Comparison of diffraction-based and light extinction techniques for particle sizing

ABSTRACT Studies of direct injection, compression ignition engines demand a particle sizing technique which can reliably retrievedroplet sizes in transient fuel sprays. Many of the existing sizing techniques cannot analyze the dense, transient fuel spraysor provide information which is awkward to interpret. This work compares the performance of a diffraction based particlesizing technique and a light extinction sizing technique on a series ofDiesel sprays. Both techniques were effective inacquiring useful data from the sprays. Under equivalent conditions, the light extinction technique provides somewhat smallersizes than the diffraction based method. KEYWORDS Particle sizing, fuel sprays, diffraction techniques, light extinction techniques 1. INTRODUCTION Current improvements ofdirect injection, compression ignition engines concentrate on reducing emissions while retaininggood performance. These efforts to both improve engine efficiency and reduce engine emissions encourage detailed study ofthe combustion. The behavior ofthe fuel spray, especially the rates ofevaporation and breakup ofthe fuel droplets,substantially affects the combustion. A thorough understanding ofthe spray behavior is thus essential to aptly describe thecombustion in the cylinder. Good spray models are integral parts of complete combustion prediction codes. Experimentalstudies are necessaiy to better understand fuel spray behavior and to develop and validate spray models. Measuring thedispersion offuel droplet sizes in sprays helps document their behavior and explore events like evaporation and breakup.Determining droplet size distributions is not trivial, because fuel sprays are typically dense and transient. Many matureparticle sizing techniques exist, but few are capable ofacquiring useful information from transient fuel sprays. Most of thequantitative techniques obtain size information at individual points in the spray over a span oftime. Assimilating this sort oftime-ranging point data to yield spatial information is awkward, because each point is at a different flow condition. Manyexisting sizing techniques count or measure individual particles; this attention to individual particles hinders the use of thesetechniques in the dense fields of droplets found in fuel sprays.This work compares two distinctly different particle sizing techniques capable ofproviding spatial information in transientsprays. The first technique, diffraction particle sizing (DPs), determines size distributions by analyzing the diffraction patternproduced by images ofparticles' . Obtainingsize measurements with DPS requires capturing images ofparticles, interrogatingthe particle images to yield diffraction patterns, and inverting the intensity ofthe diffraction pattern to recover the particlesize distribution. DPS yields spatial maps ofparticle size from planar slices ofthe spray. DPS is a relatively young techniquewhich spawns from the basic diffraction based technique developed by Swithenbank, et al.2 The implementation of DPSdiffers significantly from the previous technique. DPS analyzes planes of captured images to yield spatially resolved data; theprevious diffraction based technique analyzes live particles to yield time-ranging line of sight data.The second technique, a light extinction sizing method (LEs), determines droplet sizes by analyzing the light transmittedthrough a group of droplets. Obtaining size measurements with LES involves imaging the light transmitted through thedroplets, digitizing the image to quantify the intensity distribution, and using the resulting transmitted intensity to iterativelycalculate the mean droplet size. LES yields cross-section averaged droplet size at axial locations in the spray. Otherinvestigators have previously used light extinction techniques to study fuel sprays3'4'5.540/SPIE