Synergistic effect of swirl flow and prechamber jet on the combustion of a natural gas-diesel dual-fuel marine engine

Abstract

The increasing demands on decarbonization and emission reduction for marine engines are urgently driving the development of efficient and clean combustion technologies for low-carbon fuels. In this work, the combustion of a natural gas (NG)-diesel dual-fuel marine engine was numerically investigated, with particular interest on the synergistic effect of swirl flow and prechamber jet on the mixture formation and flame propagation. Results show that the scavenging port angle governs in-cylinder NG stratification. Specifically, at a small scavenging port angle, NG mainly distributes on the piston top and exhibits a “top lean, bottom rich” stratification pattern; at a large scavenging port angle, NG mainly distributes on the lateral of the cylinder and forms a “inner lean, outer rich” stratification pattern, which is preferred since prechamber ignition favors rich mixture, provided that the swirl ratio is not excessively large to yield reduced scavenging efficiency. Moreover, the prechamber layout significantly affects the swirl motion and the high-speed jet flame propagation. Utilizing multiple prechambers shortens the flame propagation distance and suppresses the sensitivity of ignition on the non-uniform mixture distribution. With the increase of the prechamber channel angle, flame propagation is speeded up due to enhanced swirl flow, while the wall heat loss is also increased, yielding a maximum combustion rate at an intermediate prechamber channel angle of approximately 20°. By synergistically coordinating the scavenging swirl and the prechamber ignition jet, the indicated specific gas consumption was optimized within the Tier III NOx emission limit. The present results are helpful for the development of the combustion system for dual-fuel marine engines.