performed in a reduced gravity environment at the NASA Glenn 2.2-s Drop Tower. Depending on the testing environment, various imaging and diagnostic techniques are employed to interrogate flame behavior: flame photographs, chemiluminescence measurements, Planar Laser Induced Fluorescence, and Schlieren. Results present evidence of a relationship between the measured ion current and CH* chemiluminescence, illuminate the impact of soot on ion current measurements, as well as the limitations of the ion current as a representative measurement of chemi-ionization. In addition, numerical models developed in OpenFOAM contribute toward an understanding of ion formation in non-premixed jet and coflowing flames.