Fundamental Investigation Of Unsteady Cavitation Process Inside Diesel Injection Nozzles And Its Effects On The Jet Atomization

It is well known that the atomization process of high-pressure diesel jets issuing from micro-hole nozzles has a decisive impact on air-fuel mixing rates and combustion processes in diesel engines. More recently, a series of experiments and phenomenological analyses have also verified that cavitation phenomenon inside the nozzle holes is one of dominant factors causing diesel jet atomization. In order to further clarify the nature of diesel jet atomization under realistic fuel injection conditions and provide a solid theoretical basis for control strategy optimization of in-cylinder low temperature combustion (LTC), some key phenomena (such as injection pressure fluctuaiton, cavitation, turbulence and jet atomization) related to diesel fuel injections and the interaction mechanisms among these phenomena have been systematically investigated in this study by using improved experimental and 3D computational approaches.A reasonable experimental schmeme has been firstly designed to measure pressure fluctuations close to the inlet of diesel nozzle hole. The measured results show that the pressure close to the inlet of diesel nozzle hole fluctuates more dramatically as fuel injection pressure increases. Moreover, statistics results indicate that the maximum amplitude and frequency of pressure fluctuation can even be around 10% of the average pressure and 40 kHz, respectively.The result of cavitating flow analysis by similarity theory based method reveals that the cavitation bubble length scale determined by bubble number density must match well with the hydrodynamic length scale determined by different flow conditions in modeling of cavitating flows, and accordingly cavitation bubble number density being assumed to be a constant for different cavitating flows is unreasonable. A modeling idea for initial cavitation bubble number density, which is originated from combination of cavitation bubble dynamics and internal flow characteristics of nozzle hole, has been brought forward in this study. Based on this modeling idea, a novel formula of initial cavitation bubble number density has also been developed. Model validation results verify that this formula together with a two-fluid model can predict well cavitating flow behavior under high-pressure injection conditions.Some important conclusions can be drawn from numerical analysis of cavitating flows inside diesel nozzle holes. Firstly, cavitation processes within the nozzle hole are very unsteady due to the inet pressure fluctuations, and traditional steady analysis of nozzle cavitation is hard to reveal its realistic behavior. Secondly, Amplitude of the Dpin /Dt (time derivative of inlet pressure) wave can be regarded as an index to evaluate the unsteadiness of cavitation process inside a diesel nozzle hole. Thirdly, cavitation content in the recirculation flow region does not show an obvious change as inlet pressure fluctuates due to enough liquid tension in this region. But cavitation content within the wake of recirculation flow is very sensitive to the variation of inlet pressure because of interactions between shedding vortices and cavitation bubbles. In addition, compared to cavitation content in a symmetrical nozzle, cavitation content in an asymmetrical nozzle shows less change under the same inlet pressure fluctuation because of geometry effect.Unsteady cavition processes induce transient flow conditions at nozzle exit. On the exit section, the maximum turbulence kinetic energy of liquid phase appears at"boundary zone", and its value increases as local cavitation content increases. The mass flow rate of liquid phase mainly depends on the total cavitation content in the nozzle hole, and it usually decreases as total cavitation content increases.An unique"two-stage simulation scheme", together with an improved primary breakup model that can take into account cavitation effect, has been applied to numerically evaluate the impact of nozzle hole internal flow corresponding to unsteady cavitation and inlet pressure fluctuation on diesel high-pressure jet atomization. Analysis results indicate that cavitation phenomenon enhances the atomization process close to nozzle exit, and original primary breakup model seriously underestimates the breakup rate of near-field spray. Furthermore, the internal flow of nozzle hole corresponding to unsteady cavitation and inlet pressure fluctuation is a big disturbance source both for gas phase and liquid phase in the spray field, and the level of breakup rate of near-field spray closely depends on the unsteadiness of nozzle hole internal flow.