Convection Heat Transfer
Convection heat transfer involves the transfer of thermal energy from a surface to a fluid. Convection heat transfer can be divided into four distinct categories that describe distinct processes. These four categories are: 1) forced convection, 2) natural convection, 3) boiling, and 4) evaporation/condensation. The figure shows the fluid temperatures around a hot soldering iron. The heat is traveling upward due to buoyancy effects (hot air rises). If the air is moving purely due to buoyancy effects it is a natural convection problem. If the air is forced to move over the object it is a forced convection problem. The primary focus of this publication is on forced convection in which a fluid is forced to move across a surface by an external mechanism (i.e. fan, pump, etc.). Thermal energy transfer by forced convection is controlled by a thin layer of fluid close to the surface of an object. This thin layer is called the boundary layer. There are two types of boundary layers: 1) velocity, and 2) thermal. A significant portion of this publication deals with determining the velocity and temperature gradients in this thin layer. Three primary thermal energy transfer mechanisms exist in this thin layer: 1) conduction from the surface to the fluid, 2) conduction within the fluid, and 3) advection or transport of the energy downstream by the motion of the fluid. The equations that govern the flow in a boundary layer are based on an approximation to the continuity, momentum and energy equations. An analytical solution to the boundary layer equations exists for laminar flow over a flat plate. This solution, confirmed by experimental data, provides insight into the relationships between various non-dimensional parameters and serves as a basis for fitting data to form experimental correlations.
Theory associated with force convection heat transfer coefficients.