The Importance of Newtonian and Non-Newtonian Fluid Dynamics in Processing

All plant and processing personnel understand that maintaining product integrity is critical.Infood processing, handling can change the texture, taste, and appearance of product, and inpharmaceutical processing, maintaining delicate cell structure is vital. The behavior of foods,beveragesand pharmaceuticals under processing conditions is a key factor to consider.

In this post — written for plant personnel, engineers, quality specialists, R&D departments, and other processing nerds — we focus on products that respond differently to forces that come into play during processing. Those forces include system pressure and velocity and friction that occurs at pipe or tube surfaces. For example,pumpscreate pressure for moving fluids through systems, and that pressure acts as a force on the fluids.

Products fall into two categories –Newtonian fluids and Non-Newtonian fluids。我们在处理过程中讨论其行为以及与每个流体类型相关的常见处理问题。

一个关键的牛顿和Non-Newtoni之间的区别an fluids is their reaction to the forces they experience during processing.

运动中的流体发育由彼此移动的颗粒引起的剪切应力。For a fluid flowing in a pipe, fluid velocity at the pipe wall is nearly zero, but the velocity of fluids increases with distance from the pipe wall.

根据系统内的流体的粘度和温度，流体颗粒相对于管壁的相对运动更快或较慢。

Higher shear rate = less viscosity (thinner)

Some fluids become thinner as the rate of shear increases. These fluids are referred to as shear-thinning or thixotropic. Fluids that experience shear-thinning include:ketchup, lotions, and blood, for example.

Shear stress is caused by the flow of fluid across a surface.剪切应力的结果是管表面和管的中心的流体速度的差异。

剪切应变

Shear-thickening fluids act like solids under a fast-moving shear force. That’s why pumps running at high RPMs can generate fast-moving shear forces and high pressure causing an increase in the viscosity of product.Under high pressure, molecules inshear-thickeningproducts crowd each other and resist flow.

However, with lower pressure and lower velocity processes, molecules have time to get out of each other’s way, so their viscosity decreases allowing shear-thickening products to act like liquids because molecules don’t bunch up.

Shear-sensitive products such as ketchup, shampoos, and egg whites must behandled gentlythroughout the system to preserve product integrity.

While a slower RPM pump can prevent product from being affected inside pump casings, pump speed is only one variable in your product handling. Pipe diameter and product temperature also have a role.

Shear in pumps

Shear-sensitive liquids change viscosity when under stress or pressure.When flowing by the impeller inside a pump, some liquids become less viscous, a phenomenon called shear thinning. Other fluids become more viscous with increased force, which is called shear thickening or dilatant.

The distinctive feature of dilatants is that they get thicker in proportion to the shear forces they encounter.

通过泵搅拌水，一切都流过;通过泵搅拌玉米淀粉/水混合物，取决于泵产生的剪切应力的量，您的流体具有基本上更高的粘度。

关于摩擦损失，通过管道的流体同样地改变粘度，这取决于管壁附近产生的剪切应力的量，速度和体积在管壁附近减小，并在管道中心增加。

Newtonian liquids such as water do not change viscosity when force is applied. Non-Newtonian fluids such as ketchup change viscosity when force is applied.

Pump efficiency at the given duty point is an important factor in achieving gentle product handling.泵中的效率被测量为进入泵的流体量之间的比率和退出的量。

Factors such as velocity, energy, pump efficiency, and tube diameter all determine how product is handled. In some cases when product is sensitive to processing parameters, engineers can slow velocity by increasing line size and shortening distances to minimize pressure requirements.

The role of temperature

Temperature control is an important factor in maintaining optimum viscosity because product viscosity can vary significantly with temperature.

Some food sauces, for example, have low viscosity when heated but thicken to high viscosity when cooled. Similarly, heat honey, and it pours faster than when it cools down to room temperature.