Characterization of near-wall and free-stream in-cylinder turbulence during the intake phase of a direct-injection engine

HP Lien, C Welch, A Pati, M Hasenzahl… - Available at SSRN …, 2023 - papers.ssrn.com
HP Lien, C Welch, A Pati, M Hasenzahl, B Böhm, C Hasse, S Jakirlic
Available at SSRN 4519647, 2023papers.ssrn.com
The aim of the present study is to investigate the flow characteristics inside a direct-injection
engine during the intake phase, focusing on the turbulence behavior in the near-wall
regions and the core flow affected by the pressure field variation, using a wall-resolved large
eddy simulation. The considered engine flow bench operating under stationary conditions,
implying the non-moving piston and continuously open valves, is adopted with the aim of
appropriately reducing the geometric complexity of the engine configuration, allowing a …
Abstract
The aim of the present study is to investigate the flow characteristics inside a direct-injection engine during the intake phase, focusing on the turbulence behavior in the near-wall regions and the core flow affected by the pressure field variation, using a wall-resolved large eddy simulation. The considered engine flow bench operating under stationary conditions, implying the non-moving piston and continuously open valves, is adopted with the aim of appropriately reducing the geometric complexity of the engine configuration, allowing a more detailed insight into the structural properties of the flow during the intake phase. Special attention is paid to the flow near the inlet valves and the impingement region on the liner wall. The mean velocity and turbulent kinetic energy fields are first evaluated and compared with experimental data measured by particle image velocimetry diagnostics. The anisotropic turbulence analysis on the centerline of the intake jet discharging from its margin reveals some similarities with the free shear jet flow. In the near-wall region of the curved intake valve, the turbulence anisotropy initially behaves as that characterizing canonical channel flow, complying with two-component anisotropic turbulence, and transitions to a state induced by longitudinal curvature-induced acceleration approaching the one-component turbulence condition, corresponding to the axisymmetric expansion process projected within the anisotropy invariant map. However, due to the strong influence of the inlet flow, the level of turbulence anisotropy is maintained in terms of the magnitude of the Lumley’s two componentality parameter. Many studies have shown that the wall-function relevant treatment of wall proximity derived from the canonical channel flow is inadequate for engine applications due to the simplified assumptions of near-wall flow, such as the zero pressure gradient assumption. However, it remains to be determined which non-equilibrium effects dominate the near-wall region. Analysis of the viscosity-affected layer at the inlet valve and the impingement region at the liner wall shows that the streamwise pressure gradient strongly influences the near-wall behavior. Furthermore, a turbulent kinetic energy budget analysis at the inlet valve demonstrates that near-wall turbulence generation is primarily influenced by pressure-related diffusion rather than viscous dissipation and turbulent diffusion.
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