Turbulence modeling of gaseous injection and mixing in DI engines

With increasing interest in alternative fuel technology, natural gas has become an attractive fuel for reciprocating engines. In this work, a modified version of the Los Alamos KIVA3 code has been used to model gaseous injection and mixing, by establishing the necessary boundary conditions at the injector/cylinder interface. It has been shown that gaseous injection models have to be accompanied by sufficient grid refinement to capture length scales of the order of the injector diameter (Abraham, 1997). In addition, high fidelity turbulence models are needed to monitor the plume evolution away from the injector.; Turbulence modeling is explored in this work by considering variants of the k-{dollar}epsilon{dollar} and LES models. The standard k-{dollar}epsilon{dollar} model (Launder and Spalding, 1972) was found to underpredict the recirculation length in the backward-facing step geometry, overpredict dissipation in the confined co-flow jet geometry and underpredict penetration histories in all three gaseous jet configurations considered. These jet configurations included downward injection of methane in a pressurized vessel (Aesoy, 1996), horizontal injection of hydrogen in a confined chamber (Tomita et al., 1997) and upward impinging transient acetylene jet in a confined box (Fujimoto et al., 1997). The implementation of the nonlinear k-{dollar}epsilon{dollar} model (Speziale, 1986) either provided moderate corrections in the geometries of the backward-facing step and confined co-flow jets, or produced convergence problems in engine calculations. The implementation of the RNG based k-{dollar}epsilon{dollar} model (Han and Reitz, 1995) provided better agreement with experiments for both the recirculating (backward-facing step) and gaseous injection cases. The implementation of the LES model was based on the formulation of Moin et al. (1991), and the evaluation of the sub-grid scale stresses proposed by Lily, (1992). It was shown that this model was significantly affected by the high numerical diffusivity of KTVA-3.; Based on the better predictions for recirculating flows and closer agreement with measured penetration data, the RNG k-{dollar}epsilon{dollar} model was selected for engine calculations. Parametric studies of injector and chamber geometries indicated that an optimum injection strategy could lead to as much as 100% flammable mixture near top dead center.