Flame Dynamics and Chemi-Ion Flows Driven by Applied Electric Fields
The enhancement to laminar, non-premixed flames by the application of an external electric field is studied. It has long been known that naturally occurring ions are produced through chemical reactions during the combustion of hydrocarbon flames. Chemi-ionization has been noted in the literature as the source of naturally occurring flame ions and electrons. Because chemi-ions exist in very small quantities within the flame (1e9 - 1e10 ions/cm^3), they contribute very little to the overall natural combustion process and can usually be neglected in conventional combustion applications/studies. However, with the application of an external electric field, the chemi-ions/electrons can have a dramatic effect on combustion through ion driven winds.
Ion current measurements are characterized for conical-type methane flames stabilized on axially symmetric burners with longitudinal electric fields applied. The ion currents are used to correlate the flux of chemi-ions to measured flame characteristics. The manipulation of the flame by ion winds is analyzed through CH* chemiluminescence where flame geometry and light emission are the main targets to be studied. The modification to the thermal flow field is analyzed through schlieren imagery. The visualization through schlieren demonstrates how ion winds enhance the exhaust gas and provides a means of analyzing the interaction between ion fluxes and surrounding neutral flow. Numerical CFD models are described and validated. Ion wind body forces are generalized to simple body forces to test the extent to which ion winds can be used to describe the experimental observations. The numerical models provide a means of studying more in-depth complicated interactions of the flame and gaseous flows to enhanced body forces. Finally, a theoretical model is developed which highlights the important mechanisms involved in the modification of flame and gas flow dynamics by body forces due to gravity and external electric fields.
Ion winds are shown to be the dominant mechanism in external electric field enhanced combustion. Ion winds explain nearly all of the observed effects on flames by external fields. The relatively small disparities that do exist between the experimental results and numerical results are likely from the uncertainties associated with chemical kinetics of minor species with extremely small concentrations and the use of simplified transport in the simulation.