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With the heightened pressure on car manufacturers to increase the efficiency and reduce the carbon emissions of their fleets, more challenging engine operation has become a viable option. Highly dilute, boosted, and stratified charge, among others, promise engine efficiency gains and emissions reductions. At such demanding engine conditions, the spark-ignition process is a key factor for the flame initiation propagation and the combustion event. From a computational standpoint, there exist multiple spark-ignition models that perform well under conventional conditions but are not truly predictive under strenuous engine operation modes, where the underlying physics needs to be expanded. In this paper, a hybrid Lagrangian–Eulerian spark-ignition (LESI) model is coupled with different turbulence models, grid sizes, and combustion models. The ignition model, previously developed, relies on coupling Eulerian energy deposition with a Lagrangian particle evolution of the spark channel, at every time-step. The spark channel is attached to the electrodes and allowed to elongate at a speed derived from the flow velocity. The LESI model is used to simulate spark ignition in a nonquiescent crossflow environment at engine-like conditions, using converge commercial computational fluid dynamics (CFD) solver. The results highlight the consistency, robustness, and versatility of the model in a range of engine-like setups, from typical with Reynolds-averaged Navier–Stokes (RANS) and a larger grid size to high fidelity with large-eddy simulation (LES) and a finer grid size. The flame kernel growth is then evaluated against Schlieren images from an optical constant volume ignition chamber with a focus on the performance of flame propagation models, such as G-equation and thickened flame model, versus the baseline well-stirred reactor model. Finally, future development details are discussed.

In the United States transportation sector, Light-Duty Vehicles (LDVs) are the largest energy consumers and CO2 emitters. Electrification of LDVs is posed as a potential solution, but SI engines can still contribute to decarbonization. Car manufacturers have turned to unconventional engine operation to increase the efficiency of Spark-Ignition (SI) engines and reduce the carbon emissions of their fleets. Dilute, lean, and stratified-charge engine operation has the potential for engine efficiency improvements at the expense of increased cyclic variability and combustion instability. At such demanding engine conditions, the spark ignition event is key for flame initiation and propagation and for enhanced combustion stability. Reliable and accurate spark ignition models can help design ignition systems that reduce cyclic variability. Multiple computational spark-ignition models exist that perform well under conventional conditions, but the underlying physics needs to be expanded, for unconventional engine operation. In this paper, a hybrid Lagrangian–Eulerian Spark-Ignition (LESI) model is coupled with different turbulent flame propagation models for engine simulations. LESI relies on Lagrangian arc tracking and Eulerian energy deposition. The LESI model is coupled with the Well-Stirred Reactor (WSR), Thickened Flame Model (TFM), and g-equation model and used to simulate several cycles of a Direct-Injection Spark-Ignition (DISI) engine using a commercial Computational Fluid Dynamics (CFD) engine solver. The results showcase the successful coupling of LESI with the combustion models. Global engine metrics, such as pressure and Apparent Heat Release Rate (AHRR), for each simulation setup are compared to experimental engine results, for validation. In addition, results highlight the successful prediction of spark channel movement by comparing simulation images to experimental optical engine images. Finally, the successful coupling of LESI to combustion models, making it a usable model in the engine modeling community, is emphasized and future development details are discussed.

An unstructured, three-dimensional, moving-mesh algorithm is developed for free-surface flows with the capability of simulating large deformation. A robust edge swapping algorithm and an optimization-based mesh smoothing algorithm are used to improve mesh quality as the interface deforms. The edge swapping algorithm uses an optimal swap sequence to transform configurations with several tetrahedra sharing a common edge. The optimization-based mesh smoothing relocates vertex position so that the minimum quality of the incident cells is maximized. Also, edge contraction and bisection are used to adjust the mesh resolution according to surface curvature. These local refinements effectively maintain good mesh quality and appropriate mesh size while avoiding global re-meshing. This scheme's robustness and accuracy are demonstrated with deforming liquid ligaments, periodic jets, and colliding droplets.

The effect of droplet oscillation on internal heat transfer was investigated using a transient, three-dimensional Navier-Stokes solver for free-surface flows. The code solves conservation of momentum and energy on a three-dimensional deforming domain. The investigation sought to explore the effect that oscillations might have on temperature non-uniformity within droplets. Biot numbers of 0.1 and 0.25 were investigated for three different degrees of distortion: no perturbation, 13% and 35%. The droplets were given an initial perturbation and allowed to relax back to a spherical shape. The results show that the effect of the oscillation is minimal. As expected, the effect was greatest at large distortion and large Biot number. However, it appears that distortion does not contribute much to heat transfer within droplets.Copyright © 2002 by ASME

Numerical simulation of head-on collision of two equal-size droplets is conducted to observe the effect of viscosity on the maximum deformation amplitude using a moving-mesh finite-volume method. Recent experimental results by Willis and Orme [Exp. Fluids 34, 28 (2003)] have shown that the maximum deformation amplitude depends on the viscosity coefficient, and thus the percentage of energy that is dissipated until the instant of maximum deformation increases with the increasing fluid viscosity. This observation contradicts previous results by Jiang, Umemura, and Law [J. Fluid Mech. 234, 171 (1992)]. The numerical results in this Letter show that the dissipated energy and the maximum deformation depend on the collision Reynolds number, which is consistent with Willis and Orme (2003). However, this dependence decreases with increasing Reynolds number, which suggests that the effect caused by viscosity on maximum deformation becomes insignificant at sufficiently high Reynolds number.

Fuel injectors often experience cavitation due to regions of extremely low pressure. In this work, a cavitation modeling method is implemented in the CONVERGE computational fluid dynamics (CFD) code in order to model the flow in fuel injectors. The CONVERGE code includes a Cartesian mesh based flow solver. In this solver, a volume of fluid (VOF) method is used to simulate the multiphase flow. The cavitation model is based on a flash-boiling method with rapid heat transfer between the liquid and vapor phases. In this method, a homogeneous relaxation model is used to describe the rate at which the instantaneous quality, the mass fraction of vapor in a two-phase mixture, will tend towards its equilibrium value. The model is first validated with the nozzle flow case of Winklhofer by comparing the mass flow rate with experimentally measured values at different outlet pressures. The cavitation contour shape is also compared with the experimental observations. Flow in the Engine Combustion Network Spray-A nozzle configuration is simulated. The mesh dependency is also studied in this work followed by validation against discharge coefficient data. Finally, calculations of a five-hole injector, including moving needle effects, are compared to experimental measurements.