A Numerical Study On The Flow And Combustion Of A Natural Gas Engine With Pre-chamber

With the increasing stringent NO_X emission standards for marine engines by the International Maritime Organization(IMO),the research activities on low NO_X emissions from gas engine are drawing more and more attention.Although lean burn can effectively reduce NO_X emissions and increase the thermal efficiency,it can cause misfiring and poor combustion stability,increase cyclical changes and hydrocarbons emissions.Pre-chamber combustion systems which can form stratified lean combustion between pre-and main chamber is applied to solve this problem.On the premise of low emissions and high thermal efficiency,the combustion reliability is achieved.In this dissertation,the 12V190 type gas engine of Shengli Power Co.,Ltd.was selected as the research object to investigate the influences of pre-combustion chamber structure and parameters on gas flow and combustion.In this paper,a series of researches are carried out by means of simulation calculations using 3D CFD package CONVERGE.Firstly,the one-dimensional model of work processes for the prototype was established by using the one-dimensional engine software AVL BOOST,thus providing a more accurate initial and boundary conditions for the three-dimensional CFD simulation of in-cylinder processes.To understand the effects of the spark plug location on initial combustion,the offset and neutralization spark plug schemes were calculated separately.Due to the air flow from the main chamber to pre-chamber in the compression stroke,a thin mixture area appeared in the center of the pre-chamber at ignition timing.In order to ensure reliable ignition,the spark plug was set on one side of the pre-chamber to avoid the lean mixture region.To study the influences of the pre-chamber shape on the concentration field and combustion characteristics,four types of pre-chambers were designed.The results showed that various shapes of the pre-chamber will generate different turbulence intensity in pre-chamber,the turbulence can affect the concentration distributions of the mixture in pre-chamber,and then affect the flame propagation rate and the entire combustion duration.The option 3 achieved the best pre-chamber shape,which can obtain the most uniform concentration distribution of pre-chamber mixture,further acquiring the fastest combustion rate and the shortest combustion duration.To get the influences of the orifice number on the air motion and flame propagation rate,various number of orifices(4 to 8)were designed by keeping the rest of the conditions unchanged,such as the enriched injection strategy of pre-chamber,the ignition advance angle(20°CA),and the constantβvalue(0.3).The results showed that at a constantβvalue,excessive orifices will result in insufficient jet velocity,slowing the radial flame propagation in main combustion chamber and prolonging the combustion duration;less orifices will result in insufficient circumferential flame propagation in main chamber,although the flame was equipped with a strong penetration force,the entire combustion duration was still longer.The 6-orifice scheme was optimal for the fastest combustion rate and lower NO_X emissions.To discuss the effects of orifice diameter on flow velocity and flame propagation characteristics,various orifice diameters(1.0 mm to 4.0 mm)were designed based on6-orifice.The results showed that larger or smaller orifice diameter can affect the air motion and combustion characteristics.With the smaller orifice diameter,the flame dissipation rate in pre-chamber was larger due to the quickly temperature drop caused by heat exchange.As the flame jet from pre-chamber passed through the orifice,the flame was quenched,and the hot combustible mixture which was injected into the main chamber could not ignite the cold lean mixture in main chamber.While the orifice diameter is larger,the velocity of flame jets from pre-chamber was not enough,resulting in a very slow radial flame propagation speed,and the entire combustion duration was much longer.The optimal orifice diameter was 2.5mm,which achieved the fastest combustion rate and low NO_X emissions.