Local momentum flux measurement: An effective way for GDI spray targeting in flash boiling conditions

Fuel injection and spray development are crucial processes for gasoline direct injected engines as an extremely accurate control of mixture formation is mandatory in order to attain high thermal efficiency targets while limiting pollutant emissions formation (mainly soot and NOx). In particular detecting the actual spray targeting is challenging as the spray evolution is significantly affected by the wide range of injection operating conditions typically implemented in GDI engines to allow a proper combustion process control. Peculiar injection strategies (rail pressure level, number of injection events) and combustion chamber pressure/temperature levels can result in flash boiling conditions, in which both the atomization process is enhanced and the spray structure is heavily modified with respect to the nominal one, with the different jets possibly collapsing in one single plume. The spray structure experimental analysis is problematic in flash boiling conditions, being conventional optical techniques as imaging and phase Doppler anemometry often unable to provide quantitative data about the inner spray structure, particularly very close to the nozzle. In this study, the actual spray evolution in flashing and non-flashing conditions was investigated by imaging and by momentum local distribution measurement, a methodology based on the spray-impact force method applied to small portions of the fluid structure to detect the local contribution of both the liquid and the gaseous phases to the jet momentum flux. The resulting momentum flux maps can be obtained very close to the nozzle and are not affected by the injector and test chamber operating conditions, offering a significant insight in the jet-to-jet interaction and spray evolution in flash boiling conditions. The obtained results evidenced the progressive modification of the spray structure of a 6-hole symmetrical GDI nozzle from standard to flare flash-boiling conditions, passing from transitional state. The plumes fingerprint gets larger when the ambient to saturation pressure ratio decreases and, when flare flash-boiling occurs, the plumes interaction becomes so intense that the spray structure collapses with a strong reduction of the global cone angle and an increased tip penetration. The obtained results confirmed momentum distribution maps to be an effective way to quantitatively analyze the actual spray targeting, the hole-to-hole flow distribution and the overall spray evolution under a wide range of operating conditions, overcoming severe limitations of other diagnostics like spray visualization and phase Doppler anemometry.