Modeling of droplet evaporation, flash-boiling, and mixture preparation in internal combustion engines

The evolving regulation on internal combustion engine emissions as well as the rising expectation on the engine efficiency and performance pose challenges in the development of the future engine technology. Computational methods are needed to understand the mechanism of the advanced engine combustion concepts and to facilitate the development of the future clean and efficient engine technology. This study examines numerical models essential to the simulation of mixture preparation, a vital process that determines the combustion outcome. Combustion models are also developed for compression-ignition and spark-ignition engines, and are used to simulate the advanced engine operation concepts. In this study, a modular multi-component droplet evaporation model is developed based on the existing model by the author’s lab. The updated model is capable of estimating the thermal and transport properties of real mixture of known compositions. Also added is a vapor-liquid equilibrium solver based on the fugacity coefficient of the Peng-Robinson equation of state. The modular droplet evaporation model is integrated into a customized engine CFD software, KIVA, to simulate the droplet evaporation in fuel spray. Flash-boiling of spray generates ultra-fine fuel mist and is potentially beneficial to the mixture preparation. The author examines the existing model for the droplet flashing breakup. The existing model is reformulated and merged with the Taylor analogy breakup model, an aerodynamic droplet breakup model. The unified droplet breakup model is capable of simulating the droplet breakup under the combined effects of aerodynamic excitation and internal flashing bubble growth. Flashing spray simulations are conducted with the unified droplet breakup model. For the compression-ignition engine operation, the chemical kinetics translates the result of mixture preparation into combustion outcome. An efficient chemical kinetics solver is developed for the chemical reaction calculation in engine CFD. The solver exploits an estimated Jacobian matrix of the chemical kinetics problem and reduces the computational time without compromising the accuracy of the solution. Studies of various advanced compression-ignition engine concepts are conducted with the efficient chemical kinetics solver. Spark-ignition engine simulation requires accurate flame propagation prediction. In this study, an efficient G-equation model is developed to model the flame propagation without resolving the flame structure. An advantage over the existing G-equation models is the addition of a knock prediction method inspired by the Livengood-Wu integral. The combination of the G-equation model and the spontaneous ignition calculation results in an unified combustion model for the simulation of spark-ignition engine operation with end-gas ignition, a.k.a., knocking. The model is tested by simulating the operation of a swirl-dominant gasoline direct-injection engine. As the concluding remark, this study develops not only mixture preparation models that predict the mixture preparation outcome, but also combustion models that assess the combustion’s dependency on the mixture preparation. CFD simulations demonstrate the usefulness of these models in the discovery of the future engine technologies