Phase boundary detection in transient, evaporating high-pressure fuel sprays by rainbow schlieren deflectometry

The rainbow schlieren deflectometry (RSD) technique is used to simultaneously identify the liquid and vapor boundaries during transient evolution of evaporating high-pressure fuel sprays. Traditionally, liquid and vapor phases require separate diagnostics, whereas this work relies upon a single technique to identify each phase, in addition to the previously demonstrated capability of RSD to measure local fuel-air mixing in the vapor zone. The proposed RSD methodology is a significant improvement over a previously published technique, i.e., it is applicable to the transient period, and much less data and hence, computational effort, is required. Experiments are conducted in a constant pressure flow rig for multiple fuels and test conditions to demonstrate the capability of the proposed RSD diagnostic. For each test condition, multiple injections are performed in quick succession to obtain statistically significant data sets to depict the spray evolution at a spatial resolution of 100 μm, a camera framing rate up to 40 kHz, and an exposure time down to 1 μs. The method and the results are compared and discussed within the context of prior work using RSD, Mie-scattering, and Siebers' liquid length scaling law.