HIGH-SPEED, CONTINUOUS MONITORING OF LIQUID SPRAYS BY MULTI-WAVELENGTH ABSORPTION SPECTROSCOPY

Abstract

A fiber-optic array was created by arranging four Corning lensed fibers in a parallel, horizontal plane. It has been shown that through the use of multiwavelength spectroscopy, this fiber array is capable of probing a spray in four discrete locations. Two wavelengths of laser-sourced light were selected based on the optical properties of water, the fluid under investigation. Water was used as the working fluid only to evaluate the measurement techniques described here; the intent is to apply the techniques, once developed, to fuel sprays. One wavelength that is absorbed by water (1450nm) and one that is not (1310nm) were used for the spectroscopic analysis. A super-continuum (1300nm-2000nm) and diode lasers were independently used as the source of light for this experiment. In the case of ii the super-continuum source, the absorbing and non-absorbing wavelengths were selected using fiber Bragg gratings or a custom filter based on a profile gauge. By generating two pulses of different wavelengths and temporally separating them by various delay fibers, it was possible to make each wavelength distinct when received by a photodiode (detector). The dual-wavelength signal was split four ways to service each branch of the lensed-fiber array. Additional delay fibers were placed after the splitter and before the lensed fibers with the purpose of making each branch?s signal distinct. By keeping each delay described to the minimum detectable amount of time, it was possible to ?freeze? the spray in time. In this situation, the dynamics of the spray are much slower than the dynamics of the optical system, a key requirement facilitating quantitative measurements even though the amount of scattering in the spray varies dramatically. After the light has been transmitted through the spray, the light was collected by a detector. By comparing the transmitted strength of the absorbing and nonabsorbing wavelengths, one may begin to make quantitative statements about the spray. Using the quantifiable measurements of droplet size, spray-plume geometry and fluid properties, complete measurements of the quantity of liquid can be made. Through the use of the multi-point measurements described in this work, one can produce a map of liquid and vapor throughout the spray plume at a given distance from the nozzle tip. iii The results of trials using the various sources of light are consistent, as the diode lasers were tuned to produce similar characteristics to the super-continuum source. Further development of the processes described here depend on light sourced from super-continua, as this method provides greater flexibility and expandability. Single-point measurements were also conducted using careful spatial filtering of the transmitted light. Through this method, it was possible to determine whether the light has contacted liquid only, vapor only, or both phases. The method of collecting light based on its direction of incidence at the detector is discussed in the body of this work. Future iterations of the multi-point technique may incorporate advanced spatial filtering techniques.