| As downsizing and alternative fuels have become the trends of development ofgasoline engines, the applications of flash boiling spray are attracting more and moreattentions. However, the characteristics of flash boiling spray and the mechanism ofspray collapse during flash boiling have not been fully understood. In the presentthesis, comprehensive optical diagnostics on both macroscopic and microscopiccharacteristics of flash boiling spray were conducted a in constant volume chamberand an optical engine with high-speed photography, Glare Point Velocimetry andSizing (GPVS) and Particle Image Velocimetry (PIV).A constant volume chamber bench, an optical engine bench as well as a set ofspray optical diagnostic system were first designed and built up. An opposite incidentlaser sheet method was used to expand the applicability of GPVS method. Two newparameters (i.e., ratio of collapse and spray span) and a new definition (i.e., coneangle of spray) of macroscopic characteristics for flash boiling spray were proposed,which could quantify the transition of spray morphology, reveal the law of flashboiling spray and clarify its mechanism.In the macroscopic experiments of gasoline flash boiling spray, the front viewwith the bottom view of spray was first compared and the close relationships betweenthe process of collapse and overlap was revealed. On the basis of this finding, thetransition of spray pattern and the variation of spray macroscopic characteristics wereanalyzed. The results demonstrated the existence of low pressure core and overlapregion. Then, the spray pattern of single-hole, two-hole and six-hole injection werecompared and the effects of low pressure core, overlap region and superheat on thespray morphology were decoupled. The low pressure core was found to influence thedirection of spray plume via gas entrainment, which contracted the spray pattern inthe radial direction. In the overlap region, the droplet number density was increasedand the collision and coalescence of droplets were enhanced, which would lead tohigher penetrating momentum downwards and weaken the decelerating effect of airdrag force on spray penetration and thus elongated the spray pattern axially.Additionally, the low pressure core and the overlap region are mutually promotive.Therefore, the mechanism of flash boiling spray collapse was fully revealed.In the flash boiling experiments of alkanes and their mixtures, the sprayconditions were divided into low, medium and high degree of superheat stages and thevariations of the collapse ratio at different stages were quantitatively analyzed. Theresults showed that at the low degree of superheat stage, the collapse ratio was around1.3and the superheat degree was found to have little effect on the spray pattern. Atthe medium degree of superheat stage, the collapse ratio was between1.3and3.2and increased significantly with the increasing of the superheat degree and the fueltemperature was found to have the dominant effect on the spray pattern. At the highdegree of superheat stage, the collapse ratio was between2.9and4.9as the spraycollapsed completely and the ambient pressure was found to have a significant effecton the spray pattern. The effects of superheat degree and blend ratio on spraymicro-explosion and collapse were clarified by using mixtures of n-pentane (boilingpoint36.1℃) and n-heptane (boiling point98.5℃) with various blend ratios.In the microscopic experiments of flash boiling spray, a spray droplet imageprocessing was developed. The deviation of droplet diameter calculation was3.8%,which meets the requirement of the droplet diameter statistics. The results showed thatwith the superheat increased, the distribution range of droplet diameter reduced from10μm-25μm at low degree of superheat stage to10μm-15μm at high degree ofsuperheat stage. Additionally, at the location of30mm under the nozzle, theincomplete secondary breakup results at the high probability of large droplets.Moreover, due to the flow entrainment, a large amount of small droplets entrainedinto this location, thus the probability of small droplets was also high. Fueltemperature, ambient pressure and test location were found to have significant effectson the local Sauter Mean Diameter (SMD) of the spray. Generally, the local SMDdecreased as the fuel temperature increased or the ambient pressure decreased, and theeffect of test location could be analyzed by the factors of airflow, spray breakup andevaporation. The results also showed that spray stage, test location and density ofambient gas significantly impacted the probability distribution of droplet velocity.Specifically, at medium degree of superheat stage, the velocity of spray droplets wasbecoming smaller with narrower range. In the conditions of0.1bar ambient pressure,the low ambient pressure weakened the decelerated effect of airflow on droplets, thusmuch more droplets with high velocity appeared.In the experiments for the investigation of the effects of in-cylinder air motion onflash boiling spray in an optical engine, fuel temperature was found to play a majorrole on the variations of spray pattern and velocity field. Specifically, at low degree ofsuperheat stage, the biggest spray SMD was observed, air motion didn’t sign faintlyaffect the spray characteristics. At medium degree of superheat stage, SMD, spraypenetration and width decreased, and the directions of droplets velocity becameconsistent. At high degree of superheat stage, the spray collapse ratio and penetrationincreased with fuel temperature increasing, and the consistency of the direction ofdroplets velocity was much better. Meanwhile, the effect of in-cylinder air motion onthe spray was weakened. Furthermore, the in-cylinder tumble motion of air was alsofound to have somewhat effects on the spray pattern and velocity field. In the lowtumble ratio conditions, the direction of droplets during the early stage of injection are consistent with the direction of intake airflow, thus the maximum speeds of dropletduring the early stage of injection in the low tumble ratio conditions were higher thanthose in the high tumble ratio conditions. Before middle stage of spray injection,spray tended to penetrate towards the piston at low tumble ratios, but towards the leftcylinder wall at high tumble ratios.Orally, by conducting a series of optical diagnostics on both macroscopic andmicroscopic characteristics of flash boiling spray, the variation of spray patterns atdifferent superheat degree stages were systematically demonstrated. Additionally, theconcept of overlap region was proposed and the understanding of the mechanism offlash boiling spray collapse was promoted. Moreover, the effects of superheat degreeand blend ratio on the flash boiling characteristics of test fuels, as well as thefeasibility of controlling the extent of flash boiling by adjusting the blend ratio of low-and high-boiling point components were analyzed. The knowledge of microscopiccharacteristics of flash boiling spray was expanded. At last, the technical route ofapplication of flash boiling spray on direct injection gasoline engines was discussed. |
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