배기가스 재순환이 적용된 선박용 엔진에서 예혼합 가스의 유동 특성에 관한 수치해석

초록

This study investigates the scavenging and flow characteristics of premixed gas composed of air, recirculated exhaust gas, and CH4 in a marine two-stroke engine equipped with an exhaust gas recirculation (EGR) system using computational fluid dynamics (CFD). A three-dimensional symmetric model and dynamic mesh technique were applied to reproduce piston motion and scavenging flow under realistic operating conditions. Measured in-cylinder pressure data obtained from shop trials were used as boundary conditions to improve the reliability of the numerical analysis. The species transport model was employed to analyze the spatial distribution and mixing behavior of CH4 under various engine load conditions. The numerical results show that the overall scavenging flow maintains a typical uniflow scavenging structure regardless of CH4 concentration. However, as the CH4 concentration increases, CH4 tends to accumulate in the central recirculation region of the cylinder, resulting in increased mixture non-uniformity. To quantitatively evaluate mixture homogeneity, the coefficient of variation (COV) of CH4 mass fraction was calculated. The COV values were 0.14, 0.18, and 0.33 for lean, representative, and CH4-rich conditions, respectively. These results indicate that local concentration gradients become more pronounced under CH4-rich conditions due to recirculation flow structures and low-velocity regions inside the cylinder. Mixture non-uniformity increases with both engine load and CH4 concentration. This behavior is mainly caused by stronger local concentration gradients in the central and low-velocity recirculation regions. The results suggest that sufficient premixing before scavenging is necessary under high-load and CH4-rich conditions to improve mixture uniformity and combustion stability in EGR-equipped CH4 gas engines.

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