| High speed liquid jet is widely used in today’s engine combustion systems toatomize and vaporize liquid fuel. Understanding the physical mechanisms involved inthe atomization and vaporization of high speed liquid jet is crucial to combustionefficiency improvement and emission reduction. High speed liquid jet can becategorized to three different regions based on the thermodynamic state of the liquidduring injection, namely subcool liquid jet, superheat flash boiling jet and supercriticaljet. The current study only covers the liquid and superheat region since extremely hightemperature and pressure are needed to reach supercritical conditions, which greatlylimits its application in practical combustion systems. Due to the differentthermodynamic states of injected liquid, the atomization and vaporization mechanismsof liquid and superheat jet are remarkably different. Generally speaking, the liquid jet isin the single phase region where the atomization is primarily determined by the forcesapplying to it, including inertia force, viscous force, surface tension force andaerodynamic force. The vaporization is determined by the heat and mass transfer on thedroplet surface. On the other hand, the atomization and vaporization of superheat liquidjet takes place in the liquid/vapor two-phase region. Due to the thermodynamicallyinstability, large amount of vapor bubbles are generated inside the bulk liquid. Themicro-explosion of bubbles efficiently breaks up the liquid and releases the vaporwithin the bubble directly to the environment, resulting prompt atomization and fastvaporization. Comparing with liquid jet, the atomization and vaporization of superheatjet are much more complicated. To understand the physical mechanisms we started fromthe macroscopic characteristics and generated accurate data on parameter that is crucialfor the atomization and vaporization process. Based on those data a comprehensive andsystematic study was carried out.Advanced laser diagnostic techniques are needed to understand the atomization andvaporization of high speed liquid jet, which is decided by the transient and random nature associated with those processes. In particular, diagnostics on the vaporization ofhigh speed liquid jet are difficult, which becomes especially challenging when the liquidis superheated and large amount of liquid and vapor co-exists. To solve those issues thecurrent study developed advanced laser diagnostic techniques to measure the crucialparameters for spray vaporization, including liquid/vapor distribution, quantitativevapor concentration distribution and liquid temperature distribution. The separatedliquid and vapor distribution measurement was based on laser induced exciplexfluorescence (LIEF) technique. To achieve quantitative vapor concentrationmeasurements, a series of calibration and image correction procedures were developedto calibrate the signal interference from liquid phase on vapor phase (crosstalk), thelaser energy attenuation, as well as the concentration and temperature dependence of thevapor fluorescence. To get2D liquid temperature distribution, we proposed a novelthermometry technique that combined the LIEF and two color thermometry. Throughsystematic spectroscopic studies and calibration experiments, the accuracy of thetechnique was confirmed. The generation of liquid/vapor distribution, vaporconcentration and liquid temperature data greatly enhances the current knowledge onliquid jet vaporization, which enables an insightful investigation on the jet vaporization.The development of novel laser diagnostic technique enables a detail study on theatomization and vaporization of high speed liquid jet. For atomization study, we appliedplanar Mie scattering imaging to obtain the macroscopic structure of liquid jet. Imageprocessing algorithms were developed to calculate the jet characteristics such as tippenetration and cone angle. The atomization of liquid jet is determined by thecompetition between inertia force, surface tension force, viscous force and aerodynamicforce, which could be quantitatively expressed using dimensionless numbers. Based onthis idea, Weber number (We), Reynolds number (Re) and air-to-liquid density ratio(ρ0/ρ1) were chosen to describe the relative importance among those forces. Themechanisms of liquid jet atomization was revealed by building temporal correlationsbetween the jet characteristics and dimensionless numbers. Results show that thebreakup of liquid jet consists of two temporal stages, namely initial stage and fullydeveloped stage. During the initial stage, a continuous liquid core moves away from thenozzle exit with a steady velocity that is primary determined by the competitionbetween inertia force and surface tension force (We). The aerodynamic force only havesecondary effects on the liquid core velocity while the viscous force only shows minor impact when the Reynolds number is low. The existing time of the liquid core isdecided by the inertia force, viscous force and aerodynamic force. When the liquid corebreaks up to discrete droplet, fully developed stage is reached where the effects ofaerodynamic force increases and the influence of other forces decreases. When theliquid is superheated, the effects of above forces on jet breakup become secondary whilethe effects of bubble explosion become dominant. We chose the ambient-to-saturationpressure ratio (Pa/Ps) as an indicator of the degree of superheat and bubble behavior.The atomization of superheat liquid jet was analyzed by building correlations betweenthe above dimensionless numbers and jet characteristics. Results shows under flare flashboiling conditions (0.3＜Pa/Ps＜1.0) the jet breakup is determined by the vapor with a “gasjet” structure presented in the spray field. Under transition region (Pa/Ps＜0.3)the initial stage and fully developed stage also exists during the jet breakup while thebubbles have strong effects on jet breakup of each stage. Besides, for multi-hole liquidjet we observed severe plume width enlargement under superheat conditions, whichcould lead to strong plume-to-plume interaction and result to unique structure. To revealthe effects of plume-to-plume interaction, we designed injectors with1,2,3,4,5,6holes and investigated the spray structure of each injector. The results show that theplume interaction of multi-hole spray may result to a “low pressure” region formed inthe spray centerline. The spray mass is pushed towards the centerline by the “lowpressure” region and “collapsed” spray is formed. Based on the above analysis thebreakup mechanism can be summarized: in the liquid region, inertia force, surfacetension force, viscous force and aerodynamic force determine the jet breakup; intransition flash boiling region bubble growth is the main factor to influence the jetbehavior while the plume interaction decides the breakup in flare flash boiling region.To understand the evaporation of liquid and superheat jets, the current studyfocused on the effects of liquid/vapor distribution, vapor concentration and liquidtemperature. We first analyzed the liquid and vapor distribution under various operationconditions to have a qualitative understanding. Then the quantitative vaporconcentration was investigated and the evaporation rates were calculated based on thosedata to quantitatively reveal the contribution of surface evaporation and inner boiling tothe spray evaporation. Finally, the liquid temperature distribution was investigated toreveal the driven force of evaporation. The mechanisms of spray evaporation weresummarized based on the systematic data set. The results show that in liquid region surface evaporation is dominant. However the long time scale of surface evaporationand relatively simple flow direction inside liquid jets leads to slow evaporation. Insuperheated region the evaporation is dominated by inner boiling. The short time scaleof boiling greatly enhances the evaporation rate. The complicated flow induced by the“low pressure” region also greatly accelerates the heat and mass transfer. Under thesame liquid temperatures evaporate rate of flash boiling sprays are60%faster than thatof liquid sprays.The current study covers the complete process from the continuous liquid jetatomizes into discrete droplets and the droplets transforms to vapor through heat andmass transfer. The physical mechanisms of each process are revealed. The resultsprovide a solid foundation for the combustion system design of modern liquid fuelcombustion engines. |
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