Chapter 9, Problem 1CP

What is natural convection? How does it differ from forced convection? What force causes natural convection currents?

To determine

The meaning of natural convection, the difference between the natural and forced convections and the force causing natural convection currents.

Explanation of Solution

Introduction:

There are three modes of heat transfer:

1. Conduction
2. Convection
3. Radiation

The convection heat transfer is associated to heat transfer in fluids (liquid and gases) in which there is physical movement of fluid particles. On the basis offorce responsible for the physical movement of fluid particles, the convection is classified as natural and forced convection.

Natural Convection: The convection heat transfer process in which the bulk movement of fluid particles is observed due to density variations arising out of temperature difference is called the natural convection. The difference in temperature causes variation in different fluid layers, and this density variation facilitates movement of fluid particles without any aid of external force. There is no external force involved for the fluid motion.That is why it is referred as natural convection.

Forced Convection: When the movement of fluid particles is assisted by some external media such as fan, blower, pump, suction device, the heat transfer occurring in fluid is termed as forced convection. The forced convection results in better mixing and hence is faster and more efficient. The mild forces like buoyancy and gravity do not play any role in forced convection and the heat transfer process is dominated by the external force.

Forced convection caused by fan

The main difference between the natural and forced convection is the presence of external force. If the external force is present in heat transfer process, it is termed as forced convection, otherwise the convection heat transfer process is natural convection.

The force which play role in natural convection are buoyancy and gravity. The gravity becomes dominant in liquids which helps in producing the natural currents of heat transfer. The heated particles become less dense, they tend to move upward, and cold heavy particles move downward due to gravity, resulting in formation of natural currents. These natural currents are responsible for natural convection.