Introduction to Convective Heat Transfer

12Free Convection Heat Transfer

12.1 Introduction

We have discussed forced convection heat transfer due to greater scope of its application in engineering. However, free convection is also an equally important means to transfer energy. For this reason, we will discuss its characteristics and present its analysis. Heat transfer resulting from a body force field, such as gravitational field, is called natural or free convection and will be studied in this chapter. The motion of the fluid in free convection is created by the buoyancy forces imposed on the fluid due to density variations. For example, density variations may be created between the fluid and the heated solid wall. There are two main characteristics of free convection flows:

1. (1) There is a strong coupling between heat transfer and flow.
2. (2) Buoyancy forces are in general so weak that the resulting velocities are small.

Typical examples of free convection heat transfer are: heat transfer from a radiator to heat the room through which hot water circulates, heat transfer from pipes, electric transformers, electric motors, electronic equipment, etc.

The accurate prediction of the Nusselt number is very important in engineering. Based on the analysis, we see that the Nusselt number Nu is a function of the Prandtl number Pr and the Rayleigh number Ra:

(12.1a)

The power law is often used to correlate the experimental data ...