Free convection (also known as natural convection) is fluid motion induced by buoyancy forces. The key difference between this and forced convection is that the motion is not caused by any external forcing condition - for example a fan or a pump.
Free convection is induced when a body force acts on a fluid in which there are density gradients. The body force, in most common cases, will due to the gravitational field of the Earth. The density gradient is due to a temperature gradient throughout the fluid. The combination of these two effects creates the buoyancy force. In this revision guide, the focus will be on problems in which the density gradient is due to a temperature gradient and the body force is gravity.
A good example of free convection is heating a pan of water. If the water in the pan is assumed to be of uniform temperature initially, when heat is applied to the pan then a temperature gradient forms in the fluid. This generates a density gradient - due to the expansion of fluids with increasing temperature - and leads to convection currents due to the action of gravity. To put this more explicitly: when water gets warmer it becomes less dense and therefore gravity acts less strongly on these regions. This means that they rise to the top of the pan, while the cooler and denser regions of water sink to the bottom under a larger gravitational force. This effect is what generates free convection.
Free convection velocities and transfer coefficients are generally much lower than in forced convection. However, free convection should not be ignored and is a major design consideration as it often provides a large resistance to heat transfer.
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