Abstract
Liquid-tip penetration for diesel spray from a common rail diesel injection system is measured using diffuse backlight imaging (DBI) and Mie-scattering techniques under evaporating conditions. Experiments were conducted for three injection pressures of 500 bar, 1000 bar, and 1500 bar, while the ambient nitrogen gas is maintained at 50 bar pressure and 900 K temperature in a high-pressure spray chamber with optical access. Spray images are recorded using a high-speed camera and are post-processed using two methods found in the literature. The simpler of the two involves using a grey level threshold (threshold method) to convert the raw images into binary images and identify the liquid-tip, whereas the other consists of utilizing a line-fit for the time-averaged center-line intensity variation near the spray-tip (line-fit method). The results show that for images with good contrast, steady-state liquid lengths obtained from both the recording techniques are similar. However, the line-fit method applied to ensemble-averaged spray images fails to capture the transient variation of liquid-tip penetration due to significant waviness in the intensity. A comparison of liquid lengths obtained from DBI and Mie-scattering methods showed that the line-fit method underestimates the average liquid length by 9.4% for DBI method and 4.8% for Mie-scattering method, as it does not take into account the low-intensity signal near the tip of the spray. The transient variation of liquid-tip penetration is simpler to obtain using the threshold method in comparison with the line-fit method. Overall, the threshold method of post-processing for either DBI or Mie-scattering images is observed to be advantageous over the line-fit method for obtaining the transient variation of liquid-tip penetration and average liquid length.