Measuring Droplet Arrival Times Using Phase Doppler Interferometry
J F. Widmann, Cary Presser
The difficulty of accurately measuring droplet arrival time statistics is demonstrated using phase Doppler interferometry (PDI) measurements obtained in water sprays produced by pressure-swirl atomizers. It is shown that Doppler burst splitting, in which single Doppler bursts are erroneously interpreted as multiple droplets, is difficult to avoid and can negatively impact arrival time data. The influence of optical and sampling parameters on the probability of Doppler bursts being split is discussed. Due to the variation of droplet velocity at a given location within a spray, it is difficult to avoid split Doppler bursts without further biasing the measurements. It is found that using a mixer frequency, fMIXER, equal to the shift frequency of the Bragg cell, fSHIFT, can result in both under- and over-sampling of Doppler bursts when measuring droplet velocity distributions. It is also shown that in many regions of the spray there is overlap of the PDI gate times and the droplet interarrival times, further questioning the validity of the measured arrival time statistics.
May 14-17, 2002
Conference on Liquid Atomization and Spray Systems