4.1 Principle of convection
Till now we have understood and solved the problem where heat transfer coefficient was known. Now, we would emphasize on finding the convective heat transfer coefficient h. Finding out the heat transfer coefficient is not so easy in a given situation as it is a very complex term and depends in many physical properties of the fluid like, thermal conductivity, viscosity, density etc. Moreover, the flow field has a great impact on the convective heat transfer. The convective heat transfer requires a good knowledge of fluid dynamics, and we expect that the reader has adequate knowledge of that.

We have seen that it is easy to form a differential heat balance equations. However, for practical cases, it becomes tedious or impossible to solve the differential equations. Therefore, for practical situations, the heat transfer coefficient is calculated using the empirical correlations, which are developed over the years. Though these correlations have a good amount of error in most of the situations but these are indispensible for the estimation of heat transfer coefficients.

The convective heat transfer may be

• forced convection: in which the motion in the fluid medium is generated by the application of an external force, e.g. by a pump, blower, agitator etc.
• natural convection: in which the motion in the fluid is generated due to a result of density difference caused by the temperature difference.

Moreover, in many applications, heat is transferred between fluid streams without any phase change in the fluid. However, in some situations, the heat transfer is accompanied by phase change.
In this chapter we will consider the forced convection. Before we analyse the forced convection we need to know its physical mechanism.

4.2 Forced convection mechanism: flow over a flat horizontal plate
Consider the flow of a fluid over an immersed flat horizontal plate of very large width, as shown in fig. 4.1. The fig. shows that the flow is fully developed with no entrance losses. The bulk flow or free stream velocity is represented by v. Velocity of the fluid at the surface of the flat plate is zero at no-slip condition. The velocity of the fluid in the y-direction from the plate increases from zero to free stream velocity. The region above the plate surface within which this change of velocity from zero to the free stream value occurs is called the boundary layer. The thickness of this region is called the boundary layer thickness and is generally denoted by δ. It can be seen in the fig.4.1 that the boundary layer thickness increased with the distance x from the body edge. The boundary layer thickness, δ, usually is defined as the distance from the surface to the point where the velocity is within 1% of the free shear velocity. It should be noted that the y-axis of the fig. 4.1 is exaggerated greatly to have a clear picture.