| World pressure to reduce automotive fuel consumption and CO2emissionsis driving the introduction of downsized boosted direct-injection-spark-ignition(DISI) engine technologies. On the other hand, it is believed and wide consensusexists that both fossil fuels and biofuels will make significant contributions tothe energy future. Therefore, determining the physics foundations that enablebridging the gaps in applying new engine technologies and using biofuels inDISI engines is necessary to achieve the highest fuel efficiency and lowestpollutant formation. A key area is the investigation of the DISI sprays.For DISI engines, liquid fuel is directly injected into the combustionchamber. The spray formation is governed by the forces acting on the liquid jetand droplets, including inertia force, viscous force, sucface tension force and airdrag force. However, flash boiling will occur when injecting fuel into conditionsbelow the saturation pressure. The mechanism of flash-boiling atomization isthe bubble’s generation and growth depending on the superheat degree. Thedifference in breakup mechanisms between the flash-boiling spray and the liquidjet illustrates the importance of generating fundamental knowledges of thesetwo sprays indivually.Understnding of spray formation and characteristics needs to conduct laserdiagnostic experiments. Spray drop-sizing is one of the most important anddifficult diagnostics. Laser-induced-fluorescence/Mie-scattering (LIF/MIE) wasproven to be a useful diagnostic for Sauter Mean Diameter (SMD) m easurementin non-evaporating sprays, while it is not reliable for cases of an evaporatingspray due to the interference of the fluorescence signal from the vapor phase. Inthis work, simultaneous Laser-induced-exciplex-fluorescence/Mie-scattering(LIEF/MIE) imaging techniques were proposed to obtain the SMD distributionof evaporating sprays for the first time. Different from the usual LIF/MIEtechnique, the combination of LIEF and Mie techniques allows eliminating the effect of tracer fluorescence from va por phase in an evaporating spray. Inaddition, carefully selected tracers and specially designed filters were used todecrease the effects of variation of tracer concentration and temperaturedependency of fluorescence intensity during evaporation. The numericalanalysis based on geometrical optics approximation (GOA) and experimentalanalysis was conducted to reduce the measurement uncertainties and determinethe calibration coefficient K. The results show that the SMD of a flash boilingspray obtained from LIEF/MIE is accurate and the evaporation effect cannot beignored for evaporating sprays. Besides the drop size distribution, the LIEF andMie-scattering techniques were also used to investigate the spray vaporizationprocess and the spray macroscopic characteristics, repectively.The dimensionless method was used to analyze the spray characteristics toreveal the physics behind for both liquid jet and flash-boiling spray. The Webernumber, Reynolds number, and air-to-liquid density ratio were used to capturethe dominating forces associated with liquid jet breakup. T he ambient-to-saturation pressure ratio (Pa/Ps), representative of the superheated degree, wasidentified as the primary dimensionless number that describes the sprayformation under flash-boiling conditions. LIEF/MIE technique was applied toobtain the spray structure, drop size and vaporization information over a broadrange of fuel temperature conditions for different fuels, providing a large rangeof dimensionless numbers. The spray similarity was revealed to verify theavailability of the dimensionless analysis and the spray characteristicsdependence on the dimensionless numbers was described for both liquid jet andflash-boiling spray.For liquid jet, a number of previous studies have already investigated thecharacteristics under various injection conditions, providing a set of empiricalformulations that describes the spray penetration, spray-plume angle, etc.However, the empirical correlations and models usually were developed for thespecific applications. There have not been any general spray correlations andmodels to cover all the applications. These formulations were based on dieselsprays that may not be directly applicable to gasoline and increasingly usedgasoline substitutes, such as methanol and ethanol. In this study, the spraycharacteristics were quantified using dimensionless analysis by correlating the spray characteristics with Weber number, R eynolds number and air-to-liquiddensity ratio. The good correlations between these dimensionless numbers andspray characteristics have yielded a set of dimeniosnless formulations, whichare general since the three dimensionless numbers comprehend the effects of allspray control parameters and fuel type. These formulations provide importantinsight into the jet breakup and atomization processes, and could express thephysical mechanism explicitly, independent of the test conditions and fuel type.For flash-boiling sprays, the breakup and atomization mechanisms aresignificantly different compared to that for non-flash-boiling liquid jet. Previousexperimental investigations unveiled the important spray structur al changesunder flash-boiling conditions by providing visual descriptions of the spraytransformation. However, the mechanisms responsible for the spray behaviorover the range of superheated degrees require further investigations. In thiswork, the flash-boiling spray was characterized using LIEF/MIE under a broadrange of superheat degrees. Then, the spray characteristics were analyzed usingdimensionless method and found that the flash-boiling spray formation andcharacteristics parimarily depend on the superheat degree with secondar ydependences on other dimensionless numbers. The observed trends among thespray structure, spray Sauter Mean Diameter and vapor quantity are anticipatedto have strong inter-dependence. The correlations of the spray structural change,SMD and extent of vaporization with increasing superheated degree haveyielded dimensionless formulations and provided good insight into themechanisms responsible for the observed behaviors during flash-boilingconditions. Finally, the flash-boiling spray behavours were proven to be quiteattractive for the existing and future DISI engines. |
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