Numerical Analysis Of Redispersing Process On Segregative Emulsified Asphalt Particles During Stirring

In recent years, owing to its simple construction, good workability and advantages of materials, emulsified asphalt in road engineering is constantly used in road construction. Before the commencement of work, emulsified asphalt must be guaranteed that exists in the state of stable and uniform emulsion, namely the asphalt in the storage tanks of finished emulsified asphalt is dispersed homogeneously with particles in the emulsifier aqueous solution. Usually there is a churn-dasher in the storage tank and stirring can make the emulsified asphalt in the storage tanks continuously move circularly to keep its steady and uniform state and avoid appearing lay-divided, ensuring its normal use of emulsified asphalt. But because of various factors, emulsified asphalt will inevitably have segregation.When it occurs dissociation in the emulsified asphalt storage tanks, this thesis will proceed a virtual experiment for researching the process of anew diffusing of sedimentary asphalt particles after the dissociation under the stir intervention. The study takes the bi-layer and three pitched turbine-type agitator used as an example, applying the Fluent14.0software, choosing the Multiple Reference Frames to dispose the agitator blade area and adopting numerical methods such as the Eulerian two-phase flow model and standard k-ε model, we proceed a simulation analysis to the stirring process. Through the simulated result we analyze the diffusing process of asphalt particles, the distribution regularities in the velocity field of storage tanks and the time of asphalt particles diffusing. Then, we improve the impeller, analyze the change of flow field in the agitator bath under the strengthening of stirring and calculate the impact of improving the agitator blade on the mixing time. The size of asphalt particles is between1u m-5u m usually in the manufacturing technique of emulsified asphalt. Therewith, we set the size of asphalt particles1μ m and5μ m to simulate as a comparison and analyze the impact of the size of asphalt particles on the mixing time. Compositing analysis on the different virtual experiment results:First, take the bi-layer and three pitched turbine-type agitator used in the study for example. When the span of impeller is half of the stirred tank’s diameter, agitator can also reach an ideal suspension effect in mixing process even rotating in low speed and calculation can suggest that the resultant velocity of fluid near the blade is the fastest. The farther it is away from the blade, the slower the velocity becomes gradually. Meanwhile, the liquid in the whole stirred tank shows a trend that the liquid in upper blade area moves circularly from up to down while that in lower blade area is just the reverse and the axial flow field drives the rise of asphalt particles which have been segregated and precipitated, forming the even emulsified asphalt mixture required. But the time needed for this mixed mode is about20minutes which is longer.Second, considering the limitation of the stirring speed, the paper will increase the length of stirring paddle’s blade from9cm to11cm, which can enlarge the contact area between the blade and emulsifier aqueous solution in the same rotating speed. It can be analyzed from the simulations that the emulsifier aqueous solution in the agitation tanks after lengthening the blade forms a higher flow velocity and four eddy current regions formed by the rotation of bi-layer impeller are more apparent and have wider range. Especially, the flow velocity of the liquid phase area driven by lower blade rotation is faster than that of the stirring of short blade and after the blade enhanced, circulation flow field also shortens the time of stir and mixing.Third, when the size of asphalt particle is changed to1μ m in virtual test, it can be found after simulation that the mixing time required takes longer than that when the particle size is5u m, needing to extend about200s’mixing time. When the precipitation amount of asphalt accounts for70%, the mixed time is longer than before from459s to642s; when that accounts for80%, mixed time is lengthened from552s to754s.Virtual test results will not only provide certain reference values for the design, optimization, improvement and enlarge of the impeller in emulsified asphalt storage tanks, but also for preventing serious segregation and needing stirring time to mix evenly in the process of emulsified asphalt storage, having a certain guiding significance.