In diaphragm pump products, the ability to pump high-viscosity liquids is often mentioned. So, what is viscosity? How does it affect the performance of the pump, and what applications does it have in various fields of life? Today, Hapoin will help you understand this better in the following article.
What is Viscosity?
Viscosity, also known as the thickness of a liquid or the resistance to the flow of a liquid, is measured in English as Viscosity. Different types of liquids with different densities will have different viscosities; for example, water has low viscosity, while honey has high viscosity.
In chemistry, interactions or friction between different particles in a liquid produce viscosity. This means that if the molecules inside vibrate and rub against each other quickly, the attractive force between them increases, the repulsive force decreases, and the resulting high molecular friction causes the liquid to have high viscosity, and vice versa.
In the case of friction between solid particles, viscosity will determine the amount of energy needed to move the solid material inside the pump.
In the industrial pump industry, viscosity significantly affects pump selection and pump performance.
What is Viscosity? What is the viscosity of a liquid? (Photo: Internet)
Classification of Viscosity
When exploring what viscosity is, we find that there are two types of viscosity: dynamic viscosity (absolute viscosity) and kinematic viscosity.
What is the Difference Between Dynamic Viscosity and Kinematic Viscosity?
Here is a comparison table between dynamic viscosity and kinematic viscosity:
| Property | Dynamic Viscosity | Kinematic Viscosity |
|---|---|---|
| Definition | Measures the internal resistance of a fluid to flow. | Measures the fluid’s resistance to flow under the influence of gravity. |
| Units | Pascal-seconds (Pa·s) or Poise (P) | Square meters per second (m²/s) or centistokes (cSt) |
| Dependence on Density | Independent of fluid density. | Dependent on the fluid’s density. |
| Formula | τ = μ (du/dy) where τ is shear stress, μ is dynamic viscosity, and du/dy is the velocity gradient. | ν = μ/ρ where ν is kinematic viscosity, μ is dynamic viscosity, and ρ is fluid density. |
| Applications | Important in studying fluid flow in pipes, lubrication, and other processes where shear stress is a factor. | Used in applications such as oil flow in engines and other situations where gravity affects the fluid flow. |
| Measurement Methods | Typically measured using a viscometer or rheometer. | Measured using a capillary viscometer or other methods that account for fluid density. |
Understanding the differences and applications of these types of viscosities helps in selecting the right measurement techniques and equipment for various industrial and scientific applications.
Formula for Calculating Liquid Viscosity
Dynamic viscosity and kinematic viscosity are calculated using the following formulas:
η = k x p x t
V = k x t
Where:
- η: Dynamic viscosity (units: cP or mPas)
- V: Kinematic viscosity (units: cSt or mm²/s)
- k: Instrument constant
- p: Density of the test liquid (units: g/cm³)
- t: Flow time (units: s)
Alternatively, using the simplified formula often applied to calculate the flow rate of a liquid through a pipe or conduit:
η = (F/A) x (L/V)
Where:
- η: Viscosity of the liquid (units: Centipoise or Centistokes)
- F: Force applied (units: Newton or dyne)
- A: Cross-sectional area of the pipe (units: cm²)
- L: Length of the pipe (units: cm)
- V: Velocity of the liquid flow through the pipe (units: cm/s)
What is relative viscosity? Classification of viscosity (Photo: Internet)
Classification of Liquids and Their Viscosities
Liquids are categorized into two main types: Newtonian liquids and non-Newtonian liquids. Non-Newtonian liquids are further divided into thixotropic fluids and dilatant fluids.
Newtonian Liquids
These liquids do not change their viscosity regardless of deformation or stirring. Newtonian liquids have a linear relationship between viscosity and shear stress, with viscosity only affected by temperature. Common examples of Newtonian liquids are water and alcohol.
Non-Newtonian Liquids
- Thixotropic Fluids: Also known as shear-thinning fluids, these liquids decrease in viscosity when stirred or mixed. An example of a thixotropic fluid is paint.
