Injector Modeling

Nozzle Exit Flow Modeling: One-Way Coupling (OWC) Model

Improved GDI (Gasoline Direct Injection) spray modeling

In this work, we introduce a new approach for achieving more realistic modeling of GDI sprays. The method enables predictive characterization of spray plume evolution, particularly in the near-nozzle region where conventional Lagrangian injector models have struggled. To realize this advancement, we established a workflow that integrates several independent experimental and simulation techniques, as outlined below.

Advanced measurement using X-ray provides near-nozzle spray characteristics of real fuels (Argonne APS experiment)
Detailed in-nozzle flow simulation (high-fidelity): Eulerian two-phase flow simulation using volume-of-fluid (VoF) method was carried out to validate against the near-field X-ray measurements.
One-Way Coupling (OWC) implementation verifies the far-field spray morphology with VoF results in constant-volume chamber (CVC) setup.
In-cylinder Lagrangian spray simulation evaluates realistic spray performance subject to engine flow.

Tested GDI injector

The employed injector is a commercially available side-mounted 6-hole GDI injector. Detailed geometrical features of the injector were characterized with X-ray tomography at the Advanced Photon Source (APS) of the Argonne National Laboratory. Figure on the left shows the morphology of the internal geometry of the injector, with the needle sitting in the sac volume, and displays the 6-hole nozzles that are arranged to reflect the planar symmetry with respect to the plane passing through holes 1 and 4. Each nozzle is connected to a counter-bore orifice outlet with a larger diameter than that of the internal nozzle diameter.

Benchmarking injector operating conditions

Two target conditions were considered to benchmark a relevant range of light-duty GDI engine with distinctive spray behaviors: flash-boiling and non-flashing conditions. The corresponding injection conditions as well as steady-state ambient thermal conditions are listed on the left.

OWC (One-Way Coupling) method for capturing detailed boundary conditions at the nozzle exit

To practice the advanced OWC method, a comprehensive analysis procedure involving a chain of validations needs to be followed. Figure shown below illustrates the key steps sequentially. Firstly, high-resolution X-ray measurements of real fuel sprays were obtained under conditions of interest within a constant volume chamber (CVC). Internal nozzle flow simulations were then performed using Eulerian VoF two-phase flow modeling and validated against X-ray measurements. Lastly, Lagrangian turbulence levels needed to be properly matched to achieve realistic OWC implementation, where the high-fidelity VoF simulation results could serve as the calibration source.

Using the commercial CFD software CONVERGE, 3D Eulerian multiphase nozzle flow simulations based on the VoF method were first carried out to characterize the multiphase flow in in-nozzle and near-nozzle (i.e. close vicinity downstream the nozzle exit) regions. A proper CFD domain for internal nozzle flow was generated by utilizing the X-ray characterized high-fidelity injector geometry (shown in the Figure). The injector was then incorporated into an open-chamber domain to represent the nozzle exit flow. For the nozzle flow simulations, a compressible single-fluid mixture model was employed with a set of transport equations with homogeneous relaxation model to accomodate the phase change owing to cavitation, condensation, and flash-boiling regimes.

A key process for OWC implementation is to properly transfer the high-fidelity Eulerian information to a Lagrangian framework. The major multiphase flow features obtained from the Eulerian nozzle flow simulations were sampled over a set of user-prescribed coupling interfaces, which in this case were defined at the counter-bore outlets as shown in the figure (right). The flow field at the exits were discretized with a spatial resolution matching the Eulerian computational grid. A map file representing the quasisteady flow field at the nozzle outlets was then extracted in plain text format. The map file contains spatial profiles of liquid volume fraction, velocity, turbulence intensity, and temperature.

OWC model validation in a constant-volume chamber (CVC) setup

The developed OWC model coupling workflow was assessed against the high-fidelity Eulerian simulation (VoF) and the conventional Lagrangian injector model (named ROI model). Figures displayed below present the spray morphology reproduced by the Eulerian VoF nozzle flow simulations and the corresponding Lagrangian simulations under the two test conditions of interest. The contour images present the spray density distributions through a virtual planar slice downstream of the injector tip (0.51mm for non-flashing condition and 1.0mm for flash-boiling condition), which identify spray plume spreading and the degree of spray collapsing. From the results shown here, the advanced OWC modeling enables to reproduce the realistic spray plume structure with low computational load, which is ultimately beneficial for fuel screening analysis and practical engine design process.

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