| Spray impingement is one of the main sources of the particle emission and oil dilution in GDI engines,and the study on spray impingement has received extensive attention from researchers.However,the characteristics of the deposited fuel on hot surfaces and oil surfaces have not been well studied.The existing models of the spray impingement are usually based on single drop impact experiments,but the drops in the spray are quite dense and the severe interaction among the impinging drops makes the it a question whether the spray impingement process could be described as the simple addition of the single drop impact.In order to deepen the understanding of the spray impingement process,I studied the quantitative characteristics of the deposited fuel and the fundamental theories of the impingement process of multiple drops on a rigid surface.Laser induced fluorescence is used to measure the mass,ratio and area of the deposited fuel.The surface was heated to simulate the temperature of a piston in real engines.The deposited fuel was found to have completely different morphologies under different surface temperatures.Continuous fuel deposited film was found on a 25? surface and the film shrunk to liquid strings due to enhanced evaporation when the surface is heated to 100?,and only scattered isolated drops could be observed when the surface temperature exceeded 140?.Deposited fuel mass decreases monotonously as the surface temperature climbs and the Leidenfrost phenomenon does not lead to an increase of the deposited fuel mass although it impedes fuel evaporation.On the contrary,it generates air cushion between incident drops and the surface which makes the incident drops easier to splash and more difficult to deposit on the surface.The spray impingement process on oil films was also investigated,the depth of the film varies from 10?m to 50?m and the viscosity was set to 10 cSt and 100 cSt separately.It was found that a thicker oil film thickness leads to a larger fuel deposited mass while the higher injection pressure could decrease the fuel deposited mass.The effects of oil viscosity on the spray impingement process is rather complicated,and different effects were found under different injection pressures.The interaction between the multiple drops during impingement was studied using two steams of mono-disperse drops with high speed shadow graph.Interaction between two wall films was observed during impingement process and found to be the reason for the different outcomes from the single and multiple drop impingement.Based on this observation,I constructed the “central rising sheet” model and the “central accumulated pool” model for different initial wall film velocities and stream distances.The models were fitted to the experimental results and the entire interaction process was final separated into five regimes,which are “no obvious interaction” “severe splash”“mild splash” “column rising” and “simple accumulation”.The characteristics of the secondary droplets of the severe splash regime were further studied.An edge-detection-based algorithm was established to identify the secondary droplets in the high-speed shadow graphs.It was found that the secondary droplets from multiple drop impingement were obviously larger than those from single drop impingement,the size distribution of the multiple-drop impingement is also more dispersed.The probability density function of the secondary droplet diameter was established based on three-dimensional Weibull distribution function.In summary,this thesis focuses on the deposited fuel characteristics and multiple drop interaction of spray impingement process,both quantitative experimental results and theoretical models were obtained and other researchers and engineers who are interested in the spray strategy design and spray impingement model development could find useful information in this work. |
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