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Schlieren/shadowgraphy has been adopted in the combustion research as a standard technique for tip penetration analysis of sprays under diesel-like engine conditions. When dealing with schlieren images of reacting sprays, the combustion process and the subsequent light emission from the soot within the flame have revealed both limitations as well as considerations that deserve further investigation. Seeking for answers to such concerns, the current work reports an experimental study with this imaging technique where, besides spatial filtering at the Fourier plane, both short exposure time and chromatic filtering were performed to improve the resulting schlieren image, as well as the reliability of the subsequent tip penetration measurement. The proposed methodology has reduced uncertainties caused by artificial pixel saturation (blooming).

Visualization of single-hole nozzles into quiescent ambient has been used extensively in the literature to characterize spray mixing and combustion. However in-cylinder flow may have some meaningful impact on the spray evolution. In the present work, visualization of direct diesel injection spray under both non-reacting and reacting operating conditions was conducted in an optically accessible two-stroke engine equipped with a single-hole injector. Two different high-speed imaging techniques, Schlieren and UV-Light Absorption, were applied here to quantify vapor penetration for non-reacting spray. Meanwhile, Mie-scattering was used to measure the liquid length. As for reacting conditions, Schlieren and OH* chemiluminescence were simultaneously applied to obtain the spray tip penetration and flame lift-off length under the same TDC density and temperature. Additionally, PIV was used to characterize in-cylinder flow motion.

A study was conducted to investigate the evolution of liquid phase penetration of evaporating sprays under engine-like conditions, with diesel and biodiesel fuel blends. This study has been performed in a facility based on a single cylinder two-stroke direct injection Diesel engine operating at low rotational speed which provides a quiescent thermodynamic environment around TDC, when fuel is injected, realistic for current D.I. Diesel engines. Due to the absence of inlet or exhaust valves, very easy optical access to the combustion chamber can be provided through the cylinder head. Pure nitrogen is supplied to the engine as intake gas, in order to avoid combustion. The injection event is carried out by an electronically controlled common rail system. The injector is equipped with real 8-hole nozzles, with a hole diameter of 0.115 mm. Injection pressures in this study ranged between 30 and 160 MPa and different in-cylinder peak pressure and temperature values were considered.