A piezoelectric droplet generator for use in wind tunnels

Abstract

This article describes a compact piezoelectric droplet generator. A unique feature of this instrument is the small size of its nozzle assembly which includes a cylindrical piezoelectric ceramic transducer, an interchangeable orifice, a charging ring, and deflection plates. The nozzle assembly is housed in an acrylic tube, 19 mm in outside diameter and 150 mm in length, which can be placed inside a wind tunnel without causing a significant blockage. The main mechanical components consist of a pressurized system which discharges liquid through the orifice to create a free liquid jet in the nozzle assembly. The transducer causes this jet to break up into uniform droplets by exciting the instability mechanism of symmetric varicose deformations. Droplets tend to be closely spaced because of this method of generation so increased separation is obtained by selectively charging and deflecting some of the droplets from the main stream to a drain. One droplet out of 1–9999 droplets can be selected to exit from the nozzle assembly. The electronic circuitry consists of a transducer driver, and circuits for charging and deflecting the droplets. Power metal-oxide-semiconductor field-effect transistors are used in an H bridge and a push-pull configuration to provide rapid operation of the transducer-driver and droplet-charging circuitry. A transient analysis is performed to obtain the gain factor between the applied voltage and the corresponding flow modulation of the liquid jet. This analysis provides operating characteristics of the transducer and may be used to calculate the breakup length of the jet for a typical applied voltage. Uniform droplets can be produced reliably with this instrument over a large range of size and speed. For the current research application, involving droplet drag in turbulent flows, droplets with diameters from 150 to 1000 μm are required at speeds from 3 to 20 m/s. For other studies, the range of size can be changed by using different orifices.