- Dilatant Fluids: Also known as shear-thickening fluids, these liquids increase in viscosity when stirred. An example of a dilatant fluid is quicksand, which is a mixture of sand, water, and clay.
Viscosity Table for Common Liquids
| Parameter | Dynamic Viscosity (Độ nhớt động lực học) | Kinematic Viscosity (Độ nhớt động học) |
|---|---|---|
| Representation | Viscous force of the liquid | Both inertia and viscous force |
| Symbol | μ | v |
| Unit | Centipoise (Cps) | Centistokes (Cst) |
| Ratio | Shear stress to deformation | Dynamic viscosity to density |
| Density | Independent | Dependent |
| Also known as | Absolute viscosity | Diffusion of momentum |
Viscosity is measured in units of Centipoise (Cps), Centistokes (Cst), and Saybolt Second Universal (SSU). The calculations are as follows:
- SSU = cSt x 4.55 (When cSt > 50)
- cSt = cPs / SG (According to specific gravity)
Table of Absolute Viscosity for Some Substances at Room Temperature:
| Substance | Absolute Viscosity (Cps) | Kinematic Viscosity (Cst) | SSU |
|---|---|---|---|
| Water | 1.002 | 1.004 | 4.56 |
| Olive Oil | 81 | 84 | 382.2 |
| Glycerin | 1412 | 1437 | 6538.5 |
| Motor Oil (SAE 10W-30) | 75 | 80 | 364 |
(Note: These values are approximations and can vary slightly based on specific conditions and measurements.)
Impact of Viscosity
When selecting a pump, an essential factor to consider is the viscosity of the liquid to be pumped. Viscosity significantly affects the pump’s operation and performance. More concerning is the fact that the viscosity of a liquid can vary under different operating conditions and environments.
Theoretically, viscosity inversely affects the flow rate of the pump, causing the flow inside the pump to slow down or become sluggish. High-viscosity, sticky liquids will have a lower flow rate, whereas low-viscosity, less sticky liquids will have a higher flow rate.
Typically, centrifugal pumps are only suitable for low-viscosity liquids because their operation can deform the liquid. Therefore, when dealing with high-viscosity liquids, it is necessary to adjust the performance of the centrifugal pump to prevent liquid deformation. This can be managed by reducing pressure, decreasing flow rate and head, or increasing the pump’s power.
For efficient handling of viscous liquids, choosing a positive displacement pump is the most optimal method. Positive displacement pumps operate at lower speeds, causing less deformation to the liquid compared to centrifugal pumps. Thus, viscosity is a crucial factor in determining and selecting auxiliary pump equipment such as filters, pipelines, and valves.
Viscosity has an important influence in equipment selection (Photo: Internet)
Applications of Viscosity Measurement in Various Fields
Design of Motion Equipment (Pumps, Valves, Engines, Tractors, etc.)
The viscosity of a liquid is a crucial parameter that ensures efficient operation of equipment. By using the viscosity formula, one can estimate the frictional forces between moving surfaces within devices. This allows for more accurate evaluation of the equipment’s efficiency and longevity.
Quality Control of Products
The formula for calculating the viscosity of a liquid is essential for controlling the quality and consistency of products. It helps in maintaining product standards and ensuring uniformity in production.
Applications in Scientific Research
In the scientific field, this formula aids in evaluating the physical properties of liquids. It also helps determine the impact of chemical and physical factors on the viscosity of liquids, thus contributing to a deeper understanding of fluid dynamics.
The viscosity calculation formula is commonly applied in the process of diagnosing cardiovascular diseases (Photo: Internet)
Applications in Medicine
Measuring the viscosity of fluids, such as blood, is used to diagnose and treat cardiovascular diseases and strokes. This evaluation helps assess the impact of various factors on blood viscosity.
>>> Read more: Solder Paste Viscosity Measurement Devices
Conclusion
We have explored what viscosity is, how to calculate it, and its applications across various fields. It is evident that viscosity plays a crucial role in determining and selecting valves, pipe sizes, and pump engines. Understanding viscosity allows users to make accurate analyses and calculations for the processing and transportation of fluids.
Jasmine Wu – Hapoin
