분무응용 기술 : Gaseous sphere 분사모델을 이용한 CNG 직분사 엔진 모델링

Abstract

This paper describes the modeling of CNG (compressed natural gas) direct injection engine using gaseous sphere injection model. Numerical modeling was conducted using KIVA-3V Release 2 code with some modifications. Three dimensional mesh included 4 valves was used for computational grid. Gaseous sphere injection model could be integrated in KIVA-3V Release 2 code with some modification of liquid injection model and RNG (re-normalization group) k-ε turbulence model. This model could simulate gaseous sphere injection using coarse mesh which saves calculation time. The fine mesh is not required to resolve the inflow boundary for gaseous sphere injection. Likewise with liquid injection model, gaseous spheres are injected as parcels which represent a group of gaseous spheres and these parcels evaporate at a time. The evaporation of gaseous spheres occurs without energy change. Particularly, poppet type injector was used for CNG direct injection therefore hollow cone type injection model was modified. The RNG k-ε turbulence model need some modifications. Since this model is known to over-predict gas jet diffusion, turbulence kinetic energy and turbulence length scale values were adjusted depend on grid location. The modified RNG k-ε turbulence model is applied only for the injection period. After the fuel injection was finished, the conventional RNG k-ε turbulence model is applied. Chemkin chemistry solver 2 was coupled with KIVA-3V Release 2 code to simulate combustion process of CNG fuel. CNG is represented by methane and GRI 3.0 mechanism which optimized for combustion process of natural gas was used. In order to calculate the turbulent flame speed, G-equation model was used in which flame front is specified by zero level set of G. In addition, experiments of gaseous fuel injection was performed for gas-jet visualizations using PLIF (planar laser induced fluorescence) method. For safety reasons, compressed nitrogen was used instead of compressed natural gas in the experiments. The tracer which plays an important role in PLIF experiments was acetone that has a very low boiling point, a high saturation pressure, a good fluorescence and low toxicity. Furthermore, experiments of CNG combustion was performed using single cylinder SI (spark ignition) engine. For CNG direct injection, poppet type gaseous fuel injector was used and this injector was centrally mounted. In this study, the simulation results of CNG direct injection engine were compared to experimental results. The gaseous sphere injection model can reliably predict CNG direct injection. Furthermore, the results of ignition and combustion process agreed well with experiment results.