TURBULENT FLOW CHARACTERISTICS OF HCCI COMBUSTION WITH INCREASED SWIRL − A COMPUTATIONAL STUDY
Auto-ignition of cylinder contents in volumetric combustion is an attractive feature of HCCI combustion. The paper deals with a thorough understanding of auto-ignition in HCCI engines in terms of turbulent flow characteristics. A computational study is carried out to relate in-cylinder fluid flows and turbulent characteristics at four different swirl ratios of a single cylinder reentrant type piston bowl CI engine running in HCCI mode. For this purpose a Three Zone Extended Coherent Flame Combustion Model (ECFM-3Z, Compression Ignition) of STAR-CD is used. The analysis is focused on the outcome of the swirl ratio effect on the flow and turbulent characteristics in terms turbulent kinetic energy, turbulence dissipation rate, and turbulent kinematic viscosity, turbulence length scale and intensity with respect to the crank angle. The study revealed that among the five turbulent characteristics the turbulent kinetic energy and turbulence dissipation rate are observed to play a vital role in enhancing the burn time and flame speed and are increasing with increase in swirl ratio. The value of turbulent kinetic energy at 712 CA has increased from 1.181m2/s2 to 4.964 m2/s2 at 712 CA with increase in swirl ratio from 1-4 respectively Also, higher induction induced swirl is observed to play a vital role in achieving near complete and efficient combustion aided by turbulent characteristics. Higher turbulent kinetic energy and velocity magnitude levels are realized at higher swirl ratios. The study revealed a provision of inlet manifold that enhances swirl is sufficient enough rather than to vary piston bowl geometry to achieve complete combustion in HCCI mode under lean mixture operation.