Natural convection

Definition

Natural convection is the transfer of heat through a fluid (liquid or gas) caused by molecular motion and density differences within the fluid. It occurs without any external forces like fans or pumps.

5 Must Know Facts For Your Next Test

1. Natural convection is driven by buoyancy forces that occur due to temperature-induced density variations in the fluid.
2. In natural convection, warmer, less dense fluid rises while cooler, denser fluid sinks.
3. The efficiency of natural convection depends on the temperature gradient and properties of the fluid such as viscosity and thermal conductivity.
4. Examples of natural convection include the circulation of air in the atmosphere and ocean currents.
5. The Rayleigh number ($Ra$) is a dimensionless number used to predict the onset of natural convection; it combines effects of buoyancy, thermal expansion, thermal diffusivity, and viscosity.

Review Questions

• What causes natural convection to occur in fluids?
• How does temperature variation influence natural convection?
• Why is the Rayleigh number important in studying natural convection?

Related terms

Thermal Conductivity: A physical property that describes how easily heat is conducted through a material.

Buoyancy: The upward force exerted by a fluid that opposes the weight of an object immersed in it.

Rayleigh Number: $$Ra$$ is a dimensionless number that characterizes the flow regime in natural convection; it is proportional to buoyancy forces and inversely proportional to viscous forces.