Study On The High-speed Transient Diesel Spray Mixture Formation And Combustion Characteristics

Under the background of reducing greenhouse gas emissions in the shipping industry,the application of advanced injection and combustion strategies to continuously optimize the performance of diesel engines and the use of zero-carbon/low-carbon fuels are two important ways for ocean-going ships to achieve energy conservation and emission reduction.To the multi-stage injection,the low-temperature combustion strategy,and the zero-carbon/low-carbon fuels such as natural gas,ammonia,and methanol operated with the dual-fuel model,the pulse width of diesel injection is much smaller than that of traditional diesel internal combustion engines.It is necessary to conduct in-depth research on the mixture formation in the high-speed and strong transient process of spray injection for the realization of clean combustion of diesel engines and dual-fuel engines using low-carbon or zero-carbon fuels.However,related research is rarely reported.Laser-based optical diagnostic techniques are widely used in diesel spray mixture formation studies.For the transient process of spray,limited by the frequency of the laser,the existing laser testing methods cannot provide sufficient time resolution accuracy.To further understand the mixing process during the transient,The research carried out in this paper with the high-speed testing technology as the starting point is as follows:（1）The high-speed micro-PTV system was firstly established.The transient air entrainment velocity of diesel spray was measured,which elucidated the enhanced mechanism of air entrainment during transients.For quasi-steady state and transient state,the entrainment velocity increase with the injection pressure.The increase of ambient density will not reduce the air entrainment velocity in the breakup length,that is,the volume flow of air entrained into the spray under different ambient densities is the same,so increasing the ambient density can effectively increase the mass flow of air entrainment.（2）During SOI and EOI,the movement of the needle valve may strengthen the cavitation in the nozzle hole,which will increase the primary breakup and the cone angle,leading to higher entrainment velocity because more ambient air needs to be entrained to compensate for the air transported downstream.（3）High-speed photography experiment was conducted to study the high-speed and strong transient short-pulse-width spray.Based on the existing zero-dimensional model of spray tip penetration,a model suitable for high-speed and strong transient short-pulse-width spray is established.Then,the model was tested and verified.The spray tip penetration after injection is dominated by the maximum injection rate at the moment of the t_srd.By introducing the characteristic time t_srd（the time when the fuel injection rate drops rapidly）,a spray tip penetration model for the short-injection duration spray is established.The new model is suitable for the prediction of tip penetration length of short-pulse-width and long-pulse-width spray at the same time.（4）The UV-LAS system with the high temporal and spatial resolution was established in this paper to realize the fuel concentration distribution test in the transient process of evaporation spray.For the spray gas phase in the near-nozzle area,during the end of the injection,not only does not appear too lean,but the local equivalence area is relatively high due to the accelerated evaporation rate.（5）OH^*chemiluminescence was used to record the high-temperature reaction area of spray combustion,and the two-color method was used to measure the flame temperature and soot concentration.Then,the influence of air entrainment and concentration distribution on soot formation was analyzed.The research results show that reducing the orifice diameter and increasing the injection pressure can reduce the soot generation,but the mechanisms for reducing soot generation are different.Increasing the injection pressure has little effect on the distribution of the equivalence ratio in the spray,but the air entrainment velocity is fast at high injection pressure,and more oxygen can pass through the high-temperature reaction zone to the inside of the spray,so the soot generated per unit mass of fuel under high injection pressure is lower.（6）The high-temperature reaction of the spray in the near-nozzle area is affected by the Damkohler number,the lower spray velocity during the end of the injection is conducive to the high-temperature reaction.However,the high-concentration area will decrease rapidly due to mixing.Therefore,as long as the ignition delay time of self-ignition is shorter than the over-lean time,combustion recession can occur.For the 0#diesel spray studied in this paper,the occurrence of flame retraction after injection is dominated by the ambient temperature.