| In order to meet increasingly stringent emission regulations,various new combustion methods have been proposed in succession,and higher requirements have been put forward for the development of diesel fuel injection systems.At present,the fuel injection pressure can be as high as 300 MPa,and the orifice diameter is only 0.1 mm,making it easier to form cavitation in the nozzle hole.On the one hand,cavitation can promote the spray to improve combustion,but on the other hand,cavitation will suppress the flow and cause cavitation erosion in the nozzle.In severe cases,the injector may even fail.Aiming at the cavitation and erosion phenomena in the diesel model nozzles,this paper has carried out a large number of visualization studies on the two important influencing factors of the taper and roughness of the nozzle.At the same time,durability cavitation erosion tests were performed on a real injector,and numerical simulations were conducted to study the distribution and causes of cavitation erosion in the nozzle.The main contents and innovations are as follows:(1)Based on the self-circulation internal flow test bench,a cavitation visualization experiment device was built.According to different research purposes,three model nozzles were designed and processed,and rapid cavitation erosion tests can be realized.(2)The cavitation erosion distribution in different taper nozzles was compared under the same injection pressure.The study finds that the cavitation has been developed to the outlet of the divergent orifice and collapsed at the outlet,so that the cavitation pits are mainly distributed at the outlet of the orifice.For non-tapered and convergent nozzles,the cavitation areas are mainly located at the hole entrance,which is due to the collapse of the cavitation condensation and the collapse of rupture at the entrance.(3)The effects of three different wall roughness of 1.6?m,6.3?m and 30?m on the cavitation and flow in the hole were studied.The experimental results show that with the increase of the roughness of the wall surface,the degree of cavitation in the nozzle hole weakens first and then increases,which is the result of the wall resistance and the gas nucleus content together.The change of flow rate in different roughness nozzles is affected not only by the wall resistance but also by the degree of cavitation.In the single-phase flow and the initial stage of cavitation,the influence of wall resistance plays a leading role.As the roughness increases,the flow rate in the nozzle hole gradually decreases.In the development stage of cavitation and super-cavitation,the effect of cavitation is more important.Therefore,the flow rate in the 6.3?m spray hole is the highest,and the flow rate in the 30?m spray hole is the lowest.(4)The main shedding frequency of cloud cavitation in the nozzles with different roughness was analyzed by the time slices of the relative length of the sheet cavitation and the frequency domain diagram of the FFT.It was found that at the same injection pressure,the main shedding frequency is lower in the 30?m orifice,but the difference is not large.And the overall difference of the sheet cavitation length can be obtained from the time-domain diagrams in which the relative length of the sheet cavitation in the roughness nozzle holes varies with time,which are same with cavitation average length analysis results.(5)Using X-ray CT scanning non-destructive testing technology,the experimental nozzle was observed,and it was found that the cavitation erosion is easy to occur at the entrance of the nozzle holes.The open nozzle was detected by a scanning electron microscope and it was found that an annular erosion zone exists on the needle body wall downstream of the needle seal line.Combined with numerical simulation,the cause of erosion formation was analyzed.And research has found that changing the step transition type needle valve to a multi-cone transition type needle valve will effectively reduce the cavitation erosion in the nozzle. |
PDF Full Text Request