Effect of Gasoline Fuel Additives on Combustion and Engine Performance

2.3.3
 
Droplet Sizing Methods 
The following section introduces optical methods used for determining droplet 
size and droplet size distribution. The three most commonly used methods described 
here are Particle/Droplet Image Analysis, Phase Doppler Anemometry and Laser 
Diffraction. 
2.3.3.1
 
Particle/Droplet Image Analysis 
Particle/droplet image analysis (PDIA) is an image based method of 
determining the droplet size. Particle detection algorithms allow for usage of digital 
imaging of small regions of the spray. Kashdan et al. [148, 149] applied PDIA and 
Phase Doppler Anemometry techniques to sprays produced by a pressure-swirl GDI 
injector with droplets in the region of 5-30 µm. They found good agreement between 
the results. They also found good response of the PDIA system to large and non-
spherical droplets as well as coping with greater droplet size ranges. According to 
Anand et al. [109], this makes the PDIA system suitable to PFI sprays where large and 
often non-spherical droplets are expected.
Downsides to the PDIA system include inaccuracies caused by incorrect 
distance between particle and the focal plane that makes detection and contour 
defining of unfocused droplets difficult [150]. Also, small region imaging results in 
Figure 2.6: PDIA image with identified droplets [109] 

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2.3 Spray Characterisation 
58 
only a very few droplets being measured, meaning a distortion in droplet size 
distribution is possible. A sample image from a PDIA system can be seen in Figure 
2.6. 
2.3.3.2
 
Phase Doppler Anemometry 
Phase Doppler anemometry (PDA) uses a split light source directed through a 
small volume of the spray. The interference caused by the spray creates a scattered 
light signal that can be translated into spatial and temporal data on droplet size, 
location and velocity [151]. A typical PDA set-up can be seen in Figure 2.7. Two 
detectors are positioned on the y-z plane out of the incident beam at an angle φ, and at 
symmetric ±ψ degrees from the off-axis angle. However, as pointed out by Damaschke 
et al. [152], PDA systems can produce unreliable results if non-spherical droplets are 
present and determination of local droplet number density and volume flux can be 
difficult. Non-spherical droplets can further force the PDA system to reject data, 
meaning often large numbers of repeat spray events are necessary [153]. Due to the 
very small sampling volume of the PDA system, large numbers of repeats are always 
necessary for a statistically relevant result [154].