Experimental study on the effect of nozzle geometry on string cavitation in real-size optical diesel nozzles and spray characteristics

Cavitation is quite important for the diesel spray atomization and the combustion of air-fuel mixture. In this study, a high-speed CMOS camera equipped with a long-distance microscope was utilized to capture the transient cavitating flow and spray characteristics in real-size optical nozzles with needle motion. The transient cavitation images, including geometry-induced cavitation and vortex-induced string cavitation, were captured clearly in cylindrical-orifice nozzles and tapered-orifice nozzles, respectively. Besides, the agglomerated phenomenon of geometry-induced cavitation was visually captured and analyzed for the first time. It was found that the string cavitation in nozzle excites the instability of spray cone angle and it is synchronized with increase of spray cone angle. In addition, it is the string cavitation but not geometry-induced cavitation has a much larger contribution to the increase of spray cone angle. It is interesting that the influence of agglomerated geometry-induced cavitation on spray cone angle was prominent. Furthermore, both the nozzle orifice L/D ratio and sac types have significant influences on string cavitation and spray characteristics. The smaller L/D ratio and VCO-type nozzles are prone to incur the stronger string cavitation, and then spray cone angle is obviously larger.