| The near-wall region is an important formation zone of exhaust emissions such as soot,unburned hydrocarbons(UHC),and carbon monoxide in the process of gasoline compression ignition(GCI).It also significantly affects the combustion efficiency and thermal efficiency of internal combustion engines.Therefore,it is of great significance to conduct further studies of spray mixing,combustion and pollutant generation and evolution mechanism in the near-wall region of GCI for improving the efficiency,reducing exhaust emissions,and achieving efficient and clean combustion of internal combustion engines.In this study,multiple optical diagnostic methods were applied to study the droplet dynamics,spray mixing characteristics,combustion process and soot evolution process in the near-wall region of GCI in a constant volume combustion bomb.Firstly,the particle image analysis(PIA)and laser-induced fluorescence(LIF)technology were combined to study the droplet dynamic characteristics in the near-wall region after the gasoline spray impinged on the oil film.The results showed that the droplet splashing process after the spray impinged on the oil film was roughly divided into two stages,the prompt splash in the early stage and the crown splash in the later stage.The droplet size was smaller and the speed was faster in the prompt splash stage,while the droplet size was larger and the speed was slower in the crown splash stage.The crown structure was mainly formed from the oil film.The diameter distribution curve of the splash droplet and the velocity direction distribution curve were both unimodal,but the droplet velocity distribution was bimodal,corresponding to the two different droplet splashing stages.The greater the viscosity of the oil film was,the larger the size and the stronger the stability of the crown structure had.The oil film viscosity had no significant effect on the inclination angle of the crown structure,but the increase in viscosity produced greater vorticity intensity,which distorted the crown structure.When the viscosity of the oil film was greater,the more large-size droplets were produced when the crown structure was broken.The Sauter mean diameter(SMD)of the droplets increased with the increase of the oil film viscosity.When the thickness of oil film increased,the size of crown structure increased,the inclination angle of crown structure increased,the number and size of splashed droplets were larger,the SMD of droplets increased,and the angle between the droplet velocity direction and the wall surface also increased.The influence degree of oil film thickness was greater than that of oil film viscosity.Secondly,laser-induced exciplex fluorescence(LIEF)technology was used to study the spatial distribution and development process of the vapor and liquid phases of gasoline sprays in the near-wall region.During the gasoline injection process,the penetration distance of the liquid phase quickly stabilized and did not change much.The liquid phase did not directly impinged on the wall surface.After the vapor phase impinged on the wall,it developed along the wall surface and produced a head vortex structure.Compared with the free spray head,the vapor equivalence ratio in the wall-jet zone of wall-impinging spray was relatively low.In the free spray zone,there was little difference in the liquid phase fluorescence and vapor phase concentration distributions between the wall-impinging spray and free spray.The spray volume and entrained air mass of wall-impinging spray and free spray had a roughly linear trend with respect to the injected fuel mass,while the average equivalence ratio gradually decreased and the mass-weighted average temperature changed a little.Compared with the free spray,the spray volume and entrained air mass of the wall-impinging spray were smaller,while the average equivalence ratio and mass-weighted average temperature were larger.Compared with the diesel spray under similar conditions,the liquid phase penetration distance of gasoline spray was significantly shorter,but the difference in vapor phase penetration distance was smaller.An empirical formula for the penetration distance and cone angle of gasoline free spray was proposed.With the increase of ambient pressure,the penetration distances of the liquid and vapor phases both decreased,and the average equivalent ratio and spray volume also decreased.On the contrary,the entrained air mass and mass-weighted average temperature both increased.As the injection pressure increased,the liquid phase penetration distance remaind almost unchanged.Under the condition of the same injected fuel mass,as the injection pressure increased,the penetration distance of the spray vapor phase along the wall showed a downward trend.At the same time,the spray volume decreased,but the entrained air mass increased,which led to the decrease of average equivalence ratio.The mass-weighted average temperature increased with the increase of injection pressure,while the wall temperature had a little effect on the spray mixing characteristics.Finally,formaldehyde laser-induced fluorescence(LIF)technology,flame natural luminosity and hydroxyl(OH~*)chemiluminescence imaging,and wavelength integral two-color method were used to study the low-temperature reaction process high-temperature ignition process,flame shape and development process and soot evolution process of gasoline spray in the near-wall region.The formaldehyde-LIF signal of the gasoline free spray flame first appeard at the downstream edge of the spray.The diesel free spray flame had a similar initial location of formaldehyde-LIF signal,but the formaldehyde-LIF distribution was more fragmented and irregular,and the signal intensity was also smaller.Compared with the gasoline free spray flame,the low temperature ignition delay of the wall-impinging spray flame was shortened,and the formaldehyde-LIF distribution was closer to the injector.The high-temperature ignition of the gasoline free spray flame first appeard in the spray head,and that of the gasoline wall-impinging spray flame first appeard in the spray head of the wall-jet region.The ignition delay of wall-impinging spray flame was similar to that of free spray flame,but the ignition position of the former was closer to the fuel injector.After the high temperature ignition,the lower boundary of formaldehyde-LIF and OH~* chemiluminescence under larger cycle injection mass(10.71 mg)condition were closer to the upstream of spray than smaller cycle injection mass(6.55 mg)condition,and the intensity of formaldehyde-LIF was also smaller.Under smaller cycle injection mass condition,the wall-impinging spray flame generated a large amount of formaldehyde in the region near the impingement point,which was not easy to enter into the high temperature reaction zone,and was probably quenched and deposited on the wall surface,resulting in UHC emission.Compared with smaller cycle injection mass condition,the ignition delay under larger cycle injection mass condition was relatively shorter,and the ignition position was farther from the injector.Gasoline free and wall-impinging spray flames produced almost no soot under smaller cycle injection mass condition.The spatially integrated natural luminosity(SINL)of the former was greater than that of the latter.Under larger cycle injection mass condition,the gasoline free and wall-impinging spray flames both showed yellow flames,and the SINL of the former was smaller than that of the latter,indicating that the spray/wall impingement would increase the amount of soot.In the later stage of combustion process,the soot formed in the gasoline free spray flame continuously accumulated at the upstream boundary of the flame and was difficult to be oxidized.The impingement on the wall made the time of soot generation of gasoline spray flame advance.The soot accumulated in the peripheral area at the later stage of combustion process,which was difficult to be oxidized and probably caused the final soot emission. |
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