String flash-boiling in gasoline direct injection simulations with transient needle motion

Abstract

A computational study was performed to investigate the influence of transient needle motion on gasoline direct injection (GDI) internal nozzle flow and near-field sprays. Simulations were conducted with a compressible Eulerian flow solver modeling liquid, vapor, and non-condensable gas phases with a diffuse interface. Variable rate generation and condensation of fuel vapor were captured using the homogeneous relaxation model (HRM). The non-flashing (spray G) and flashing (spray G2) conditions specified by the Engine Combustion Network were modeled using the nominal spray G nozzle geometry. Transient needle lift and wobble were based upon ensemble averaged X-ray imaging preformed at Argonne National Lab. The minimum needle lift simulated was 5 µm and dynamic mesh motion was achieved with Laplacian smoothing. The results were qualitatively validated against experimental imaging and the experimental rate of injection profile was captured accurately using pressure boundary conditions and needle motion to actuate the injection. Low needle lift is shown to result in vapor generation near the injector seat. Finally, the internal injector flow is shown to be highly complex, containing many transient and interacting vortices which result in perturbations in the spray angle and fluctuations in the mass flux. This complex internal flow also results in intermittent string flash-boiling when a strong vortex is injected and the resulting swirling spray contains a thermal * Corresponding author

Internal and near nozzle measurements of Engine Combustion Network “Spray G” gasoline direct injectors

“…Moon et al [69] have observed that multi-hole GDIs experience more pronounced transient fluctuations than diesel injectors, and propose that the low needle lift may be responsible for this phenomenon. Recent simulations [70] which reveal the complexity of the flow past the needle seat and in the sac support the hypothesis that GDIs exhibit strong and persistent transients in ROI under more realistic operating conditions. This is 175 supported by the observation that the transients can vary significantly between different injectors with the same nominal geometry, while the transients for a particular injector are highly repeatable during subsequent tests.…”

“…• C and 70 20 MPa, as per Table 1. Evaporative, non-combusting conditions were desired, so an ambient at 573K / 300 • C and 6 bar was specified as the Spray G standard.…”

“…However, the motion may lead to asymmetric and time-varying inflow to the sac. This is a matter of ongoing investigation [58].…”

“…A range of models for injector internal and 45 near-nozzle flow continue to be developed, including volume-of-fluid methods [54,55], mixed-fluid models [56,57], and others. Recent simulations of Spray G suggest that complex transients in the vorticity of the sac flow play an important role in dictating the unsteadiness of the exit flow conditions and the mass balance between the plumes [58].…”

“…The mechanism behind this variation is not yet fully understood, as there is presently insufficient information to be able to quantify the individual holes' discharge coefficients. Recent modeling efforts suggest that strong vortical structures in the sac may play an important role [58]. Further investigation with a larger sample size (more injectors) will be 265 required to determine whether a significant correlation exists.…”