Quantitative Mixing Measurements in a Vaporizing Diesel Spray by Rayleigh Imaging

This paper details the development and application of a Rayleigh imaging technique for quantitative mixing measurements in a vaporizing diesel spray under engine conditions. Experiments were performed in an optically accessible constant-volume combustion vessel that simulated the ambient conditions in a diesel engine. Two-dimensional imaging of Rayleigh scattering from a diesel spray of n-heptane and well-characterized ambient was accomplished by using a 532 nm Nd:YAG laser sheet and a low-noise back-illuminated CCD camera. Methods to minimize interference from unwanted elastic scattering sources (e.g. windows, particles) were investigated and are discussed in detail. The simultaneous measurement of Rayleigh scattering signal from the ambient and from the diesel spray provides important benefits towards making the technique quantitative and accurate. The Rayleigh scattering signal from the uniform ambient provides an in-situ calibration for the spatial non-uniformity in laser sheet intensity on a shot-to-shot basis caused by energy variation in the delivery beam as well as beam steering in the high-pressure combustion vessel. The diagnostic technique was validated by investigating the effect of equivalence ratio on soot formation in diesel jets. Consistent with previous studies, the diesel jet becomes non-sooting when the equivalence ratio distribution in the premixed-burn region is less than two.