Numerical Study of Multi-Component Spray Combustion with a Discrete Multi- Component Fuel Model

A numerical investigation of fuel composition effects on spray combustions is presented. A new discrete multicomponent (DMC) fuel model was used to represent the properties and composition of multi-component fuels. A multi-dimensional CFD code, KIVA-ERC-Chemkin, that is coupled with improved sub-models and the Chemkin library, was employed for the simulations. A large-bore, optically accessible, DI diesel engine operating in a low temperature combustion (LTC) regime was simulated with primary reference fuels for validation of the fuel models. Then, a small-bore, high-speed DI diesel engine operating in low temperature combustion (LTC) regime was simulated with two different diesel fuels using a 6-component fuel model. The oxidation chemistry was calculated using a reduced mechanism for primary reference fuel, with the reaction rate coefficients adjusted to account for the Cetane number (CN) variation of the fuels of interest. The major property differences of the fuels include volatility, viscosity, and autoignitability. The predicted pressure and heat release rate are compared with experimental data available in the literature. The results show that the present multi-component fuel model performs reliably, and captures the effects of fuel composition differences on combustion. Introduction In most multi-dimensional models of internal combustion engines the fuel is represented for simplicity as a single-component fuel. However, single-component fuel models are not able to predict the complex behavior of the vaporization of practical fuels, such gasoline and diesel. The preferential vaporization of light-end components in these multi-component fuels affects greatly the fuel distribution near the spray and cannot be represented adequately with a single-component [1-3]. Studies have been performed on the vaporization of multi-component fuels [1-6]. Multi-component fuel models are classified into two types, i.e., discrete multi-component (DMC) models and continuous multi-component (CMC) models. The continuous multi-component model, which is based on the continuous thermodynamics method [4], represents the fuel composition as a continuous distribution function with respect to an appropriate parameter such as molecular weight. This enables a reduction of computational load while maintaining the predictability of the complex behavior of the vaporization of multi-component fuels. However, when this model is applied to combustion simulations, especially with detailed chemistry, describing the multi-component features of the fuel is inevitably limited, making it difficult to model the consumption of individual components appropriately. On the contrary, the DMC approach tracks the individual components of the fuel during the evaporation process and allows coupling with the reaction kinetics of the individual fuel components. Although the DMC approach can have a high computational overhead due to the additional transport equations that must be solved when it is used for fuels with a large number of components, it is becoming more affordable as computational capacity and numerical solution techniques have improved substantially. In order to simulate spray vaporization in engine combustion, a robust model that is applicable in a wide range of operating temperatures and pressures, including normal, boiling and trans-critical evaporation regimes, is desirable. Ra and Reitz [3] developed a robust multi-component evaporation model that is applicable to both normal and boiling vaporization modes using a discrete composition distribution of the fuel. The model was applied to simulate evaporation under various drop interior, surface and surrounding gas temperature scenarios. In the model, the physical mechanism of droplet heating/cooling was treated as heat transfer from the surface/interior to the interior/surface of the droplet that is at different temperatures. One of the crucial characteristics of fuels considered in diesel engines is autoignitability, which depends on the detailed chemical composition of the fuel, as well as the evolution of the thermal and compositional state of the charge mixture. Risberg et al. [7] tested diesel fuels of different Cetane number (CN) and volatility characteristics in an engine and showed that the CN describes the autoignition quality of diesel-like fuels in homogeneous charge compression ignition (HCCI) combustion. Kalghatgi et al. [8,9] and Weall and Collings [10] also reported that