ID 211

Ultra-short Pulse Off-axis Digital Holography Under Realistic Diesel Spray Conditions

 

Abstract:

A single-shot Ultra-Short Pulse Off-axis Digital Holography system is used to image details of an atomizing dodecane spray produced by a diesel common-rail injection system. This technique takes advantage of multiple scattering noise rejection accomplished by combining coherence filtering with ultra-short laser pulses (400 nm, 100 fs FWHM) while providing the 3D reconstruction capability unique to digital holography; furthermore, being single shot, it can capture dynamic high-pressure sprays without any blur. The method allows the user to image a large portion of the spray with a single 100 fs pulse, and to then numerically focus on the features of interest. This paper presents results obtained after testing the technique on a diesel common rail fuel injection system under realistic spray conditions to provide evidence of the suitability of this technique for imaging the optically dense region of high pressure atomizing fuel sprays. The spray system, designed and built at Colorado School of Mines, includes a common rail with a single orifice Bosch LBZ fuel injector housed in a pressure vessel capable of reaching up to 30 bar and temperatures up to 650 °C. Dodecane or diesel fuel is injected at pressures up to 1700 bar. The results show how ballistic holography is unaffected by thick windows, as well as by pressure and temperature gradients. The system achieves an in-plane resolution of 30 µm while resolving ligaments and droplets which reside in depth planes several centimeters apart within the transient spray field.