Research On Mechanisms Of Impinging Atomization And Droplet Breakup Of Gelled Kerosene

Kerosene gel is formed by adding gelling agent in the kerosene and is a kind of gel fuel with a stable gel structure.With respect to the general kerosene,kerosene gel is hard to leak and volatile whose energy density can be further increased by adding enriched particles having a very high application prospects in gel ramjet.However,stable combustion and performance of the ramjet are restricted because of the low atomization quality of the gelled kerosene resulting from its high viscosity.Therefore,it is of great significance to promote engineering application of kerosene gel in ramjet engine through studying the atomization mechanism of kerosene gel and thereby obtaining the technical method to enhance the atomization quality.In this thesis,atomization problem of the gel propellant in ramjet engine is taken as the basic research background.In order to explore and study systematically the mechanisms of impinging atomization and droplet breakup of gelled kerosene,this thesis combines the experimental and theoretical analysis,establishes the gel impinging atomization and droplet breakup test platform,develops a dual-mode linear method for power-law impinging sheet and a deformation and vibration model based on TAB model,puts forward a calculation model of the shear rate control coefficient and corresponding mode division method.The main research work includes:(1)Impinging atomization experiment of gelled kerosene is carried out under different conditions,and the effects of eccentric impact and jet length on impinging atomization are analyzed for the first time.The shape and its development process of impinging sheet are photographed,and the breakup length,disturbance wavelength and droplet size distribution after atomization are measured.The calculation formula of the sheet deflection is deduced in eccentric impact,and the jet disturbance is analyzed to explore its effect on impinging atomization.The results show that the increase of jet velocity is beneficial to impinging atomization when jet velocity is lower than 27.56m/s.Increasing impact angle can also improving atomization quality,but much gelled kerosene appears in the rear area of the spray which goes against combustion organization when impact angle exceeds 60°.The eccentric impact has little influence on the shape of the sheet,but atomization quality is always better than the absolute impact.There is an optimal value of jet length which is equal to 25/3 making the atomization quality best,but the value may disappear with the increase of jet velocity resulting in a monotone decrease of atomization quality with jet length.(2)A dual-mode linear analysis method for the disturbance of impinging sheet of the gelled kerosene is established based on the power-law constitutive equation.Corresponding dispersion equation is deduced,and the influence of the physical and dynamic parameters on the instability of the sheet is obtained.The limitation of incapacity in dual-mode analysis for power-law sheet at present is broken through.A linear correlation between the jet disturbance and the sheet disturbance is proposed,and thereby a linear analysis method of the jet length to the stability of the sheet is founded.The influence mechanism of the jet length on the impinging atomization of gelled kerosene as well as the disturbance inheritance relationship between jet and sheet are revealed theoretically which files the study blank of the integrated linear analysis for the jet and sheet in impinging atomization.The results indicate that the calculated length and disturbance wavelength of the linear analysis method in this thesis are in good agreement with the results of the classical dispersion equation and are also consistent with the experimental results on the trend,and hence the theoretical model and derivation process proposed in this thesis are effective.(3)The experimental analysis of motion and deformation of kerosene and gelled kerosene droplet under different conditions is carried out,and the variation rules of deformation,period and amplitude of the droplets are obtained.The viscosity calculation model of the gelled droplets is constructed by introducing shear rate control coefficient whose application range is greatly improved compared with the early calculation model.Furthermore,the deformation and oscillation model of gelled droplet is established based on TAB model.The effect law of the physical and kinematic parameters on the droplet deformation and oscillation is analyzed by the model.The results show that drag characteristics of gelled droplet is similar to Newtonian droplet and the drag coefficient of the gelled droplet can be calculated by the classical formula given in the literature.The maximum deformation of the droplet remains essentially unchanged when Weber number is greater than 60.Compared with the experiments,results calculated by TAB model displays an accordance for amplitude on the trend but leaves a quantitative error implying that the model need to be farther improved.(4)The breakup characteristics of kerosene and gelled kerosene droplet are studied and analyzed in experiments.A calculation model of the shear rate control coefficient in sub-region and corresponding mode division method are put forward.The breakup mode distribution image of the gelled droplets is obtained.It is revealed that the gelled kerosene droplet still conforms to the Newtonian fluid droplet mode distribution law from high to low viscosity range.The initial and total breakup time,the spatial distribution of daughter droplet and their variation with the experimental conditions are measured and explored.The results indicate that breakup behaviors of the kerosene and gelled kerosene droplets are similar,and five common breakup modes are obtained:Oscillation,Bag,Multimodal mode,Shear-stripping and Catastrophic breakup mode.Comparison of the initial and total breakup time between experiments and related literatures is carried out under different Oh(Ohnesorge number)numbers revealing that the calculation model in sub-region in this thesis achieves higher precisions than the method in whole region at present.The calculation model raised in this thesis establishes a theoretical foundation for more precise spray design and numerical simulation in the ramjet and hence improving the performance of the engine.(5)The breakup characteristics of kerosene and gelled kerosene droplets in turbulent gas flow are preliminarily studied.The effect pattern of turbulence on droplets is given based on the Kolmogoroff micro-scale length,and the breakup behavior and process are photographed.Droplet breakup time and spatial distribution of daughter droplets are measured.The transition conditions and effect rules of turbulent parameters on the droplet breakup are obtained.The results show that droplet breakup behavior has a certain randomness in turbulent flow,and the breakup mode under higher air velocity may be experienced at low air velocity.In laminar flow,transition We number is 80?104 in Shear-stripping mode and is 323?423 in Catastrophic mode.While in turbulent flow,transition Weave number is 37.4-46.6 in Shear-stripping mode and is 79~123 in Catastrophic mode Compared with laminar flow,the transition values are greatly reduced revealing that the turbulence promotes the gas momentum transfer to droplet,and is beneficial to mode conversion and droplet breakup.Therefore,the gel atomization quality can be promoted by increasing artificially the turbulence intensity in the local spray field through the design of spray device and internal profiles of the engine,and the combustion efficiency and performance are further enhanced.The work of this thesis is focus on mechanism of impinging atomization and droplet breakup,and is expected to provide the theoretical and practical basis for improving atomization quality and combustion organization of gelled kerosene and developing design procedures of related spray equipment in ramjet.