ABSTRACT	

In Diesel engines, the internal flow characteristics in the fuel injection nozzles, such as the turbulence level and distribution, the cavitation pattern and the velocity profile affect significantly the air-fuel mixture in the spray and subsequently the combustion process. Since the possibility to observe experimentally and measure the flow inside real size Diesel injectors is very limited, Computational Fluid Dynamics (CFD) calculations are generally used to obtain the relevant information.

The work presented within this thesis is focused on the study of cavitation in real size automotive injectors by using a commercial CFD code. It is divided in three major phases, each corresponding to a different complementary objective.
 
The first objective of the current work is to assess the ability of the cavitation model included in the CFD code to predict cavitating flow conditions. For this, the model is validated for an injector-like study case defined in the literature, and for which experimental data is available in different operating conditions, before and after the start of cavitation. Preliminary studies are performed to analyze the effects on the solution obtained of various numerical parameters of the cavitation model itself and of the solver, and to determine the adequate setup of the model. It may be concluded that overall the cavitation model is able to predict the onset and development of cavitation accurately. Indeed, there is satisfactory agreement between the experimental data of injection rate and choked flow conditions and the corresponding numerical solution. This study serves as the basis for the physical and numerical understanding of the problem.
 
Next, using the model configuration obtained from the previous study, unsteady flow calculations are performed for real-size single and multi-hole sac type Diesel injectors, each one with two types of nozzles, tapered and cylindrical. The objective is to validate the model with real automotive cases and to understand in what way some physical factors, such as geometry, operating conditions and needle position affect the inception of cavitation and its development in the nozzle holes. These calculations are made at full needle lift and for various values of injection pressure and back-pressure. The results obtained for injection rate, momentum flux and effective injection velocity at the exit of the nozzles are compared with available CMT-Motores Termicos in-house experimental data. Also, the cavitation pattern inside the nozzle and its effect on the internal nozzle flow is analyzed. The model predicts with reasonable accuracy the effects of geometry and operating conditions.
 
Finally, the onset and development of the cavitating flow and its effect on the internal flow and at the nozzle exit of the real size Diesel injectors is studied in relation with the needle movement and position. For this, two types of calculations are performed to analyze the flow during the unsteady regime, some with fixed meshes at various needle lift positions, and others with a moving mesh to simulate the needle aperture and closure motion. The objective is to determine the validity of using one approach or the other to predict the features of the flow during the needle movement phase, especially at the nozzle exit. The methodology developed to fully automate the three-dimensional mesh generation and its motion is explained in detail. The differences in the predicted solutions obtained with the fully transient (moving mesh) and the pseudo-steady (fixed meshes) simulations are analyzed and the cavitation process characterized during the whole transient of the injection, inside the nozzle and at the exit. It is concluded that the modeling approach chosen may be critical for the prediction of nozzle exit flow conditions, especially if there is cavitation. The calculations with moving mesh boundaries, though slower and more complex, provide valuable information about the transient phase of injection that may not be neglected, especially at low needle lift.