Ion generation and ionic wind heat transfer at millimeter and micrometer scales
Ionic wind engines are air cooling enhancement devices that operate on the principles of electrohydrodynamics. Positive ions in air (N+ 2 or O+2), generated by impact between accelerated electrons and neutral air molecules (N2 or O2), are pulled through the air and collide with neutral air molecules causing a wind. In the presence of a bulk flow, an ionic wind can modulate the boundary layer at a wall and enhance heat transfer. Microscale ionic wind engines are devices that are designed at the microscale (∼ 10 μm) in order to operate at applied voltages on the order of 100 V. The development of microscale ionization and ionic wind generation devices has potential applications in a number of fields, including electronics cooling—the focus of the present work. This dissertation studies ion generation and ionic wind heat transfer in atmospheric air over scales from millimeters to micrometers. Experiments at the millimeter-scale demonstrate that ionic winds in the presence of a bulk flow can enhance convective heat transfer coefficients by more than 200%. A relationship between the ionic wind heat transfer coefficient, and the ion current is analytically determined to follow a 4th power relationship, which experimental studies also confirm. A continuum–scale model of ion transport is used to understand the electrohydrodynamics and heat transfer, and lends further insight into the nature of the physical interactions including the local boundary layer disruption and the role of Joule heating. At the microscale, a planar ionization device is designed and studied both experimentally and computationally. Methods for fabricating titanium and highly graphitic polycrystalline diamond (HGPD) electrodes on both silicon and quartz wafer have been developed. Experiments with HGPD demonstrate the robustness of the material compared to titanium. Low–voltage, moderate–current ionization with HGPD is experimentally confirmed. A direct simulation Monte Carlo (DSMC) model reveals that the operating voltage of a planar ionization device has a physical lower limit near 70 V, and that planar devices are inherently inefficient due to particle loss at dielectric boundaries. A body force map is extracted from the DSMC simulation, and implemented into a continuum-scale model to study the fluid dynamics and heat transfer, and the results illustrate that localization of the body force impedes the realization of any significant electrohydrodynamic benefit.
Garimella, Purdue University.
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