Physical Process And Model Of High-Temperature Molten Aluminium Droplet Impacting On The Solid Wall

The phenomenon of aluminium/aluminium droplets impacting on the wall is a special multiple phase flow phenomenon in the rocket motor within aluminium composite propellant.The results of droplet impacting on the wall will affect the characteristics of thermal protection of insulation layer,nozzle,the slag deposition in the back wall of embedded nozzle,and two-phase flow characteristic in the chamber.In order to ensure the safety and stability of the insulation layer and nozzle in motor design,and improving the accuracy of the simulation results in the internal flow field of chamber,it is necessary to reveal the mechanism of aluminum/aluminium droplets impacting on the wall and establish the model of high surface tension droplets-wall impingement.The molten aluminium droplets impactiong on the wall were studied by the experiment,numerical analysis and theoretical research in this paper.Based on the electromagnetic induction heating method and pulse gas pressure driving method,an experimental device for the molten aluminiumdroplet impacting on the wall was built,and an amplified light path system for shooting the impacting process of micro-aluminium droplet was established by shadowing method.The image processing software of droplet impingement was developed by the GUI based on Matlab,and the data of aluminium droplet impingement was extracted and analyzed.Based on VOF/PLIC method and the adaptive mesh technology,the open source program Gerris is used to simulate the process of aluminium droplet impacting on the wall,and the process of pressure,velocity and energy translation in droplet impacting on the wall is analyzed.Based on the physical mechanism of droplet impingement on the wall,the model of high surface tension droplet impacting on the wall was built,according to the experimental results and numerical analysis.Through this research,the main conclusions are as follows:?1?In the process of producing aluminum droplets based on pulse pneumatic driving method,when the the driving gas is given,the volume of the ejected droplets can be increased by increasing the pulse peak value and the pulse width,and a single aluminum droplet with larger particle size can be generated.When the working time of driving gas is given,the pulse width is determined.With the increase pressure of driving gas,the spraying states of droplets are non-droplet,single droplet and jet-flow respectively,and increasing the supply pressure can also improve the initial velocity of aluminum droplets from the nozzle.In order to generate a single small droplet with high-speed,it is necessary to minimize the pluse width and increase the pulse peak with a certain pulse waveform.The smallest size of aluminum droplet can reach 230?m,which was generated by the droplet generation device in this paper.?2?After aluminium droplet ejecting from the nozzle of graphite crucible,the larger the droplet size is,the slower the temperature decreases at the same initial temperature;the smaller the droplet size is,the faster the temperature decreases;when the initial temperature is1280K,the temperature of 200?m aluminium droplets in argon atmosphere decreases 15.6%after 100ms falling,while the temperature of the droplet is 1080K still higher than the melting point of aluminium when it is reaching the solid wall;the Bi number of the experimental aluminium droplets in argon atmosphere is much lower than 0.033 by the heat transfer theoretical calculation,so it can be considered that the temperature in the droplet is uniformly distributed during the falling process;the melted aluminium droplets generated by the system can meet the needs of the aluminium droplet-wall impingement experiment.?3?During the rebound of molten aluminium droplets impacting on the wall,the normal rebound coefficient e_n decreases with the increase of the normal incident velocity in the range of 0.63-2.51m/s,and the e_n varies from 0.49 to 0.81,and tangential rebound coefficient e_tmaintains between 0.74 and 0.92;the normal rebound coefficient increase with the increase of incident angle,and the normal rebound coefficient e_n increases from 0.6 to 0.8,when the incident angle?_iincreased from 0°to 50°,while the tangent rebound coefficient e_t basically keeps changing in a certain range;the normal rebound coefficient decreases with the increase of the incident size of aluminum droplet,the normal rebound coefficient e_n decreases from0.85 to 0.76,when the droplet size D₀ increases from 239?m to 950?m,while the tangential rebound e_t coefficient with little loss,and basically maintains between 0.85 and 0.96.?4?The bigger Oh number aluminum droplet?micron-size?is mainly rebounded from the wall,and the smaller Oh number aluminum droplet?millimeter-size?is mainly stick on the wall in the aluminum droplet-wall impingement experiment.With the increasing of the We number of aluminum droplet,the loss of kinetic energy and the viscous dissipation energy of droplets will increase during the process of droplet impacting on the wall,which resulting the decrease of the normal rebound coefficient of droplet,and it will eventually approach a certain value.The kinetic energy loss?E_kand normal kinetic energy loss?E_knof droplets are linearly related to D² of We numbered and normal We_ni number respectively.With the increasing of We numbere and We_ni numbered,the kinetic energy loss?E_k and normal kinetic energy loss?E_kn of droplets will keep increasing.?5?It will occur a thin gas film between the bottom and the wall,when the small and lower We aluminum droplet impacting on the wall.Meanwhile,during the impacting process,only the middle part of the droplet bottom breaks through the gas film and contacts the wall,while the other bottom edges of the droplet are separated on the wall by the thin gas film,which reduces the actual contact area between the droplet and the wall,and causing the droplet to retract and rebound from the wall.However,the gas film between the droplet and the wall will be extruded by the larger inertia and the wider spreading range in bigger droplet impacting on the wall,and unextruded gas would be wrapped to form a single bubble in the bottom of droplet,which increased the droplet-wall contact areas and viscous dissipation energy,so the large droplet is easy to stick on the wall.?6?In the droplet-wall impacting process,the higher surface tension coefficient,the larger the contact angle between droplet and wall.Comparing with the low surface tension coefficient of droplet impacting on the wall,the higher surface tension coefficient droplet would have smaller contact area with the wall,and the lower energy loss,which leading to the higher kinetic energy after retraction and easier to rebound from the wall.The critical K value of rebound and stick of aluminium droplet after impacting on the wall is0.85 4?1-cos?_c?Re,which obtained by the model of high surface tension coefficient of aluminium droplet impacting on the wall.The relationship between the rebound coefficient of aluminium droplet-wall impingment and the incident velocity,angle and size of the droplet is also established.