Numerical investigations on HCCI engine with increased induction induced swirl and engine speed

Homogeneous charge compression ignition(HCCI) mode of combustion is popularly known for achieving simultaneous reduction of NOx as well as soot emissions as it combines the compression ignition(CI) and spark ignition(SI) engine features. In this work, a CI engine was simulated to work in HCCI mode and was analyzed to study the effect of induction induced swirl under varying speeds using three-zone extended coherent flame combustion model(ECFM-3Z, compression ignition) of STAR-CD. The analysis was done considering speed ranging from 800 to 1600 r/min and swirl ratios from 1 to 4. The present study reveals that ECFM-3Z model has well predicted the performance and emissions of CI engine in HCCI mode. The simulation predicts reduced in-cylinder pressures, temperatures, wall heat transfer losses, and piston work with increase in swirl ratio irrespective of engine speed. Also, simultaneous reduction in CO2 and NOx emissions is realized with higher engine speeds and swirl ratios. Low speeds and swirl ratios are favorable for low CO2 emissions. It is observed that increase in engine speed causes a marginal reduction in in-cylinder pressures and temperatures. Also, higher turbulent energy and velocity magnitude levels are obtained with increase in swirl ratio, indicating efficient combustion necessitating no modifications in combustion chamber design. The investigations reveal a total decrease of 38.68% in CO2 emissions and 12.93% in NOx emissions when the engine speed increases from 800 to 1600 r/min at swirl ratio of 4. Also an increase of 14.16% in net work done is obtained with engine speed increasing from 800 to 1600 r/min at swirl ratio of 1. The simulation indicates that there is a tradeoff observed between the emissions and piston work. It is finally concluded that the HCCI combustion can be regarded as low temperature combustion as there is significant decrease in in-cylinder temperatures and pressures at higher speeds and higher swirl ratios.

Effect of ramp-cavity on hydrogen fueled scramjet combustor

Sustained combustion and optimization of combustor are the two challenges being faced by combustion scientists working in the area of supersonic combustion.Thorough mixing,lower stagnation pressure losses,positive thrust and sustained combustion are the key issues in the field of supersonic combustion.Special fluid mechanism is required to achieve good mixing.To induce such mechanisms in supersonic inflows,the fuel injectors should be critically shaped incurring less flow losses.Present investigations are focused on the effect of fuel injection scheme on a model scramjet combustor performance.Ramps at supersonic flow generate axial vortices that help in macro-mixing of fuel with air.Interaction of shocks generated by ramps with the fuel stream generates boro-clinic torque at the air&liquid fuel interface,enhancing micro-mixing.Recirculation zones present in cavities increase the residence time of the combustible mixture.Making use of the advantageous features of both,a ramp-cavity combustor is designed.The combustor has two sections.First,constant height section consists of a backward facing step followed by ramps and cavities on both the top and bottom walls.The ramps are located alternately on top and bottom walls.The complete combustor width is utilized for the cavities.The second section of the combustor is diverging area section.This is provided to avoid thermal choking.In the present work gaseous hydrogen is considered as fuel.This study was mainly focused on the mixing different fuel injection locations.It was found that injecting fuel upstream of the ramp was beneficial from fuel spread point of view.