7/31/2023 0 Comments Example of high viscosityHeat is typically a by-product of the stress created between particles. ![]() It is a phenomenon that occurs because of the differing velocities of the various streamlines. Is Shear Beneficial or Deleterious? Shear usually occurs at low Reynolds numbers. Polymeric fluids are the largest group in this class and may display the Weissenberg Effect, in which the fluid climbs up the shaft rotating in the fluid.įocusing on which of these characteristics fit your product profile, what conditions they create and how that may affect the mixing process can help you select the appropriate mixer. Viscoelastic fluids exhibit elastic recovery from deformations caused by the flow of a mixer blade.Examples include water suspensions of rock, grain or sewer sludge. Bingham plastic fluids, the simplest of all, require a minimum shear stress before any flow will occur. In time-independent systems, pseudoplastic products decrease in viscosity with increasing shear and dilatant products increase in viscosity with increasing shear.In time-dependent systems, thixotropic products decrease in viscosity and rheopectic products increase in viscosity at constant shear rates.Here are the characteristics of three types of non-Newtonian fluids you are likely to encounter. Non-Newtonian fluids typically have viscosities above 500 centipoise. ![]() What is the Rheological Behavior of the Product? Newtonian fluids have low, constant viscosity. Remember to carefully review the effectiveness of the whole mixing system, not just the performance of the mixer. Some of these factors include both the times and techniques of loading, mixing, cleaning, discharge, maintenance downtime, power consumption and mixer design (including vacuum, pressure and temperature control). That being said, consider the major factors in how uniformity is achieved and the consequences of achieving it. That uniformity (or lack thereof) can affect the reaction, stability, durability, performance and other parameters of the resultant product. What Are You Trying to Achieve? Uniformly blending two or more ingredients is usually the reason for mixing. With this in mind, here are a few pointers to help select the best viscous mixer for a given purpose. A low Reynolds number and a low velocity differential between streamlines further confirm this. Accordingly, the high energy levels imparted by the mixing blade dissipate within a very short distance, resulting in localized heat build-up and poor heat exchange. ![]() ![]() The mix tends to flow primarily in streamlines and stops flowing a short distance from the blade there is very little turbulent flow to promote mixing. In the viscous range, the formation of eddy currents is limited. Laminar impellers generally fill the entire mixing tank. Figure 3 shows various impeller-design configurations. But there is more to designing a proper high-viscosity mixing system.įirst of all, viscous mixing systems should use laminar impellers, not turbulent impellers. Figures 1 and 2 provide some general guidelines of which mixer types are usually used for various fluids and mixtures. One of the fascinating aspects of high-viscosity mixing is that the mixer must accurately and intensely move the product. Switching to Viscous Flow How then does one go about solving the problem of rapid mixing under viscous-flow conditions? And what insight does this provide that may help in selecting the proper high-viscosity mixer for a particular application?
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