Jet Pulse Spray System Design And Uniform Droplets Genertation

Recently, spherical particles have become requisite in many applications, such as powder metallurgy, chemical engineering, pharmacy, electronics packaging and rapid prototyping. Traditional methods for generation of uniform droplets, including atomization, wire cutting-remelting, emulsification and uniform droplets spray, have not yet met satisfactorily the ever-increasingly strict requirements both in economic and technological aspects. A novel method called Jet Pulse Spray (JPS) for producing mono-sized droplets was then initiated. After establishing the JPS experimental set-up, the influence of the processing parameters on the droplet size and its distribution has been evaluated by using paraffin wax. The mathematic models to account for the mechanism of droplet generation during JPS process were put forward. It shows that JPS is promising in broad applications by virtue of narrow size distribution, simplicity, compactness, easy to control, low cost and high productivity.The principle of JPS process lies in that a continuous fluid stream is transformed into a rationed pulsed jet which stimulates to spray into mono-disperse droplets by employing a mechanism where the periodical switch-on between the nozzle and component with pore by rotating. The JPS process is divided into two working styles; the direct mode means that the melt is distributed directly through such a mechanism, while the indirect mode means to ration the melt by a gas pulse which is generated through the mechanism. A set of experimental JPS set-up was designed and established with a full consideration of the functions of components and assemblies, meeting the demands of the experiments quite well.In the direct JPS process, the droplet size dd decreased with the increase of rotation speed n and with the decrease of jetting velocity uj; the uniformity was enhanced by increasing the jetting velocity and decreasing the thickness H of the rotating membrane. A coupling model considered the rationed cutting, droplet growing and Taylor-Couette vortices has been developed. Using this coupling model, the predicted values showed good agreement with the experimental results than the individual models. The appropriate conditions in our tests for generation of narrow-sized droplets with an average diameter of about 520μm and a CV of 6.7% was found to be H=3 mm, n=350 rpm and uj=2.3~2.7 m/s.In the indirect JPS process, the higher jetting pressure and rotation speed enhanced the uniformity and productivity and extended the particles collection field. Two models based on gas pressure and flux were developed; the former gives more consistent results with the experiments than the latter. By analyzing the relationship between the fracture length and optimum disturbance in the fluid stream, it was found that the Weber instability model can successfully explain the phenomenon of the indirect JPS process. The existence of the pulsation of liquid jet and the periodicity of disturbance as well as the correlation between which and the size distribution were clearly demonstrated by high velocity video recording. Additionally, the trajectory path of flying droplets was estimated, useful for deciding the collection range. Under appropriate conditions (P=0.04 MPa and n=350 rpm), most uniform droplets with an average diameter of about 525μm and a CV of 3.3~4.2% can be generated by the indirect JPS. The indirect JPS process is even superior to the direct mode in size uniformity, materials flexibility and process stability resulted from its non-contact style of perturbation excitation, permitting great potential of applications.