Study On The Injection And Mixing Characteristics Of Dual Fuel Spray Of A Pilot Ignited Direct Injection Natural Gas Engine

Due to energy crisis and the more strict emission regulations,alternative fuels for engines receive more and more attentions.As a clean and cheap energy with abundant reserves,natural gas is promising to be an alternative fuel.Compress ignition direct injection natural gas engine operating with directly injected natural gas and pilot diesel provides better fuel economy and less emissions,maintaining the power output at the same time.With this system,small quantities of diesel auto-ignite,and natural gas is then ignited by the pilot diesel flame.The injection characteristics and mixing process play a vital role in the optimization process for combustion and emission characteristics,therefore further research should be conducted.The experimental apparatus utilize both PIV and High Speed Schlieren to study the jet characteristics and mixing process of dual fuel spray,which include a constant volume chamber,dual fuel injection system,PIV,Schlieren and control system.Both natural gas and pilot diesel are directly injected into the constant volume chamber,so the visualization research of direct injection dual fuel spray can be realized in this paper.Firstly,natural gas is tested by PIV and Schlieren,and the velocity field of ambient gas and the picture of natural gas jet are obtained.Interaction between natural gas and ambient gas is investigated.Then the dual fuel spray is studied using Schlieren.The effects of natural gas injection pressure on natural gas jet and dual fuel spray is investigated.The research show that:(1)At single fuel condition,natural gas tip penetration increases and jet angle decreases with the increase of natural gas injection pressure.At the tip of natural gas jet,the jet impacts on the ambient gas produces high turbulence intensity,while at the middle of natural gas jet,there is a big vorticity,leading to a strong entrainment.At the bottom,a low pressure area is formed as a result of the high-speed natural gas jet,and the air is entrained in the jet.(2)At dual fuel condition,natural gas jet is interfered by diesel spray as it passes through the diesel spray.A part of diesel spray is entrained and move forward with natural gas jet.The diesel is slower than natural gas,therefore the speed of natural gas decreases,leading to less entrainment and the decrease of natural gas tip penetration and jet angle.(3)With the increase of injection interval,the interaction between natural gas jet and diesel spray decreases.As the injection interval more than 1ms,diesel spray has little effects on natural gas jet.Further simulation research on dual fuel spray is conducted with CONVERGE.Based on the concentration field and velocity field,the mechanism of the injection and mixing process of dual fuel spray is studied.It is found that: The entrainment at the side of natural gas jet has dominant effects on the mixing process of diesel and natural gas.A part of natural gas is blocked at the tip,as a result of the resistance from the diesel,therefore the concentration field of natural gas jet is different at dual fuel model and single fuel model.