Table of Contents
What Is Water Viscosity?
Viscosity is a measure of a fluid's resistance to deformation under shear stress. Dynamic viscosity (mu) describes the internal friction of a fluid, while kinematic viscosity (nu) is the ratio of dynamic viscosity to density. Water viscosity decreases with temperature because thermal energy reduces intermolecular cohesive forces.
At 20°C, water has a dynamic viscosity of approximately 1.002 centipoise (cP) or 1.002 × 10&supmin;³ Pa·s. This is one of the fundamental reference values used in fluid mechanics, chemical engineering, and hydraulic design.
Empirical Formula
The Vogel equation approximates water viscosity, where A = 2.414 × 10&supmin;&sup5; Pa·s, B = 247.8 K, C = 140 K, and T is the absolute temperature in Kelvin. This correlation is accurate for temperatures between 0°C and 100°C.
Viscosity vs Temperature Table
| Temp (°C) | Dynamic (cP) | Kinematic (cSt) | Density (kg/m³) |
|---|---|---|---|
| 0 | 1.793 | 1.787 | 999.8 |
| 20 | 1.002 | 1.004 | 998.2 |
| 40 | 0.653 | 0.658 | 992.2 |
| 60 | 0.467 | 0.475 | 983.2 |
| 80 | 0.355 | 0.365 | 971.8 |
| 100 | 0.282 | 0.294 | 958.4 |
Applications
- Pipe flow design: Viscosity determines Reynolds number and whether flow is laminar or turbulent.
- Heat exchanger sizing: Viscosity affects convective heat transfer coefficients.
- Pump selection: Higher viscosity fluids require more powerful pumps.
- Chemical processes: Reaction rates in aqueous solutions depend on mass transport, which is governed by viscosity.
Frequently Asked Questions
How does pressure affect water viscosity?
For most engineering applications at moderate pressures (below 100 atm), pressure has negligible effect on water viscosity. At very high pressures (hundreds of MPa), viscosity increases slightly due to compression of molecular structure.
Why does viscosity decrease with temperature?
As temperature rises, water molecules gain kinetic energy, which weakens the hydrogen bonds holding them together. With less intermolecular cohesion, the fluid flows more easily, resulting in lower viscosity. This is the opposite of gases, where viscosity increases with temperature.
What is the difference between dynamic and kinematic viscosity?
Dynamic viscosity measures the force needed to move one layer of fluid over another. Kinematic viscosity divides dynamic viscosity by density, representing the fluid's resistance to flow under gravity. Kinematic viscosity is used in Reynolds number calculations.