Droplet sizing interferometry: a comparison of the visibility and phase/Doppler techniques.

Spatially resolved measurements of droplet size and velocity are desirable to aid in matching fuel injectors to combustor flow fields and to support development of two-phase-flow modeling. Interferometric laser-based techniques have been available since the early 1970s. Successful application to practical sprays, however, has been hampered by numerous difficulties. In this paper, two interferometric techniques (visibility/intensity validation and phase/Doppler) are critically examined in characterizing the spray of an air-assist nozzle with Sauter mean diameter < 35 microm. The two techniques are compared to each other and evaluated against a Malvern diffraction unit. With the use of a rotating grating for frequency shifting, the interferometric techniques compare well with each other and to the diffraction method. Due to its broadened size and velocity ranges, the phase/Doppler technique is more easily applied to the spray than is visibility/intensity validation. The consistency of the interferometric results raises questions with regard to the use of Malvern's most frequently applied distribution model.