Although both Mie-scattering and diffused back-illumination (DBI) imaging may be used for liquid-phase measurements, the recommendation is to use DBI. This is also recommended by the SAE J2715 standard for gasoline spray characterization. DBI uses the extinction produced by the spray droplets to provide a measure related to the liquid volume fraction along the path of the light. The advantage for quantification is that there is self-calibration for the initial intensity. In engine (high-pressure, high-temperature) environments, care must taken to ensure that the diagnostics is not sensitive to vapor-phase beam steering from temperature/refractive index gradients. The diffuser angle and collection angle need to be quantified for a given setup, and the camera needs to be configures to accurately measure background intensity (with no transmitted DBI light). An example setup is shown below.
Fig. Diffused back-illumination for high-speed extinction imaging
Recommendations for liquid-phase penetration measurements
Extinction data are to be processed for “projected liquid volume” in the cross-stream direction, or integral(liquid volume fraction * dy), as described in the ECN5.9 presentation of Pickett (slides, recording at 1:38:45). Because of uncertainties associated with the experiment, two different criteria should be used to define the liquid-phase penetration:
- 0.2 mm3 liquid / mm2
- 2.0 mm3 liquid / mm2
Please see example comparisons of Mie scatter and DBI imaging for more technical details regarding the Mie-scattering and DBI measurements.
Liquid-phase penetration length measurements using DBI for the gasoline Spray G condition are available to download as documented by Manin et al., SAE 2015-01-1894. The liquid penetration was defined as that with an optical thickness of 0.6 in this work.