A multiple drop-size droplet generator was developed for Lycoming to calibrate spray measurement instruments with respect to drop diameter. The droplet generator can simultaneously produce one, two, or three drop diameters in the range of 50 to 450 μm. Drop diameters have temporal variations less than 1%, and each drop-size species is monodisperse to within 1%. Measurements of the spray produced by the multiple-size drop generator made with a properly aligned phase-Doppler particle analyzer, whose gain setting was optimized to reduce spurious validations about the largest diameter, gave accurate diameter readings within one or two measurement size classes or cohorts (2 to 4%). Transmitter lens spherical aberrations gave errors typically less than 3%.
Stimulated breakup of liquid jets, commonly called Rayleigh breakup, creates the droplets. Liquid, flowing from a pressurized manifold through orifices in a laminated nickel and copper plate, forms the jets. Piezoelectric transducers perturb the jets' velocity, breaking the jets into drops at the frequency of the velocity perturbations. Subharmonic velocity perturbations cause controlled agglomeration of the drops increasing the final droplet diameter and spacing. A stroboscope makes the droplets appear stationary for photographs and optical size measurement. Electrically charging the droplets creates a dispersion of controllable magnitude of the spray and a few reverse flow particles. Dividing the flow rate by the drop production frequency accurately determines the drop volume. The inexpensive and easily changed orifice plates contain 21 orifices spaced on a 2.54-cm line. Orifice diameters can be any size greater than 10 μm. In multiple diameter plates, the larger diameters are integer multiples of the smallest diameter.