TABLE OF CONTENTS Convective heat transfer occurs when a gas or liquid flows past a solid surface whose temperature is different from that of the fluid. Examples include an organifc heat-transfer fluid flowing inside a pipe whose wall is heated by electrical heating tape, and air flowing over the outside of a tube whose wall is chilled by evaporation of a refrigerant inside the tube. Two broad categories of convective heat transfer are distinguished, namely, forced convection and natural (or free) convection. In forced convection, the fluid motion is caused by an external agent such as a pump or blower. In natural convection, the fluid motion is the result of buoyancy forces created by temperature differences within the fluid.
In contrast to conductive heat transfer, convective heat-transfer problems are usually solved by means of empirical correlations derived from experimental data and dimensional analysis. The reason is that in order to solve a convection problem from first principles, one must solve the equations of fluid motion along with the energy balance equation. Although many important results have been obtained by solving the fundamental equations for convection problems in which the flow is laminar, no method has yet been devised to solve the turbulent flow equations entirely from first principles.
The empirical correlations are usually expressed in terms of a heat-transfer coefficient, h, which is defined by the relation:
In this equation, q is the rate of heat transfer between the solid surface and the fluid, A is the area...
