Study of Small Scale EHD Driven Flow Distribution Control and Understanding of the Effect of Temperature on EHD Conduction Pumping Performance

Talmor, Michal

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Study of Small Scale EHD Driven Flow Distribution Control and Understanding of the Effect of Temperature on EHD Conduction Pumping Performance

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Electrohydrodynamic (EHD) conduction pumping technology offers a unique way to control flow distribution in multi-scale environments. In EHD conduction pumping, the interaction between an applied electrical field and dissociated electrolyte species in a dielectric fluid generates a net body force within the fluid resulting in a net flow in the desired direction. EHD conduction pumps have remarkable potential due to their lack of moving parts, simple designs, light weight, low power consumption, and ability to operate in microgravity. The suitability of these pumps increases at small scales and they have been previously proven effective for heat transfer enhancement, with possible applications in electronics cooling and more, both terrestrially and in space. In this study, the single-phase flow distribution control with EHD conduction pumps between two parallel micro-channels is experimentally investigated. In EHD conduction pumping, a strong electric field is applied via asymmetric submerged electrodes in a dielectric liquid. The field enhances the dissociation of electrolytic impurities present within the fluid, generating ions that migrate to form heterocharge layers over each electrode. This study numerically investigates the heterocharge layer morphology of EHD conduction pumping used in flow distribution control between parallel micro-scale branches, using different pumping orientations and accounting for flow inertia effects. The results are qualitatively compared with available experimental data and serve to explain observed behaviors. Most experiments performed using EHD conduction pumps have focused on global flow rate and pressure generation measurements, but have not provided measurements of the actual flow velocity fields generated by these pumps. While these flow velocity fields can be simulated numerically, both qualitative flow iv visualization and quantitative measurements of the true flow velocity vectors are difficult to accomplish for EHD conduction due to the presence of the strong electric field. Few studies have therefore attempted to perform any kind of flow visualization of EHD conduction pumping in general, and fewer still offered velocity measurements for these devices. This study provides a comprehensive set of measurements of the flow velocity fields generated by a multi-electrode EHD conduction pump, measured using particle imaging velocimetry, with unique insulating particles as the visualization elements. These measurements are taken for several flow conditions that provide insight into the effect of external flow inertia on the EHD conduction pumping mechanism - static pressure generation with no net flow through the loop, open flow through the loop, and in the presence of externally applied flows supplied by a mechanical pump. Previous experimental works have shown that, in some cases, increasing the temperature of the fluid decreases the performance of EHD conduction pumps, while in other cases it is the opposite. In this study, these seemingly contradictory behaviors are explained by taking into account the different working regime (i.e., ohmic regime and saturation regime) of the EHD conduction pumps. Specifically, the effect of temperature on the pressure generation performance of EHD conduction pumping is fundamentally investigated and described based on the corresponding regime. In the ohmic regime there is always two heterocharge layers next to the electrodes and an electroneutral bulk, while in the saturation regime the heterocharge layers overlap and there is no electroneutral bulk. These considerations are relevant for the design of EHD conduction pumps, especially in small scales.

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