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A new class of subgrid scale models (dynamic structure models) for large eddy simulation is proposed for subgrid scalar flux and dissipation terms. The structure of the modeled terms is taken from the corresponding Leonard terms by the use of the test filter size equal to the base filter size, and a particular form of the scaling factor is proposed. The models are evaluated using available direct numerical simulation data. The evaluation results compare well with viscosity and similarity models. The dynamic structure models have been found to be robust and to work well under various conditions, including various test-to-base filter size ratios and filter skewnesses. Models for both terms employ the subgrid scalar variance as a part of the scaling factor. It is possible not to model the subgrid variance, but to find it via the evolution equation. A new form of the transport equation for subgrid scalar variance that contains only one unclosed term is presented.

Droplet collision models have been criticized for creating large mesh dependency in spray calculations. These numerical errors are very troublesome; they behave erratically and interfere with the predictive ability of physical models. The collision method used in KIVA can cause mesh dependent changes in average drop size of over 40 microns. In order to reduce mesh dependency, a new method has been developed for calculating the incidence of collision. The solution is to create a special collision mesh that is optimized for accuracy. The mesh is created automatically during the spray calculation. Additionally, a different stochastic collision sampling technique is also used. The new method, called the NTC algorithm, was incorporated into KIVA and found to be much faster than older algorithms. Calculations with 60,000 parcels required only a few CPU minutes. With the new NTC method and collision mesh, the mesh dependence of the drop size is only nine microns. This remaining mesh dependency is found to be due to the drag calculations and is not the fault of the collision algorithm. [DOI: 10.1115/1.1564066]

In this work, a hybrid breakup model tailored for direct-injection spark-ignition engine sprays is developed and implemented in the OpenFOAM CFD code. The model uses the Lagrangian–Eulerian approach whereby parcels of liquid fuel are injected into the computational domain. Atomization and breakup of the liquid parcels are described by two submodels based on the breakup mechanisms reported in the literature. Evaluation of the model has been carried out by comparing large-eddy simulation results with experimental measurements under multiple direct-injection spark-ignition engine-like conditions. Spray characteristics including liquid and vapor penetration curves, droplet velocities, and Sauter mean diameter distributions are examined in detail. The model has been found to perform well for the spray conditions considered in this work. Results also show that after the end of injection, most of the residual droplets that are still in the breakup process are driven by the bag and bag–stamen breakup mechanisms. Finally, an effort to unify the breakup length parameter is made, and the given value is tested under various ambient density and temperature conditions. The predicted trends follow the measured data closely for the penetration rates, even though the model is not specifically tuned for individual cases. [ABSTRACT FROM AUTHOR]

Natural gas and diesel dual-fuel combustion is a promising technology for efficiently utilizing natural gas in a compression ignition engine. Natural gas composition varies depending on the geographical source, which affects engine performance. The methane number is an indicator of natural gas fuel quality to assess the variation in composition. In this study, the influences of methane number on natural gas/diesel dual-fuel combustion were numerically examined using computational fluid dynamic simulations. The differences between natural gases with the same methane number but different components were also compared. Two dual-fuel combustion strategies, diesel pilot ignition, and reactivity controlled compression ignition were evaluated. The results show that for both diesel pilot ignition and reactivity controlled compression ignition, the ignition delay increases and the combustion duration decreases as the methane number is increased. The retarded trend of ignition of reactivity controlled compression ignition is more significant than that of diesel pilot ignition, while the decreased trend in combustion duration is less significant. To understand this trend, a chemical kinetics study of ignition delay characteristic of natural gas and n-heptane mixture was conducted. The result reveals that introducing ethane, propane, or an ethane–propane mixture into pure methane shortens the ignition delay in the entire temperature range. However, for the methane and n-heptane mixture, adding ethane, or propane, or an ethane–propane mixture shortens the ignition delay in the high temperature range, while increases the ignition delay in the low temperature range. These observations in combination with the analysis of air–fuel mixture formation and combustion provide the evidence to interpret the different ignition and combustion behaviors between diesel pilot ignition and reactivity controlled compression ignition combustion. In addition, a temperature A-factor sensitivity study was carried out to explain the result of the chemical kinetics study. Furthermore, the responses of emissions to methane number were also investigated. The results show that for diesel pilot ignition, the hydrocarbon and carbon monoxide emissions decrease with the decreased methane number. However, for reactivity controlled compression ignition, the variations of hydrocarbon and carbon monoxide emissions with the methane number are not so obvious as for diesel pilot ignition combustion. For both diesel pilot ignition and reactivity controlled compression ignition combustion, the nitrogen oxides emissions show a strong dependence on combustion phasing rather than natural gas composition. Overall, to control diesel pilot ignition combustion, the methane number should be considered together with other parameters. However, attention should be paid to other control parameters for the reactivity controlled compression ignition combustion. The engine performance of reactivity controlled compression ignition is not sensitive to the variation of natural gas composition, so it can adapt to the natural gas from different sources. [ABSTRACT FROM AUTHOR]

In this study, natural gas and diesel dual-fuel combustion under a medium load was numerically optimized for a heavyduty engine using a genetic algorithm optimization approach. This approach employed a micro-genetic algorithm optimization code coupled with an engine computational fluid dynamics code to perform the optimization. Initially, an optimization using premixed natural gas with double direct injection of diesel fueling strategy was conducted using both the stock piston bowl shape and a bathtub shaped piston. Low emissions and moderate combustion with thermal efficiency close to 50% were achieved for both piston configurations by optimizing the premixed natural gas to diesel fuel ratio, exhaust gas recirculation fraction, diesel injection timing, injection pressure, and injection split ratio. Based on this optimum point, a parametric study was performed to understand the effects of the optimization parameters. The results show that high efficiency and clean combustion (indicated thermal efficiency ≥45%, NOx ≤ 0.4 g/kWh, peak pressure rise rate ≤ 15 bar/°) can be achieved over a wide range of parameters. The optimized result of a single diesel injection strategy was also evaluated and compared with the double injection strategy. The results indicate that the single injection strategy is also able to yield near 50% thermal efficiency and clean combustion. Compared to the double injection strategy, the single injection strategy shows increased combustion losses due to reduced diesel fuel near the cylinder centerline. In addition, the bathtub shaped piston has lower heat transfer losses relative to the stock piston, leading to improved fuel efficiency. However, the piston shape shows less impact than other parameters on the overall combustion and emission characteristics for both injection strategies. [ABSTRACT FROM AUTHOR]