| It is of great significance to accelerate the atomization and evaporation process of fuel jet to achieve an efficient combustion in liquid fuel combustion system,and the particle emission is also strongly affected by above processes.In order to improve the atomization and evaporation characteristics and meet the increasingly strict emission regulation,high pressure injection technique is widely adopted.However,the effect of ever-increasing pressure becomes neglected when exceeding a certain level but the cost and efficiency of ultrahigh injection system become unacceptable.Compared with traditional injection system,which is adopted with ever-increasing high injection pressure to improve the atomization and evaporation quality of non-superheated liquid jet.Injection system using superheated liquid jet could accelerate the atomization and evaporation process significantly owing to its thermodynamically instability since plenty of bubbles nucleated and expanded inside the nozzle when superheated liquid jet undergoes the two-phase region.Therefore,this advanced injection strategy is promising to increase the fuel economy and lower the emission significantly at the same time.However,owing to the complexity,transient and multiphase property of superheated liquid jet,it is very difficult to visualize and quantify its characteristics by traditional methods.Therefore,the atomization and evaporation mechanism of superheated liquid jet is not clear jet,which limit the wide application of superheated liquid jet since no accurate model was established.In order to investigate the atomization and evaporation characteristics of superheated liquid jet as well as its dominate mechanism,different kinds of experiment apparatus and measurement systems were developed to visualize the superheated liquid jet at different stages.Specifically,a 2-D transparent nozzle was developed to implement simultaneous and quantitative observation on primary breakup of superheated liquid jet especially inside the nozzle and near the nozzle exit;a droplet generator was developed to implement a micro-observation on morphology and breakup of single superheated droplet,or rather the secondary breakup of superheated liquid jet;a scavenging HPHT constant volume chamber with five optical access as well as its accessary control system was developed for evaporation mechanism investigation of superheated liquid jet.In addition,advanced laser diagnostic measurement techniques were applied,and based on ultraviolet/visible laser absorption/scattering(UV-LAS)technique,a modified measurement system with advanced image processing tools were developed to quantitatively investigate the liquid/vapor phase distribution as well as the evaporation characteristics of superheated liquid jet.In addition,UV-LAS technique is a promising method for two phase measurements in the practical application compared with LIEF method,this is because it has several advantages,such as greater S/N ratio,applicable to spray in a higher ambient temperature and pressure,without oxygen-quenching problem,without crosstalk in signals from vapor and liquid phases.For primary breakup study,A unique optically-transparent slit nozzle with a high-speed micro-imaging system were utilized for quantifying the bubble formation inside and the liquid jet breakup outside the nozzle.Correlation between bubble number density and breakup of superheated liquid jet was obtained for Methanol and Ethanol fuels and therefore fuel properties were examined.Three flashing regimes were divided based on the characteristics of both inlet and outlet flow,namely initial flashing regime(SD*>0.3,SD<25~oC),transitional flashing regime(0.2≤SD*≤0.3,SD≈30~oC)and flare flashing regime(SD*<0.2,SD>35~oC),and transitional regime is a critical point at which the bubble formation rate and the atomization characteristics of the superheated jet changes significantly.Later,by introducing a parameter K represented bubble burst force,it is easier to correlate the primary breakup of superheated liquid jet to the bubble number density as?f=K?n,and K is related with the non-dimensional superheat degree(P_a/P_s),Reynolds number,Weber number,and the ratio of liquid to ambient gas viscosities.And finally,the mechanism of primary breakup of superheated liquid jet was revealed.For secondary breakup study,A unique superheated droplet generator was developed for observing the droplet morphology variation and the breakup process resulting from the vapor bubbles inside a superheated droplet by microscopic imaging.It was found that the droplet morphology is mainly influenced by droplet temperature,but micro bubbles formation and the breakup of the superheated droplet are dominated by superheat degree,and the superheat degree of 25~oC is an important critical point at which the droplet breakup occurs resulted from the ever-increasing void fraction exceeding a value of approximately 50%and the breakup mode shifts from aerodynamic mode to thermodynamic mode,and thus eventually influenced the primary breakup of superheated liquid jet.Based on the characteristics of superheated droplet,the initial flashing regime was subdivided into initial flashing regime(SD=0~oC)and partially flashing regime(0~oC<SD<25~oC).Besides,surface tension of superheated droplet was also evaluated by the droplet morphology,and the results show that the maximum reduction in surface tension reaches 70%as superheat degree increases to approximately 25~oC,and this explains the sharp decrease in SMD for a flash boiling spray when the superheat degree approaches this level.These results provide insightful information for understanding the secondary breakup mechanism of superheated droplets and liquid jet and its modeling.To understand the evaporation mechanism of superheated liquid jet,this study focused on the characteristics of both liquid/vapor distribution and total vapor concentration.Based on the distribution of two phase spray,the phenomenon of bubble burst was validated to exist when entering the transitional flashing regime(SD≈30~oC).Flashing evaporation was first proposed to be a new route for evaporation of superheated liquid jet.Based on analysis,the surface evaporation dominates the evaporation characteristics within initial and partially flashing regime,and changes a little within transitional and flare flashing regime.On the contrary,the flashing evaporation starts to increase at transitional regime but increases dramatically and becomes the main route of evaporation of superheated liquid jet beyond the surface evaporation at relative high superheat degree.After that,the increase of fuel temperature and injection pressure was applied individually to verify their influence on the evaporation of superheated liquid jet.The results show that they could both improve the evaporation rate within initial flashing regime and partially flashing regime,but only the increase of fuel temperature would promote the evaporation at relative high superheat degree.This is due to the increase of injection pressure would suppress nucleation as well as bubble growth inside the nozzle and therefore slow down the following process of atomization and evaporation.These results not only reveal its evaporation mechanism but also provide insightful information for applying the superheated liquid jet as well as its modeling.The current study covers the complete process including the two-phase flow inside the nozzle,breakup process(including the primary breakup and secondary breakup)of liquid jet outside the nozzle as well as the evaporation process.Based on this systematic investigation,both physical mechanisms of atomization and evaporation process of superheated liquid jet are fully revealed,which provide insight information for combustion system design and its modeling. |
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