Spatial and Temporal Evolution of an Electrohydrodynamic Flow Generated Off the Surface of a Piezoelectric Transformer

In this work, the spatial and temporal evolution of an electrohydrodynamic (EHD) flow (also known as ionic or electric wind) generated from the surface of a piezoelectric transformer (PT) was characterized by schlieren imaging. EHD flows have several industrial purposes that make studying their behavior a matter of interest. Producing the EHD flow, however, requires the formation of an atmospheric plasma which can be sustained at high electric fields (3 V/μm). Obtaining fields of this magnitude requires a high input voltage, and due to the size and cost of the electrical supply needed to accomplish this, it is often not practical. Here, a PT is utilized to circumvent this challenge. A piezoelectric transformer is a non-centrosymmetric crystal that converts low-voltage AC input to high-voltage AC output through innate electromechanical coupling, enabling the formation of plasmas using only ~10-30 V. This experiment utilizes a lead zirconate titanate (PZT) Rosen-type PT actuated at its second harmonic frequency in atmospheric air leading to a discharge forming at its corners. From this discharge, an EHD flow is produced due to the positive and negative ions being driven away from the PT due to the local electric field, and bombarding air molecules in their path creating a bulk flow. EHD jets were characterized using schlieren imaging for varying input voltages (magnitude of EHD force) and duty cycles (duration that EHD force acts). Results show that the maximum flow speeds increased linearly with increasing input voltage, and surprisingly, decreased with increasing duty cycle. The jet width initially tends to increase with increasing input voltage and eventually saturates for all input voltages as the duty cycle increases above approximately 20%. The flow velocity, as a function of centerline distance, tends to fall off more steeply than a plane laminar jet subject to a constant initial momentum. Still, the EHD flow does have aspects comparable to a laminar jet.