(BQ) Part 2 book "Holman heat transfer" has contents: Natural convection systems, radiation heat transfer, condensation and boiling heat transfer, heat exchangers, mass transfer, mass transfer. | hol29362_Ch07 11/3/2008 15:0 C H A P T E R 7 7-1 Natural Convection Systems INTRODUCTION Our previous discussions of convection heat transfer have considered only the calculation of forced-convection systems where the fluid is forced by or through the heat-transfer surface. Natural, or free, convection is observed as a result of the motion of the fluid due to density changes arising from the heating process. A hot radiator used for heating a room is one example of a practical device that transfers heat by free convection. The movement of the fluid in free convection, whether it is a gas or a liquid, results from the buoyancy forces imposed on the fluid when its density in the proximity of the heat-transfer surface is decreased as a result of the heating process. The buoyancy forces would not be present if the fluid were not acted upon by some external force field such as gravity, although gravity is not the only type of force field that can produce the free-convection currents; a fluid enclosed in a rotating machine is acted upon by a centrifugal force field, and thus could experience free-convection currents if one or more of the surfaces in contact with the fluid were heated. The buoyancy forces that give rise to the free-convection currents are called body forces. 7-2 FREE-CONVECTION HEAT TRANSFER ON A VERTICAL FLAT PLATE Consider the vertical flat plate shown in Figure 7-1. When the plate is heated, a freeconvection boundary layer is formed, as shown. The velocity profile in this boundary layer is quite unlike the velocity profile in a forced-convection boundary layer. At the wall the velocity is zero because of the no-slip condition; it increases to some maximum value and then decreases to zero at the edge of the boundary layer since the “free-stream” conditions are at rest in the free-convection system. The initial boundary-layer development is laminar; but at some distance from the leading edge, depending on the fluid properties and the temperature difference .
