Shear Rate

Understanding Shear Rate

shear rate refers to how fast layers of a liquid move past each other. Imagine water flowing through a pipe. Different layers of the water move at different speeds because of things like how thick the liquid is and how close it is to the sides of the pipe.

Think of a spot in the flowing water where nearby layers are moving at different speeds. When these layers rub against each other, they cause what's called shear, which is like when two things slide against each other, but really tiny in the liquid.

Shear rate is a number that shows how fast the liquid is moving compared to how far apart these layers are. If the shear rate is high, it means the liquid is getting really squished and stretched.

Knowing about shear rate is super important for engineers, especially in stuff like making things flow smoothly, working with chemicals, and building machines. With this knowledge, engineers can make things work better, prevent problems like turbulence or things getting stuck, and choose the right equipment for the liquid they're dealing with.

In factories and other places, understanding shear rate helps keep things running smoothly, make products consistently good, and keep machines from breaking. Engineers use shear rate to decide how to handle materials, mix stuff together, pump liquids, and more, making sure everything meets standards and works well.

Shear Rate and Pump Selection

The behavior of shear-sensitive liquids varies significantly under shear. Some liquids need shear to achieve the desired viscosity for transfer or application, while others may be temporarily or permanently damaged by shear. It's essential to understand the fluid's characteristics through discussions with the manufacturer or by conducting rheology tests to determine how the liquid behaves under shear over time.

From the moment the liquid leaves the vessel and enters the suction piping, shear begins as the fluid in the center of the pipe moves relative to the pipe walls. The pump itself introduces shear between its rotating elements and stationary casing. Calculating these shear rates involves using the shear rate equation in conjunction with parameters such as flow rate, pipe diameters, pump rotational speed, and internal pump dimensions.

Combining these calculations with the liquid’s rheology data helps in making informed decisions about pump selection and sizing. This approach ensures that the chosen pump meets the specific needs of the liquid being handled, maintaining optimal performance and preventing damage to shear-sensitive fluids.

Shear Rate of Different Fluids

Newtonian Fluids: Newtonian fluids are fluids that follow Newton's law of viscosity, meaning their viscosity remains constant regardless of the shear rate applied. In simpler terms, their resistance to flow doesn't change as you change how fast you're trying to make them flow. Examples include water and most gases.

1. Couette Flow: Shear rate = (2 * Velocity) / Gap distance
2. Capillary Flow: Shear rate = (Pressure * Radius) / (4 * Viscosity)
3. Parallel Plate Flow: Shear rate = (6 * Velocity) / (Gap distance)

Non-Newtonian Fluids: Non-Newtonian fluids, unlike Newtonian fluids, don't have a constant viscosity. Instead, their viscosity changes with the applied shear rate. This means they can behave differently under different conditions, such as becoming thicker or thinner when they're stirred or squeezed. Examples include ketchup, toothpaste, and blood.

1. Power-Law Fluid: Shear rate = (n * Stress) / (K * (n+1))
2. Bingham Plastic: Shear rate = (Stress - Yield Stress) / Viscosity
3. Herschel-Bulkley Fluid: Shear rate = [(Stress - Yield Stress) / (K * (n+1))]^(1/n)

Rotational Viscometry: Rotational viscometry is a technique used to measure the viscosity of fluids by applying shear stress using a rotating spindle or cone. By observing how the fluid responds to different shear rates, engineers can characterize its flow behavior and determine if it's Newtonian or non-Newtonian.

1. Cone-and-Plate: Shear rate = (Radius * Angular velocity) / Gap distance
2. Parallel-Disk: Shear rate = (Angular velocity * Radius) / Gap distance
3. Searle's Method: Shear rate = (6 * Angular velocity) / (π * Gap distance)

Pipe Flow: Pipe flow refers to the movement of fluids through pipes or conduits. Understanding shear rate in pipe flow is crucial for designing efficient piping systems, as it affects factors such as pressure drop, flow rate, and energy consumption.

1. Hagen–Poiseuille Equation: Shear rate = (8 * Velocity) / Diameter
2. Darcy–Weisbach Equation: Shear rate = (Velocity * Reynolds number) / (Diameter * Density)

Extrusion Processes: Extrusion processes involve forcing a material through a shaped die to create products with a specific cross-sectional profile. Shear rate plays a key role in extrusion, influencing factors like the quality and consistency of the extruded product, the required force to push the material through the die, and the cooling or curing of the material.

1. Ram Extrusion: Shear rate = (π * Radius * Speed) / (π * Radius²)
2. Screw Extrusion: Shear rate = (π * Radius * Speed) / (π * Radius² + Channel Width²)

Melt Spinning: Melt spinning is a manufacturing process used to produce fibers from melted polymers. Shear rate influences the fiber's properties, such as diameter, strength, and orientation. Controlling the shear rate during melt spinning is essential for achieving desired fiber characteristics and ensuring product quality.

1. Rotational Speed: Shear rate = (π * Spinneret Diameter * Rotational Speed) / (π * Spinneret Diameter²)
2. Linear Speed: Shear rate = (Spinneret Linear Speed) / Spinneret Diameter

Rheometers: Rheometers are instruments used to measure the flow and deformation properties of fluids and soft solids. They apply controlled stresses or strains to the material and measure the resulting response, including shear rate. Rheometers are invaluable tools in characterizing the flow behavior of complex fluids, such as polymer melts, food products, and cosmetics.

1. Cone-and-Plate Rheometer: Shear rate = (2 * Angular velocity * Radius) / Gap distance
2. Couette Rheometer: Shear rate = (Velocity of outer cylinder - Velocity of inner cylinder) / (Outer cylinder radius - Inner cylinder radius)

Polymer Processing: Polymer processing involves various techniques for shaping and forming polymer materials into desired products. Shear rate is a critical parameter in polymer processing, influencing processes such as injection molding, blow molding, and extrusion. Understanding the relationship between shear rate and polymer behavior is essential for optimizing processing conditions and product performance.

1. Injection Molding: Shear rate = (Flow rate) / (Channel Width * Channel Height)
2. Blow Molding: Shear rate = (Flow rate) / (Parison thickness * 2π * Parison Radius)
3. Compression Molding: Shear rate = (Plunger Speed) / (Area of the Melt Flow Path)

What is Good & Bad Shear Rate

Whether a shear rate is considered good or bad relies on the specific task at hand and how the fluid behaves in that situation. There's no one-size-fits-all answer. It's all about the context.

So, when you're looking at shear rates, think about what you're trying to accomplish. Are you aiming for smooth flow, or do you need the fluid to be thicker or thinner? Consider what the fluid is like and what you need it to do.

Ultimately, the goal is to make sure things run smoothly and efficiently, while still getting the results you want. So, when you're figuring out shear rates, keep in mind what's best for the job and what will help you reach your goals.