| As requirements for clean and efficient power sources gradually increase,the thermal efficiency and emission performance of diesel engines must be further improved.Fuel atomization is a key factor affecting fuel combustion and engine emissions.Past decades,researchers mainly focused on cavitation when considering the influence of fuel flow in the nozzle on atomization,while ignoring the effect of stagnant bubbles on spray when the fuel was primary broken.It have been found that multiple fuel injection is beneficial to improve the combustion and emission performance of diesel engines,making the unstable injection phase more important.To explore the influence of the fuel injection interval on the bubble ingestion and the primary breakage of the fuel,this paper used a combination of experiment and simulation to study the fuel flow in the injector and the fuel atomization outside the orifice during multiple injections.The experiment used acrylic transparent material to make a nozzle with a coneshaped sac and a single straight orifice,built a fuel injection visualization experimental platform and conducted multiple tests,and photographed the fuel flow in the nozzle and the near-field spray pattern at different injection intervals.According to the analysis of the test results,it is found that the fuel atomization forms of the two fuel injections are quite different.There are 4 spray forms in the first fuel injection,while only 3 fuel spray forms in the second fuel injection.The spray patterns of the two fuel injections are different before and after.The injection interval has a significant effect on the spray pattern of the second injection.The shorter the injection interval,the higher the frequency of liquid film spray;the longer the interval,the higher the frequency of slender liquid filament spray.Besides,when the fuel is injected twice before and after,the spray cone angle during the first injection at the same time is larger,the fuel penetration distance of the second injection is longer.And when the injection interval is shorter,the phenomenon is more obvious.In terms of numerical simulation,the physical model of volume of fluid(VOF)/k-ε fuel cavitation under the Euler framework was used to simulate the fuel flow pattern in the injector during multiple injections,and the images were taken with the experiment.In contrast,it verifies the accuracy of numerical calculations.Through numerical calculation and analysis,in the process of multiple fuel injection,the bubbles in the fuel injection nozzle has not reached a stable state before the second fuel injection.As the fuel injection interval increases,the bubble volume ratio in the sac first increases rapidly and then gradually decreases.It shows that the bubbles are still in the process of continuous movement and aggregation.Under different injection intervals,there is no significant difference in the total amount of ingestion bubbles in the nozzle before the second injection,but the bubble distribution positions are different.The shorter the injection interval,the more ingestion bubbles in the sac.When the needle valve is lifted,there is a short-term backflow of outside air,which further increases the total amount of ingestion bubbles in the fuel injection nozzle.During the fuel injection process,the air bubbles sucked in from the nozzle are sprayed out of the nozzle multiple times,and will cause severe disturbance to the fuel flow,causing the turbulent kinetic energy to have multiple peaks.The larger the fuel injection interval angle,the later the ingestion bubbles are completely ejected from the fuel injection nozzle.Finally,the compressible VOF/LES coupling model was used to calculate the primary breakage of the fuel outside the orifice and near-field spray during the second injection,and the adaptive mesh refinement technology was used to optimize the simulation results.The results show that the ingestion bubbles in the sac have a positive effect on fuel atomization when the nozzles are ejected.When the initial bubble surface ratio is relatively large,the fuel spray surface ratio has a strong volatility,but the fuel spray quality tends to be stable with the increase of the initial bubble surface ratio.The distribution of ingestion bubbles has an important influence on the fuel primary breakage and near-field atomization.The fuel liquid film spray is mainly caused by multiple ingestion bubbles stacked in the sac,and when the bubbles are all at the same height(the direction from the needle valve to the orifice),a liquid filament spray will be produced. |
