In this work the influence of needle movement and “off-axis” motion on the three-dimensional transient flow development in a Diesel injector is studied under realistic engine operating conditions. The transient flow characteristics, such as turbulence and cavitation at the injector exit are important for accurately modeling the spray atomization and fuel-air mixture formation, and thus the combustion in the engine cylinder. Hence, it is critical to accurately predict the high-speed internal nozzle flow characterized by high-pressure gradients and two-phase flow phenomena in such small-scale devices. In this study, the two-phase cavitating flow is described by a mixture-based model in the commercial code CONVERGE. The needle movement profiles in “in-” and “off-” axial directions are measured using an X-ray phase-contrast technique at Argonne National Laboratory. These profiles are imposed as moving boundary conditions in the simulations. Model validation and grid-convergence studies are performed first. Single-hole Spray A (using n-dodecane fuel) from the Engine Combustion Network (ECN) is studied. Simulations are also performed on a five-hole nozzle by imposing the needle movement profiles. The influence of three-dimensional transient needle movement on the in-nozzle flow and cavitation development is investigated for single and multi-hole nozzles. The internal flow characteristics are compared between the needle off-axis and in-axis conditions, and the effects are discussed. The quantitative distributions of turbulence and flow parameters at the nozzle exit are also studied. The main conclusion from this study is that the needle off-axis motion may not be important in single orifice injectors, however, it can influence the flow in the sac region in multi-hole injectors thus causing differences in mass flow rates between different holes. This study highlights the importance of needle movement effects on flow development in single and multi-hole nozzles.

Three-dimensional Simulations of the Transient Internal Flow in a Diesel Injector: Effects of Needle Movement

BATTISTONI, MICHELE;
2013

Abstract

In this work the influence of needle movement and “off-axis” motion on the three-dimensional transient flow development in a Diesel injector is studied under realistic engine operating conditions. The transient flow characteristics, such as turbulence and cavitation at the injector exit are important for accurately modeling the spray atomization and fuel-air mixture formation, and thus the combustion in the engine cylinder. Hence, it is critical to accurately predict the high-speed internal nozzle flow characterized by high-pressure gradients and two-phase flow phenomena in such small-scale devices. In this study, the two-phase cavitating flow is described by a mixture-based model in the commercial code CONVERGE. The needle movement profiles in “in-” and “off-” axial directions are measured using an X-ray phase-contrast technique at Argonne National Laboratory. These profiles are imposed as moving boundary conditions in the simulations. Model validation and grid-convergence studies are performed first. Single-hole Spray A (using n-dodecane fuel) from the Engine Combustion Network (ECN) is studied. Simulations are also performed on a five-hole nozzle by imposing the needle movement profiles. The influence of three-dimensional transient needle movement on the in-nozzle flow and cavitation development is investigated for single and multi-hole nozzles. The internal flow characteristics are compared between the needle off-axis and in-axis conditions, and the effects are discussed. The quantitative distributions of turbulence and flow parameters at the nozzle exit are also studied. The main conclusion from this study is that the needle off-axis motion may not be important in single orifice injectors, however, it can influence the flow in the sac region in multi-hole injectors thus causing differences in mass flow rates between different holes. This study highlights the importance of needle movement effects on flow development in single and multi-hole nozzles.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11391/1213904
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