Energy-saving discharge needle shape for electrohydrodynamic airflow generation

Abstract

Electrohydrodynamics (EHD) is a way to produce low energy-consuming airflow without moving components. The basis of airflow by EHD is corona discharge. A way to generate corona discharge is done, among others, via needle-type emitter electrodes whose shape and arrangement play a crucial role in the effectiveness of the discharge. Until now, the needle shape was chosen somewhat arbitrarily, although it impacts the energy consumption of the EHD process. We lack systematic studies on the impact of needle shape on the EHD discharge process and associated airflow to help engineers and scientists choose the best shape. This in-silico study screens the impact of the needle shape parameters on EHD performance in terms of electrical power consumption and airflow generation. The study aims to find the ideal EHD needle shape for unrestricted and confined flow. For this purpose, we test three different geometrical configurations. The first configuration is a free-flow single-needle configuration. The second configuration adds a dielectric nearby, which represents a needle enclosure. Lastly, a configuration including a dielectric and a converging nozzle is examined. All studies use a 2D-axisymmetric, fully automatized EHD physics-based model. The first set of parametric studies explores the inherent geometrical properties of the needle shape, like tip radii (10–250 $\mu m$), needle cone angle (10–70°), and needle diameters (0.5–2 mm). The second set of parametric studies investigates the operation conditions, such as the emitter-collector distance (10–40 mm), the nozzle contraction ratio (0.04–1), and the operating voltage (6–32 kV). The results of the free-flow configuration show qualitative agreement with experiments on existing needle products. The ideal energy-saving needle shape for free flow configuration features a short conical tip length (i.e., a large angle $\ge 30°$), a diameter of 2 mm, and a needle rip radius of 100 $\mu m$. The situation changes when a dielectric is present, and a sharp angle of 10° is favorable to reduce energy consumption. Since a dielectric inverts the optimal needle shape, it makes sense to customize it for a particular application in EHD airflow generation. We provide performance maps that can be used as design charts. This study is a guideline to optimize EHD devices further to reduce energy consumption and increase airflow speed.